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Contents
C++ Interview Questions ............................................................................................................................................5
C++ Object-Oriented Interview Questions And Answers .................................................................................... 35
C++ programming on UNIX ............................................................................................................................... 45
C++ Networking Interview Questions and Answers ............................................................................................ 47
C++ Algorithm Interview Questions and Answers .............................................................................................. 48
Java Interview Questions and Answers................................................................................................................... 51
JDBC Interview Questions and Answers .............................................................................................................. 144
Servlet Interview Questions and Answers ............................................................................................................. 229
Data Structures Interview Questions and Answers ............................................................................................... 234
Networking Interview Questions and Answers........................................................................................................ 245
Operating System Interview Questions and Answers ............................................................................................ 267
Linux Interview Questions and Answers............................................................................................................... 275
Unix Interview Questions and Answers................................................................................................................... 292
Manual Testing Interview Questions and Answers ............................................................................................... 314
Microprocessor Interview Questions and Answers .................................................................................................. 341
Oracle Interview Questions and Answers ............................................................................................................. 348
C Aptitude Questions ............................................................................................................................................. 517
Bibliography .......................................................................................................................................................... 610
Material referenced from :................................................................................................................................... 610
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 Preface
After the huge success of our E-guide : ‗The Complete Placement Guide‘, we
would now like to bring to you the Second Revised Edition of ‗The Complete
Placement Guide 2012‘. With the overwhelming response of the first edition (with
over 5000+ downloads) the demand for the revised edition was on high. A lot of
students had requested an update for the revised edition as the campus placements in
majority colleges will be commencing soon.
In our second edition, we have included a few subjects which was not covered in the
prior version, like – Servlets, Testing, Microprocessors, Linux and Unix based
Interview Questions. With our guides students would not require any more references
for technical preparation. The guide covers all possible subjects for Technical Aptitude
and interviews in campus placements.‘The Complete Placement Guide ‘ is a
complete book for technical preparation. The presentation of the contents of this guide
is in a lucid format and hence none of you would face any difficulty in understanding
the contents of this guide.
If you find any errors in our guide or you would like to give us any suggestion, we
would be glad to hear it from you. Please feel free to contact us on our forum or you
can directly mail to us at [email protected].
Finally, our team expresses our sincere gratitude to 5 main websites for referencing
the contents requires for this guide namely- www.techpreparation.com,
www.indiabix.com
,
www.stackoverflow.com,
www.roseindia.com
and
www.geekinterview.com .
Regards Team : IT Engg Portal
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C++
Interview
Questions
&
Answers
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C++ Interview Questions
What is C++?
Released in 1985, C++ is an object-oriented programming language created by Bjarne
Stroustrup. C++ maintains almost all aspects of the C language, while simplifying
memory management and adding several features - including a new datatype known as
a class (you will learn more about these later) - to allow object-oriented programming.
C++ maintains the features of C which allowed for low-level memory access but also
gives the programmer new tools to simplify memory management.
C++ used for:
C++ is a powerful general-purpose programming language. It can be used to create
small programs or large applications. It can be used to make CGI scripts or consoleonly DOS programs. C++ allows you to create programs to do almost anything you
need to do. The creator of C++, Bjarne Stroustrup, has put together a partial list of
applications written in C++.
How do you find out if a linked-list has an end? (i.e. the list is not a cycle)
You can find out by using 2 pointers. One of them goes 2 nodes each time. The second
one goes at 1 nodes each time. If there is a cycle, the one that goes 2 nodes each time
will eventually meet the one that goes slower. If that is the case, then you will know
the linked-list is a cycle.
What is the difference between realloc() and free()?
The free subroutine frees a block of memory previously allocated by the malloc
subroutine. Undefined results occur if the Pointer parameter is not a valid pointer. If
the Pointer parameter is a null value, no action will occur. The realloc subroutine
changes the size of the block of memory pointed to by the Pointer parameter to the
number of bytes specified by the Size parameter and returns a new pointer to the
block. The pointer specified by the Pointer parameter must have been created with the
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malloc, calloc, or realloc subroutines and not been deallocated with the free or realloc
subroutines. Undefined results occur if the Pointer parameter is not a valid pointer.
What is function overloading and operator overloading?
Function overloading: C++ enables several functions of the same name to be defined,
as long as these functions have different sets of parameters (at least as far as their
types are concerned). This capability is called function overloading. When an
overloaded function is called, the C++ compiler selects the proper function by
examining the number, types and order of the arguments in the call. Function
overloading is commonly used to create several functions of the same name that
perform
similar
tasks
but
on
different
data
types.
Operator overloading allows existing C++ operators to be redefined so that they work
on objects of user-defined classes. Overloaded operators are syntactic sugar for
equivalent function calls. They form a pleasant facade that doesn't add anything
fundamental to the language (but they can improve understandability and reduce
maintenance costs).
What is the difference between declaration and definition?
The declaration tells the compiler that at some later point we plan to present the
definition of this declaration.
E.g.: void stars () //function declaration
The definition contains the actual implementation.
E.g.: void stars () // declarator
{
for(int j=10; j > =0; j--) //function body
cout << *;
cout << endl; }
What are the advantages of inheritance?
It permits code reusability. Reusability saves time in program development. It
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encourages the reuse of proven and debugged high-quality software, thus reducing
problem after a system becomes functional.
How do you write a function that can reverse a linked-list?
void reverselist(void)
{
if(head==0)
return;
if(head->next==0)
return;
if(head->next==tail)
{
head->next = 0;
tail->next = head;
}
else
{
node* pre = head;
node* cur = head->next;
node* curnext = cur->next;
head->next = 0;
cur-> next = head;
for(; curnext!=0; )
{
cur->next = pre;
pre = cur;
cur = curnext;
curnext = curnext->next;
}
curnext->next = cur;
}
}
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What do you mean by inline function?
The idea behind inline functions is to insert the code of a called function at the point
where the function is called. If done carefully, this can improve the application's
performance in exchange for increased compile time and possibly (but not always) an
increase in the size of the generated binary executables.
Write a program that ask for user input from 5 to 9 then calculate the average
#include "iostream.h"
int main() {
int MAX = 4;
int total = 0;
int average;
int numb;
for (int i=0; i<MAX; i++) {
cout << "Please enter your input between 5 and 9: ";
cin >> numb;
while ( numb<5 || numb>9) {
cout << "Invalid input, please re-enter: ";
cin >> numb;
}
total = total + numb;
}
average = total/MAX;
cout << "The average number is: " << average << "\n";
return 0;
}
Write a short code using C++ to print out all odd number from 1 to 100 using a
for loop
for( unsigned int i = 1; i < = 100; i++ )
if( i & 0x00000001 )
cout << i << \",\";
What is public, protected, private?
Public, protected and private are three access specifier in C++.
Public data members and member functions are accessible outside the class.
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Protected data members and member functions are only available to derived classes.
Private data members and member functions can‘t be accessed outside the class.
However there is an exception can be using friend classes.
Write a function that swaps the values of two integers, using int* as the argument
type.
void swap(int* a, int*b) {
int t;
t = *a;
*a = *b;
*b = t;
}
Tell how to check whether a linked list is circular.
Create two pointers, each set to the start of the list. Update each as follows:
while (pointer1) {
pointer1 = pointer1->next;
pointer2 = pointer2->next; if (pointer2) pointer2=pointer2->next;
if (pointer1 == pointer2) {
print (\"circular\n\");
}
}
OK, why does this work?
If a list is circular, at some point pointer2 will wrap around and be either at the item
just before pointer1, or the item before that. Either way, it‘s either 1 or 2 jumps until
they meet.
What is virtual constructors/destructors?
Answer1
Virtual destructors:
If an object (with a non-virtual destructor) is destroyed explicitly by applying the
delete operator to a base-class pointer to the object, the base-class destructor function
(matching the pointer type) is called on the object.
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There is a simple solution to this problem declare a virtual base-class destructor.
This makes all derived-class destructors virtual even though they don‘t have the same
name as the base-class destructor. Now, if the object in the hierarchy is destroyed
explicitly by applying the delete operator to a base-class pointer to a derived-class
object, the destructor for the appropriate class is called. Virtual constructor:
Constructors cannot be virtual. Declaring a constructor as a virtual function is a syntax
error.
Answer2
Virtual destructors: If an object (with a non-virtual destructor) is destroyed explicitly
by applying the delete operator to a base-class pointer to the object, the base-class
destructor function (matching the pointer type) is called on the object.
There is a simple solution to this problem – declare a virtual base-class destructor.
This makes all derived-class destructors virtual even though they don‘t have the same
name as the base-class destructor. Now, if the object in the hierarchy is destroyed
explicitly by applying the delete operator to a base-class pointer to a derived-class
object, the destructor for the appropriate class is called.
Virtual constructor: Constructors cannot be virtual. Declaring a constructor as a
virtual function is a syntax error. Does c++ support multilevel and multiple
inheritance?
Yes.
What are the advantages of inheritance?
• It permits code reusability.
• Reusability saves time in program development.
• It encourages the reuse of proven and debugged high-quality software, thus reducing
problem after a system becomes functional.
What is the difference between declaration and definition?
The declaration tells the compiler that at some later point we plan to present the
definition of this declaration.
E.g.: void stars () //function declaration
The definition contains the actual implementation.
E.g.: void stars () // declarator
{
for(int j=10; j>=0; j--) //function body
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cout<<‖*‖;
cout<<endl; }
What is the difference between an ARRAY and a LIST?
Answer1
Array is collection of homogeneous elements.
List is collection of heterogeneous elements.
For Array memory allocated is static and continuous.
For List memory allocated is dynamic and Random.
Array: User need not have to keep in track of next memory allocation.
List: User has to keep in Track of next location where memory is allocated.
Answer2
Array uses direct access of stored members, list uses sequencial access for members.
//With Array you have direct access to memory position 5
Object x = a[5]; // x takes directly a reference to 5th element of array
//With the list you have to cross all previous nodes in order to get the 5th node:
list mylist;
list::iterator it;
for( it = list.begin() ; it != list.end() ; it++ )
{
if( i==5)
{
x = *it;
break;
}
i++;
}
Does c++ support multilevel and multiple inheritance?
Yes.
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What is a template?
Templates allow to create generic functions that admit any data type as parameters and
return value without having to overload the function with all the possible data types.
Until certain point they fulfill the functionality of a macro. Its prototype is any of the
two following ones:
template <class indetifier> function_declaration; template <typename indetifier>
function_declaration;
The only difference between both prototypes is the use of keyword class or typename,
its use is indistinct since both expressions have exactly the same meaning and behave
exactly the same way.
Define a constructor - What it is and how it might be called (2 methods).
Answer1
constructor is a member function of the class, with the name of the function being the
same as the class name. It also specifies how the object should be initialized.
Ways of calling constructor:
1) Implicitly: automatically by complier when an object is created.
2) Calling the constructors explicitly is possible, but it makes the code unverifiable.
Answer2
class Point2D{
int x; int y;
public Point2D() : x(0) , y(0) {} //default (no argument) constructor
};
main(){
Point2D MyPoint; // Implicit Constructor call. In order to allocate memory on stack,
the default constructor is implicitly called.
Point2D * pPoint = new Point2D(); // Explicit Constructor call. In order to allocate
memory on HEAP we call the default constructor.
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You have two pairs: new() and delete() and another pair : alloc() and free().
Explain differences between eg. new() and malloc()
Answer1
1.) ―new and delete‖ are preprocessors while ―malloc() and free()‖ are functions. [we
dont use brackets will calling new or delete].
2.) no need of allocate the memory while using ―new‖ but in ―malloc()‖ we have to
use ―sizeof()‖.
3.) ―new‖ will initlize the new memory to 0 but ―malloc()‖ gives random value in the
new alloted memory location [better to use calloc()]
Answer2
new() allocates continous space for the object instace
malloc() allocates distributed space.
new() is castless, meaning that allocates memory for this specific type,
malloc(), calloc() allocate space for void * that is cated to the specific class type
pointer.
What is the difference between class and structure?
Structure: Initially (in C) a structure was used to bundle different type of data types
together to perform a particular functionality. But C++ extended the structure to
contain functions also. The major difference is that all declarations inside a structure
are by default public.
Class: Class is a successor of Structure. By default all the members inside the class are
private.
What is RTTI?
Runtime type identification (RTTI) lets you find the dynamic type of an object when
you have only a pointer or a reference to the base type. RTTI is the official way in
standard C++ to discover the type of an object and to convert the type of a pointer or
reference (that is, dynamic typing). The need came from practical experience with
C++. RTTI replaces many Interview Questions - Homegrown versions with a solid,
consistent approach.
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What is encapsulation?
Packaging an object‘s variables within its methods is called encapsulation.
Explain term POLIMORPHISM and give an example using eg. SHAPE object: If
I have a base class SHAPE, how would I define DRAW methods for two objects
CIRCLE and SQUARE
Answer1
POLYMORPHISM : A phenomenon which enables an object to react differently to
the same function call.
in C++ it is attained by using a keyword virtual
Example
public class SHAPE
{
public virtual void SHAPE::DRAW()=0;
}
Note here the function DRAW() is pure virtual which means the sub classes must
implement the DRAW() method and SHAPE cannot be instatiated
public class CIRCLE::public SHAPE
{
public void CIRCLE::DRAW()
{
// TODO drawing circle
}
}
public class SQUARE::public SHAPE
{
public void SQUARE::DRAW()
{
// TODO drawing square
}
}
now from the user class the calls would be like
globally
SHAPE *newShape;
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When user action is to draw
public void MENU::OnClickDrawCircle(){
newShape = new CIRCLE();
}
public void MENU::OnClickDrawCircle(){
newShape = new SQUARE();
}
the when user actually draws
public void CANVAS::OnMouseOperations(){
newShape->DRAW();
}
Answer2
class SHAPE{
public virtual Draw() = 0; //abstract class with a pure virtual method
};
class CIRCLE{
public int r;
public virtual Draw() { this->drawCircle(0,0,r); }
};
class SQURE
public int a;
public virtual Draw() { this->drawRectangular(0,0,a,a); }
};
Each object is driven down from SHAPE implementing Draw() function in its own
way.
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What is an object?
Object is a software bundle of variables and related methods. Objects have state and
behavior.
How can you tell what shell you are running on UNIX system?
You can do the Echo $RANDOM. It will return a undefined variable if you are from
the C-Shell, just a return prompt if you are from the Bourne shell, and a 5 digit random
numbers if you are from the Korn shell. You could also do a ps -l and look for the
shell with the highest PID.
What do you mean by inheritance?
Inheritance is the process of creating new classes, called derived classes, from existing
classes or base classes. The derived class inherits all the capabilities of the base class,
but can add embellishments and refinements of its own.
Describe PRIVATE, PROTECTED and PUBLIC – the differences and give
examples.
class Point2D{
int x; int y;
public int color;
protected bool pinned;
public Point2D() : x(0) , y(0) {} //default (no argument) constructor
};
Point2D MyPoint;
You cannot directly access private data members when they are declared (implicitly)
private:
MyPoint.x = 5; // Compiler will issue a compile ERROR
//Nor yoy can see them:
int x_dim = MyPoint.x; // Compiler will issue a compile ERROR
On the other hand, you can assign and read the public data members:
MyPoint.color = 255; // no problem
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int col = MyPoint.color; // no problem
With protected data members you can read them but not write them: MyPoint.pinned =
true; // Compiler will issue a compile ERROR
bool isPinned = MyPoint.pinned; // no problem
What is namespace?
Namespaces allow us to group a set of global classes, objects and/or functions under a
name. To say it somehow, they serve to split the global scope in sub-scopes known as
namespaces.
The form to use namespaces is:
namespace identifier { namespace-body }
Where identifier is any valid identifier and namespace-body is the set of classes,
objects and functions that are included within the namespace. For example:
namespace general { int a, b; } In this case, a and b are normal variables integrated
within the general namespace. In order to access to these variables from outside the
namespace we have to use the scope operator ::. For example, to access the previous
variables we would have to put:
general::a general::b
The functionality of namespaces is specially useful in case that there is a possibility
that a global object or function can have the same name than another one, causing a
redefinition error.
What is a COPY CONSTRUCTOR and when is it called?
A copy constructor is a method that accepts an object of the same class and copies it‘s
data members to the object on the left part of assignement:
class Point2D{
int x; int y;
public int color;
protected bool pinned;
public Point2D() : x(0) , y(0) {} //default (no argument) constructor
public Point2D( const Point2D & ) ;
};
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Point2D::Point2D( const Point2D & p )
{
this->x = p.x;
this->y = p.y;
this->color = p.color;
this->pinned = p.pinned;
}
main(){
Point2D MyPoint;
MyPoint.color = 345;
Point2D AnotherPoint = Point2D( MyPoint ); // now AnotherPoint has color = 345
What is Boyce Codd Normal form?
A relation schema R is in BCNF with respect to a set F of functional dependencies if
for all functional dependencies in F+ of the form a-> , where a and b is a subset of R,
at least one of the following holds:
* a- > b is a trivial functional dependency (b is a subset of a)
* a is a superkey for schema R
What is virtual class and friend class?
Friend classes are used when two or more classes are designed to work together and
need access to each other's implementation in ways that the rest of the world shouldn't
be allowed to have. In other words, they help keep private things private. For instance,
it may be desirable for class DatabaseCursor to have more privilege to the internals of
class Database than main() has.
What is the word you will use when defining a function in base class to allow this
function to be a polimorphic function?
virtual
What do you mean by binding of data and functions?
Encapsulation.
What are 2 ways of exporting a function from a DLL?
1.Taking a reference to the function from the DLL instance.
2. Using the DLL ‘s Type Library
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What is the difference between an object and a class?
Classes and objects are separate but related concepts. Every object belongs to a class
and every class contains one or more related objects.
- A Class is static. All of the attributes of a class are fixed before, during, and after the
execution of a program. The attributes of a class don't change.
- The class to which an object belongs is also (usually) static. If a particular object
belongs to a certain class at the time that it is created then it almost certainly will still
belong to that class right up until the time that it is destroyed.
- An Object on the other hand has a limited lifespan. Objects are created and
eventually destroyed. Also during that lifetime, the attributes of the object may
undergo significant change.
Suppose that data is an array of 1000 integers. Write a single function call that
will sort the 100 elements data [222] through data [321].
quicksort ((data + 222), 100);
What is a class?
Class is a user-defined data type in C++. It can be created to solve a particular kind of
problem. After creation the user need not know the specifics of the working of a class.
What is friend function?
As the name suggests, the function acts as a friend to a class. As a friend of a class, it
can access its private and protected members. A friend function is not a member of the
class. But it must be listed in the class definition.
Which recursive sorting technique always makes recursive calls to sort subarrays
that are about half size of the original array?
Mergesort always makes recursive calls to sort subarrays that are about half size of the
original array, resulting in O(n log n) time.
What is abstraction?
Abstraction is of the process of hiding unwanted details from the user.
What are virtual functions?
A virtual function allows derived classes to replace the implementation provided by
the base class. The compiler makes sure the replacement is always called whenever the
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object in question is actually of the derived class, even if the object is accessed by a
base pointer rather than a derived pointer. This allows algorithms in the base class to
be replaced in the derived class, even if users don't know about the derived class.
What is the difference between an external iterator and an internal iterator?
Describe an advantage of an external iterator.
An internal iterator is implemented with member functions of the class that has items
to step through. .An external iterator is implemented as a separate class that can be
"attach" to the object that has items to step through. .An external iterator has the
advantage that many difference iterators can be active simultaneously on the same
object.
What is a scope resolution operator?
A scope resolution operator (::), can be used to define the member functions of a class
outside the class.
What do you mean by pure virtual functions?
A pure virtual member function is a member function that the base class forces derived
classes to provide. Normally these member functions have no implementation. Pure
virtual functions are equated to zero.
class Shape { public: virtual void draw() = 0; };
What is polymorphism? Explain with an example?
"Poly" means "many" and "morph" means "form". Polymorphism is the ability of an
object (or reference) to assume (be replaced by) or become many different forms of
object.
Example: function overloading, function overriding, virtual functions. Another
example can be a plus ‗+‘ sign, used for adding two integers or for using it to
concatenate two strings.
What‟s the output of the following program? Why?
#include <stdio.h>
main()
{
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typedef union
{
int a;
char b[10];
float c;
}
Union;
Union x,y = {100};
x.a = 50;
strcpy(x.b,\"hello\");
x.c = 21.50;
printf(\"Union x : %d %s %f \n\",x.a,x.b,x.c );
printf(\"Union y :%d %s%f \n\",y.a,y.b,y.c);
}
Given inputs X, Y, Z and operations | and & (meaning bitwise OR and AND,
respectively)
What is output equal to in
output = (X & Y) | (X & Z) | (Y & Z)
Why are arrays usually processed with for loop?
The real power of arrays comes from their facility of using an index variable to
traverse the array, accessing each element with the same expression a[i]. All the is
needed to make this work is a iterated statement in which the variable i serves as a
counter, incrementing from 0 to a.length -1. That is exactly what a loop does.
What is an HTML tag?
Answer: An HTML tag is a syntactical construct in the HTML language that
abbreviates specific instructions to be executed when the HTML script is loaded into a
Web browser. It is like a method in Java, a function in C++, a procedure in Pascal, or a
subroutine in FORTRAN.
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Explain which of the following declarations will compile and what will be
constant - a pointer or the value pointed at: * const char *
* char const *
* char * const
Note: Ask the candidate whether the first declaration is pointing to a string or a single
character. Both explanations are correct, but if he says that it‘s a single character
pointer, ask why a whole string is initialized as char* in C++. If he says this is a string
declaration, ask him to declare a pointer to a single character. Competent candidates
should not have problems pointing out why const char* can be both a character and a
string declaration, incompetent ones will come up with invalid reasons.
You‟re given a simple code for the class Bank Customer. Write the following
functions:
* Copy constructor
* = operator overload
* == operator overload
* + operator overload (customers‟ balances should be added up, as an example of
joint account between husband and wife)
Note:Anyone confusing assignment and equality operators should be dismissed from
the interview. The applicant might make a mistake of passing by value, not by
reference. The candidate might also want to return a pointer, not a new object, from
the addition operator. Slightly hint that you‘d like the value to be changed outside the
function, too, in the first case. Ask him whether the statement customer3 = customer1
+ customer2 would work in the second case.
What problems might the following macro bring to the application?
#define sq(x) x*x
Anything wrong with this code?
T *p = new T[10];
delete p;
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Everything is correct, Only the first element of the array will be deleted‖, The entire
array will be deleted, but only the first element destructor will be called.
Anything wrong with this code?
T *p = 0;
delete p;
Yes, the program will crash in an attempt to delete a null pointer.
How do you decide which integer type to use?
It depends on our requirement. When we are required an integer to be stored in 1 byte
(means less than or equal to 255) we use short int, for 2 bytes we use int, for 8 bytes
we use long int.
A char is for 1-byte integers, a short is for 2-byte integers, an int is generally a 2-byte
or 4-byte integer (though not necessarily), a long is a 4-byte integer, and a long long is
a 8-byte integer.
What does extern mean in a function declaration?
Using extern in a function declaration we can make a function such that it can used
outside the file in which it is defined.
An extern variable, function definition, or declaration also makes the described
variable or function usable by the succeeding part of the current source file. This
declaration does not replace the definition. The declaration is used to describe the
variable that is externally defined.
If a declaration for an identifier already exists at file scope, any extern declaration of
the same identifier found within a block refers to that same object. If no other
declaration for the identifier exists at file scope, the identifier has external linkage.
What can I safely assume about the initial values of variables which are not
explicitly initialized?
It depends on complier which may assign any garbage value to a variable if it is not
initialized.
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What is the difference between char a[] = “string”; and char *p = “string”;?
In the first case 6 bytes are allocated to the variable a which is fixed, where as in the
second case if *p is assigned to some other value the allocate memory can change.
What‟s the auto keyword good for?
Answer1
Not much. It declares an object with automatic storage duration. Which means the
object will be destroyed at the end of the objects scope. All variables in functions that
are not declared as static and not dynamically allocated have automatic storage
duration by default.
For example
int main()
{
int a; //this is the same as writing ―auto int a;‖
}
Answer2
Local variables occur within a scope; they are ―local‖ to a function. They are often
called automatic variables because they automatically come into being when the scope
is entered and automatically go away when the scope closes. The keyword auto makes
this explicit, but local variables default to auto auto auto auto so it is never necessary
to declare something as an auto auto auto auto.
What is the difference between char a[] = “string”; and char *p = “string”; ?
Answer1
a[] = ―string‖;
char *p = ―string‖;
The difference is this:
p is pointing to a constant string, you can never safely say
p[3]=‘x';
however you can always say a[3]=‘x';
char a[]=‖string‖; - character array initialization.
char *p=‖string‖ ; - non-const pointer to a const-string.( this is permitted only in the
case of char pointer in C++ to preserve backward compatibility with C.)
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Answer2
a[] = ―string‖;
char *p = ―string‖;
a[] will have 7 bytes. However, p is only 4 bytes. P is pointing to an adress is either
BSS or the data section (depending on which compiler — GNU for the former and CC
for the latter).
Answer3
char a[] = ―string‖;
char *p = ―string‖;
for char a[]…….using the array notation 7 bytes of storage in the static memory block
are taken up, one for each character and one for the terminating nul character.
But, in the pointer notation char *p………….the same 7 bytes required, plus N bytes
to store the pointer variable ―p‖ (where N depends on the system but is usually a
minimum of 2 bytes and can be 4 or more)……
How do I declare an array of N pointers to functions returning pointers to
functions returning pointers to characters?
Answer1
If you want the code to be even slightly readable, you will use typedefs.
typedef char* (*functiontype_one)(void);
typedef functiontype_one (*functiontype_two)(void);
functiontype_two myarray[N]; //assuming N is a const integral
Answer2 - pto
char* (* (*a[N])())()
Here a is that array. And according to question no function will not take any parameter
value.
What does extern mean in a function declaration?
It tells the compiler that a variable or a function exists, even if the compiler hasn‘t yet
seen it in the file currently being compiled. This variable or function may be defined in
another file or further down in the current file.
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How do I initialize a pointer to a function?
This is the way to initialize a pointer to a function
void fun(int a)
{
}
void main()
{
void (*fp)(int);
fp=fun;
fp(1);
}
How do you link a C++ program to C functions?
By using the extern "C" linkage specification around the C function declarations.
Explain the scope resolution operator.
It permits a program to reference an identifier in the global scope that has been hidden
by another identifier with the same name in the local scope.
What are the differences between a C++ struct and C++ class?
The default member and base-class access specifier are different.
How many ways are there to initialize an int with a constant?
Two.
There are two formats for initializers in C++ as shown in the example that follows.
The first format uses the traditional C notation. The second format uses constructor
notation.
int foo = 123;
int bar (123);
How does throwing and catching exceptions differ from using setjmp and
longjmp?
The throw operation calls the destructors for automatic objects instantiated since entry
to the try block.
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What is a default constructor?
Default constructor WITH arguments class B { public: B (int m = 0) : n (m) {} int n;
}; int main(int argc, char *argv[]) { B b; return 0; }
What is a conversion constructor?
A constructor that accepts one argument of a different type.
What is the difference between a copy constructor and an overloaded assignment
operator?
A copy constructor constructs a new object by using the content of the argument
object. An overloaded assignment operator assigns the contents of an existing object to
another existing object of the same class.
When should you use multiple inheritance?
There are three acceptable answers: "Never," "Rarely," and "When the problem
domain cannot be accurately modeled any other way."
Explain the ISA and HASA class relationships. How would you implement each
in a class design?
A specialized class "is" a specialization of another class and, therefore, has the ISA
relationship with the other class. An Employee ISA Person. This relationship is best
implemented with inheritance. Employee is derived from Person. A class may have an
instance of another class. For example, an employee "has" a salary, therefore the
Employee class has the HASA relationship with the Salary class. This relationship is
best implemented by embedding an object of the Salary class in the Employee class.
When is a template a better solution than a base class?
When you are designing a generic class to contain or otherwise manage objects of
other types, when the format and behavior of those other types are unimportant to their
containment or management, and particularly when those other types are unknown
(thus, the generosity) to the designer of the container or manager class.
What is a mutable member?
One that can be modified by the class even when the object of the class or the member
function doing the modification is const.
What is an explicit constructor?
A conversion constructor declared with the explicit keyword. The compiler does not
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use an explicit constructor to implement an implied conversion of types. It‘s purpose is
reserved explicitly for construction.
What is the Standard Template Library (STL)?
A library of container templates approved by the ANSI committee for inclusion in the
standard C++ specification.
A programmer who then launches into a discussion of the generic programming
model, iterators, allocators, algorithms, and such, has a higher than average
understanding of the new technology that STL brings to C++ programming.
Describe run-time type identification.
The ability to determine at run time the type of an object by using the typeid operator
or the dynamic_cast operator.
What problem does the namespace feature solve?
Multiple providers of libraries might use common global identifiers causing a name
collision when an application tries to link with two or more such libraries. The
namespace feature surrounds a library‘s external declarations with a unique namespace
that eliminates the potential for those collisions.
This solution assumes that two library vendors don‘t use the same namespace
identifier, of course.
Are there any new intrinsic (built-in) data types?
Yes. The ANSI committee added the bool intrinsic type and its true and false value
keywords.
Will the following program execute?
void main()
{
void *vptr = (void *) malloc(sizeof(void));
vptr++;
}
Answer1
It will throw an error, as arithmetic operations cannot be performed on void pointers.
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Answer2
It will not build as sizeof cannot be applied to void* ( error ―Unknown size‖ )
Answer3
How can it execute if it won‘t even compile? It needs to be int main, not void main.
Also, cannot increment a void *.
Answer4
According to gcc compiler it won‘t show any error, simply it executes. but in general
we can‘t do arthematic operation on void, and gives size of void as 1
Answer5
The program compiles in GNU C while giving a warning for ―void main‖. The
program runs without a crash. sizeof(void) is ―1? hence when vptr++, the address is
incremented by 1.
Answer6
Regarding arguments about GCC, be aware that this is a C++ question, not C. So gcc
will compile and execute, g++ cannot. g++ complains that the return type cannot be
void and the argument of sizeof() cannot be void. It also reports that ISO C++ forbids
incrementing a pointer of type ‗void*‘.
Answer7
in C++
voidp.c: In function `int main()‘:
voidp.c:4: error: invalid application of `sizeof‘ to a void type
voidp.c:4: error: `malloc‘ undeclared (first use this function)
voidp.c:4: error: (Each undeclared identifier is reported only once for each function it
appears in.)
voidp.c:6: error: ISO C++ forbids incrementing a pointer of type `void*‘
But in c, it work without problems
void main()
{
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char *cptr = 0?2000;
long *lptr = 0?2000;
cptr++;
lptr++;
printf(” %x %x”, cptr, lptr);
}
Will it execute or not?
Answer1
For Q2: As above, won‘t compile because main must return int. Also, 0×2000 cannot
be implicitly converted to a pointer (I assume you meant 0×2000 and not 0?2000.)
Answer2
Not Excute.
Compile with VC7 results following errors:
error C2440: ‗initializing‘ : cannot convert from ‗int‘ to ‗char *‘
error C2440: ‗initializing‘ : cannot convert from ‗int‘ to ‗long *‘
Not Excute if it is C++, but Excute in C.
The printout:
2001 2004
Answer3
In C++
[$]> g++ point.c
point.c: In function `int main()‘:
point.c:4: error: invalid conversion from `int‘ to `char*‘
point.c:5: error: invalid conversion from `int‘ to `long int*‘
in C
———————————–
[$] etc > gcc point.c
point.c: In function `main‘:
point.c:4: warning: initialization makes pointer from integer without a cast
point.c:5: warning: initialization makes pointer from integer without a cast
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[$] etc > ./a.exe
2001 2004
What is the difference between Mutex and Binary semaphore?
semaphore is used to synchronize processes. where as mutex is used to provide
synchronization between threads running in the same process.
In C++, what is the difference between method overloading and method
overriding?
Overloading a method (or function) in C++ is the ability for functions of the same
name to be defined as long as these methods have different signatures (different set of
parameters). Method overriding is the ability of the inherited class rewriting the virtual
method of the base class.
What methods can be overridden in Java?
In C++ terminology, all public methods in Java are virtual. Therefore, all Java
methods can be overwritten in subclasses except those that are declared final, static,
and private.
What are the defining traits of an object-oriented language?
The defining traits of an object-oriented langauge are:
* encapsulation
* inheritance
* polymorphism
Write a program that ask for user input from 5 to 9 then calculate the average
int main()
{
int MAX=4;
int total =0;
int average=0;
int numb;
cout<<"Please enter your input from 5 to 9";
cin>>numb;
if((numb <5)&&(numb>9))
cout<<"please re type your input";
else
for(i=0;i<=MAX; i++)
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{
total = total + numb;
average= total /MAX;
}
cout<<"The average number is"<<average<<endl;
return 0;
}
Assignment Operator - What is the diffrence between a "assignment operator"
and a "copy constructor"?
Answer1.
In assignment operator, you are assigning a value to an existing object. But in copy
constructor, you are creating a new object and then assigning a value to that object.
For example:
complex c1,c2;
c1=c2; //this is assignment
complex c3=c2; //copy constructor
Answer2.
A copy constructor is used to initialize a newly declared variable from an existing
variable. This makes a deep copy like assignment, but it is somewhat simpler:
There is no need to test to see if it is being initialized from itself.
There is no need to clean up (eg, delete) an existing value (there is none).
A reference to itself is not returned.
RTTI - What is RTTI?
Answer1.
RTTI stands for "Run Time Type Identification". In an inheritance hierarchy, we can
find out the exact type of the objet of which it is member. It can be done by using:
1) dynamic id operator
2) typecast operator
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Answer2.
RTTI is defined as follows: Run Time Type Information, a facility that allows an
object to be queried at runtime to determine its type. One of the fundamental
principles of object technology is polymorphism, which is the ability of an object to
dynamically change at runtime.
STL Containers - What are the types of STL containers?
There are 3 types of STL containers:
1. Adaptive containers like queue, stack
2. Associative containers like set, map
3. Sequence containers like vector, deque
What is the need for a Virtual Destructor ?
Destructors are declared as virtual because if do not declare it as virtual the base class
destructor will be called before the derived class destructor and that will lead to
memory leak because derived class’s objects will not get freed.Destructors are
declared virtual so as to bind objects to the methods at runtime so that appropriate
destructor is called.
What is "mutable"?
Answer1.
"mutable" is a C++ keyword. When we declare const, none of its data members can
change. When we want one of its members to change, we declare it as mutable.
Answer2.
A "mutable" keyword is useful when we want to force a "logical const" data member
to have its value modified. A logical const can happen when we declare a data member
as non-const, but we have a const member function attempting to modify that data
member. For example:
class Dummy {
public:
bool isValid() const;
private:
mutable int size_ = 0;
mutable bool validStatus_ = FALSE;
// logical const issue resolved
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};
bool Dummy::isValid() const
// data members become bitwise const
{
if (size > 10) {
validStatus_ = TRUE; // fine to assign
size = 0; // fine to assign
}
}
Answer2.
"mutable" keyword in C++ is used to specify that the member may be updated or
modified even if it is member of constant object. Example:
class Animal {
private:
string name;
string food;
mutable int age;
public:
void set_age(int a);
};
void main() {
const Animal Tiger(’Fulffy’,'antelope’,1);
Tiger.set_age(2);
// the age can be changed since its mutable
}
Differences of C and C++
Could you write a small program that will compile in C but not in C++ ?
In C, if you can a const variable e.g.
const int i = 2;
you can use this variable in other module as follows
extern const int i;
C compiler will not complain.
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But for C++ compiler u must write
extern const int i = 2;
else error would be generated.
Bitwise Operations - Given inputs X, Y, Z and operations | and & (meaning
bitwise OR and AND, respectively), what is output equal to in?
output = (X & Y) | (X & Z) | (Y & Z);
C++ Object-Oriented Interview Questions And Answers
What is a modifier?
A modifier, also called a modifying function is a member function that changes the
value of at least one data member. In other words, an operation that modifies the state
of an object. Modifiers are also known as ‗mutators‘. Example: The function mod is a
modifier in the following code snippet:
class test
{
int x,y;
public:
test()
{
x=0; y=0;
}
void mod()
{
x=10;
y=15;
}
};
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What is an accessor?
An accessor is a class operation that does not modify the state of an object. The
accessor functions need to be declared as const operations
Differentiate between a template class and class template.
Template class: A generic definition or a parameterized class not instantiated until the
client provides the needed information. It‘s jargon for plain templates. Class template:
A class template specifies how individual classes can be constructed much like the
way a class specifies how individual objects can be constructed. It‘s jargon for plain
classes.
When does a name clash occur?
A name clash occurs when a name is defined in more than one place. For example.,
two different class libraries could give two different classes the same name. If you try
to use many class libraries at the same time, there is a fair chance that you will be
unable to compile or link the program because of name clashes.
Define namespace.
It is a feature in C++ to minimize name collisions in the global name space. This
namespace keyword assigns a distinct name to a library that allows other libraries to
use the same identifier names without creating any name collisions. Furthermore, the
compiler uses the namespace signature for differentiating the definitions.
What is the use of „using‟ declaration. ?
A using declaration makes it possible to use a name from a namespace without the
scope operator.
What is an Iterator class ?
A class that is used to traverse through the objects maintained by a container class.
There are five categories of iterators: input iterators, output iterators, forward iterators,
bidirectional iterators, random access. An iterator is an entity that gives access to the
contents of a container object without violating encapsulation constraints. Access to
the contents is granted on a one-at-a-time basis in order. The order can be storage
order (as in lists and queues) or some arbitrary order (as in array indices) or according
to some ordering relation (as in an ordered binary tree). The iterator is a construct,
which provides an interface that, when called, yields either the next element in the
container, or some value denoting the fact that there are no more elements to examine.
Iterators hide the details of access to and update of the elements of a container class.
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The simplest and safest iterators are those that permit read-only access to the contents
of a container class.
What is an incomplete type?
Incomplete types refers to pointers in which there is non availability of the
implementation of the referenced location or it points to some location whose value is
not available for modification.
int *i=0x400 // i points to address 400
*i=0; //set the value of memory location pointed by i.
Incomplete types are otherwise called uninitialized pointers.
What is a dangling pointer?
A dangling pointer arises when you use the address of an object after
its lifetime is over. This may occur in situations like returning
addresses of the automatic variables from a function or using the
address of the memory block after it is freed. The following
code snippet shows this:
class Sample
{
public:
int *ptr;
Sample(int i)
{
ptr = new int(i);
}
~Sample()
{
delete ptr;
}
void PrintVal()
{
cout << "The value is " << *ptr;
}
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};
void SomeFunc(Sample x)
{
cout << "Say i am in someFunc " << endl;
}
int main()
{
Sample s1 = 10;
SomeFunc(s1);
s1.PrintVal();
}
In the above example when PrintVal() function is
called it is called by the pointer that has been freed by the
destructor in SomeFunc.
Differentiate between the message and method.
Message:
* Objects communicate by sending messages to each other.
* A message is sent to invoke a method.
Method
* Provides response to a message.
* It is an implementation of an operation.
What is an adaptor class or Wrapper class?
A class that has no functionality of its own. Its member functions hide the use of a
third party software component or an object with the non-compatible interface or a
non-object-oriented implementation.
What is a Null object?
It is an object of some class whose purpose is to indicate that a real object of that class
does not exist. One common use for a null object is a return value from a member
function that is supposed to return an object with some specified properties but cannot
find such an object.
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What is class invariant?
A class invariant is a condition that defines all valid states for an object. It is a logical
condition to ensure the correct working of a class. Class invariants must hold when an
object is created, and they must be preserved under all operations of the class. In
particular all class invariants are both preconditions and post-conditions for all
operations or member functions of the class.
What do you mean by Stack unwinding?
It is a process during exception handling when the destructor is called for all local
objects between the place where the exception was thrown and where it is caught.
Define precondition and post-condition to a member function.
Precondition: A precondition is a condition that must be true on entry to a member
function. A class is used correctly if preconditions are never false. An operation is not
responsible for doing anything sensible if its precondition fails to hold. For example,
the interface invariants of stack class say nothing about pushing yet another element
on a stack that is already full. We say that isful() is a precondition of the push
operation. Post-condition: A post-condition is a condition that must be true on exit
from a member function if the precondition was valid on entry to that function. A class
is implemented correctly if post-conditions are never false. For example, after pushing
an element on the stack, we know that isempty() must necessarily hold. This is a postcondition of the push operation.
What are the conditions that have to be met for a condition to be an invariant of
the class?
* The condition should hold at the end of every constructor.
* The condition should hold at the end of every mutator (non-const) operation.
What are proxy objects?
Objects that stand for other objects are called proxy objects or surrogates.
template <class t="">
class Array2D
{
public:
class Array1D
{
public:
T& operator[] (int index);
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const T& operator[] (int index)const;
};
Array1D operator[] (int index);
const Array1D operator[] (int index) const;
};
The following then becomes legal:
Array2D<float>data(10,20);
cout<<data[3][6]; // fine
Here data[3] yields an Array1D object and the operator [] invocation on that object
yields the float in position(3,6) of the original two dimensional array. Clients of the
Array2D class need not be aware of the presence of the Array1D class. Objects of this
latter class stand for one-dimensional array objects that, conceptually, do not exist for
clients of Array2D. Such clients program as if they were using real, live, twodimensional arrays. Each Array1D object stands for a one-dimensional array that is
absent from a conceptual model used by the clients of Array2D. In the above example,
Array1D is a proxy class. Its instances stand for one-dimensional arrays that,
conceptually, do not exist.
Name some pure object oriented languages.
Smalltalk, Java, Eiffel, Sather.
What is an orthogonal base class?
If two base classes have no overlapping methods or data they are said to be
independent of, or orthogonal to each other. Orthogonal in the sense means that two
classes operate in different dimensions and do not interfere with each other in any
way. The same derived class may inherit such classes with no difficulty.
What is a node class?
A node class is a class that,
* relies on the base class for services and implementation,
* provides a wider interface to the users than its base class,
* relies primarily on virtual functions in its public interface
* depends on all its direct and indirect base class
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* can be understood only in the context of the base class
* can be used as base for further derivation
* can be used to create objects.
A node class is a class that has added new services or functionality beyond the
services inherited from its base class.
What is a container class? What are the types of container classes?
A container class is a class that is used to hold objects in memory or external storage.
A container class acts as a generic holder. A container class has a predefined behavior
and a well-known interface. A container class is a supporting class whose purpose is to
hide the topology used for maintaining the list of objects in memory. When a container
class contains a group of mixed objects, the container is called a heterogeneous
container; when the container is holding a group of objects that are all the same, the
container is called a homogeneous container.
How do you write a function that can reverse a linked-list?
Answer1:
void reverselist(void)
{
if(head==0)
return;
if(head-<next==0)
return;
if(head-<next==tail)
{
head-<next = 0;
tail-<next = head;
}
else
{
node* pre = head;
node* cur = head-<next;
node* curnext = cur-<next;
head-<next = 0;
cur-<next = head;
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for(; curnext!=0; )
{
cur-<next = pre;
pre = cur;
cur = curnext;
curnext = curnext-<next;
}
curnext-<next = cur;
}
}
Answer2:
node* reverselist(node* head)
{
if(0==head || 0==head->next)
//if head->next ==0 should return head instead of 0;
return 0;
{
node* prev = head;
node* curr = head->next;
node* next = curr->next;
for(; next!=0; )
{
curr->next = prev;
prev = curr;
curr = next;
next = next->next;
}
curr->next = prev;
head->next = 0;
head = curr;
}
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return head;
}
What is polymorphism?
Polymorphism is the idea that a base class can be inherited by several classes. A base
class pointer can point to its child class and a base class array can store different child
class objects.
How do you find out if a linked-list has an end? (i.e. the list is not a cycle)
You can find out by using 2 pointers. One of them goes 2 nodes each time. The second
one goes at 1 nodes each time. If there is a cycle, the one that goes 2 nodes each time
will eventually meet the one that goes slower. If that is the case, then you will know
the linked-list is a cycle.
How can you tell what shell you are running on UNIX system?
You can do the Echo $RANDOM. It will return a undefined variable if you are from
the C-Shell, just a return prompt if you are from the Bourne shell, and a 5 digit random
numbers if you are from the Korn shell. You could also do a ps -l and look for the
shell with the highest PID.
What is Boyce Codd Normal form?
A relation schema R is in BCNF with respect to a set F of functional dependencies if
for all functional dependencies in F+ of the form a->b, where a and b is a subset of R,
at least one of the following holds:
* a->b is a trivial functional dependency (b is a subset of a)
* a is a superkey for schema R
What is pure virtual function?
A class is made abstract by declaring one or more of its virtual functions to be pure. A
pure virtual function is one with an initializer of = 0 in its declaration
Write a Struct Time where integer m, h, s are its members
struct Time
{
int m;
int h;
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43
int s;
};
How do you traverse a Btree in Backward in-order?
Process the node in the right subtree
Process the root
Process the node in the left subtree
What is the two main roles of Operating System?
As a resource manager
As a virtual machine
In the derived class, which data member of the base class are visible?
In the public and protected sections.
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C++ programming on UNIX
Could you tell something about the Unix System Kernel?
The kernel is the heart of the UNIX openrating system, it‘s reponsible for controlling
the computer‘s resouces and scheduling user jobs so that each one gets its fair share of
resources.
What are each of the standard files and what are they normally associated with?
They are the standard input file, the standard output file and the standard error file.
The first is usually associated with the keyboard, the second and third are usually
associated with the terminal screen.
Detemine the code below, tell me exectly how many times is the operation sum++
performed ?
for ( i = 0; i < 100; i++ )
for ( j = 100; j > 100 - i; j–)
sum++;
(99 * 100)/2 = 4950
The sum++ is performed 4950 times.
Give 4 examples which belongs application layer in TCP/IP architecture?
FTP, TELNET, HTTP and TFTP
What‟s the meaning of ARP in TCP/IP?
The "ARP" stands for Address Resolution Protocol. The ARP standard defines two
basic message types: a request and a response. a request message contains an IP
address and requests the corresponding hardware address; a replay contains both the IP
address, sent in the request, and the hardware address.
What is a Makefile?
Makefile is a utility in Unix to help compile large programs. It helps by only
compiling the portion of the program that has been changed.
A Makefile is the file and make uses to determine what rules to apply. make is useful
for far more than compiling programs.
What is deadlock?
Deadlock is a situation when two or more processes prevent each other from running.
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Example: if T1 is holding x and waiting for y to be free and T2 holding y and waiting
for x to be free deadlock happens.
What is semaphore?
Semaphore is a special variable, it has two methods: up and down. Semaphore
performs atomic operations, which means ones a semaphore is called it can not be
inturrupted.
The internal counter (= #ups - #downs) can never be negative. If you execute the
―down‖ method when the internal counter is zero, it will block until some other thread
calls the ―up‖ method. Semaphores are use for thread synchronization.
Is C an object-oriented language?
C is not an object-oriented language, but limited object-oriented programming can be
done in C.
Name some major differences between C++ and Java.
C++ has pointers; Java does not. Java is platform-independent; C++ is not. Java has
garbage collection; C++ does not. Java does have pointers. In fact all variables in Java
are pointers. The difference is that Java does not allow you to manipulate the
addresses of the pointer
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C++ Networking Interview Questions and Answers
What is the difference between Stack and Queue?
Stack is a Last In First Out (LIFO) data structure.
Queue is a First In First Out (FIFO) data structure
Write a fucntion that will reverse a string.
char *strrev(char *s)
{
int i = 0, len = strlen(s);
char *str;
if ((str = (char *)malloc(len+1)) == NULL)
/*cannot allocate memory */
err_num = 2;
return (str);
}
while(len)
str[i++]=s[–len];
str[i] = NULL;
return (str);
}
What is the software Life-Cycle?
The software Life-Cycle are
1) Analysis and specification of the task
2) Design of the algorithms and data structures
3) Implementation (coding)
4) Testing
5) Maintenance and evolution of the system
6) Obsolescence
What is the difference between a Java application and a Java applet?
The difference between a Java application and a Java applet is that a Java application
is a program that can be executed using the Java interpeter, and a JAVA applet can be
transfered to different networks and executed by using a web browser (transferable to
the WWW).
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Name 7 layers of the OSI Reference Model?
-Application layer -Presentation layer
-Session layer
-Transport layer
-Network layer
-Data Link layer
-Physical layer
C++ Algorithm Interview Questions and Answers
What are the advantages and disadvantages of B-star trees over Binary trees?
Answer1
B-star trees have better data structure and are faster in search than Binary trees, but it‘s
harder to write codes for B-start trees.
Answer2
The major difference between B-tree and binary tres is that B-tree is a external data
structure and binary tree is a main memory data structure. The computational
complexity of binary tree is counted by the number of comparison operations at each
node, while the computational complexity of B-tree is determined by the disk I/O, that
is, the number of node that will be loaded from disk to main memory. The
comparision of the different values in one node is not counted.
Write the psuedo code for the Depth first Search.
dfs(G, v) //OUTLINE
Mark v as "discovered"
For each vertex w such that edge vw is in G:
If w is undiscovered:
dfs(G, w); that is, explore vw, visit w, explore from there as much as possible, and
backtrack from w to v. Otherwise:
"Check" vw without visiting w. Mark v as "finished".
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Describe one simple rehashing policy.
The simplest rehashing policy is linear probing. Suppose a key K hashes to location i.
Suppose other key occupies H[i]. The following function is used to generate
alternative locations:
rehash(j) = (j + 1) mod h
where j is the location most recently probed. Initially j = i, the hash code for K. Notice
that this version of rehash does not depend on K.
Describe Stacks and name a couple of places where stacks are useful.
A Stack is a linear structure in which insertions and deletions are always made at one
end, called the top. This updating policy is called last in, first out (LIFO). It is useful
when we need to check some syntex errors, such as missing parentheses.
Suppose a 3-bit sequence number is used in the selective-reject ARQ, what is the
maximum number of frames that could be transmitted at a time?
If a 3-bit sequence number is used, then it could distinguish 8 different frames. Since
the number of frames that could be transmitted at a time is no greater half the numner
of frames that could be distinguished by the sequence number, so at most 4 frames can
be transmitted at a time.
-x-x-x-x-x-x-x-x-x-x-x-x-x-xx-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-
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Java
Interview Questions
&
Answers
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Java Interview Questions and Answers
What is Collection API ?
The Collection API is a set of classes and interfaces that support operation on
collections of objects. These classes and interfaces are more flexible, more
powerful, and more regular than the vectors, arrays, and hashtables if effectively
replaces.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and
TreeMap.
Example of interfaces: Collection, Set, List and Map.
Is Iterator a Class or Interface? What is its use?
Answer: Iterator is an interface which is used to step through the elements of a
Collection.
What is similarities/difference between an Abstract class and Interface?
Differences are as follows:
Interfaces provide a form of multiple inheritance. A class can extend only one
other class. Interfaces are limited to public methods and constants with no
implementation. Abstract classes can have a partial implementation, protected
parts, static methods, etc.
A Class may implement several interfaces. But in case of abstract class, a class
may extend only one abstract class. Interfaces are slow as it requires extra
indirection to to find corresponding method in in the actual class. Abstract classes
are fast.
Similarities:
Neither Abstract classes or Interface can be instantiated.
Java Interview Questions - How to define an Abstract class?
A class containing abstract method is called Abstract class. An Abstract class
can't be instantiated.
Example of Abstract class:
abstract class testAbstractClass {
protected String myString;
public String getMyString() {
return myString;
}
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public abstract string anyAbstractFunction();
}
How to define an Interface in Java ?
In Java Interface defines the methods but does not implement them. Interface can
include constants. A class that implements the interfaces is bound to implement
all the methods defined in Interface.
Emaple of Interface:
public interface sampleInterface {
public void functionOne();
public long CONSTANT_ONE = 1000;
}
If a class is located in a package, what do you need to change in the OS
environment to be able to use it?
You need to add a directory or a jar file that contains the package directories to
the CLASSPATH environment variable. Let's say a class Employee belongs to a
package com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.java.
In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class
contains the method main(), you could test it from a command prompt window as
follows:
c:\>java com.xyz.hr.Employee
How many methods in the Serializable interface?
There is no method in the Serializable interface. The Serializable interface acts as
a marker, telling the object serialization tools that your class is serializable.
How many methods in the Externalizable interface?
There are two methods in the Externalizable interface. You have to implement
these two methods in order to make your class externalizable. These two methods
are readExternal() and writeExternal().
What is the difference between Serializalble and Externalizable interface?
When you use Serializable interface, your class is serialized automatically by
default. But you can override writeObject() and readObject() two methods to
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control more complex object serailization process. When you use Externalizable
interface, you have a complete control over your class's serialization process.
What is a transient variable in Java?
A transient variable is a variable that may not be serialized. If you don't want
some field to be serialized, you can mark that field transient or static.
Which containers use a border layout as their default layout?
The Window, Frame and Dialog classes use a border layout as their default
layout.
How are Observer and Observable used?
Objects that subclass the Observable class maintain a list of observers. When an
Observable object is updated, it invokes the update() method of each of its
observers to notify the observers that it has changed state. The Observer interface
is implemented by objects that observe Observable objects.
What is Java?
Java is an object-oriented programming language developed initially by James
Gosling and colleagues at Sun Microsystems. The language, initially called Oak
(named after the oak trees outside Gosling's office), was intended to replace C++,
although the feature set better resembles that of Objective C. Java should not be
confused with JavaScript, which shares only the name and a similar C-like
syntax. Sun Microsystems currently maintains and updates Java regularly.
What does a well-written OO program look like?
A well-written OO program exhibits recurring structures that promote abstraction,
flexibility, modularity and elegance.
Can you have virtual functions in Java?
Yes, all functions in Java are virtual by default. This is actually a pseudo trick
question because the word "virtual" is not part of the naming convention in Java
(as it is in C++, C-sharp and VB.NET), so this would be a foreign concept for
someone who has only coded in Java. Virtual functions or virtual methods are
functions or methods that will be redefined in derived classes.
Jack developed a program by using a Map container to hold key/value pairs.
He wanted to make a change to the map. He decided to make a clone of the
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map in order to save the original data on side. What do you think of it? ?
If Jack made a clone of the map, any changes to the clone or the original map
would be seen on both maps, because the clone of Map is a shallow copy. So Jack
made a wrong decision.
What is more advisable to create a thread, by implementing a Runnable
interface or by extending Thread class?
Strategically speaking, threads created by implementing Runnable interface are
more advisable. If you create a thread by extending a thread class, you cannot
extend any other class. If you create a thread by implementing Runnable
interface, you save a space for your class to extend another class now or in future.
What is NullPointerException and how to handle it?
When an object is not initialized, the default value is null. When the following
things happen, the NullPointerException is thrown:
--Calling the instance method of a null object.
--Accessing or modifying the field of a null object.
--Taking the length of a null as if it were an array.
--Accessing or modifying the slots of null as if it were an array.
--Throwing null as if it were a Throwable value.
The NullPointerException is a runtime exception. The best practice is to catch
such exception even if it is not required by language design.
An application needs to load a library before it starts to run, how to code?
One option is to use a static block to load a library before anything is called. For
example,
class Test {
static {
System.loadLibrary("path-to-library-file");
}
....
}
When you call new Test(), the static block will be called first before any
initialization happens. Note that the static block position may matter.
How could Java classes direct program messages to the system console, but
error messages, say to a file?
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The class System has a variable out that represents the standard output, and the
variable err that represents the standard error device. By default, they both point
at the system console. This how the standard output could be re-directed:
Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st);
System.setOut(st);
What's the difference between an interface and an abstract class?
An abstract class may contain code in method bodies, which is not allowed in an
interface. With abstract classes, you have to inherit your class from it and Java
does not allow multiple inheritance. On the other hand, you can implement
multiple interfaces in your class.
Name the containers which uses Border Layout as their default layout?
Containers which uses Border Layout as their default are: window, Frame and
Dialog classes.
What do you understand by Synchronization?
Synchronization is a process of controlling the access of shared resources by the
multiple threads in such a manner that only one thread can access one resource at
a time. In non synchronized multithreaded application, it is possible for one
thread to modify a shared object while another thread is in the process of using or
updating the object's value.
Synchronization prevents such type of data corruption.
E.g. Synchronizing a function:
public synchronized void Method1 () {
// Appropriate method-related code.
}
E.g. Synchronizing a block of code inside a function:
public myFunction (){
synchronized (this) {
// Synchronized code here.
}
}
What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the
access of multiple threads to shared resources. Without synchronization, it is
possible for one thread to modify a shared object while another thread is in the
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process of using or updating that object's value. This often causes dirty data and
leads to significant errors.
What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to a method or
an object. A thread only executes a synchronized method after it has acquired the
lock for the method's object or class. Synchronized statements are similar to
synchronized methods. A synchronized statement can only be executed after a
thread has acquired the lock for the object or class referenced in the synchronized
statement.
What are three ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by blocking
on IO, by unsuccessfully attempting to acquire an object's lock, or by invoking an
object's wait() method. It can also enter the waiting state by invoking its
(deprecated) suspend() method.
Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the class's Class
object.
What's new with the stop(), suspend() and resume() methods in JDK 1.2?
The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.
What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow
the component to display normally.
What's the difference between J2SDK 1.5 and J2SDK 5.0?
There's no difference, Sun Microsystems just re-branded this version.
What would you use to compare two String variables - the operator == or
the method equals()?
I'd use the method equals() to compare the values of the Strings and the == to
check if two variables point at the same instance of a String object.
What is thread?
A thread is an independent path of execution in a system.
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What is multi-threading?
Multi-threading means various threads that run in a system.
How does multi-threading take place on a computer with a single CPU?
The operating system's task scheduler allocates execution time to multiple tasks.
By quickly switching between executing tasks, it creates the impression that tasks
execute sequentially.
How to create a thread in a program?
You have two ways to do so. First, making your class "extends" Thread class.
Second, making your class "implements" Runnable interface. Put jobs in a run()
method and call start() method to start the thread.
Can Java object be locked down for exclusive use by a given thread?
Yes. You can lock an object by putting it in a "synchronized" block. The locked
object is inaccessible to any thread other than the one that explicitly claimed it.
Can each Java object keep track of all the threads that want to exclusively
access to it?
Yes. Use Thread.currentThread() method to track the accessing thread.
Does it matter in what order catch statements for FileNotFoundException
and IOExceptipon are written?
Yes, it does. The FileNoFoundException is inherited from the IOException.
Exception's subclasses have to be caught first.
What invokes a thread's run() method?
After a thread is started, via its start() method of the Thread class, the JVM
invokes the thread's run() method when the thread is initially executed.
What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(),notify(), and notifyAll() methods are used to provide an efficient way
for threads to communicate each other.
What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.
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What is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the ready state. When a task
invokes its sleep() method, it returns to the waiting state.
What happens when a thread cannot acquire a lock on an object?
If a thread attempts to execute a synchronized method or synchronized statement
and is unable to acquire an object's lock, it enters the waiting state until the lock
becomes available.
What is the difference between Process and Thread?
A process can contain multiple threads. In most multithreading operating systems,
a process gets its own memory address space; a thread doesn't. Threads typically
share the heap belonging to their parent process. For instance, a JVM runs in a
single process in the host O/S. Threads in the JVM share the heap belonging to
that process; that's why several threads may access the same object. Typically,
even though they share a common heap, threads have their own stack space. This
is how one thread's invocation of a method is kept separate from another's. This is
all a gross oversimplification, but it's accurate enough at a high level. Lots of
details differ between operating systems. Process vs. Thread A program vs.
similar to a sequential program an run on its own vs. Cannot run on its own Unit
of allocation vs. Unit of execution Have its own memory space vs. Share with
others Each process has one or more threads vs. Each thread belongs to one
process Expensive, need to context switch vs. Cheap, can use process memory
and may not need to context switch More secure. One process cannot corrupt
another process vs. Less secure. A thread can write the memory used by another
thread
Can an inner class declared inside of a method access local variables of this
method?
It's possible if these variables are final.
What can go wrong if you replace &emp;&emp; with &emp; in the following
code: String a=null; if (a!=null && a.length()>10) {...}
A single ampersand here would lead to a NullPointerException.
What is the Vector class?
The Vector class provides the capability to implement a growable array of objects
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What modifiers may be used with an inner class that is a member of an outer
class?
A (non-local) inner class may be declared as public, protected, private, static,
final, or abstract.
If a method is declared as protected, where may the method be accessed?
A protected method may only be accessed by classes or interfaces of the same
package or by subclasses of the class in which it is declared.
What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.
How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8
characters?
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character
set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters
using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.
What's the main difference between a Vector and an ArrayList?
Java Vector class is internally synchronized and ArrayList is not.
What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.
Does garbage collection guarantee that a program will not run out of
memory?
No, it doesn't. It is possible for programs to use up memory resources faster than
they are garbage collected. It is also possible for programs to create objects that
are not subject to garbage collection.
What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the
waiting or dead states or a higher priority task comes into existence. Under time
slicing, a task executes for a predefined slice of time and then reenters the pool of
ready tasks. The scheduler then determines which task should execute next, based
on priority and other factors.
Name Component subclasses that support painting ?
The Canvas, Frame, Panel, and Applet classes support painting.
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What is a native method?
A native method is a method that is implemented in a language other than Java.
How can you write a loop indefinitely?
for(;;)--for loop; while(true)--always true, etc.
Can an anonymous class be declared as implementing an interface and extending
a class?
An anonymous class may implement an interface or extend a superclass, but may not
be declared to do both.
What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform
any cleanup processing before the object is garbage collected.
When should the method invokeLater()be used?
This method is used to ensure that Swing components are updated through the eventdispatching thread.
How many methods in Object class?
This question is not asked to test your memory. It tests you how well you know Java.
Ten in total.
clone()
equals() & hashcode()
getClass()
finalize()
wait() & notify()
toString()
How does Java handle integer overflows and underflows?
It uses low order bytes of the result that can fit into the size of the type allowed by the
operation.
What is the numeric promotion?
Numeric promotion is used with both unary and binary bitwise operators. This means
that byte, char, and short values are converted to int values before a bitwise operator is
applied.
If a binary bitwise operator has one long operand, the other operand is converted to a
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long value.
The type of the result of a bitwise operation is the type to which the operands have
been promoted. For example:
short a = 5;
byte b = 10;
long c = 15;
The type of the result of (a+b) is int, not short or byte. The type of the result of (a+c)
or (b+c) is long.
Is the numeric promotion available in other platform?
Yes. Because Java is implemented using a platform-independent virtual machine,
bitwise operations always yield the same result, even when run on machines that use
radically different CPUs.
What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are
evaluated. Then the & operator is applied to the operand. When an expression
involving the && operator is evaluated, the first operand is evaluated. If the first
operand returns a value of true then the second operand is evaluated. The && operator
is then applied to the first and second operands. If the first operand evaluates to false,
the evaluation of the second operand is skipped.
Operator & has no chance to skip both sides evaluation and && operator does. If
asked why, give details as above.
When is the ArithmeticException throwQuestion: What is the
GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars.
What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
How can a subclass call a method or a constructor defined in a superclass?
Use the following syntax: super.myMethod(); To call a constructor of the superclass,
just write super(); in the first line of the subclass's constructor.
What is the Properties class?
The properties class is a subclass of Hashtable that can be read from or written to a
stream. It also provides the capability to specify a set of default values to be used.
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What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.
What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.
What is the purpose of the finally clause of a try-catch-finally statement?
The finally clause is used to provide the capability to execute code no matter whether
or not an exception is thrown or caught.
What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular
geographic, political, or cultural region.
What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass. Or, a
method that has no implementation.
What is the difference between interface and abstract class?
interface contains methods that must be abstract; abstract class may contain concrete
methods. interface contains variables that must be static and final; abstract class may
contain non-final and final variables. members in an interface are public by default,
abstract class may contain non-public members. interface is used to "implements";
whereas abstract class is used to "extends". interface can be used to achieve multiple
inheritance; abstract class can be used as a single inheritance. interface can "extends"
another interface, abstract class can "extends" another class and "implements" multiple
interfaces. interface is absolutely abstract; abstract class can be invoked if a main()
exists. interface is more flexible than abstract class because one class can only
"extends" one super class, but "implements" multiple interfaces. If given a choice, use
interface instead of abstract class.
What is a static method?
A static method is a method that belongs to the class rather than any object of the class
and doesn't apply to an object or even require that any objects of the class have been
instantiated.
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What is a protected method?
A protected method is a method that can be accessed by any method in its package and
inherited by any subclass of its class.
What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated with instances of
the class's outer class. A static inner class does not have any object instances.
What is an object's lock and which object's have locks?
An object's lock is a mechanism that is used by multiple threads to obtain
synchronized access to the object. A thread may execute a synchronized method of an
object only after it has acquired the object's lock. All objects and classes have locks. A
class's lock is acquired on the class's Class object.
When can an object reference be cast to an interface reference?
An object reference can be cast to an interface reference when the object implements
the referenced interface.
What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that can have a
menu bar.
What is the difference between a Window and a Frame?
Heavy weight components like Abstract Window Toolkit (AWT), depend on the local
windowing toolkit. For example, java.awt.Button is a heavy weight component, when
it is running on the Java platform for Unix platform, it maps to a real Motif button. In
this relationship, the Motif button is called the peer to the java.awt.Button. If you
create two Buttons, two peers and hence two Motif Buttons are also created. The Java
platform communicates with the Motif Buttons using the Java Native Interface. For
each and every component added to the application, there is an additional overhead
tied to the local windowing system, which is why these components are called heavy
weight.
Which package has light weight components?
javax.Swing package. All components in Swing, except JApplet, JDialog, JFrame and
JWindow are lightweight components.
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What are peerless components?
The peerless components are called light weight components.
What is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific properties, such as
ascent and descent, of a Font object
What is the difference between the Reader/Writer class hierarchy and the
InputStream/OutputStream class hierarchy?
The Reader/Writer class hierarchy is character-oriented, and the
InputStream/OutputStream class hierarchy is byte-oriented.
What classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that may be assigned to the Throwable
type. This includes the Error and Exception types.
What is the difference between throw and throws keywords?
The throw keyword denotes a statement that causes an exception to be initiated. It
takes the Exception object to be thrown as argument. The exception will be
caught by an immediately encompassing try-catch construction or propagated
further up the calling hierarchy.
The throws keyword is a modifier of a method that designates that exceptions
may come out of the method, either by virtue of the method throwing the
exception itself or because it fails to catch such exceptions that a method it calls
may throw.
If a class is declared without any access modifiers, where may the class be
accessed?
A class that is declared without any access modifiers is said to have package or
friendly access. This means that the class can only be accessed by other classes
and interfaces that are defined within the same package.
What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate
keys with values.
Does a class inherit the constructors of its super class?
A class does not inherit constructors from any of its superclasses.
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Name primitive Java types.
The primitive types are byte, char, short, int, long, float, double, and Boolean.
Which class should you use to obtain design information about an object?
The Class class is used to obtain information about an object's design.
How can a GUI component handle its own events?
A component can handle its own events by implementing the required eventlistener interface and adding itself as its own event listener.
How are the elements of a GridBagLayout organized?
The elements of a GridBagLayout are organized according to a grid. However,
the elements are of different sizes and may occupy more than one row or column
of the grid. In addition, the rows and columns may have different sizes.
What advantage do Java's layout managers provide over traditional
windowing systems?
Java uses layout managers to lay out components in a consistent manner across
all windowing platforms. Since Java's layout managers aren't tied to absolute
sizing and positioning, they are able to accommodate platform-specific
differences among windowing systems.
What are the problems faced by Java programmers who don't use layout
managers?
Without layout managers, Java programmers are faced with determining how
their GUI will be displayed across multiple windowing systems and finding a
common sizing and positioning that will work within the constraints imposed by
each windowing system.
What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific
instances of a class. Non-static variables take on unique values with each object
instance.
What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method
is used to cause paint() to be invoked by the AWT painting thread.
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What is the purpose of the File class?
The File class is used to create objects that provide access to the files and
directories of a local file system.
Why would you use a synchronized block vs. synchronized method?
Synchronized blocks place locks for shorter periods than synchronized methods.
What restrictions are placed on method overriding?
Overridden methods must have the same name, argument list, and return type. The
overriding method may not limit the access of the method it overrides. The overriding
method may not throw any exceptions that may not be thrown by the overridden
method.
What is casting?
There are two types of casting, casting between primitive numeric types and casting
between object references. Casting between numeric types is used to convert larger
values, such as double values, to smaller values, such as byte values. Casting between
object references is used to refer to an object by a compatible class, interface, or array
type reference.
Explain the usage of the keyword transient?
This keyword indicates that the value of this member variable does not have to be
serialized with the object. When the class will be de-serialized, this variable will be
initialized with a default value of its data type (i.e. zero for integers).
What class allows you to read objects directly from a stream?
The ObjectInputStream class supports the reading of objects from input streams.
How are this() and super() used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a
superclass constructor.
How is it possible for two String objects with identical values not to be equal
under the == operator? How are this() and super() used with constructors?
The == operator compares two objects to determine if they are the same objects in
memory. It is possible for two String objects to have the same value, but located in
different areas of memory.
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What is an IO filter?
An IO filter is an object that reads from one stream and writes to another, usually
altering the data in some way as it is passed from one stream to another.
What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical
set. Sets do not allow duplicate elements.
How can you force garbage collection?
You can't force GC, but could request it by calling System.gc(). JVM does not
guarantee that GC will be started immediately.
What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular object.
Normally, an event is enabled when a listener is added to an object for a particular
event. The enableEvents() method is used by objects that handle events by overriding
their event-dispatch methods.
What is the difference between the File and RandomAccessFile classes?
The File class encapsulates the files and directories of the local file system. The
RandomAccessFile class provides the methods needed to directly access data
contained in any part of a file.
What interface must an object implement before it can be written to a stream as
an object?
An object must implement the Serializable or Externalizable interface before it can be
written to a stream as an object.
What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded
by a program to tailor the program's appearance to the particular locale in which it is
being run.
How do you know if an explicit object casting is needed?
If you assign a superclass object to a variable of a subclass's data type, you need to do
explicit casting. For example:
Object a; Customer b; b = (Customer) a;
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When you assign a subclass to a variable having a supeclass type, the casting is
performed automatically.
What is a Java package and how is it used?
A Java package is a naming context for classes and interfaces. A package is used to
create a separate name space for groups of classes and interfaces. Packages are also
used to organize related classes and interfaces into a single API unit and to control
accessibility to these classes and interfaces.
How do you restrict a user to cut and paste from the html page?
Using Servlet or client side scripts to lock keyboard keys. It is one of solutions.
What are the Object and Class classes used for?
The Object class is the highest-level class in the Java class hierarchy. The Class
class is used to represent the classes and interfaces that are loaded by a Java
program.
What is Serialization and deserialization ?
Serialization is the process of writing the state of an object to a byte stream.
Deserialization is the process of restoring these objects.
Explain the usage of Java packages.
This is a way to organize files when a project consists of multiple modules. It also
helps resolve naming conflicts when different packages have classes with the
same names. Packages access level also allows you to protect data from being
used by the non-authorized classes.
Does the code in finally block get executed if there is an exception and a
return statement in a catch block?
If an exception occurs and there is a return statement in catch block, the finally
block is still executed. The finally block will not be executed when the
System.exit(1) statement is executed earlier or the system shut down earlier or the
memory is used up earlier before the thread goes to finally block.
Is Java a super set of JavaScript?
No. They are completely different. Some syntax may be similar.
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What is a Container in a GUI?
A Container contains and arranges other components (including other containers)
through the use of layout managers, which use specific layout policies to
determine where components should go as a function of the size of the container.
How the object oriented approach helps us keep complexity of software
development under control?
We can discuss such issue from the following aspects:
Objects allow procedures to be encapsulated with their data to reduce potential
interference.
Inheritance allows well-tested procedures to be reused and enables changes to
make once and have effect in all relevant places.
The well-defined separations of interface and implementation allow constraints to
be imposed on inheriting classes while still allowing the flexibility of overriding
and overloading.
What is polymorphism?
Polymorphism means "having many forms". It allows methods (may be variables)
to be written that needn't be concerned about the specifics of the objects they will
be applied to. That is, the method can be specified at a higher level of abstraction
and can be counted on to work even on objects of un-conceived classes.
What is design by contract?
The design by contract specifies the obligations of a method to any other methods
that may use its services and also theirs to it. For example, the preconditions
specify what the method required to be true when the method is called. Hence
making sure that preconditions are. Similarly, postconditions specify what must
be true when the method is finished, thus the called method has the responsibility
of satisfying the post conditions.
In Java, the exception handling facilities support the use of design by contract,
especially in the case of checked exceptions. The assert keyword can be used to
make such contracts.
What are use cases?
A use case describes a situation that a program might encounter and what
behavior the program should exhibit in that circumstance. It is part of the analysis
of a program. The collection of use cases should, ideally, anticipate all the
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standard circumstances and many of the extraordinary circumstances possible so
that the program will be robust.
What is scalability and performance?
Performance is a measure of "how fast can you perform this task." and scalability
describes how an application behaves as its workload and available computing
resources increase.
What is the benefit of subclass?
Generally: The sub class inherits all the public methods and the implementation.
The sub class inherits all the protected methods and their implementation.
The sub class inherits all the default(non-access modifier) methods and their
implementation.
The sub class also inherits all the public, protected and default member variables
from the super class.
The constructors are not part of this inheritance model.
How to add menushortcut to menu item?
If you have a button instance called aboutButton, you may add menu short cut by
calling aboutButton.setMnemonic('A'), so the user may be able to use Alt+A to
click the button.
In System.out.println(),what is System,out and println,pls explain?
System is a predefined final class,out is a PrintStream object acting as a field
member and println is a built-in overloaded method in the out object.
Can you write a Java class that could be used both as an applet as well as an
application?
A. Yes. Add a main() method to the applet.
Can you make an instance of an abstract class? For example java.util.Calender is an abstract class with a method getInstance() which
returns an instance of the Calender class.
No! You cannot make an instance of an abstract class. An abstract class has to be
sub-classed. If you have an abstract class and you want to use a method which has
been implemented, you may need to subclass that abstract class, instantiate your
subclass and then call that method.
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What is the output of x > y? a:b = p*q when x=1,y=2,p=3,q=4?
When this kind of question has been asked, find the problems you think is
necessary to ask back before you give an answer. Ask if variables a and b have
been declared or initialized. If the answer is yes. You can say that the syntax is
wrong. If the statement is rewritten as: x
What is the difference between Swing and AWT components?
AWT components are heavy-weight, whereas Swing components are lightweight.
Heavy weight components depend on the local windowing toolkit. For example,
java.awt.Button is a heavy weight component, when it is running on the Java
platform for Unix platform, it maps to a real Motif button.
Why Java does not support pointers?
Because pointers are unsafe. Java uses reference types to hide pointers and
programmers feel easier to deal with reference types without pointers. This is
why Java and C-sharp shine.
Parsers? DOM vs SAX parser
Parsers are fundamental xml components, a bridge between XML documents and
applications that process that XML. The parser is responsible for handling xml
syntax, checking the contents of the document against constraints established in a
DTD or Schema.
DOM
1. Tree of nodes
2. Memory: Occupies more memory, preffered for small XML documents
3. Slower at runtime
4. Stored as objects
5. Programmatically easy
6. Ease of navigation
SAX
1. Sequence of events
2. Doesn't use any memory preferred for large documents
3. Faster at runtime
4. Objects are to be created
5. Need to write code for creating objects
6. Backward navigation is not possible as it sequentially processes the document
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Can you declare a class as private?
Yes, we can declare a private class as an inner class. For example,
class MyPrivate {
private static class MyKey {
String key = "12345";
}
public static void main(String[] args) {
System.out.println(new MyKey().key);//prints 12345
}
}
What is the difference between shallow copy and deep copy?
Shallow copy shares the same reference with the original object like cloning,
whereas the deep copy get a duplicate instance of the original object. If the
shallow copy has been changed, the original object will be reflected and vice
versa.
Can one create a method which gets a String and modifies it?
No. In Java, Strings are constant or immutable; their values cannot be changed
after they are created, but they can be shared. Once you change a string, you
actually create a new object. For example:
String s = "abc"; //create a new String object representing "abc"
s = s.toUpperCase(); //create another object representing "ABC"
Why is multiple inheritance not possible in Java?
It depends on how you understand "inheritance". Java can only "extends" one
super class, but can "implements" many interfaces; that doesn't mean the multiple
inheritance is not possible. You may use interfaces to make inheritance work for
you. Or you may need to work around. For example, if you cannot get a feature
from a class because your class has a super class already, you may get that class's
feature by declaring it as a member field or getting an instance of that class. So
the answer is that multiple inheritance in Java is possible.
What's the difference between constructors and other methods?
Constructors must have the same name as the class and can not return a value.
They are only called once while regular methods could be called many times.
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What is the relationship between synchronized and volatile keyword?
The JVM is guaranteed to treat reads and writes of data of 32 bits or less as
atomic.(Some JVM might treat reads and writes of data of 64 bits or less as atomic in
future) For long or double variable, programmers should take care in multi-threading
environment. Either put these variables in a synchronized method or block, or declare
them volatile.
This class (IncrementImpl) will be used by various threads concurrently; can you
see the inherent flaw(s)? How would you improve it?
public class IncrementImpl {
private static int counter = 0;
public synchronized void increment() {
counter++;
}
public int getCounter() {
return counter;
}
}
The counter is static variable which is shared by multiple instances of this class. The
increment() method is synchronized, but the getCounter() should be synchronized too.
Otherwise the Java run-time system will not guarantee the data integrity and the race
conditions will occur. The famous producer/consumer example listed at Sun's thread
tutorial site will tell more.
one of solutions
public class IncrementImpl {
private static int counter = 0;
public synchronized void increment() {
counter++;
}
public synchronized int getCounter() {
return counter;
}
}
What are the drawbacks of inheritance?
Since inheritance inherits everything from the super class and interface, it may make
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the subclass too clustering and sometimes error-prone when dynamic overriding or
dynamic overloading in some situation. In addition, the inheritance may make peers
hardly understand your code if they don't know how your super-class acts and add
learning curve to the process of development.
Usually, when you want to use a functionality of a class, you may use subclass to
inherit such function or use an instance of this class in your class. Which is better,
depends on your specification.
Is there any other way that you can achieve inheritance in Java?
There are a couple of ways. As you know, the straight way is to "extends" and/or
"implements". The other way is to get an instance of the class to achieve the
inheritance. That means to make the supposed-super-class be a field member. When
you use an instance of the class, actually you get every function available from this
class, but you may lose the dynamic features of OOP
Two methods have key words static synchronized and synchronized separately.
What is the difference between them?
Both are synchronized methods. One is instance method, the other is class method.
Method with static modifier is a class method. That means the method belongs to class
itself and can be accessed directly with class name and is also called Singleton design.
The method without static modifier is an instance method. That means the instance
method belongs to its object. Every instance of the class gets its own copy of its
instance method.
When synchronized is used with a static method, a lock for the entire class is obtained.
When synchronized is used with a non-static method, a lock for the particular object
(that means instance) of the class is obtained.
Since both methods are synchronized methods, you are not asked to explain what is a
synchronized method. You are asked to tell the difference between instance and class
method. Of course, your explanation to how synchronized keyword works doesn't
hurt. And you may use this opportunity to show your knowledge scope.
How do you create a read-only collection?
The Collections class has six methods to help out here:
1. unmodifiableCollection(Collection c)
2. unmodifiableList(List list)
3. unmodifiableMap(Map m)
4. unmodifiableSet(Set s)
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5. unmodifiableSortedMap(SortedMap m)
6. unmodifiableSortedSet(SortedSet s)
If you get an Iterator from one of these unmodifiable collections, when you call
remove(), it will throw an UnsupportedOperationException.
Can a private method of a superclass be declared within a subclass?
Sure. A private field or method or inner class belongs to its declared class and hides
from its subclasses. There is no way for private stuff to have a runtime overloading or
overriding (polymorphism) features.
Why Java does not support multiple inheritance ?
This is a classic question. Yes or No depends on how you look at Java. If you focus on
the syntax of "extends" and compare with C++, you may answer 'No' and give
explanation to support you. Or you may answer 'Yes'. Recommend you to say 'Yes'.
Java DOES support multiple inheritance via interface implementation. Some people
may not think in this way. Give explanation to support your point.
What is the difference between final, finally and finalize?
Short answer:
final - declares constant
finally - relates with exception handling
finalize - helps in garbage collection
If asked to give details, explain:
final field, final method, final class
try/finally, try/catch/finally
protected void finalize() in Object class
What kind of security tools are available in J2SE 5.0?
There are three tools that can be used to protect application working within the
scope of security policies set at remote sites.
keytool -- used to manage keystores and certificates.
jarsigner -- used to generate and verify JAR signatures.
policytool -- used for managing policy files.
There are three tools that help obtain, list and manage Kerberos tickets.
kinit -- used to obtain Kerberos V5 tickets.
tklist -- used to list entries in credential cache and key tab.
ktab -- used to help manage entries in the key table.
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How to make an array copy from System?
There is a method called arraycopy in the System class. You can do it:
System.arraycopy(sourceArray, srcOffset, destinationArray, destOffset,
numOfElements2Copy);
When you use this method, the destinationArray will be filled with the elements
of sourceArray at the length specified.
Can we use System.arraycopy() method to copy the same array?
Yes, you can. The source and destination arrays can be the same if you want to
copy a subset of the array to another area within that array.
What is shallow copy or shallow clone in array cloning?
Cloning an array invloves creating a new array of the same size and type and
copying all the old elements into the new array. But such copy is called shallow
copy or shallow clone because any changes to the object would be reflected in
both arrays.
When is the ArrayStoreException thrown?
When copying elements between different arrays, if the source or destination
arguments are not arrays or their types are not compatible, an
ArrayStoreException will be thrown.
How to check two arrays to see if contents have the same types and contain
the same elements?
One of options is to use the equals() method of Arrays class.
Arrays.equals(a, b);
If the array types are different, a compile-time error will happen.
Can you call one constructor from another if a class has multiple
constructors?
Yes. Use this() syntax.
What are the different types of inner classes?
There are four different types of inner classes in Java. They are: a)Static member
classes , a static member class has access to all static methods of the parent, or
top-level, class b) Member classes, the member class is instance specific and has
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access to any and all methods and members, even the parent's this reference c)
Local classes, are declared within a block of code and are visible only within that
block, just as any other method variable. d) Anonymous classes, is a local class
that has no name
In which case would you choose a static inner class?
Interesting one, static inner classes can access the outer class's protected and
private fields. This is both a positive and a negative point for us since we can, in
essence, violate the encapsulation of the outer class by mucking up the outer
class's protected and private fields. The only proper use of that capability is to
write white-box tests of the class -- since we can induce cases that might be very
hard to induce via normal black-box tests (which don't have access to the internal
state of the object). Second advantage,if I can say, is that, we can this static
concept to impose restriction on the inner class. Again as discussed in earlier
point, an Inner class has access to all the public, private and protected members of
the parent class. Suppose you want to restrict the access even to inner class, how
would you go ahead? Making the inner class static enforces it to access only the
public static members of the outer class( Since, protected and private members
are not supposed to be static and that static members can access only other static
members). If it has to access any non-static member, it has to create an instance
of the outer class which leads to accessing only public members.
What is weak reference in Java
A weak reference is one that does not prevent the referenced object from being
garbage collected. You might use them to manage a HashMap to look up a cache
of objects. A weak reference is a reference that does not keep the object it refers
to alive. A weak reference is not counted as a reference in garbage collection. If
the object is not referred to elsewhere as well, it will be garbage collected.
What is the difference between final, finally and finalize?
final is used for making a class no-subclassable, and making a member variable
as a constant which cannot be modified. finally is usually used to release all the
resources utilized inside the try block. All the resources present in the finalize
method will be garbage collected whenever GC is called. Though finally and
finalize seem to be for a similar task there is an interesting tweak here, usually I
prefer finally than finalize unless it is unavoidable. This is because the code in
finally block is guaranteed of execution irrespective of occurrence of exception,
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while execution of finalize is not guarenteed.finalize method is called by the
garbage collector on an object when the garbage collector determines that there
are no more references to the object. Presumably the garbage collector will, like
its civil servant namesake, visit the heap on a regular basis to clean up resources
that are no longer in use. Garbage collection exists to prevent programmers from
calling delete. This is a wonderful feature. For example, if you can't call delete,
then you can't accidentally call delete twice on the same object. However,
removing delete from the language is not the same thing as automatically
cleaning up. To add to it, Garbage collection might not ever run. If garbage
collection runs at all, and an object is no longer referenced, then that object's
finalize will run. Also, across multiple objects, finalize order is not predictable.
The correct approach to resource cleanup in Java language programs does not rely
on finalize. Instead, you simply write explicit close methods for objects that wrap
native resources. If you take this approach, you must document that the close
method exists and when it should be called. Callers of the object must then
remember to call close when they are finished with a resource.
What's the difference between the methods sleep() and wait()
The code sleep(1000); puts thread aside for exactly one second. The code
wait(1000), causes a wait of up to one second. A thread could stop waiting earlier
if it receives the notify() or notifyAll() call. The method wait() is defined in the
class Object and the method sleep() is defined in the class Thread.
The following statement prints true or false, why?
byte[] a = { 1, 2, 3 };,
byte[] b = (byte[]) a.clone();
System.out.println(a == b);
The false will be printed out. Because the two arrays have distinctive memory
addresses. Starting in Java 1.2, we can use java.util.Arrays.equals(a, b) to
compare whether two arrays have the same contents.
Why do we need to use getSystemResource() and getSystemResources()
method to load resources?
Because we want to look for resources strictly from the system classpath, These
methods use the system ClassLoader to locate resources, which gives you stricter
control of the resources used by the application.
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ArithmeticException?
The ArithmeticException is thrown when integer is divided by zero or taking the
remainder of a number by zero. It is never thrown in floating-point operations.
What is a transient variable?
A transient variable is a variable that may not be serialized.
Which containers use a border Layout as their default layout?
The window, Frame and Dialog classes use a border layout as their default layout.
Why do threads block on I/O?
Threads block on I/O (that is enters the waiting state) so that other threads may
execute while the I/O Operation is performed.
What is the output from System.out.println("Hello"+null);?
Hellonull
What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the
access of multiple threads to shared resources. Without synchronization, it is
possible for one thread to modify a shared object while another thread is in the
process of using or updating that object's value. This often leads to significant
errors.
Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the class's Class
object.
What's new with the stop(), suspend() and resume() methods in JDK 1.2?
The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.
Is null a keyword?
The null value is not a keyword.
What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow
the component to display normally.
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What method is used to specify a container's layout?
The setLayout() method is used to specify a container's layout.
Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.
What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.
What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on
collections of objects.
Which characters may be used as the second character of an identifier, but not as
the first character of an identifier?
The digits 0 through 9 may not be used as the first character of an identifier but they
may be used after the first character of an identifier.
What is the List interface?
The List interface provides support for ordered collections of objects.
How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type allowed
by the operation.
What is the Vector class?
The Vector class provides the capability to implement a growable array of objects
What modifiers may be used with an inner class that is a member of an outer
class?
A (non-local) inner class may be declared as public, protected, private, static, final, or
abstract.
What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.
What is the difference between the >> and >>> operators?
The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that
have been shifted out.
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Which method of the Component class is used to set the position and size of a
component?
setBounds()
How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8
characters?
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set
uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using
8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.
What is the difference between yielding and sleeping?
When a task invokes its yield() method, it returns to the ready state. When a task
invokes its sleep() method, it returns to the waiting state.
Which java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.
Is sizeof a keyword?
The sizeof operator is not a keyword.
What are wrapper classes?
Wrapper classes are classes that allow primitive types to be accessed as objects.
Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is
possible for programs to use up memory resources faster than they are garbage
collected. It is also possible for programs to create objects that are not subject to
garbage collection.
What restrictions are placed on the location of a package statement within a
source code file?
A package statement must appear as the first line in a source code file (excluding
blank lines and comments).
Can an object's finalize() method be invoked while it is reachable?
An object's finalize() method cannot be invoked by the garbage collector while the
object is still reachable. However, an object's finalize() method may be invoked by
other objects.
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What is the immediate superclass of the Applet class?
Panel
What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the
waiting or dead states or a higher priority task comes into existence. Under time
slicing, a task executes for a predefined slice of time and then reenters the pool of
ready tasks. The scheduler then determines which task should execute next, based on
priority and other factors.
Name three Component subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.
What value does readLine() return when it has reached the end of a file?
The readLine() method returns null when it has reached the end of a file.
What is the immediate superclass of the Dialog class?
Window.
What is clipping?
Clipping is the process of confining paint operations to a limited area or shape.
What is a native method?
A native method is a method that is implemented in a language other than Java.
Can a for statement loop indefinitely?
Yes, a for statement can loop indefinitely. For example, consider the following: for(;;)
;
What are order of precedence and associativity, and how are they used?
Order of precedence determines the order in which operators are evaluated in
expressions. Associatity determines whether an expression is evaluated left-to-right or
right-to-left
When a thread blocks on I/O, what state does it enter?
A thread enters the waiting state when it blocks on I/O.
To what value is a variable of the String type automatically initialized?
The default value of a String type is null.
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What is the catch or declare rule for method declarations?
If a checked exception may be thrown within the body of a method, the method must
either catch the exception or declare it in its throws clause.
What is the difference between a MenuItem and a CheckboxMenuItem?
The CheckboxMenuItem class extends the MenuItem class to support a menu item that
may be checked or unchecked.
What is a task's priority and how is it used in scheduling?
A task's priority is an integer value that identifies the relative order in which it should
be executed with respect to other tasks. The scheduler attempts to schedule higher
priority tasks before lower priority tasks.
What class is the top of the AWT event hierarchy?
The java.awt.AWTEvent class is the highest-level class in the AWT event-class
hierarchy.
When a thread is created and started, what is its initial state?
A thread is in the ready state after it has been created and started.
Can an anonymous class be declared as implementing an interface and extending
a class?
An anonymous class may implement an interface or extend a superclass, but may not
be declared to do both.
What is the range of the short type?
The range of the short type is -(2^15) to 2^15 - 1.
What is the range of the char type?
The range of the char type is 0 to 2^16 - 1.
In which package are most of the AWT events that support the event-delegation
model defined?
Most of the AWT-related events of the event-delegation model are defined in the
java.awt.event package. The AWTEvent class is defined in the java.awt package.
What is the immediate super class of Menu?
What is the immediate super class of Menu? MenuItem
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What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform
any cleanup processing before the object is garbage collected.
Which class is the immediate super class of the MenuComponent class.
Object
What invokes a thread's run() method?
After a thread is started, via its start() method or that of the Thread class, the JVM
invokes the thread's run() method when the thread is initially executed.
What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are
evaluated. Then the & operator is applied to the operand. When an expression
involving the && operator is evaluated, the first operand is evaluated. If the first
operand returns a value of true then the second operand is evaluated. The && operator
is then applied to the first and second operands. If the first operand evaluates to false,
the evaluation of the second operand is skipped.
Name three subclasses of the Component class.
Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or
TextComponent
What is the GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars.
Which Container method is used to cause a container to be laid out and
redisplayed?
validate()
What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.
How many times may an object's finalize() method be invoked by the garbage
collector?
An object's finalize() method may only be invoked once by the garbage collector.
What is the purpose of the finally clause of a try-catch-finally statement? garbage
collector?
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The finally clause is used to provide the capability to execute code no matter whether
or not an exception is thrown or caught.
What is the argument type of a program's main() method?
A program's main() method takes an argument of the String[] type.
Which Java operator is right associative?
The = operator is right associative.
What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular
geographic, political, or cultural region.
Can a double value be cast to a byte?
Yes, a double value can be cast to a byte.
What is the difference between a break statement and a continue statement?
A break statement results in the termination of the statement to which it applies
(switch, for, do, or while). A continue statement is used to end the current loop
iteration and return control to the loop statement.
What must a class do to implement an interface?
It must provide all of the methods in the interface and identify the interface in its
implements clause.
What method is invoked to cause an object to begin executing as a separate
thread?
The start() method of the Thread class is invoked to cause an object to begin executing
as a separate thread.
Name two subclasses of the TextComponent class.
TextField and TextArea
What is the advantage of the event-delegation model over the earlier eventinheritance model?
The event-delegation model has two advantages over the event-inheritance model.
First, it enables event handling to be handled by objects other than the ones that
generate the events (or their containers). This allows a clean separation between a
component's design and its use. The other advantage of the event-delegation model is
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that it performs much better in applications where many events are generated. This
performance improvement is due to the fact that the event-delegation model does not
have to repeatedly process unhandled events, as is the case of the event-inheritance
model.
Which containers may have a MenuBar?
Frame
How are commas used in the initialization and iteration parts of a for statement?
Commas are used to separate multiple statements within the initialization and iteration
parts of a for statement.
What is the purpose of the wait(), notify(), and notifyAll() methods?
The wait(),notify(), and notifyAll() methods are used to provide an efficient way for
threads to wait for a shared resource. When a thread executes an object's wait()
method, it enters the waiting state. It only enters the ready state after another thread
invokes the object's notify() or notifyAll() methods.
What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.
How are Java source code files named?
A Java source code file takes the name of a public class or interface that is defined
within the file. A source code file may contain at most one public class or interface. If
a public class or interface is defined within a source code file, then the source code file
must take the name of the public class or interface. If no public class or interface is
defined within a source code file, then the file must take on a name that is different
than its classes and interfaces. Source code files use the .java extension.
What is the relationship between the Canvas class and the Graphics class?
A Canvas object provides access to a Graphics object via its paint() method.
What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.
What value does read() return when it has reached the end of a file?
The read() method returns -1 when it has reached the end of a file.
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Can a Byte object be cast to a double value?
No, an object cannot be cast to a primitive value.
What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated with instances of
the class's outer class. A static inner class does not have any object instances.
What is the difference between the String and StringBuffer classes?
String objects are constants. StringBuffer objects are not.
If a variable is declared as private, where may the variable be accessed?
A private variable may only be accessed within the class in which it is declared.
What is an object's lock and which objects have locks?
An object's lock is a mechanism that is used by multiple threads to obtain
synchronized access to the object. A thread may execute a synchronized method
of an object only after it has acquired the object's lock. All objects and classes
have locks. A class's lock is acquired on the class's Class object.
What is the Dictionary class?
The Dictionary class provides the capability to store key-value pairs.
How are the elements of a BorderLayout organized?
The elements of a BorderLayout are organized at the borders (North, South, East,
and West) and the center of a container.
What is the % operator?
It is referred to as the modulo or remainder operator. It returns the remainder of
dividing the first operand by the second operand.
When can an object reference be cast to an interface reference?
An object reference be cast to an interface reference when the object implements
the referenced interface.
What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that can
have a menu bar.
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Which class is extended by all other classes?
The Object class is extended by all other classes.
Can an object be garbage collected while it is still reachable?
A reachable object cannot be garbage collected. Only unreachable objects may be
garbage collected..
Is the ternary operator written x : y ? z or x ? y : z ?
It is written x ? y : z.
What is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific properties, such
as ascent and descent, of a Font object.
How is rounding performed under integer division?
The fractional part of the result is truncated. This is known as rounding toward
zero.
What happens when a thread cannot acquire a lock on an object?
If a thread attempts to execute a synchronized method or synchronized statement
and is unable to acquire an object's lock, it enters the waiting state until the lock
becomes available.
What is the difference between the Reader/Writer class hierarchy and the
InputStream/OutputStream class hierarchy?
The Reader/Writer class hierarchy is character-oriented, and the
InputStream/OutputStream class hierarchy is byte-oriented.
What classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that may be assigned to the Throwable
type. This includes the Error and Exception types.
If a class is declared without any access modifiers, where may the class be
accessed?
A class that is declared without any access modifiers is said to have package
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access. This means that the class can only be accessed by other classes and
interfaces that are defined within the same package.
What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate
keys with values.
Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its super classes.
For which statements does it make sense to use a label?
The only statements for which it makes sense to use a label are those statements
that can enclose a break or continue statement.
What is the purpose of the System class?
The purpose of the System class is to provide access to system resources.
Which TextComponent method is used to set a TextComponent to the readonly state?
setEditable()
How are the elements of a CardLayout organized?
The elements of a CardLayout are stacked, one on top of the other, like a deck of
cards.
Is &&= a valid Java operator?
No, it is not.
Name the eight primitive Java types.
The eight primitive types are byte, char, short, int, long, float, double, and
boolean.
Which class should you use to obtain design information about an object?
The Class class is used to obtain information about an object's design.
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What is the relationship between clipping and repainting?
When a window is repainted by the AWT painting thread, it sets the clipping
regions to the area of the window that requires repainting.
Is "abc" a primitive value?
The String literal "abc" is not a primitive value. It is a String object.
What is the relationship between an event-listener interface and an eventadapter class?
An event-listener interface defines the methods that must be implemented by an
event handler for a particular kind of event. An event adapter provides a default
implementation of an event-listener interface.
What restrictions are placed on the values of each case of a switch
statement?
During compilation, the values of each case of a switch statement must evaluate
to a value that can be promoted to an int value.
What modifiers may be used with an interface declaration?
An interface may be declared as public or abstract.
Is a class a subclass of itself?
A class is a subclass of itself.
What is the highest-level event class of the event-delegation model?
The java.util.EventObject class is the highest-level class in the event-delegation
class hierarchy.
What event results from the clicking of a button?
The ActionEvent event is generated as the result of the clicking of a button.
How can a GUI component handle its own events?
A component can handle its own events by implementing the required eventlistener interface and adding itself as its own event listener.
What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the next loop
iteration should occur. A do statement checks at the end of a loop to see whether
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the next iteration of a loop should occur. The do statement will always execute
the body of a loop at least once.
How are the elements of a GridBagLayout organized?
The elements of a GridBagLayout are organized according to a grid. However,
the elements are of different sizes and may occupy more than one row or column
of the grid. In addition, the rows and columns may have different sizes.
What advantage do Java's layout managers provide over traditional
windowing systems?
Java uses layout managers to lay out components in a consistent manner across
all windowing platforms. Since Java's layout managers aren't tied to absolute
sizing and positioning, they are able to accommodate platform-specific
differences among windowing systems.
What is the Collection interface?
The Collection interface provides support for the implementation of a
mathematical bag - an unordered collection of objects that may contain
duplicates.
What modifiers can be used with a local inner class?
A local inner class may be final or abstract.
What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific
instances of a class. Non-static variables take on unique values with each object
instance.
What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method
is used to cause paint() to be invoked by the AWT painting thread.
What is the purpose of the File class?
The File class is used to create objects that provide access to the files and
directories of a local file system.
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Can an exception be rethrown?
Yes, an exception can be rethrown.
Which Math method is used to calculate the absolute value of a number?
The abs() method is used to calculate absolute values.
How does multithreading take place on a computer with a single CPU?
The operating system's task scheduler allocates execution time to multiple tasks.
By quickly switching between executing tasks, it creates the impression that tasks
execute sequentially.
When does the compiler supply a default constructor for a class?
The compiler supplies a default constructor for a class if no other constructors are
provided.
When is the finally clause of a try-catch-finally statement executed?
The finally clause of the try-catch-finally statement is always executed unless the
thread of execution terminates or an exception occurs within the execution of the
finally clause.
Which class is the immediate superclass of the Container class?
Component
If a method is declared as protected, where may the method be accessed?
A protected method may only be accessed by classes or interfaces of the same
package or by subclasses of the class in which it is declared.
How can the Checkbox class be used to create a radio button?
By associating Checkbox objects with a CheckboxGroup.
Which non-Unicode letter characters may be used as the first character of an
identifier?
The non-Unicode letter characters $ and _ may appear as the first character of an
identifier
What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return
types.
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What happens when you invoke a thread's interrupt method while it is
sleeping or waiting?
When a task's interrupt() method is executed, the task enters the ready state. The
next time the task enters the running state, an InterruptedException is thrown.
What is the return type of a program's main() method?
A program's main() method has a void return type.
Name four Container classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane
What is the difference between a Choice and a List?
A Choice is displayed in a compact form that requires you to pull it down to see
the list of available choices. Only one item may be selected from a Choice. A List
may be displayed in such a way that several List items are visible. A List supports
the selection of one or more List items.
What class of exceptions are generated by the Java run-time system?
The Java runtime system generates RuntimeException and Error exceptions.
What class allows you to read objects directly from a stream?
The ObjectInputStream class supports the reading of objects from input streams.
What is the difference between a field variable and a local variable?
A field variable is a variable that is declared as a member of a class. A local
variable is a variable that is declared local to a method.
Under what conditions is an object's finalize() method invoked by the
garbage collector?
The garbage collector invokes an object's finalize() method when it detects that
the object has become unreachable.
How are this () and super () used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a
superclass constructor.
What is the relationship between a method's throws clause and the
exceptions that can be thrown during the method's execution?
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A method's throws clause must declare any checked exceptions that are not
caught within the body of the method.
What is the difference between the JDK 1.02 event model and the eventdelegation model introduced with JDK 1.1?
The JDK 1.02 event model uses an event inheritance or bubbling approach. In
this model, components are required to handle their own events. If they do not
handle a particular event, the event is inherited by (or bubbled up to) the
component's container. The container then either handles the event or it is
bubbled up to its container and so on, until the highest-level container has been
tried. In the event-delegation model, specific objects are designated as event
handlers for GUI components. These objects implement event-listener interfaces.
The event-delegation model is more efficient than the event-inheritance model
because it eliminates the processing required to support the bubbling of
unhandled events.
How is it possible for two String objects with identical values not to be equal
under the == operator?
The == operator compares two objects to determine if they are the same object in
memory. It is possible for two String objects to have the same value, but located
indifferent areas of memory.
Why are the methods of the Math class static?
So they can be invoked as if they are a mathematical code library.
What Checkbox method allows you to tell if a Checkbox is checked?
getState()
What state is a thread in when it is executing?
An executing thread is in the running state.
What are the legal operands of the instanceof operator?
The left operand is an object reference or null value and the right operand is a
class, interface, or array type.
How are the elements of a GridLayout organized?
The elements of a GridBad layout are of equal size and are laid out using the
squares of a grid.
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What an I/O filter?
An I/O filter is an object that reads from one stream and writes to another, usually
altering the data in some way as it is passed from one stream to another.
If an object is garbage collected, can it become reachable again?
Once an object is garbage collected, it ceases to exist. It can no longer become
reachable again.
What are E and PI?
E is the base of the natural logarithm and PI is mathematical value pi.
Are true and false keywords?
The values true and false are not keywords.
What is a void return type?
A void return type indicates that a method does not return a value.
What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular object.
Normally, an event is enabled when a listener is added to an object for a
particular event. The enableEvents() method is used by objects that handle events
by overriding their event-dispatch methods.
What is the difference between the File and RandomAccessFile classes?
The File class encapsulates the files and directories of the local file system. The
RandomAccessFile class provides the methods needed to directly access data
contained in any part of a file.
What happens when you add a double value to a String?
The result is a String object.
What is your platform's default character encoding?
If you are running Java on English Windows platforms, it is probably Cp1252. If
you are running Java on English Solaris platforms, it is most likely 8859_1..
Which package is always imported by default?
The java.lang package is always imported by default.
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What interface must an object implement before it can be written to a
stream as an object?
An object must implement the Serializable or Externalizable interface before it
can be written to a stream as an object.
How are this and super used?
this is used to refer to the current object instance. super is used to refer to the
variables and methods of the superclass of the current object instance.
What is the purpose of garbage collection?
The purpose of garbage collection is to identify and discard objects that are no
longer needed by a program so that their resources may be reclaimed and reused.
What is a compilation unit?
A compilation unit is a Java source code file.
What interface is extended by AWT event listeners?
All AWT event listeners extend the java.util.EventListener interface.
What restrictions are placed on method overriding?
Overridden methods must have the same name, argument list, and return type.
The overriding method may not limit the access of the method it overrides. The
overriding method may not throw any exceptions that may not be thrown by the
overridden method.
How can a dead thread be restarted?
A dead thread cannot be restarted.
What happens if an exception is not caught?
An uncaught exception results in the uncaughtException() method of the thread's
ThreadGroup being invoked, which eventually results in the termination of the
program in which it is thrown.
What is a layout manager?
A layout manager is an object that is used to organize components in a container.
Which arithmetic operations can result in the throwing of an
ArithmeticException?
Integer / and % can result in the throwing of an ArithmeticException.
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What are three ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by blocking
on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking
an object's wait() method. It can also enter the waiting state by invoking its
(deprecated) suspend() method.
Can an abstract class be final?
An abstract class may not be declared as final.
What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be
loaded by a program to tailor the program's appearance to the particular locale in
which it is being run.
What happens if a try-catch-finally statement does not have a catch clause to
handle an exception that is thrown within the body of the try statement?
The exception propagates up to the next higher level try-catch statement (if any)
or results in the program's termination.
What is numeric promotion?
Numeric promotion is the conversion of a smaller numeric type to a larger
numeric type, so that integer and floating-point operations may take place. In
numerical promotion, byte, char, and short values are converted to int values. The
int values are also converted to long values, if necessary. The long and float
values are converted to double values, as required.
What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A
ScrollPane handles its own events and performs its own scrolling.
What is the difference between a public and a non-public class?
A public class may be accessed outside of its package. A non-public class may
not be accessed outside of its package.
To what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is false.
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Can try statements be nested?
Try statements may be tested.
What is the difference between the prefix and postfix forms of the ++
operator?
The prefix form performs the increment operation and returns the value of the
increment operation. The postfix form returns the current value all of the
expression and then performs the increment operation on that value.
What is the purpose of a statement block?
A statement block is used to organize a sequence of statements as a single
statement group.
What is a Java package and how is it used?
A Java package is a naming context for classes and interfaces. A package is used
to create a separate name space for groups of classes and interfaces. Packages are
also used to organize related classes and interfaces into a single API unit and to
control accessibility to these classes and interfaces.
What modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.
What are the Object and Class classes used for?
The Object class is the highest-level class in the Java class hierarchy. The Class
class is used to represent the classes and interfaces that are loaded by a Java
program.
How does a try statement determine which catch clause should be used to
handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses
of the try statement are examined in the order in which they appear. The first
catch clause that is capable of handling the exception is executed. The remaining
catch clauses are ignored.
Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the
object's finalize() method is invoked and the object performs an operation which
causes it to become accessible to reachable objects.
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When is an object subject to garbage collection?
An object is subject to garbage collection when it becomes unreachable to the
program in which it is used.
What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread
or implement the Runnable interface.
What methods are used to get and set the text label displayed by a Button
object?
getLabel() and setLabel()
Which Component subclass is used for drawing and painting?
Canvas
What are the two basic ways in which classes that can be run as threads may
be defined?
A thread class may be declared as a subclass of Thread, or it may implement the
Runnable interface.
What are the problems faced by Java programmers who don't use layout
managers?
Without layout managers, Java programmers are faced with determining how
their GUI will be displayed across multiple windowing systems and finding a
common sizing and positioning that will work within the constraints imposed by
each windowing system.
What is the difference between an if statement and a switch statement?
The if statement is used to select among two alternatives. It uses a Boolean
expression to decide which alternative should be executed. The switch statement
is used to select among multiple alternatives. It uses an int expression to
determine which alternative should be executed.
Can there be an abstract class with no abstract methods in it?
yes.
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Can an Interface be final?
yes.
Can an Interface have an inner class?
Yes public interface abc { static int i=0; void dd(); class a1 { a1() { int j;
System.out.println("in interfia"); }; public static void main(String a1[]) {
System.out.println("in interfia"); } } }
Can we define private and protected modifiers for variables in interfaces?
Yes.
What is Externalizable?
Externalizable is an Interface that extends Serializable Interface. And sends data
into Streams in Compressed Format. It has two methods,
writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
What is a local, member and a class variable?
Variables declared within a method are "local" variables.
Variables declared within the class i.e not within any methods are "member"
variables (global variables).
Variables declared within the class i.e not within any methods and are defined as
"static" are class variables
I made my class Cloneable but I still get 'Can't access protected method
clone. Why?
Yeah, some of the Java books, in particular "The Java Programming Language",
imply that all you have to do in order to have your class support clone() is
implement the Cloneable interface. Not so. Perhaps that was the intent at some
point, but that's not the way it works currently. As it stands, you have to
implement your own public clone() method, even if it doesn't do anything special
and just calls super.clone().
What are the different identifier states of a Thread?
The different identifiers of a Thread are:
R - Running or runnable thread
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S - Suspended thread
CW - Thread waiting on a condition variable
MW - Thread waiting on a monitor lock
MS - Thread suspended waiting on a monitor lock
What are some alternatives to inheritance?
Delegation is an alternative to inheritance. Delegation means that you include an
instance of another class as an instance variable, and forward messages to the
instance. It is often safer than inheritance because it forces you to think about
each message you forward, because the instance is of a known class, rather than a
new class, and because it doesn't force you to accept all the methods of the super
class: you can provide only the methods that really make sense. On the other
hand, it makes you write more code, and it is harder to re-use (because it is not a
subclass).
Why isn't there operator overloading?
Because C++ has proven by example that operator overloading makes code
almost impossible to maintain. In fact there very nearly wasn't even method
overloading in Java, but it was thought that this was too useful for some very
basic methods like print(). Note that some of the classes like DataOutputStream
have unoverloaded methods like writeInt() and writeByte().
What does it mean that a method or field is "static"?
Static variables and methods are instantiated only once per class. In other words
they are class variables, not instance variables. If you change the value of a static
variable in a particular object, the value of that variable changes for all instances
of that class.
Static methods can be referenced with the name of the class rather than the name
of a particular object of the class (though that works too). That's how library
methods like System.out.println() work. out is a static field in the
java.lang.System class.
Why do threads block on I/O?
Threads block on i/o (that is enters the waiting state) so that other threads may
execute while the i/o Operation is performed.
What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the
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access of multiple threads to shared resources. Without synchronization, it is
possible for one thread to modify a shared object while another thread is in the
process of using or updating that object's value. This often leads to significant
errors.
Is null a keyword?
The null value is not a keyword.
Which characters may be used as the second character of an identifier,but
not as the first character of an identifier?
The digits 0 through 9 may not be used as the first character of an identifier but
they may be used after the first character of an identifier.
What is the difference between notify() and notifyAll()?
notify() is used to unblock one waiting thread; notifyAll() is used to unblock all
of them. Using notify() is preferable (for efficiency) when only one blocked
thread can benefit from the change (for example, when freeing a buffer back into
a pool). notifyAll() is necessary (for correctness) if multiple threads should
resume (for example, when releasing a "writer" lock on a file might permit all
"readers" to resume).
Why can't I say just abs() or sin() instead of Math.abs() and Math.sin()?
The import statement does not bring methods into your local name space. It lets
you abbreviate class names, but not get rid of them altogether. That's just the way
it works, you'll get used to it. It's really a lot safer this way.
However, there is actually a little trick you can use in some cases that gets you
what you want. If your top-level class doesn't need to inherit from anything else,
make it inherit from java.lang.Math. That *does* bring all the methods into your
local name space. But you can't use this trick in an applet, because you have to
inherit from java.awt.Applet. And actually, you can't use it on java.lang.Math at
all, because Math is a "final" class which means it can't be extended.
Why are there no global variables in Java?
Global variables are considered bad form for a variety of reasons: · Adding state
variables breaks referential transparency (you no longer can understand a
statement or expression on its own: you need to understand it in the context of the
settings of the global variables).
· State variables lessen the cohesion of a program: you need to know more to
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understand how something works. A major point of Object-Oriented
programming is to break up global state into more easily understood collections
of local state.
· When you add one variable, you limit the use of your program to one instance.
What you thought was global, someone else might think of as local: they may
want to run two copies of your program at once.
For these reasons, Java decided to ban global variables.
What does it mean that a class or member is final?
A final class can no longer be subclassed. Mostly this is done for security reasons
with basic classes like String and Integer. It also allows the compiler to make
some optimizations, and makes thread safety a little easier to achieve. Methods
may be declared final as well. This means they may not be overridden in a
subclass.
Fields can be declared final, too. However, this has a completely different
meaning. A final field cannot be changed after it's initialized, and it must include
an initializer statement where it's declared. For example,
public final double c = 2.998;
It's also possible to make a static field final to get the effect of C++'s const
statement or some uses of C's #define, e.g. public static final double c = 2.998;
What does it mean that a method or class is abstract?
An abstract class cannot be instantiated. Only its subclasses can be instantiated.
You indicate that a class is abstract with the abstract keyword like this:
public abstract class Container extends Component {
Abstract classes may contain abstract methods. A method declared abstract is not
actually implemented in the current class. It exists only to be overridden in
subclasses. It has no body. For example,
public abstract float price();
Abstract methods may only be included in abstract classes. However, an abstract
class is not required to have any abstract methods, though most of them do.
Each subclass of an abstract class must override the abstract methods of its
superclasses or itself be declared abstract.
What is the main difference between Java platform and other platforms?
The Java platform differs from most other platforms in that it's a software-only
platform that runs on top of other hardware-based platforms.
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The Java platform has three elements:
Java programming language
The Java Virtual Machine (Java VM)
The Java Application Programming Interface (Java API)
What is the Java Virtual Machine?
The Java Virtual Machine is a software that can be ported onto various hardwarebased platforms.
What is the Java API?
The Java API is a large collection of ready-made software components that
provide many useful capabilities, such as graphical user interface (GUI) widgets.
What is the package?
The package is a Java namespace or part of Java libraries. The Java API is
grouped into libraries of related classes and interfaces; these libraries are known
as packages.
What is native code?
The native code is code that after you compile it, the compiled code runs on a
specific hardware platform.
Explain the user defined Exceptions?
User defined Exceptions are the separate Exception classes defined by the user
for specific purposed. An user defined can created by simply sub-classing it to the
Exception class. This allows custom exceptions to be generated (using throw) and
caught in the same way as normal exceptions.
Example:
class myCustomException extends Exception {
// The class simply has to exist to be an exception
}
Is Java code slower than native code?
Not really. As a platform-independent environment, the Java platform can be a bit
slower than native code. However, smart compilers, well-tuned interpreters, and
just-in-time bytecode compilers can bring performance close to that of native
code without threatening portability.
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Can main() method be overloaded?
Yes. the main() method is a special method for a program entry. You can
overload main() method in any ways. But if you change the signature of the main
method, the entry point for the program will be gone.
What is the serialization?
The serialization is a kind of mechanism that makes a class or a bean persistence
by having its properties or fields and state information saved and restored to and
from storage.
Explain the new Features of JDBC 2.0 Core API?
The JDBC 2.0 API includes the complete JDBC API, which includes both core
and Optional Package API, and provides inductrial-strength database computing
capabilities.
New Features in JDBC 2.0 Core API:
Scrollable result sets- using new methods in the ResultSet interface allows
programmatically move the to particular row or to a position relative to its current
position
JDBC 2.0 Core API provides the Batch Updates functionality to the java
applications.
Java applications can now use the ResultSet.updateXXX methods.
New data types - interfaces mapping the SQL3 data types
Custom mapping of user-defined types (UTDs)
Miscellaneous features, including performance hints, the use of character streams,
full precision for java.math.BigDecimal values, additional security, and support
for time zones in date, time, and timestamp values.
How you can force the garbage collection?
Garbage collection automatic process and can't be forced.
Explain garbage collection?
Garbage collection is one of the most important feature of Java. Garbage
collection is also called automatic memory management as JVM automatically
removes the unused variables/objects (value is null) from the memory. User
program cann't directly free the object from memory, instead it is the job of the
garbage collector to automatically free the objects that are no longer referenced
by a program. Every class inherits finalize() method from java.lang.Object, the
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finalize() method is called by garbage collector when it determines no more
references to the object exists. In Java, it is good idea to explicitly assign null into
a variable when no more in use. I Java on calling System.gc() and Runtime.gc(),
JVM tries to recycle the unused objects, but there is no guarantee when all the
objects will garbage collected.
Describe the principles of OOPS.
There are three main principals of oops which are called Polymorphism,
Inheritance and Encapsulation.
Explain the Encapsulation principle.
Encapsulation is a process of binding or wrapping the data and the codes that
operates on the data into a single entity. This keeps the data safe from outside
interface and misuse. One way to think about encapsulation is as a protective
wrapper that prevents code and data from being arbitrarily accessed by other code
defined outside the wrapper.
Explain the Inheritance principle.
Inheritance is the process by which one object acquires the properties of another
object.
Explain the Polymorphism principle.
The meaning of Polymorphism is something like one name many forms.
Polymorphism enables one entity to be used as as general category for different
types of actions. The specific action is determined by the exact nature of the
situation. The concept of polymorphism can be explained as "one interface,
multiple methods".
Explain the different forms of Polymorphism.
From a practical programming viewpoint, polymorphism exists in three distinct
forms in Java:
Method overloading
Method overriding through inheritance
Method overriding through the Java interface
What are Access Specifiers available in Java?
ccess specifiers are keywords that determines the type of access to the member of
a class. These are:
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Public
Protected
Private
Defaults
Describe the wrapper classes in Java.
Wrapper class is wrapper around a primitive data type. An instance of a wrapper
class contains, or wraps, a primitive value of the corresponding type.
Following table lists the primitive types and the corresponding wrapper classes:
Primitive Wrapper
boolean java.lang.Boolean
byte java.lang.Byte
char java.lang.Character
double java.lang.Double
float java.lang.Float
int java.lang.Integer
long java.lang.Long
short java.lang.Short
void java.lang.Void
Question: Read the following program:
public class test {
public static void main(String [] args) {
int x = 3;
int y = 1;
if (x = y)
System.out.println("Not equal");
else
System.out.println("Equal");
}
}
What is the result?
A. The output is “Equal”
B. The output in “Not Equal”
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C. An error at " if (x = y)" causes compilation to fall.
D. The program executes but no output is show on console.
Answer: C
Use the Externalizable interface when you need complete control over your
Bean's serialization (for example, when writing and reading a specific file
format).
No. Earlier order is maintained.
The superclass constructor runs before the subclass constructor. The
subclass's version of the overridable method will be invoked before the
subclass's constructor has been invoked. If the subclass's overridable method
depends on the proper initialization of the subclass (through the subclass
constructor), the method will most likely fail. Is that true?
Yes. It is true
Why are the interfaces more flexible than abstract classes?
--An interface-defined type can be implemented by any class in a class hierarchy
and can be extended by another interface. In contrast, an abstract-class-defined
type can be implemented only by classes that subclass the abstract class.
--An interface-defined type can be used well in polymorphism. The so-called
interface type vs. implementation types.
--Abstract classes evolve more easily than interfaces. If you add a new concrete
method to an abstract class, the hierarchy system is still working. If you add a
method to an interface, the classes that rely on the interface will break when
recompiled.
--Generally, use interfaces for flexibility; use abstract classes for ease of
evolution (like expanding class functionality).
What are new language features in J2SE 5.0?
Generally:
1. generics
2. static imports
3. annotations
4. typesafe enums
5. enhanced for loop
6. autoboxing/unboxing
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7. varargs
8. covariant return types
What is covariant return type?
A covariant return type lets you override a superclass method with a return type
that subtypes the superclass method's return type. So we can use covariant return
types to minimize upcasting and downcasting.
class Parent {
Parent foo () {
System.out.println ("Parent foo() called");
return this;
}
}
class Child extends Parent {
Child foo () {
System.out.println ("Child foo() called");
return this;
}
}
class Covariant {
public static void main(String[] args) {
Child c = new Child();
Child c2 = c.foo(); // c2 is Child
Parent c3 = c.foo(); // c3 points to Child
}
}
What is the result of the following statement?
int i = 1, float f = 2.0f;
i += f; //ok, the cast done automatically by the compiler
i = i + f; //error
The compound assignment operators automatically include cast operations in
their behaviors.
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What is externalization? Where is it useful?
Use the Externalizable interface when you need complete control over your
Bean's serialization (for example, when writing and reading a specific file
format).
What will be the output on executing the following code.
public class MyClass {
public static void main (String args[] ) {
int abc[] = new int [5];
System.out.println(abc);
}
}
A Error array not initialized
B5
C null
D Print some junk characters
Answer : D
It will print some junk characters to the output. Here it will not give any compile
time or runtime error because we have declared and initialized the array properly.
Event if we are not assigning a value to the array, it will always initialized to its
defaults.
What will be the output on executing the following code.
public class MyClass {
public static void main (String args[] ) {
int abc[] = new int [5];
System.out.println(abc[0]);
}
}
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A Error array not initialized
B5
C0
D Print some junk characters
Answer : C.
What is a marker interface ?
An interface that contains no methods. E.g.: Serializable, Cloneable,
SingleThreadModel etc. It is used to just mark java classes that support certain
capability.
What are tag interfaces?
Tag interface is an alternate name for marker interface.
What are the restrictions placed on static method ?
We cannot override static methods. We cannot access any object variables inside
static method. Also the this reference also not available in static methods.
What is JVM?
JVM stands for Java Virtual Machine. It is the run time for java programs. All are
java programs are running inside this JVM only. It converts java byte code to OS
specific commands. In addition to governing the execution of an application's
byte codes, the virtual machine handles related tasks such as managing the
system's memory, providing security against malicious code, and managing
multiple threads of program execution.
What is JIT?
JIT stands for Just In Time compiler. It compiles java byte code to native code.
What are ClassLoaders?
A class loader is an object that is responsible for loading classes. The class
ClassLoader is an abstract class. Given the name of a class, a class loader should
attempt to locate or generate data that constitutes a definition for the class. A
typical strategy is to transform the name into a file name and then read a "class
file" of that name from a file system.
Every Class object contains a reference to the ClassLoader that defined it.
Class objects for array classes are not created by class loaders, but are created
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automatically as required by the Java runtime. The class loader for an array class,
as returned by Class.getClassLoader() is the same as the class loader for its
element type; if the element type is a primitive type, then the array class has no
class loader.
Applications implement subclasses of ClassLoader in order to extend the manner
in which the Java virtual machine dynamically loads classes.
What is Service Locator pattern?
The Service Locator pattern locates J2EE (Java 2 Platform, Enterprise Edition)
services for clients and thus abstracts the complexity of network operation and
J2EE service lookup as EJB (Enterprise JavaBean) Interview Questions - Home
and JMS (Java Message Service) component factories. The Service Locator hides
the lookup process's implementation details and complexity from clients. To
improve application performance, Service Locator caches service objects to
eliminate unnecessary JNDI (Java Naming and Directory Interface) activity that
occurs in a lookup operation.
What is Session Facade pattern?
Session facade is one design pattern that is often used while developing enterprise
applications. It is implemented as a higher level component (i.e.: Session EJB),
and it contains all the iteractions between low level components (i.e.: Entity EJB).
It then provides a single interface for the functionality of an application or part of
it, and it decouples lower level components simplifying the design. Think of a
bank situation, where you have someone that would like to transfer money from
one account to another. In this type of scenario, the client has to check that the
user is authorized, get the status of the two accounts, check that there are enough
money on the first one, and then call the transfer. The entire transfer has to be
done in a single transaction otherwise is something goes south, the situation has
to be restored.
As you can see, multiple server-side objects need to be accessed and possibly
modified. Multiple fine-grained invocations of Entity (or even Session) Beans
add the overhead of network calls, even multiple transaction. In other words, the
risk is to have a solution that has a high network overhead, high coupling, poor
reusability and mantainability.
The best solution is then to wrap all the calls inside a Session Bean, so the clients
will have a single point to access (that is the session bean) that will take care of
handling all the rest.
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What is Data Access Object pattern?
The Data Access Object (or DAO) pattern: separates a data resource's client
interface from its data access mechanisms adapts a specific data resource's access
API to a generic client interface
The DAO pattern allows data access mechanisms to change independently of the
code that uses the data.
The DAO implements the access mechanism required to work with the data
source. The data source could be a persistent store like an RDBMS, an external
service like a B2B exchange, a repository like an LDAP database, or a business
service accessed via CORBA Internet Inter-ORB Protocol (IIOP) or low-level
sockets. The business component that relies on the DAO uses the simpler
interface exposed by the DAO for its clients. The DAO completely hides the data
source implementation details from its clients. Because the interface exposed by
the DAO to clients does not change when the underlying data source
implementation changes, this pattern allows the DAO to adapt to different storage
schemes without affecting its clients or business components. Essentially, the
DAO acts as an adapter between the component and the data source.
Can we make an EJB singleton?
This is a debatable question, and for every answer we propose there can be
contradictions. I propose 2 solutions of the same. Remember that EJB's are
distributed components and can be deployed on different JVM's in a Distributed
environment
i) Follow the steps as given below
Make sure that your serviceLocator is deployed on only one JVM.
In the serviceLocator create a HashTable/HashMap(You are the right judge to
choose between these two)
When ever a request comes for an EJB to a serviceLocator, it first checks in the
HashTable if an entry already exists in the table with key being the JNDI name of
EJB. If key is present and value is not null, return the existing reference, else
lookup the EJB in JNDI as we do normally and add an entry into the Hashtable
before returning it to the client. This makes sure that you maintain a singleton of
EJB.
ii) In distributed environment our components/Java Objects would be running on
different JVM's. So the normal singleton code we write for maintaining single
instance works fine for single JVM, but when the class could be loaded in
multiple JVM's and Instantiated in multiple JVM's normal singleton code does
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not work. This is because the ClassLoaders being used in the different JVM's are
different from each other and there is no defined mechanism to check and
compare what is loaded in another JVM. A solution could be(Not tested yet. Need
your feedback on this) to write our own ClassLoader and pass this classLoader as
argument, whenever we are creating a new Instance and make sure that only one
instance is created for the proposed class. This can be done easily.
How can we make a class Singleton ?
A) If the class is Serializable
class Singleton implements Serializable
{
private static Singleton instance;
private Singleton() { }
public static synchronized Singleton getInstance()
{
if (instance == null)
instance = new Singleton();
return instance;
}
/**
If the singleton implements Serializable, then this
* method must be supplied.
*/
protected Object readResolve() {
return instance;
}
/**
This method avoids the object fro being cloned
*/
public Object clone() {
throws CloneNotSupportedException ;
//return instance;
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}
}
B) If the class is NOT Serializable
class Singleton
{
private static Singleton instance;
private Singleton() { }
public static synchronized Singleton getInstance()
{
if (instance == null)
instance = new Singleton();
return instance;
}
/**
This method avoids the object from being cloned
**/
public Object clone() {
throws CloneNotSupportedException ;
//return instance;
}
}
How is static Synchronization different form non-static synchronization?
When Synchronization is applied on a static Member or a static block, the lock is
performed on the Class and not on the Object, while in the case of a Non-static
block/member, lock is applied on the Object and not on class. [Trail 2: There is a
class called Class in Java whose object is associated with the object(s) of your
class. All the static members declared in your class will have reference in this
class(Class). As long as your class exists in memory this object of Class is also
present. Thats how even if you create multiple objects of your class only one
Class object is present and all your objects are linked to this Class object. Even
though one of your object is GCed after some time, this object of Class is not
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GCed untill all the objects associated with it are GCed.
This means that when ever you call a "static synchronized" block, JVM locks
access to this Class object and not any of your objects. Your client can till access
the non-static members of your objects.
What are class members and Instance members?
Any global members(Variables, methods etc.) which are static are called as Class
level members and those which are non-static are called as Instance level
members.
Name few Garbage collection algorithms?
Here they go:
Mark and Sweep
Reference counting
Tracing collectors
Copying collectors
Heap compaction
Mark-compact collectors
Can we force Garbage collection?
java follows a philosophy of automatic garbage collection, you can suggest or
encourage the JVM to perform garbage collection but you can not force it. Once a
variable is no longer referenced by anything it is available for garbage collection.
You can suggest garbage collection with System.gc(), but this does not guarantee
when it will happen. Local variables in methods go out of scope when the method
exits. At this point the methods are eligible for garbage collection. Each time the
method comes into scope the local variables are re-created.
Does Java pass by Value or reference?
Its uses Reference while manipulating objects but pass by value when sending
method arguments. Those who feel why I added this simple question in this
section while claiming to be maintaining only strong and interesting questions, go
ahead and answer following questions.
a)What is the out put of:
import java.util.*;
class TestCallByRefWithObject
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{
ArrayList list = new ArrayList(5);
public void remove(int index){
list.remove(index);
}
public void add(Object obj){
list.add(obj);
}
public void display(){
System.out.println(list);
}
public static void main(String[] args)
{
TestCallByRefWithObject test = new TestCallByRefWithObject();
test.add("1");
test.add("2");
test.add("3");
test.add("4");
test.add("5");
test.remove(4);
test.display();
}
}
b) And now what is the output of:
import java.util.*;
class TestCallByRefWithInt
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{
int i = 5;
public void decrement(int i){
i--;
}
public void increment(int i){
i++;
}
public void display(){
System.out.println("\nValue of i is : " +i);
}
public static void main(String[] args)
{
TestCallByRefWithInt test = new TestCallByRefWithInt();
test.increment(test.i);
test.display();
}
}
Why Thread is faster compare to process?
A thread is never faster than a process. If you run a thread(say there's a process
which has spawned only one thread) in one JVM and a process in another and
that both of them require same resources then both of them would take same time
to execute. But, when a program/Application is thread based(remember here there
will be multiple threads running for a single process) then definetly a thread
based appliation/program is faster than a process based application. This is
because, when ever a process requires or waits for a resource CPU takes it out of
the critical section and allocates the mutex to another process.
Before deallocating the ealier one, it stores the context(till what state did it
execute that process) in registers. Now if this deallocated process has to come
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back and execute as it has got the resource for which it was waiting, then it can't
go into critical section directly. CPU asks that process to follow scheduling
algorithm. So this process has to wait again for its turn. While in the case of
thread based application, the application is still with CPU only that thread which
requires some resource goes out, but its co threads(of same process/apllication)
are still in the critical section. Hence it directly comes back to the CPU and does
not wait outside. Hence an application which is thread based is faster than an
application which is process based.
Be sure that its not the competion between thread and process, its between an
application which is thread based or process based.
When and How is an object considered as Garbage by a GC?
An object is considered garbage when it can no longer be reached from any
pointer in the running program. The most straightforward garbage collection
algorithms simply iterate over every reachable object. Any objects left over are
then considered garbage.
What are generations in Garbage Collection terminology? What is its
relevance?
Garbage Collectors make assumptions about how our application runs. Most
common assumption is that an object is most likely to die shortly after it was
created: called infant mortality. This assumes that an object that has been around
for a while, will likely stay around for a while. GC organizes objects into
generations (young, tenured, and perm). This tells that if an object lives for more
than certain period of time it is moved from one generation to another
generations( say from young -> tenured -> permanent). Hence GC will be run
more frequently at the young generations and rarely at permanent generations.
This reduces the overhead on GC and gives faster response time.
What is a Throughput Collector?
The throughput collector is a generational collector similar to the default collector
but with multiple threads used to do the minor collection. The major collections
are essentially the same as with the default collector. By default on a host with N
CPUs, the throughput collector uses N garbage collector threads in the collection.
The number of garbage collector threads can be controlled with a command line
option.
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When to Use the Throughput Collector?
Use the throughput collector when you want to improve the performance of your
application with larger numbers of processors. In the default collector garbage
collection is done by one thread, and therefore garbage collection adds to the
serial execution time of the application. The throughput collector uses multiple
threads to execute a minor collection and so reduces the serial execution time of
the application. A typical situation is one in which the application has a large
number of threads allocating objects. In such an application it is often the case
that a large young generation is needed
What is Aggressive Heap?
The -XX:+AggressiveHeap option inspects the machine resources (size of
memory and number of processors) and attempts to set various parameters to be
optimal for long-running, memory allocation-intensive jobs. It was originally
intended for machines with large amounts of memory and a large number of
CPUs, but in the J2SE platform, version 1.4.1 and later it has shown itself to be
useful even on four processor machines. With this option the throughput collector
(-XX:+UseParallelGC) is used along with adaptive sizing (XX:+UseAdaptiveSizePolicy). The physical memory on the machines must be at
least 256MB before Aggressive Heap can be used.
What is a Concurrent Low Pause Collector?
The concurrent low pause collector is a generational collector similar to the
default collector. The tenured generation is collected concurrently with this
collector. This collector attempts to reduce the pause times needed to collect the
tenured generation. It uses a separate garbage collector thread to do parts of the
major collection concurrently with the applications threads. The concurrent
collector is enabled with the command line option XX:+UseConcMarkSweepGC. For each major collection the concurrent collector
will pause all the application threads for a brief period at the beginning of the
collection and toward the middle of the collection. The second pause tends to be
the longer of the two pauses and multiple threads are used to do the collection
work during that pause. The remainder of the collection is done with a garbage
collector thread that runs concurrently with the application. The minor collections
are done in a manner similar to the default collector, and multiple threads can
optionally be used to do the minor collection.
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When to Use the Concurrent Low Pause Collector?
Use the concurrent low pause collector if your application would benefit from
shorter garbage collector pauses and can afford to share processor resources with
the garbage collector when the application is running. Typically applications
which have a relatively large set of long-lived data (a large tenured generation),
and run on machines with two or more processors tend to benefit from the use of
this collector. However, this collector should be considered for any application
with a low pause time requirement. Optimal results have been observed for
interactive applications with tenured generations of a modest size on a single
processor.
What is Incremental Low Pause Collector?
The incremental low pause collector is a generational collector similar to the
default collector. The minor collections are done with the same young generation
collector as the default collector. Do not use either -XX:+UseParallelGC or XX:+UseParNewGC with this collector. The major collections are done
incrementally on the tenured generation. This collector (also known as the train
collector) collects portions of the tenured generation at each minor collection. The
goal of the incremental collector is to avoid very long major collection pauses by
doing portions of the major collection work at each minor collection. The
incremental collector will sometimes find that a non-incremental major collection
(as is done in the default collector) is required in order to avoid running out of
memory.
When to Use the Incremental Low Pause Collector?
Use the incremental low pause collector when your application can afford to trade
longer and more frequent young generation garbage collection pauses for shorter
tenured generation pauses. A typical situation is one in which a larger tenured
generation is required (lots of long-lived objects), a smaller young generation will
suffice (most objects are short-lived and don't survive the young generation
collection), and only a single processor is available.
How do you enable the concurrent garbage collector on Sun's JVM?
-Xconcgc options allows us to use concurrent garbage collector (1.2.2_07+)we
can also use -XX:+UseConcMarkSweepGC which is available beginning with
J2SE 1.4.1.
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What is a platform?
A platform is the hardware or software environment in which a program runs.
Most platforms can be described as a combination of the operating system and
hardware, like Windows 2000 and XP, Linux, Solaris, and MacOS.
What is transient variable?
Transient variable can't be serialize. For example if a variable is declared as
transient in a Serializable class and the class is written to an ObjectStream, the
value of the variable can't be written to the stream instead when the class is
retrieved from the ObjectStream the value of the variable becomes null.
How to make a class or a bean serializable?
By implementing either the java.io.Serializable interface, or the
java.io.Externalizable interface. As long as one class in a class's inheritance
hierarchy implements Serializable or Externalizable, that class is serializable.
What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return
types.
Name Container classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane
What is the List interface?
The List interface provides support for ordered collections of objects.
What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A
ScrollPane handles its own events and performs its own scrolling.
What is tunnelling?
Tunnelling is a route to somewhere. For example, RMI tunnelling is a way to
make RMI application get through firewall. In CS world, tunnelling means a way
to transfer data.
What is meant by "Abstract Interface"?
First, an interface is abstract. That means you cannot have any implementation in
an interface. All the methods declared in an interface are abstract methods or
signatures of the methods.
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Can Java code be compiled to machine dependent executable file?
Yes. There are many tools out there. If you did so, the generated exe file would
be run in the specific platform, not cross-platform.
Do not use the String contatenation operator in lengthy loops or other places
where performance could suffer. Is that true?
Yes.
What method is used to specify a container's layout?
The setLayout() method is used to specify a container's layout.
Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.
What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.
What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on
collections of objects.
What is the List interface?
The List interface provides support for ordered collections of objects.
Is sizeof a keyword?
The sizeof operator is not a keyword in Java.
Which class is the superclass for every class.
Object.
Which Container method is used to cause a container to be laid out and
redisplayed?
validate()
What's the difference between a queue and a stack?
Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule
What comes to mind when you hear about a young generation in Java?
Garbage collection.
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You can create an abstract class that contains only abstract methods. On the
other hand, you can create an interface that declares the same methods. So
can you use abstract classes instead of interfaces?
Sometimes. But your class may be a descendent of another class and in this case
the interface is your only option.
What comes to mind when someone mentions a shallow copy in Java?
Object cloning.
If you're overriding the method equals() of an object, which other method
you might also consider?
hashCode()
You are planning to do an indexed search in a list of objects. Which of the
two Java collections should you use: ArrayList or LinkedList?
ArrayList
How would you make a copy of an entire Java object with its state?
Have this class implement Cloneable interface and call its method clone().
How can you minimize the need of garbage collection and make the memory
use more effective?
Use object pooling and weak object references.
There are two classes: A and B. The class B need to inform a class A when
some important event has happened. What Java technique would you use to
implement it?
If these classes are threads I'd consider notify() or notifyAll(). For regular classes
you can use the Observer interface.
What access level do you need to specify in the class declaration to ensure
that only classes from the same directory can access it?
You do not need to specify any access level, and Java will use a default package
access level.
What is the difference between an Interface and an Abstract class?
An abstract class can have instance methods that implement a default behavior.
An Interface can only declare constants and instance methods, but cannot
implement default behavior and all methods are implicitly abstract. An interface
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has all public members and no implementation. An abstract class is a class which
may have the usual flavors of class members (private, protected, etc.), but has
some abstract methods.
What is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to identify and discard objects that are no
longer needed by a program so that their resources can be reclaimed and reused.
A Java object is subject to garbage collection when it becomes unreachable to the
program in which it is used.
Describe synchronization in respect to multithreading.
With respect to multithreading, synchronization is the capability to control the
access of multiple threads to shared resources. Without synchonization, it is
possible for one thread to modify a shared variable while another thread is in the
process of using or updating same shared variable. This usually leads to
significant errors.
Explain different way of using thread?
The thread could be implemented by using runnable interface or by inheriting
from the Thread class. The former is more advantageous, 'cause when you are
going for multiple inheritance..the only interface can help.
What are pass by reference and passby value?
Pass By Reference means the passing the address itself rather than passing the
value. Passby Value means passing a copy of the value to be passed.
What is HashMap and Map?
Map is Interface and Hashmap is class that implements that.
Difference between HashMap and HashTable?
The HashMap class is roughly equivalent to Hashtable, except that it is
unsynchronized and permits nulls. (HashMap allows null values as key and value
whereas Hashtable doesnt allow). HashMap does not guarantee that the order of
the map will remain constant over time. HashMap is unsynchronized and
Hashtable is synchronized.
Difference between Vector and ArrayList?
Vector is synchronized whereas arraylist is not.
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Difference between Swing and AWT?
AWT are heavy-weight components. Swings are light-weight components. Hence
swing works faster than AWT.
What is the difference between a constructor and a method?
A constructor is a member function of a class that is used to create objects of that
class. It has the same name as the class itself, has no return type, and is invoked
using the new operator. A method is an ordinary member function of a class. It
has its own name, a return type (which may be void), and is invoked using the dot
operator.
What is an Iterator?
Some of the collection classes provide traversal of their contents via a
java.util.Iterator interface. This interface allows you to walk through a collection
of objects, operating on each object in turn. Remember when using Iterators that
they contain a snapshot of the collection at the time the Iterator was obtained;
generally it is not advisable to modify the collection itself while traversing an
Iterator.
State the significance of public, private, protected, default modifiers both
singly and in combination and state the effect of package relationships on
declared items qualified by these modifiers.
public : Public class is visible in other packages, field is visible everywhere (class
must be public too) private : Private variables or methods may be used only by an
instance of the same class that declares the variable or method, A private feature
may only be accessed by the class that owns the feature. protected : Is available to
all classes in the same package and also available to all subclasses of the class
that owns the protected feature. This access is provided even to subclasses that
reside in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public
private or protected). It means that it is visible to all within a particular package.
What is an abstract class?
Abstract class must be extended/subclassed (to be useful). It serves as a template.
A class that is abstract may not be instantiated (ie, you may not call its
constructor), abstract class may contain static data. Any class with an abstract
method is automatically abstract itself, and must be declared as such.
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A class may be declared abstract even if it has no abstract methods. This prevents
it from being instantiated.
What is static in java?
Static means one per class, not one for each object no matter how many instance
of a class might exist. This means that you can use them without creating an
instance of a class. Static methods are implicitly final, because overriding is done
based on the type of the object, and static methods are attached to a class, not an
object. A static method in a super class can be shadowed by another static method
in a subclass, as long as the original method was not declared final. However, you
can't override a static method with a no static method. In other words, you can't
change a static method into an instance method in a subclass.
What is final?
A final class can't be extended ie., final class may not be subclassed. A final
method can't be overridden when its class is inherited. You can't change value of
a final variable (is a constant).
What if the main method is declared as private?
The program compiles properly but at runtime it will give "Main method not
public." message.
What if the static modifier is removed from the signature of the main
method?
Program compiles. But at runtime throws an error "NoSuchMethodError".
What if I write static public void instead of public static void?
Program compiles and runs properly.
What if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error "NoSuchMethodError".
What is the first argument of the String array in main method?
The String array is empty. It does not have any element. This is unlike C/C++
where the first element by default is the program name.
If I do not provide any arguments on the command line, then the String
array of Main method will be empty or null?
It is empty. But not null.
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How can one prove that the array is not null but empty using one line of
code?
Print args.length. It will print 0. That means it is empty. But if it would have been
null then it would have thrown a NullPointerException on attempting to print
args.length.
Can an application have multiple classes having main method?
Yes it is possible. While starting the application we mention the class name to be
run. The JVM will look for the Main method only in the class whose name you
have mentioned. Hence there is not conflict amongst the multiple classes having
main method.
Can I have multiple main methods in the same class?
No the program fails to compile. The compiler says that the main method is
already defined in the class.
Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.
Can I import same package/class twice? Will the JVM load the package
twice at runtime?
One can import the same package or same class multiple times. Neither compiler
nor JVM complains abt it. And the JVM will internally load the class only once
no matter how many times you import the same class.
What are Checked and UnChecked Exception?
A checked exception is some subclass of Exception (or Exception itself),
excluding class RuntimeException and its subclasses. Making an exception
checked forces client programmers to deal with the possibility that the exception
will be thrown. eg, IOException thrown by java.io.FileInputStream's read()
method· Unchecked exceptions are RuntimeException and any of its subclasses.
Class Error and its subclasses also are unchecked. With an unchecked exception,
however, the compiler doesn't force client programmers either to catch the
exception or declare it in a throws clause. In fact, client programmers may not
even know that the exception could be thrown. eg,
StringIndexOutOfBoundsException thrown by String's charAt() method·
Checked exceptions must be caught at compile time. Runtime exceptions do not
need to be. Errors often cannot be.
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What is Overriding?
When a class defines a method using the same name, return type, and arguments
as a method in its superclass, the method in the class overrides the method in the
superclass.
When the method is invoked for an object of the class, it is the new definition of
the method that is called, and not the method definition from superclass. Methods
may be overridden to be more public, not more private.
What are different types of inner classes?
Nested top-level classes, Member classes, Local classes, Anonymous classes
Nested top-level classes- If you declare a class within a class and specify the
static modifier, the compiler treats the class just like any other top-level class.
Any class outside the declaring class accesses the nested class with the declaring
class name acting similarly to a package. eg, outer.inner. Top-level inner classes
implicitly have access only to static variables.There can also be inner interfaces.
All of these are of the nested top-level variety.
Member classes - Member inner classes are just like other member methods and
member variables and access to the member class is restricted, just like methods
and variables. This means a public member class acts similarly to a nested toplevel class. The primary difference between member classes and nested top-level
classes is that member classes have access to the specific instance of the
enclosing class.
Local classes - Local classes are like local variables, specific to a block of code.
Their visibility is only within the block of their declaration. In order for the class
to be useful beyond the declaration block, it would need to implement a more
publicly available interface.Because local classes are not members, the modifiers
public, protected, private, and static are not usable.
Anonymous classes - Anonymous inner classes extend local inner classes one
level further. As anonymous classes have no name, you cannot provide a
constructor.
Are the imports checked for validity at compile time? e.g. will the code
containing an import such as java.lang.ABCD compile?
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Yes the imports are checked for the semantic validity at compile time. The code
containing above line of import will not compile. It will throw an error saying,can
not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;
Does importing a package imports the subpackages as well? e.g. Does
importing com.MyTest.* also import com.MyTest.UnitTests.*?
No you will have to import the subpackages explicitly. Importing com.MyTest.*
will import classes in the package MyTest only. It will not import any class in any
of it's subpackage.
What is the difference between declaring a variable and defining a variable?
In declaration we just mention the type of the variable and it's name. We do not
initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s
= "abcd"; are both definitions.
What is the default value of an object reference declared as an instance
variable?
Null unless we define it explicitly.
Can a top level class be private or protected?
No. A top level class can not be private or protected. It can have either "public" or
no modifier. If it does not have a modifier it is supposed to have a default
access.If a top level class is declared as private the compiler will complain that
the "modifier private is not allowed here". This means that a top level class can
not be private. Same is the case with protected.
What type of parameter passing does Java support?
In Java the arguments are always passed by value .
Primitive data types are passed by reference or pass by value?
Primitive data types are passed by value.
Objects are passed by value or by reference?
Java only supports pass by value. With objects, the object reference itself is
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passed by value and so both the original reference and parameter copy both refer
to the same object .
What is serialization?
Serialization is a mechanism by which you can save the state of an object by
converting it to a byte stream.
How do I serialize an object to a file?
The class whose instances are to be serialized should implement an interface
Serializable. Then you pass the instance to the ObjectOutputStream which is
connected to a fileoutputstream. This will save the object to a file.
Which methods of Serializable interface should I implement?
The serializable interface is an empty interface, it does not contain any methods.
So we do not implement any methods.
How can I customize the serialization process? i.e. how can one have a
control over the serialization process?
Yes it is possible to have control over serialization process. The class should
implement Externalizable interface. This interface contains two methods namely
readExternal and writeExternal. You should implement these methods and write
the logic for customizing the serialization process.
What is the common usage of serialization?
Whenever an object is to be sent over the network, objects need to be serialized.
Moreover if the state of an object is to be saved, objects need to be serilized.
What is Externalizable interface?
Externalizable is an interface which contains two methods readExternal and
writeExternal. These methods give you a control over the serialization
mechanism. Thus if your class implements this interface, you can customize the
serialization process by implementing these methods.
When you serialize an object, what happens to the object references included
in the object?
The serialization mechanism generates an object graph for serialization. Thus it
determines whether the included object references are serializable or not. This is a
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recursive process. Thus when an object is serialized, all the included objects are
also serialized alongwith the original obect.
What one should take care of while serializing the object?
One should make sure that all the included objects are also serializable. If any of
the objects is not serializable then it throws a NotSerializableException.
What happens to the static fields of a class during serialization?
There are three exceptions in which serialization doesnot necessarily read and
write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular
state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.
Does Java provide any construct to find out the size of an object?
No there is not sizeof operator in Java. So there is not direct way to determine the
size of an object directly in Java.
What are wrapper classes?
Java provides specialized classes corresponding to each of the primitive data
types. These are called wrapper classes. They are e.g. Integer, Character, Double
etc.
Give a simplest way to find out the time a method takes for execution
without using any profiling tool?
Read the system time just before the method is invoked and immediately after
method returns. Take the time difference, which will give you the time taken by a
method for execution.
To put it in code...
long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();
System.out.println ("Time taken for execution is " + (end - start));
Remember that if the time taken for execution is too small, it might show that it is
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taking zero milliseconds for execution. Try it on a method which is big enough, in
the sense the one which is doing considerable amount of processing.
Why do we need wrapper classes?
It is sometimes easier to deal with primitives as objects. Moreover most of the
collection classes store objects and not primitive data types. And also the wrapper
classes provide many utility methods also. Because of these reasons we need
wrapper classes. And since we create instances of these classes we can store them
in any of the collection classes and pass them around as a collection. Also we can
pass them around as method parameters where a method expects an object.
What are checked exceptions?
Checked exception are those which the Java compiler forces you to catch. e.g.
IOException are checked Exceptions.
What are runtime exceptions?
Runtime exceptions are those exceptions that are thrown at runtime because of
either wrong input data or because of wrong business logic etc. These are not
checked by the compiler at compile time.
What is the difference between error and an exception?
An error is an irrecoverable condition occurring at runtime. Such as
OutOfMemory error. These JVM errors and you can not repair them at runtime.
While exceptions are conditions that occur because of bad input etc. e.g.
FileNotFoundException will be thrown if the specified file does not exist. Or a
NullPointerException will take place if you try using a null reference. In most of
the cases it is possible to recover from an exception (probably by giving user a
feedback for entering proper values etc.).
How to create custom exceptions?
Your class should extend class Exception, or some more specific type thereof.
If I want an object of my class to be thrown as an exception object, what
should I do?
The class should extend from Exception class. Or you can extend your class from
some more precise exception type also.
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If my class already extends from some other class what should I do if I want
an instance of my class to be thrown as an exception object?
One can not do anything in this scenario. Because Java does not allow multiple
inheritance and does not provide any exception interface as well.
How does an exception permeate through the code?
An unhandled exception moves up the method stack in search of a matching
When an exception is thrown from a code which is wrapped in a try block
followed by one or more catch blocks, a search is made for matching catch block.
If a matching type is found then that block will be invoked. If a matching type is
not found then the exception moves up the method stack and reaches the caller
method. Same procedure is repeated if the caller method is included in a try catch
block. This process continues until a catch block handling the appropriate type of
exception is found. If it does not find such a block then finally the program
terminates.
What are the different ways to handle exceptions?
There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch
the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller
of the method handle those exceptions.
What is the basic difference between the 2 approaches to exception handling.
1. try catch block and
2. specifying the candidate exceptions in the throws clause?
When should you use which approach?
In the first approach as a programmer of the method, you yourself are dealing
with the exception. This is fine if you are in a best position to decide should be
done in case of an exception. Whereas if it is not the responsibility of the method
to deal with it's own exceptions, then do not use this approach. In this case use the
second approach. In the second approach we are forcing the caller of the method
to catch the exceptions, that the method is likely to throw. This is often the
approach library creators use. They list the exception in the throws clause and we
must catch them. You will find the same approach throughout the java libraries
we use.
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Is it necessary that each try block must be followed by a catch block?
It is not necessary that each try block must be followed by a catch block. It should
be followed by either a catch block OR a finally block. And whatever exceptions
are likely to be thrown should be declared in the throws clause of the method.
If I write return at the end of the try block, will the finally block still
execute?
Yes even if you write return as the last statement in the try block and no
exception occurs, the finally block will execute. The finally block will execute
and then the control return.
If I write System.exit (0); at the end of the try block, will the finally block
still execute?
No in this case the finally block will not execute because when you say
System.exit (0); the control immediately goes out of the program, and thus finally
never executes.
How are Observer and Observable used?
Objects that subclass the Observable class maintain a list of observers. When an
Observable object is updated it invokes the update() method of each of its
observers to notify the observers that it has changed state. The Observer interface
is implemented by objects that observe Observable objects.
What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the
access of multiple threads to shared resources. Without synchronization, it is
possible for one thread to modify a shared object while another thread is in the
process of using or updating that object's value. This often leads to significant
errors.
How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type
allowed by the operation.
Does garbage collection guarantee that a program will not run out of
memory?
Garbage collection does not guarantee that a program will not run out of memory.
It is possible for programs to use up memory resources faster than they are
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garbage collected. It is also possible for programs to create objects that are not
subject to garbage collection .
What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the
waiting or dead states or a higher priority task comes into existence. Under time
slicing, a task executes for a predefined slice of time and then reenters the pool of
ready tasks. The scheduler then determines which task should execute next, based
on priority and other factors.
When a thread is created and started, what is its initial state?
A thread is in the ready state after it has been created and started.
What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to
perform any cleanup processing before the object is garbage collected.
What is the Locale class?
The Locale class is used to tailor program output to the conventions of a
particular geographic, political, or cultural region.
What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the next loop
iteration should occur. A do statement checks at the end of a loop to see whether
the next iteration of a loop should occur. The do statement will always execute
the body of a loop at least once.
What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific
instances of a class. Non-static variables take on unique values with each object
instance.
How are this() and super() used with constructors?
This() is used to invoke a constructor of the same class. super() is used to invoke
a superclass constructor.
What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. A
thread only executes a synchronized method after it has acquired the lock for the
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method's object or class. Synchronized statements are similar to synchronized
methods. A synchronized statement can only be executed after a thread has
acquired the lock for the object or class referenced in the synchronized statement.
What is daemon thread and which method is used to create the daemon
thread?
Daemon thread is a low priority thread which runs intermittently in the back
ground doing the garbage collection operation for the java runtime system.
setDaemon method is used to create a daemon thread.
Can applets communicate with each other?
At this point in time applets may communicate with other applets running in the
same virtual machine. If the applets are of the same class, they can communicate
via shared static variables. If the applets are of different classes, then each will
need a reference to the same class with static variables. In any case the basic idea
is to pass the information back and forth through a static variable.
An applet can also get references to all other applets on the same page using the
getApplets() method of java.applet.AppletContext. Once you get the reference to
an applet, you can communicate with it by using its public members.
It is conceivable to have applets in different virtual machines that talk to a server
somewhere on the Internet and store any data that needs to be serialized there.
Then, when another applet needs this data, it could connect to this same server.
Implementing this is non-trivial.
What are the steps in the JDBC connection?
While making a JDBC connection we go through the following steps :
Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();
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How does a try statement determine which catch clause should be used to
handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses
of the try statement are examined in the order in which they appear. The first
catch clause that is capable of handling the exceptionis executed. The remaining
catch clauses are ignored.
Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the
object's finalize() method is invoked and the object performs an operation which
causes it to become accessible to reachable objects.
What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread
or implement the Runnable interface.
What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. A
thread only executes a synchronized method after it has acquired the lock for the
method's object or class. Synchronized statements are similar to synchronized
methods. A synchronized statement can only be executed after a thread has
acquired the lock for the object or class referenced in the synchronized statement.
What is Externalizable?
Externalizable is an Interface that extends Serializable Interface. And sends data
into Streams in Compressed Format. It has two methods,
writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
What are some alternatives to inheritance?
Delegation is an alternative to inheritance. Delegation means that you include an
instance of another class as an instance variable, and forward messages to the
instance. It is often safer than inheritance because it forces you to think about
each message you forward, because the instance is of a known class, rather than a
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new class, and because it doesn't force you to accept all the methods of the super
class: you can provide only the methods that really make sense. On the other
hand, it makes you write more code, and it is harder to re-use (because it is not a
subclass).
What does it mean that a method or field is "static"?
Static variables and methods are instantiated only once per class. In other words
they are class variables, not instance variables. If you change the value of a static
variable in a particular object, the value of that variable changes for all instances
of that class.
Static methods can be referenced with the name of the class rather than the name
of a particular object of the class (though that works too). That's how library
methods like System.out.println() work out is a static field in the
java.lang.System class.
What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until it enters the
waiting or dead states or a higher priority task comes into existence. Under time
slicing, a task executes for a predefined slice of time and then reenters the pool of
ready tasks. The scheduler then determines which task should execute next, based
on priority and other factors.
What is the catch or declare rule for method declarations?
If a checked exception may be thrown within the body of a method, the method
must either catch the exception or declare it in its throws clause.
Is Empty .java file a valid source file?
Yes, an empty .java file is a perfectly valid source file.
Can a .java file contain more than one java classes?
Yes, a .java file contain more than one java classes, provided at the most one of
them is a public class.
Is String a primitive data type in Java?
No String is not a primitive data type in Java, even though it is one of the most
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extensively used object. Strings in Java are instances of String class defined in
java.lang package.
Is main a keyword in Java?
No, main is not a keyword in Java.
Is next a keyword in Java?
No, next is not a keyword.
Is delete a keyword in Java?
No, delete is not a keyword in Java. Java does not make use of explicit
destructors the way C++ does.
Is exit a keyword in Java?
No. To exit a program explicitly you use exit method in System object.
What happens if you don't initialize an instance variable of any of the
primitive types in Java?
Java by default initializes it to the default value for that primitive type. Thus an
int will be initialized to 0, a Boolean will be initialized to false.
What will be the initial value of an object reference which is defined as an
instance variable?
The object references are all initialized to null in Java. However in order to do
anything useful with these references, you must set them to a valid object, else
you will get NullPointerException everywhere you try to use such default
initialized references.
What are the different scopes for Java variables?
The scope of a Java variable is determined by the context in which the variable is
declared. Thus a java variable can have one of the three scopes at any given point
in time.
1. Instance : - These are typical object level variables, they are initialized to
default values at the time of creation of object, and remain accessible as long as
the object accessible.
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2. Local : - These are the variables that are defined within a method. They remain
accessible only during the course of method execution. When the method finishes
execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class
is loaded in JVM for the first time and remain there as long as the class remains
loaded. They are not tied to any particular object instance.
What is the default value of the local variables?
The local variables are not initialized to any default value, neither primitives nor
object references. If you try to use these variables without initializing them
explicitly, the java compiler will not compile the code. It will complain about the
local variable not being initialized..
How many objects are created in the following piece of code? MyClass c1,
c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
Only 2 objects are created, c1 and c3. The reference c2 is only declared and not
initialized.
Can a public class MyClass be defined in a source file named
YourClass.java?
No the source file name, if it contains a public class, must be the same as the
public class name itself with a .java extension.
Can main method be declared final?
Yes, the main method can be declared final, in addition to being public static.
What will be the output of the following statement?
System.out.println ("1" + 3);
It will print 13.
What will be the default values of all the elements of an array defined as an
instance variable?
If the array is an array of primitive types, then all the elements of the array will be
initialized to the default value corresponding to that primitive type. e.g. All the
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elements of an array of int will be initialized to 0, while that of Boolean type will
be initialized to false. Whereas if the array is an array of references (of any type),
all the elements will be initialized to null.
---------------------------------------------------------------
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JDBC
Interview Questions
&
Answers
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JDBC Interview Questions and Answers
What is JDBC?
JDBC may stand for Java Database Connectivity. It is also a trade mark. JDBC is a
layer of abstraction that allows users to choose between databases. It allows you to
change to a different database engine and to write to a single API. JDBC allows you to
write database applications in Java without having to concern yourself with the
underlying details of a particular database.
What's the JDBC 3.0 API?
The JDBC 3.0 API is the latest update of the JDBC API. It contains many features,
including scrollable result sets and the SQL:1999 data types.
JDBC (Java Database Connectivity) is the standard for communication between a Java
application and a relational database. The JDBC API is released in two versions;
JDBC version 1.22 (released with JDK 1.1.X in package java.sql) and version 2.0
(released with Java platform 2 in packages java.sql and javax.sql). It is a simple and
powerful largely database-independent way of extracting and inserting data to or from
any database.
Does the JDBC-ODBC Bridge support the new features in the JDBC 3.0 API?
The JDBC-ODBC Bridge provides a limited subset of the JDBC 3.0 API.
Can the JDBC-ODBC Bridge be used with applets?
Use of the JDBC-ODBC bridge from an untrusted applet running in a browser, such as
Netscape Navigator, isn't allowed. The JDBC-ODBC bridge doesn't allow untrusted
code to call it for security reasons. This is good because it means that an untrusted
applet that is downloaded by the browser can't circumvent Java security by calling
ODBC. Remember that ODBC is native code, so once ODBC is called the Java
programming language can't guarantee that a security violation won't occur. On the
other hand, Pure Java JDBC drivers work well with applets. They are fully
downloadable and do not require any client-side configuration.
Finally, we would like to note that it is possible to use the JDBC-ODBC bridge with
applets that will be run in appletviewer since appletviewer assumes that applets are
trusted. In general, it is dangerous to turn applet security off, but it may be appropriate
in certain controlled situations, such as for applets that will only be used in a secure
intranet environment. Remember to exercise caution if you choose this option, and use
an all-Java JDBC driver whenever possible to avoid security problems.
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How do I start debugging problems related to the JDBC API?
A good way to find out what JDBC calls are doing is to enable JDBC tracing. The
JDBC trace contains a detailed listing of the activity occurring in the system that is
related to JDBC operations.
If you use the DriverManager facility to establish your database connection, you use
the DriverManager.setLogWriter method to enable tracing of JDBC operations. If you
use a DataSource object to get a connection, you use the DataSource.setLogWriter
method to enable tracing. (For pooled connections, you use the
ConnectionPoolDataSource.setLogWriter method, and for connections that can
participate in distributed transactions, you use the XADataSource.setLogWriter
method.)
What is new in JDBC 2.0?
With the JDBC 2.0 API, you will be able to do the following:
Scroll forward and backward in a result set or move to a specific row
(TYPE_SCROLL_SENSITIVE,previous(), last(), absolute(), relative(), etc.)
Make updates to database tables using methods in the Java programming language
instead of using SQL commands.(updateRow(), insertRow(), deleteRow(), etc.)
Send multiple SQL statements to the database as a unit, or batch (addBatch(),
executeBatch())
Use the new SQL3 datatypes as column values like Blob, Clob, Array, Struct, Ref.
How to move the cursor in scrollable resultset ?
a. create a scrollable ResultSet object.
Statement stmt = con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,
ResultSet.CONCUR_READ_ONLY);
ResultSet srs = stmt.executeQuery("SELECT COLUMN_1,
COLUMN_2 FROM TABLE_NAME");
b. use a built in methods like afterLast(), previous(), beforeFirst(), etc. to scroll the
resultset.
srs.afterLast();
while (srs.previous()) {
String name = srs.getString("COLUMN_1");
float salary = srs.getFloat("COLUMN_2");
//...
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c. to find a specific row, use absolute(), relative() methods.
srs.absolute(4); // cursor is on the fourth row
int rowNum = srs.getRow(); // rowNum should be 4
srs.relative(-3);
int rowNum = srs.getRow(); // rowNum should be 1
srs.relative(2);
int rowNum = srs.getRow(); // rowNum should be 3
d. use isFirst(), isLast(), isBeforeFirst(), isAfterLast() methods to check boundary
status.
How to update a resultset programmatically?
a. create a scrollable and updatable ResultSet object.
Statement stmt = con.createStatement
(ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE);
ResultSet uprs = stmt.executeQuery("SELECT COLUMN_1,
COLUMN_2 FROM TABLE_NAME");
b. move the cursor to the specific position and use related method to update data and
then, call updateRow() method.
uprs.last();
uprs.updateFloat("COLUMN_2", 25.55);//update last row's data
uprs.updateRow();//don't miss this method, otherwise,
// the data will be lost.
How can I use the JDBC API to access a desktop database like Microsoft Access
over the network?
Most desktop databases currently require a JDBC solution that uses ODBC
underneath. This is because the vendors of these database products haven't
implemented all-Java JDBC drivers.
The best approach is to use a commercial JDBC driver that supports ODBC and the
database you want to use. See the JDBC drivers page for a list of available JDBC
drivers.
The JDBC-ODBC bridge from Sun's Java Software does not provide network access
to desktop databases by itself. The JDBC-ODBC bridge loads ODBC as a local DLL,
and typical ODBC drivers for desktop databases like Access aren't networked. The
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JDBC-ODBC bridge can be used together with the RMI-JDBC bridge, however, to
access a desktop database like Access over the net. This RMI-JDBC-ODBC solution is
free.
Are there any ODBC drivers that do not work with the JDBC-ODBC Bridge?
Most ODBC 2.0 drivers should work with the Bridge. Since there is some variation in
functionality between ODBC drivers, the functionality of the bridge may be affected.
The bridge works with popular PC databases, such as Microsoft Access and FoxPro.
What causes the "No suitable driver" error?
"No suitable driver" is an error that usually occurs during a call to the
DriverManager.getConnection method. The cause can be failing to load the
appropriate JDBC drivers before calling the getConnection method, or it can be
specifying an invalid JDBC URL--one that isn't recognized by your JDBC driver.
Your best bet is to check the documentation for your JDBC driver or contact your
JDBC driver vendor if you suspect that the URL you are specifying is not being
recognized by your JDBC driver.
In addition, when you are using the JDBC-ODBC Bridge, this error can occur if one or
more the the shared libraries needed by the Bridge cannot be loaded. If you think this
is the cause, check your configuration to be sure that the shared libraries are accessible
to the Bridge.
Why isn't the java.sql.DriverManager class being found?
This problem can be caused by running a JDBC applet in a browser that supports the
JDK 1.0.2, such as Netscape Navigator 3.0. The JDK 1.0.2 does not contain the JDBC
API, so the DriverManager class typically isn't found by the Java virtual machine
running in the browser.
Here's a solution that doesn't require any additional configuration of your web clients.
Remember that classes in the java.* packages cannot be downloaded by most browsers
for security reasons. Because of this, many vendors of all-Java JDBC drivers supply
versions of the java.sql.* classes that have been renamed to jdbc.sql.*, along with a
version of their driver that uses these modified classes. If you import jdbc.sql.* in your
applet code instead of java.sql.*, and add the jdbc.sql.* classes provided by your
JDBC driver vendor to your applet's codebase, then all of the JDBC classes needed by
the applet can be downloaded by the browser at run time, including the DriverManager
class.
This solution will allow your applet to work in any client browser that supports the
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JDK 1.0.2. Your applet will also work in browsers that support the JDK 1.1, although
you may want to switch to the JDK 1.1 classes for performance reasons. Also, keep in
mind that the solution outlined here is just an example and that other solutions are
possible.
How to insert and delete a row programmatically? (new feature in JDBC 2.0)
Make sure the resultset is updatable.
1. move the cursor to the specific position.
uprs.moveToCurrentRow();
2. set value for each column.
uprs.moveToInsertRow();//to set up for insert
uprs.updateString("col1" "strvalue");
uprs.updateInt("col2", 5);
...
3. call inserRow() method to finish the row insert process.
uprs.insertRow();
To delete a row: move to the specific position and call deleteRow() method:
uprs.absolute(5);
uprs.deleteRow();//delete row 5
To see the changes call refreshRow();
uprs.refreshRow();
What are the two major components of JDBC?
One implementation interface for database manufacturers, the other implementation
interface for application and applet writers.
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What is JDBC Driver interface?
The JDBC Driver interface provides vendor-specific implementations of the abstract
classes provided by the JDBC API. Each vendor driver must provide implementations
of the java.sql.Connection,Statement,PreparedStatement, CallableStatement, ResultSet
and Driver.
How do I retrieve a whole row of data at once, instead of calling an individual
ResultSet.getXXX method for each column?
The ResultSet.getXXX methods are the only way to retrieve data from a ResultSet
object, which means that you have to make a method call for each column of a row. It
is unlikely that this is the cause of a performance problem, however, because it is
difficult to see how a column could be fetched without at least the cost of a function
call in any scenario. We welcome input from developers on this issue.
What are the common tasks of JDBC?
Create an instance of a JDBC driver or load JDBC drivers through jdbc.drivers
Register a driver
Specify a database
Open a database connection
Submit a query
Receive results
Process results
Why does the ODBC driver manager return 'Data source name not found and no
default driver specified Vendor: 0'
This type of error occurs during an attempt to connect to a database with the bridge.
First, note that the error is coming from the ODBC driver manager. This indicates that
the bridge-which is a normal ODBC client-has successfully called ODBC, so the
problem isn't due to native libraries not being present. In this case, it appears that the
error is due to the fact that an ODBC DSN (data source name) needs to be configured
on the client machine. Developers often forget to do this, thinking that the bridge will
magically find the DSN they configured on their remote server machine
How to use JDBC to connect Microsoft Access?
There is a specific tutorial at javacamp.org. Check it out.
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What are four types of JDBC driver?
Type 1 Drivers
Bridge drivers such as the jdbc-odbc bridge. They rely on an intermediary such as
ODBC to transfer the SQL calls to the database and also often rely on native code. It is
not a serious solution for an application
Type 2 Drivers
Use the existing database API to communicate with the database on the client. Faster
than Type 1, but need native code and require additional permissions to work in an
applet. Client machine requires software to run.
Type 3 Drivers
JDBC-Net pure Java driver. It translates JDBC calls to a DBMS-independent network
protocol, which is then translated to a DBMS protocol by a server. Flexible. Pure Java
and no native code.
Type 4 Drivers
Native-protocol pure Java driver. It converts JDBC calls directly into the network
protocol used by DBMSs. This allows a direct call from the client machine to the
DBMS server. It doesn't need any special native code on the client machine.
Recommended by Sun's tutorial, driver type 1 and 2 are interim solutions where direct
pure Java drivers are not yet available. Driver type 3 and 4 are the preferred way to
access databases using the JDBC API, because they offer all the advantages of Java
technology, including automatic installation. For more info, visit Sun JDBC page
Which type of JDBC driver is the fastest one?
JDBC Net pure Java driver(Type IV) is the fastest driver because it converts the jdbc
calls into vendor specific protocol calls and it directly interacts with the database.
Are all the required JDBC drivers to establish connectivity to my database part
of the JDK?
No. There aren't any JDBC technology-enabled drivers bundled with the JDK 1.1.x or
Java 2 Platform releases other than the JDBC-ODBC Bridge. So, developers need to
get a driver and install it before they can connect to a database. We are considering
bundling JDBC technology- enabled drivers in the future.
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Is the JDBC-ODBC Bridge multi-threaded?
No. The JDBC-ODBC Bridge does not support concurrent access from different
threads. The JDBC-ODBC Bridge uses synchronized methods to serialize all of the
calls that it makes to ODBC. Multi-threaded Java programs may use the Bridge, but
they won't get the advantages of multi-threading. In addition, deadlocks can occur
between locks held in the database and the semaphore used by the Bridge. We are
thinking about removing the synchronized methods in the future. They were added
originally to make things simple for folks writing Java programs that use a singlethreaded ODBC driver.
Does the JDBC-ODBC Bridge support multiple concurrent open statements per
connection?
No. You can open only one Statement object per connection when you are using the
JDBC-ODBC Bridge.
What is the query used to display all tables names in SQL Server (Query
analyzer)?
select * from information_schema.tables
Why can't I invoke the ResultSet methods afterLast and beforeFirst when the
method next works?
You are probably using a driver implemented for the JDBC 1.0 API. You need to
upgrade to a JDBC 2.0 driver that implements scrollable result sets. Also be sure that
your code has created scrollable result sets and that the DBMS you are using supports
them.
How can I retrieve a String or other object type without creating a new object
each time?
Creating and garbage collecting potentially large numbers of objects (millions)
unnecessarily can really hurt performance. It may be better to provide a way to
retrieve data like strings using the JDBC API without always allocating a new object.
We are studying this issue to see if it is an area in which the JDBC API should be
improved. Stay tuned, and please send us any comments you have on this question.
How many types of JDBC Drivers are present and what are they?
There are 4 types of JDBC Drivers
Type 1: JDBC-ODBC Bridge Driver
Type 2: Native API Partly Java Driver
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Type 3: Network protocol Driver
Type 4: JDBC Net pure Java Driver
What is the fastest type of JDBC driver?
JDBC driver performance will depend on a number of issues:
(a) the quality of the driver code,
(b) the size of the driver code,
(c) the database server and its load,
(d) network topology,
(e) the number of times your request is translated to a different API.
In general, all things being equal, you can assume that the more your request and
response change hands, the slower it will be. This means that Type 1 and Type 3
drivers will be slower than Type 2 drivers (the database calls are make at least three
translations versus two), and Type 4 drivers are the fastest (only one translation).
There is a method getColumnCount in the JDBC API. Is there a similar method
to find the number of rows in a result set?
No, but it is easy to find the number of rows. If you are using a scrollable result set, rs,
you can call the methods rs.last and then rs.getRow to find out how many rows rs has.
If the result is not scrollable, you can either count the rows by iterating through the
result set or get the number of rows by submitting a query with a COUNT column in
the SELECT clause.
I would like to download the JDBC-ODBC Bridge for the Java 2 SDK, Standard
Edition (formerly JDK 1.2). I'm a beginner with the JDBC API, and I would like
to start with the Bridge. How do I do it?
The JDBC-ODBC Bridge is bundled with the Java 2 SDK, Standard Edition, so there
is no need to download it separately.
If I use the JDBC API, do I have to use ODBC underneath?
No, this is just one of many possible solutions. We recommend using a pure Java
JDBC technology-enabled driver, type 3 or 4, in order to get all of the benefits of the
Java programming language and the JDBC API.
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Once I have the Java 2 SDK, Standard Edition, from Sun, what else do I need to
connect to a database?
You still need to get and install a JDBC technology-enabled driver that supports the
database that you are using. There are many drivers available from a variety of
sources. You can also try using the JDBC-ODBC Bridge if you have ODBC
connectivity set up already. The Bridge comes with the Java 2 SDK, Standard Edition,
and Enterprise Edition, and it doesn't require any extra setup itself. The Bridge is a
normal ODBC client. Note, however, that you should use the JDBC-ODBC Bridge
only for experimental prototyping or when you have no other driver available.
What is the best way to generate a universally unique object ID? Do I need to use
an external resource like a file or database, or can I do it all in memory?
1: Unique down to the millisecond. Digits 1-8 are the hex encoded lower 32 bits of the
System.currentTimeMillis() call.
2: Unique across a cluster. Digits 9-16 are the encoded representation of the 32 bit
integer of the underlying IP address.
3: Unique down to the object in a JVM. Digits 17-24 are the hex representation of the
call to System.identityHashCode(), which is guaranteed to return distinct integers for
distinct objects within a JVM.
4: Unique within an object within a millisecond. Finally digits 25-32 represent a
random 32 bit integer generated on every method call using the cryptographically
strong java.security.SecureRandom class.
Answer1
There are two reasons to use the random number instead of incrementing your last. 1.
The number would be predictable and, depending on what this is used for, you could
be opening up a potential security issue. This is why ProcessIDs are randomized on
some OSes (AIX for one). 2. You must synchronize on that counter to guarantee that
your number isn't reused. Your random number generator need not be synchronized,
(though its implementation may be).
Answer2
1) If your using Oracle You can create a sequence ,by which you can generate unique
primary key or universal primary key. 2) you can generate by using random numbers
but you may have to check the range and check for unique id. ie random number
generate 0.0 to 1.0 u may have to make some logic which suits your unique id 3) Set
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the maximum value into an XML file and read that file at the time of loading your
application from xml .
What happens when I close a Connection application obtained from a connection
Pool? How does a connection pool maintain the Connections that I had closed
through the application?
Answer1
It is the magic of polymorphism, and of Java interface vs. implementation types. Two
objects can both be "instanceof" the same interface type, even though they are not of
the same implementation type.
When you call "getConnection()" on a pooled connection cache manager object, you
get a "logical" connection, something which implements the java.sql.Connection
interface.
But it is not the same implementation type as you would get for your Connection, if
you directly called getConnection() from a (non-pooled/non-cached) datasource.
So the "close()" that you invoke on the "logical" Connection is not the same "close()"
method as the one on the actual underlying "physical" connection hidden by the pool
cache manager.
The close() method of the "logical" connection object, while it satisfies the method
signature of close() in the java.sql.Connection interface, does not actually close the
underlying physical connection.
Answer2
Typically a connection pool keeps the active/in-use connections in a hashtable or other
Collection mechanism. I've seen some that use one stack for ready-for-use, one stack
for in-use.
When close() is called, whatever the mechanism for indicating inuse/ready-for-use,
that connection is either returned to the pool for ready-for-use or else physically
closed. Connections pools should have a minimum number of connections open. Any
that are closing where the minimum are already available should be physically closed.
Some connection pools periodically test their connections to see if queries work on the
ready-for-use connections or they may test that on the close() method before returning
to the ready-for-use pool.
How do I insert a .jpg into a mySQL data base? I have tried inserting the file as
byte[], but I recieve an error message stating that the syntax is incorrect.
Binary data is stored and retrieved from the database using
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streams in connection with prepared statements and resultsets.
This minimal application stores an image file in the database,
then it retrieves the binary data from the database and converts
it back to an image.
import java.sql.*;
import java.io.*;
import java.awt.*;
import java.awt.Image;
/**
* Storing and retrieving images from a MySQL database
*/
public class StoreBinary {
private static String driverName = "sun.jdbc.odbc.JdbcOdbcDriver";
private Statement stmt = null;
private Connection conn = null;
public StoreBinary() {}
/**
* Strips path prefix from filenames
* @param fileName
* @return the base filename
*/
public static String getBaseName(String fileName) {
int ix=fileName.lastIndexOf("\\");
if (ix < 0) return fileName;
return fileName.substring(ix+1);
}
/**
* Store a binary (image) file in the database using a
* prepared statement.
* @param fileName
* @return true if the operation succeeds
* @throws Exception
*/
public boolean storeImageFile(String fileName) throws Exception {
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if (!connect("test", "root", "")) {
return false;
}
FileInputStream in = new FileInputStream(fileName);
int len=in.available();
String baseName=StoreBinary.getBaseName(fileName);
PreparedStatement pStmt = conn.prepareStatement
("insert into image_tab values (?,?,?)");
pStmt.setString(1, baseName);
pStmt.setInt(2,len);
pStmt.setBinaryStream(3, in, len);
pStmt.executeUpdate();
in.close();
System.out.println("Stored: "+baseName+", length: "+len);
return true;
}
/**
* Retrieve the biary file data from the DB and convert it to an image
* @param fileName
* @return
* @throws Exception
*/
public Image getImageFile(String fileName) throws Exception {
String baseName=StoreBinary.getBaseName(fileName);
ResultSet rs=stmt.executeQuery("select * from image_tab
where image_name='"+baseName+"'");
if (!rs.next()) {
System.out.println("Image:"+baseName+" not found");
return null;
}
int len=rs.getInt(2);
byte [] b=new byte[len];
InputStream in = rs.getBinaryStream(3);
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int n=in.read(b);
System.out.println("n: "+n);
in.close();
Image img=Toolkit.getDefaultToolkit().createImage(b);
System.out.println("Image: "+baseName+" retrieved ok, size: "+len);
return img;
}
/**
* Establish database connection
* @param dbName
* @param dbUser
* @param dbPassword
* @return true if the operation succeeds
*/
public boolean connect(String dbName, String dbUser, String dbPassword) {
try {
Class.forName(driverName);
}
catch (ClassNotFoundException ex) {
ex.printStackTrace();
return false;
}
try {
conn = DriverManager.getConnection("jdbc:odbc:" + dbName,
dbUser,
dbPassword);
stmt = conn.createStatement();
}
catch (SQLException ex1) {
ex1.printStackTrace();
return false;
}
return true;
}
/******************************************
* MAIN stub driver for testing the class.
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*/
public static void main(String[] args) {
String fileName="c:\\tmp\\f128.jpg";
StoreBinary sb = new StoreBinary();
try {
if (sb.storeImageFile(fileName)) {
// stored ok, now get it back again
Image img=sb.getImageFile(fileName);
}
}
catch (Exception ex) {
ex.printStackTrace();
}
}
}
How can I know when I reach the last record in a table, since JDBC doesn't
provide an EOF method?
Answer1
You can use last() method of java.sql.ResultSet, if you make it scrollable.
You can also use isLast() as you are reading the ResultSet.
One thing to keep in mind, though, is that both methods tell you that you have reached
the end of the current ResultSet, not necessarily the end of the table. SQL and
RDBMSes make no guarantees about the order of rows, even from sequential
SELECTs, unless you specifically use ORDER BY. Even then, that doesn't necessarily
tell you the order of data in the table.
Answer2
Assuming you mean ResultSet instead of Table, the usual idiom for iterating over a
forward only resultset is:
ResultSet rs=statement.executeQuery(...);
while (rs.next()) {
// Manipulate row here
}
Where can I find info, frameworks and example source for writing a JDBC
driver?
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There a several drivers with source available, like MM.MySQL, SimpleText Database,
FreeTDS, and RmiJdbc. There is at least one free framework, the jxDBCon-Open
Source JDBC driver framework. Any driver writer should also review For Driver
Writers.
How can I create a custom RowSetMetaData object from scratch?
One unfortunate aspect of RowSetMetaData for custom versions is that it is an
interface. This means that implementations almost have to be proprietary. The JDBC
RowSet package is the most commonly available and offers the
sun.jdbc.rowset.RowSetMetaDataImpl class. After instantiation, any of the
RowSetMetaData setter methods may be used. The bare minimum needed for a
RowSet to function is to set the Column Count for a row and the Column Types for
each column in the row. For a working code example that includes a custom
RowSetMetaData,
How does a custom RowSetReader get called from a CachedRowSet?
The Reader must be registered with the CachedRowSet using
CachedRowSet.setReader(javax.sql.RowSetReader reader). Once that is done, a call to
CachedRowSet.execute() will, among other things, invoke the readData method.
How do I implement a RowSetReader? I want to populate a CachedRowSet
myself and the documents specify that a RowSetReader should be used. The
single method accepts a RowSetInternal caller and returns void. What can I do in
the readData method?
"It can be implemented in a wide variety of ways..." and is pretty vague about what
can actually be done. In general, readData() would obtain or create the data to be
loaded, then use CachedRowSet methods to do the actual loading. This would usually
mean inserting rows, so the code would move to the insert row, set the column data
and insert rows. Then the cursor must be set to to the appropriate position.
How can I instantiate and load a new CachedRowSet object from a non-JDBC
source?
The basics are:
* Create an object that implements javax.sql.RowSetReader, which loads the data.
* Instantiate a CachedRowset object.
* Set the CachedRowset's reader to the reader object previously created.
* Invoke CachedRowset.execute().
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Note that a RowSetMetaData object must be created, set up with a description of the
data, and attached to the CachedRowset before loading the actual data.
The following code works with the Early Access JDBC RowSet download available
from the Java Developer Connection and is an expansion of one of the examples:
// Independent data source CachedRowSet Example
import java.sql.*;
import javax.sql.*;
import sun.jdbc.rowset.*;
public class RowSetEx1 implements RowSetReader
{
CachedRowSet crs;
int iCol2;
RowSetMetaDataImpl rsmdi;
String sCol1,
sCol3;
public RowSetEx1()
{
try
{
crs = new CachedRowSet();
crs.setReader(this);
crs.execute(); // load from reader
System.out.println(
"Fetching from RowSet...");
while(crs.next())
{
showTheData();
} // end while next
if(crs.isAfterLast() == true)
{
System.out.println(
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"We have reached the end");
System.out.println("crs row: " +
crs.getRow());
}
System.out.println(
"And now backwards...");
while(crs.previous())
{
showTheData();
} // end while previous
if(crs.isBeforeFirst() == true)
{ System.out.println(
"We have reached the start");
}
crs.first();
if(crs.isFirst() == true)
{ System.out.println(
"We have moved to first");
}
System.out.println("crs row: " +
crs.getRow());
if(crs.isBeforeFirst() == false)
{ System.out.println(
"We aren't before the first row."); }
crs.last();
if(crs.isLast() == true)
{ System.out.println(
"...and now we have moved to the last");
}
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System.out.println("crs row: " +
crs.getRow());
if(crs.isAfterLast() == false)
{
System.out.println(
"we aren't after the last.");
}
} // end try
catch (SQLException ex)
{
System.err.println("SQLException: " +
ex.getMessage());
}
} // end constructor
public void showTheData() throws SQLException
{
sCol1 = crs.getString(1);
if(crs.wasNull() == false)
{ System.out.println("sCol1: " + sCol1); }
else { System.out.println("sCol1 is null"); }
iCol2 = crs.getInt(2);
if (crs.wasNull() == false)
{ System.out.println("iCol2: " + iCol2); }
else { System.out.println("iCol2 is null"); }
sCol3 = crs.getString(3);
if (crs.wasNull() == false)
{
System.out.println("sCol3: " +
sCol3 + "\n" );
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}
else
{ System.out.println("sCol3 is null\n"); }
} // end showTheData
// RowSetReader implementation
public void readData(RowSetInternal caller)
throws SQLException
{
rsmdi = new RowSetMetaDataImpl();
rsmdi.setColumnCount(3);
rsmdi.setColumnType(1, Types.VARCHAR);
rsmdi.setColumnType(2, Types.INTEGER);
rsmdi.setColumnType(3, Types.VARCHAR);
crs.setMetaData( rsmdi );
crs.moveToInsertRow();
crs.updateString( 1, "StringCol11" );
crs.updateInt( 2, 1 );
crs.updateString( 3, "StringCol31" );
crs.insertRow();
crs.updateString( 1, "StringCol12" );
crs.updateInt( 2, 2 );
crs.updateString( 3, "StringCol32" );
crs.insertRow();
crs.moveToCurrentRow();
crs.beforeFirst();
} // end readData
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public static void main(String args[])
{
new RowSetEx1();
}
} // end class RowSetEx1
Can I set up a connection pool with multiple user IDs? The single ID we are
forced to use causes problems when debugging the DBMS.
Since the Connection interface ( and the underlying DBMS ) requires a specific user
and password, there's not much of a way around this in a pool. While you could create
a different Connection for each user, most of the rationale for a pool would then be
gone. Debugging is only one of several issues that arise when using pools.
However, for debugging, at least a couple of other methods come to mind. One is to
log executed statements and times, which should allow you to backtrack to the user.
Another method that also maintains a trail of modifications is to include user and
timestamp as standard columns in your tables. In this last case, you would collect a
separate user value in your program.
How can I protect my database password ? I'm writing a client-side java
application that will access a database over the internet. I have concerns about
the security of the database passwords. The client will have access in one way or
another to the class files, where the connection string to the database, including
user and password, is stored in as plain text. What can I do to protect my
passwords?
This is a very common question.
Conclusion: JAD decompiles things easily and obfuscation would not help you. But
you'd have the same problem with C/C++ because the connect string would still be
visible in the executable.
SSL JDBC network drivers fix the password sniffing problem (in MySQL 4.0), but not
the decompile problem. If you have a servlet container on the web server, I would go
that route (see other discussion above) then you could at least keep people from
reading/destroying your mysql database.
Make sure you use database security to limit that app user to the minimum tables that
they need, then at least hackers will not be able to reconfigure your DBMS engine.
Aside from encryption issues over the internet, it seems to me that it is bad practice to
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embed user ID and password into program code. One could generally see the text even
without decompilation in almost any language. This would be appropriate only to a
read-only database meant to be open to the world. Normally one would either force the
user to enter the information or keep it in a properties file.
Detecting Duplicate Keys I have a program that inserts rows in a table. My table
has a column 'Name' that has a unique constraint. If the user attempts to insert a
duplicate name into the table, I want to display an error message by processing
the error code from the database. How can I capture this error code in a Java
program?
A solution that is perfectly portable to all databases, is to execute a query for checking
if that unique value is present before inserting the row. The big advantage is that you
can handle your error message in a very simple way, and the obvious downside is that
you are going to use more time for inserting the record, but since you're working on a
PK field, performance should not be so bad.
You can also get this information in a portable way, and potentially avoid another
database access, by capturing SQLState messages. Some databases get more specific
than others, but the general code portion is 23 - "Constraint Violations". UDB2, for
example, gives a specific such as 23505, while others will only give 23000.
What driver should I use for scalable Oracle JDBC applications?
Sun recommends using the thin ( type 4 ) driver.
* On single processor machines to avoid JNI overhead.
* On multiple processor machines, especially running Solaris, to avoid
synchronization bottlenecks.
Can you scroll a result set returned from a stored procedure? I am returning a
result set from a stored procedure with type SQLRPGLE but once I reach the
end of the result set it does not allow repositioning. Is it possible to scroll this
result set?
A CallableStatement is no different than other Statements in regard to whether related
ResultSets are scrollable. You should create the CallableStatement using
Connection.prepareCall(String sql, int resultSetType, int resultSetConcurrency).
How do I write Greek ( or other non-ASCII/8859-1 ) characters to a database?
From the standard JDBC perspective, there is no difference between ASCII/8859-1
characters and those above 255 ( hex FF ). The reason for that is that all Java
characters are in Unicode ( unless you perform/request special encoding ). Implicit in
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that statement is the presumption that the data store can handle characters outside the
hex FF range or interprets different character sets appropriately. That means either:
* The OS, application and database use the same code page and character set. For
example, a Greek version of NT with the DBMS set to the default OS encoding.
* The DBMS has I18N support for Greek ( or other language ), regardless of OS
encoding. This has been the most common for production quality databases, although
support varies. Particular DBMSes may allow setting the encoding/code page/CCSID
at the database, table or even column level. There is no particular standard for
provided support or methods of setting the encoding. You have to check the DBMS
documentation and set up the table properly.
* The DBMS has I18N support in the form of Unicode capability. This would handle
any Unicode characters and therefore any language defined in the Unicode standard.
Again, set up is proprietary.
How can I insert images into a Mysql database?
This code snippet shows the basics:
File file = new File(fPICTURE);
FileInputStream fis = new FileInputStream(file);
PreparedStatement ps =
ConrsIn.prepareStatement("insert into dbPICTURE values (?,?)");
// ***use as many ??? as you need to insert in the exact order***
ps.setString(1,file.getName());
ps.setBinaryStream(2,fis,(int)file.length());
ps.close();
fis.close();
Is possible to open a connection to a database with exclusive mode with JDBC?
I think you mean "lock a table in exclusive mode". You cannot open a connection with
exclusive mode. Depending on your database engine, you can lock tables or rows in
exclusive mode.
In Oracle you would create a statement st and run
st.execute("lock table mytable in exclusive mode");
Then when you are finished with the table, execute the commit to unlock the table.
Mysql, Informix and SQLServer all have a slightly different syntax for this function,
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so you'll have to change it depending on your database. But they can all be done with
execute().
What are the standard isolation levels defined by JDBC?
The values are defined in the class java.sql.Connection and are:
* TRANSACTION_NONE
* TRANSACTION_READ_COMMITTED
* TRANSACTION_READ_UNCOMMITTED
* TRANSACTION_REPEATABLE_READ
* TRANSACTION_SERIALIZABLE
Update fails without blank padding. Although a particular row is present in the
database for a given key, executeUpdate() shows 0 rows updated and, in fact, the
table is not updated. If I pad the Key with spaces for the column length (e.g. if the
key column is 20 characters long, and key is msgID, length 6, I pad it with 14
spaces), the update then works!!! Is there any solution to this problem without
padding?
In the SQL standard, CHAR is a fixed length data type. In many DBMSes ( but not
all), that means that for a WHERE clause to match, every character must match,
including size and trailing blanks. As Alessandro indicates, defining CHAR columns
to be VARCHAR is the most general answer.
What isolation level is used by the DBMS when inserting, updating and selecting
rows from a database?
The answer depends on both your code and the DBMS. If the program does not
explicitly set the isolation level, the DBMS default is used. You can determine the
default using DatabaseMetaData.getDefaultTransactionIsolation() and the level for the
current Connection with Connection.getTransactionIsolation(). If the default is not
appropriate for your transaction, change it with Connection.setTransactionIsolation(int
level).
How can I determine the isolation levels supported by my DBMS?
Use DatabaseMetaData.supportsTransactionIsolationLevel(int level).
Connecting to a database through the Proxy I want to connect to remote database
using a program that is running in the local network behind the proxy. Is that
possible?
I assume that your proxy is set to accept http requests only on port 80. If you want to
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have a local class behind the proxy connect to the database for you, then you need a
servlet/JSP to receive an HTTP request and use the local class to connect to the
database and send the response back to the client.
You could also use RMI where your remote computer class that connects to the
database acts as a remote server that talks RMI with the clients. if you implement this,
then you will need to tunnel RMI through HTTP which is not that hard.
In summary, either have a servlet/JSP take HTTP requests, instantiate a class that
handles database connections and send HTTP response back to the client or have the
local class deployed as RMI server and send requests to it using RMI.
How do I receive a ResultSet from a stored procedure?
Stored procedures can return a result parameter, which can be a result set. For a
discussion of standard JDBC syntax for dealing with result, IN, IN/OUT and OUT
parameters, see Stored Procedures.
How can I write to the log used by DriverManager and JDBC drivers?
The simplest method is to use DriverManager.println(String message), which will
write to the current log.
How can I get or redirect the log used by DriverManager and JDBC drivers?
As of JDBC 2.0, use DriverManager.getLogWriter() and
DriverManager.setLogWriter(PrintWriter out). Prior to JDBC 2.0, the DriverManager
methods getLogStream() and setLogStream(PrintStream out) were used. These are
now deprecated.
What does it mean to "materialize" data?
This term generally refers to Array, Blob and Clob data which is referred to in the
database via SQL locators "Materializing" the data means to return the actual data
pointed to by the Locator.
For Arrays, use the various forms of getArray() and getResultSet().
For Blobs, use getBinaryStream() or getBytes(long pos, int length).
For Clobs, use getAsciiStream() or getCharacterStream().
Why do I have to reaccess the database for Array, Blob, and Clob data?
Most DBMS vendors have implemented these types via the SQL3 Locator type
Some rationales for using Locators rather than directly returning the data can be seen
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most clearly with the Blob type. By definition, a Blob is an arbitrary set of binary data.
It could be anything; the DBMS has no knowledge of what the data represents. Notice
that this effectively demolishes data independence, because applications must now be
aware of what the Blob data actually represents. Let's assume an employee table that
includes employee images as Blobs.
Say we have an inquiry program that presents multiple employees with department
and identification information. To see all of the data for a specific employee, including
the image, the summary row is selected and another screen appears. It is only at this
pont that the application needs the specific image. It would be very wasteful and time
consuming to bring down an entire employee page of images when only a few would
ever be selected in a given run.
Now assume a general interactive SQL application. A query is issued against the
employee table. Because the image is a Blob, the application has no idea what to do
with the data, so why bring it down, killing performance along the way, in a long
running operation?
Clearly this is not helpful in those applications that need the data everytime, but these
and other considerations have made the most general sense to DBMS vendors.
What is an SQL Locator?
A Locator is an SQL3 data type that acts as a logical pointer to data that resides on a
database server. Read "logical pointer" here as an identifier the DBMS can use to
locate and manipulate the data. A Locator allows some manipulation of the data on the
server. While the JDBC specification does not directly address Locators, JDBC drivers
typically use Locators under the covers to handle Array, Blob, and Clob data types.
How do I set properties for a JDBC driver and where are the properties stored?
A JDBC driver may accept any number of properties to tune or optimize performance
for the specific driver. There is no standard, other than user and password, for what
these properties should be. Therefore, the developer is dependent on the driver
documentation to automatically pass properties. For a standard dynamic method that
can be used to solicit user input for properties, see What properties should I supply to
a database driver in order to connect to a database?
In addition, a driver may specify its own method of accepting properties. Many do this
via appending the property to the JDBC Database URL. However, a JDBC Compliant
driver should implement the connect(String url, Properties info) method. This is
generally invoked through DriverManager.getConnection(String url, Properties info).
The passed properties are ( probably ) stored in variables in the Driver instance. This,
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again, is up to the driver, but unless there is some sort of driver setup, which is
unusual, only default values are remembered over multiple instantiations.
What is the JDBC syntax for using a literal or variable in a standard Statement?
First, it should be pointed out that PreparedStatement handles many issues for the
developer and normally should be preferred over a standard Statement.
Otherwise, the JDBC syntax is really the same as SQL syntax. One problem that often
affects newbies ( and others ) is that SQL, like many languages, requires quotes
around character ( read "String" for Java ) values to distinguish from numerics. So the
clause:
"WHERE myCol = " + myVal
is perfectly valid and works for numerics, but will fail when myVal is a String. Instead
use:
"WHERE myCol = '" + myVal + "'"
if myVal equals "stringValue", the clause works out to:
WHERE myCol = 'stringValue'
You can still encounter problems when quotes are embedded in the value, which,
again, a PreparedStatement will handle for you.
How do I check in my code whether a maximum limit of database connections
have been reached?
Use DatabaseMetaData.getMaxConnections() and compare to the number of
connections currently open. Note that a return value of zero can mean unlimited or,
unfortunately, unknown. Of course, driverManager.getConnection() will throw an
exception if a Connection can not be obtained.
Why do I get UnsatisfiedLinkError when I try to use my JDBC driver?
The first thing is to be sure that this does not occur when running non-JDBC apps. If
so, there is a faulty JDK/JRE installation. If it happens only when using JDBC, then
it's time to check the documentation that came with the driver or the driver/DBMS
support. JDBC driver types 1 through 3 have some native code aspect and typically
require some sort of client install. Along with the install, various environment
variables and path or classpath settings must be in place. Because the requirements and
installation procedures vary with the provider, there is no reasonable way to provide
details here. A type 4 driver, on the other hand, is pure Java and should never exhibit
this problem. The trade off is that a type 4 driver is usually slower.
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Many connections from an Oracle8i pooled connection returns statement closed.
I am using import oracle.jdbc.pool.* with thin driver. If I test with many
simultaneous connections, I get an SQLException that the statement is closed.
ere is an example of concurrent operation of pooled connections from the
OracleConnectionPoolDataSource. There is an executable for kicking off threads, a
DataSource, and the workerThread.
The Executable Member
package package6;
/**
* package6.executableTester
*
*/
public class executableTester {
protected static myConnectionPoolDataSource dataSource = null;
static int i = 0;
/**
* Constructor
*/
public executableTester() throws java.sql.SQLException
{
}
/**
* main
* @param args
*/
public static void main(String[] args) {
try{
dataSource = new myConnectionPoolDataSource();
}
catch ( Exception ex ){
ex.printStackTrace();
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}
while ( i++ < 10 ) {
try{
workerClass worker = new workerClass();
worker.setThreadNumber( i );
worker.setConnectionPoolDataSource
( dataSource.getConnectionPoolDataSource() );
worker.start();
System.out.println( "Started Thread#"+i );
}
catch ( Exception ex ){
ex.printStackTrace();
}
}
}
}
The DataSource Member
package package6;
import oracle.jdbc.pool.*;
/**
* package6.myConnectionPoolDataSource.
*
*/
public class myConnectionPoolDataSource extends Object {
protected OracleConnectionPoolDataSource ocpds = null;
/**
* Constructor
*/
public myConnectionPoolDataSource()
throws java.sql.SQLException {
// Create a OracleConnectionPoolDataSource instance
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ocpds = new OracleConnectionPoolDataSource();
// Set connection parameters
ocpds.setURL("jdbc:oracle:oci8:@mydb");
ocpds.setUser("scott");
ocpds.setPassword("tiger");
}
public OracleConnectionPoolDataSource
getConnectionPoolDataSource() {
return ocpds;
}
}
The Worker Thread Member
package package6;
import oracle.jdbc.pool.*;
import java.sql.*;
import javax.sql.*;
/**
* package6.workerClass .
*
*/
public class workerClass extends Thread {
protected OracleConnectionPoolDataSource ocpds = null;
protected PooledConnection pc = null;
protected Connection conn = null;
protected int threadNumber = 0;
/**
* Constructor
*/
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public workerClass() {
}
public void doWork( ) throws SQLException {
// Create a pooled connection
pc = ocpds.getPooledConnection();
// Get a Logical connection
conn = pc.getConnection();
// Create a Statement
Statement stmt = conn.createStatement ();
// Select the ENAME column from the EMP table
ResultSet rset = stmt.executeQuery ("select ename from emp");
// Iterate through the result and print the employee names
while (rset.next ())
// System.out.println (rset.getString (1));
;
// Close the RseultSet
rset.close();
rset = null;
// Close the Statement
stmt.close();
stmt = null;
// Close the logical connection
conn.close();
conn = null;
// Close the pooled connection
pc.close();
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pc = null;
System.out.println( "workerClass.thread#
"+threadNumber+" completed..");
}
public void setThreadNumber( int assignment ){
threadNumber = assignment;
}
public void setConnectionPoolDataSource
(OracleConnectionPoolDataSource x){
ocpds = x;
}
public void run() {
try{
doWork();
}
catch ( Exception ex ){
ex.printStackTrace();
}
}
}
The OutPut Produced
Started Thread#1
Started Thread#2
Started Thread#3
Started Thread#4
Started Thread#5
Started Thread#6
Started Thread#7
Started Thread#8
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Started Thread#9
Started Thread#10
workerClass.thread# 1 completed..
workerClass.thread# 10 completed..
workerClass.thread# 3 completed..
workerClass.thread# 8 completed..
workerClass.thread# 2 completed..
workerClass.thread# 9 completed..
workerClass.thread# 5 completed..
workerClass.thread# 7 completed..
workerClass.thread# 6 completed..
workerClass.thread# 4 completed..
The oracle.jdbc.pool.OracleConnectionCacheImpl class is another subclass of the
oracle.jdbc.pool.OracleDataSource which should also be looked over, that is what you
really what to use. Here is a similar example that uses the
oracle.jdbc.pool.OracleConnectionCacheImpl. The general construct is the same as the
first example but note the differences in workerClass1 where some statements have
been commented ( basically a clone of workerClass from previous example ).
The Executable Member
package package6;
import java.sql.*;
import javax.sql.*;
import oracle.jdbc.pool.*;
/**
* package6.executableTester2
*
*/
public class executableTester2 {
static int i = 0;
protected static myOracleConnectCache
connectionCache = null;
/**
* Constructor
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*/
public executableTester2() throws SQLException
{
}
/**
* main
* @param args
*/
public static void main(String[] args) {
OracleConnectionPoolDataSource dataSource = null;
try{
dataSource = new OracleConnectionPoolDataSource() ;
connectionCache = new myOracleConnectCache( dataSource );
}
catch ( Exception ex ){
ex.printStackTrace();
}
while ( i++ < 10 ) {
try{
workerClass1 worker = new workerClass1();
worker.setThreadNumber( i );
worker.setConnection( connectionCache.getConnection() );
worker.start();
System.out.println( "Started Thread#"+i );
}
catch ( Exception ex ){
ex.printStackTrace();
}
}
}
protected void finalize(){
try{
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connectionCache.close();
} catch ( SQLException x) {
x.printStackTrace();
}
this.finalize();
}
}
The ConnectCacheImpl Member
package package6;
import javax.sql.ConnectionPoolDataSource;
import oracle.jdbc.pool.*;
import oracle.jdbc.driver.*;
import java.sql.*;
import java.sql.SQLException;
/**
* package6.myOracleConnectCache
*
*/
public class myOracleConnectCache extends
OracleConnectionCacheImpl {
/**
* Constructor
*/
public myOracleConnectCache( ConnectionPoolDataSource x)
throws SQLException {
initialize();
}
public void initialize() throws SQLException {
setURL("jdbc:oracle:oci8:@myDB");
setUser("scott");
setPassword("tiger");
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//
// prefab 2 connection and only grow to 4 , setting these
// to various values will demo the behavior
//clearly, if it is not
// obvious already
//
setMinLimit(2);
setMaxLimit(4);
}
}
The Worker Thread Member
package package6;
import oracle.jdbc.pool.*;
import java.sql.*;
import javax.sql.*;
/**
* package6.workerClass1
*
*/
public class workerClass1 extends Thread {
// protected OracleConnectionPoolDataSource
ocpds = null;
// protected PooledConnection pc = null;
protected Connection conn = null;
protected int threadNumber = 0;
/**
* Constructor
*/
public workerClass1() {
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}
public void doWork( ) throws SQLException {
// Create a pooled connection
// pc = ocpds.getPooledConnection();
// Get a Logical connection
// conn = pc.getConnection();
// Create a Statement
Statement stmt = conn.createStatement ();
// Select the ENAME column from the EMP table
ResultSet rset = stmt.executeQuery
("select ename from EMP");
// Iterate through the result
// and print the employee names
while (rset.next ())
// System.out.println (rset.getString (1));
;
// Close the RseultSet
rset.close();
rset = null;
// Close the Statement
stmt.close();
stmt = null;
// Close the logical connection
conn.close();
conn = null;
// Close the pooled connection
// pc.close();
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// pc = null;
System.out.println( "workerClass1.thread#
"+threadNumber+" completed..");
}
public void setThreadNumber( int assignment ){
threadNumber = assignment;
}
// public void setConnectionPoolDataSource
(OracleConnectionPoolDataSource x){
// ocpds = x;
// }
public void setConnection( Connection assignment ){
conn = assignment;
}
public void run() {
try{
doWork();
}
catch ( Exception ex ){
ex.printStackTrace();
}
}
}
The OutPut Produced
Started Thread#1
Started Thread#2
workerClass1.thread# 1 completed..
workerClass1.thread# 2 completed..
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Started Thread#3
Started Thread#4
Started Thread#5
workerClass1.thread# 5 completed..
workerClass1.thread# 4 completed..
workerClass1.thread# 3 completed..
Started Thread#6
Started Thread#7
Started Thread#8
Started Thread#9
workerClass1.thread# 8 completed..
workerClass1.thread# 9 completed..
workerClass1.thread# 6 completed..
workerClass1.thread# 7 completed..
Started Thread#10
workerClass1.thread# 10 completed.. DB2 Universal claims to support JDBC 2.0,
But I can only get JDBC 1.0 functionality. What can I do?
DB2 Universal defaults to the 1.0 driver. You have to run a special program to enable
the 2.0 driver and JDK support. For detailed information, see Setting the Environment
in Building Java Applets and Applications. The page includes instructions for most
supported platforms.
How do I disallow NULL values in a table?
Null capability is a column integrity constraint, normally applied at table creation
time. Note that some databases won't allow the constraint to be applied after table
creation. Most databases allow a default value for the column as well. The following
SQL statement displays the NOT NULL constraint:
CREATE TABLE CoffeeTable (
Type VARCHAR(25) NOT NULL,
Pounds INTEGER NOT NULL,
Price NUMERIC(5, 2) NOT NULL
)
How to get a field's value with ResultSet.getxxx when it is a NULL? I have tried
to execute a typical SQL statement:
select * from T-name where (clause);
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But an error gets thrown because there are some NULL fields in the table.
You should not get an error/exception just because of null values in various columns.
This sounds like a driver specific problem and you should first check the original and
any chained exceptions to determine if another problem exists. In general, one may
retrieve one of three values for a column that is null, depending on the data type. For
methods that return objects, null will be returned; for numeric ( get Byte(), getShort(),
getInt(), getLong(), getFloat(), and getDouble() ) zero will be returned; for
getBoolean() false will be returned. To find out if the value was actually NULL, use
ResultSet.wasNull() before invoking another getXXX method.
How do I insert/update records with some of the columns having NULL value?
Use either of the following PreparedStatement methods:
public void setNull(int parameterIndex, int sqlType) throws SQLException
public void setNull(int paramIndex, int sqlType, String typeName) throws
SQLException
These methods assume that the columns are nullable. In this case, you can also just
omit the columns in an INSERT statement; they will be automatically assigned null
values.
Is there a way to find the primary key(s) for an Access Database table? Sun's
JDBC-ODBC driver does not implement the getPrimaryKeys() method for the
DatabaseMetaData Objects.
// Use meta.getIndexInfo() will
//get you the PK index. Once
// you know the index, retrieve its column name
DatabaseMetaData meta = con.getMetaData();
String key_colname = null;
// get the primary key information
rset = meta.getIndexInfo(null,null, table_name, true,true);
while( rset.next())
{
String idx = rset.getString(6);
if( idx != null)
{
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//Note: index "PrimaryKey" is Access DB specific
// other db server has diff. index syntax.
if( idx.equalsIgnoreCase("PrimaryKey"))
{
key_colname = rset.getString(9);
setPrimaryKey( key_colname );
}
}
}
Why can't Tomcat find my Oracle JDBC drivers in classes111.zip?
TOMCAT 4.0.1 on NT4 throws the following exception when I try to connect to
Oracle DB from JSP.
javax.servlet.ServletException : oracle.jdbc.driver.OracleDriver
java.lang.ClassNotFoundException: oracle:jdbc:driver:OracleDriver
But, the Oracle JDBC driver ZIP file (classes111.zip)is available in the system
classpath.
Copied the Oracle Driver class file (classes111.zip) in %TOMCAT_Home Home%\lib directory and renamed it to classess111.jar.
Able to connect to Oracle DB from TOMCAT 4.01 via Oracle JDBC-Thin Driver.
I have an application that queries a database and retrieves the results into a
JTable. This is the code in the model that seems to be taken forever to execute,
especially for a large result set:
while ( myRs.next() ) {
Vector newRow =new Vector();
for ( int i=1;i<=numOfCols;i++ )
{
newRow.addElement(myRs.getObject(i));
}
allRows.addElement(newRow);
}
fireTableChanged(null);
newRow stores each row of the resultset and allRows stores all the rows.
Are the vectors here the problem?
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Is there another way of dealing with the result set that could execute faster?
java.util.Vector is largely thread safe, which means that there is a greater overhead in
calling addElement() as it is a synchronized method. If your result set is very large,
and threading is not an issue, you could use one of the thread-unsafe collections in
Java 2 to save some time. java.util.ArrayList is the likeliest candidate here.
Do not use a DefaultTableModel as it loads all of your data into memory at once,
which will obviously cause a large overhead - instead, use an AbstractTableModel and
provide an implementation which only loads data on demand, i.e. when (if) the user
scrolls down through the table.
How does one get column names for rows returned in a ResultSet?
ResultSet rs = ...
...
ResultSetMetaData rsmd = rs.getMetaData();
int numCols = rsmd.getColumnCount();
for (int i = 1; i <= numCols; i++)
{
System.out.println("[" + i + "]" +
rsmd.getColumnName(i) + " {" +
rsmd.getColumnTypeName(i) + "}");
}
What are the considerations for deciding on transaction boundaries?
Transaction processing should always deal with more than one statement and a
transaction is often described as a Logical Unit of Work ( LUW ). The rationale for
transactions is that you want to know definitively that all or none of the LUW
completed successfully. Note that this automatically gives you restart capability.
Typically, there are two conditions under which you would want to use transactions:
* Multiple statements involving a single file - An example would be inserting all of a
group of rows or all price updates for a given date. You want all of these to take effect
at the same time; inserting or changing some subset is not acceptable.
* Multiple statements involving multiple files - The classic example is transferring
money from one account to another or double entry accounting; you don't want the
debit to succeed and the credit to fail because money or important records will be lost.
Another example is a master/detail relationship, where, say, the master contains a total
column. If the entire LUW, writing the detail row and updating the master row, is not
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completed successfully, you A) want to know that the transaction was unsuccessful
and B) that a portion of the transaction was not lost or dangling.
Therefore, determining what completes the transaction or LUW should be the deciding
factor for transaction boundaries.
How can I determine where a given table is referenced via foreign keys?
DatabaseMetaData.getExportedKeys() returns a ResultSet with data similar to that
returned by DatabaseMetaData.getImportedKeys(), except that the information relates
to other tables that reference the given table as a foreign key container.
How can I get information about foreign keys used in a table?
DatabaseMetaData.getImportedKeys() returns a ResultSet with data about foreign key
columns, tables, sequence and update and delete rules.
Can I use JDBC to execute non-standard features that my DBMS provides?
The answer is a qualified yes. As discussed under SQL Conformance: "One way the
JDBC API deals with this problem is to allow any query string to be passed through to
an underlying DBMS driver. This means that an application is free to use as much
SQL functionality as desired, but it runs the risk of receiving an error on some
DBMSs. In fact, an application query may be something other than SQL, or it may be
a specialized derivative of SQL designed for specific DBMSs (for document or image
queries, for example)."
Clearly this means either giving up portability or checking the DBMS currently used
before invoking specific operations.
What is DML?
DML is an abbreviation for Data Manipulation Language. This portion of the SQL
standard is concerned with manipulating the data in a database as opposed to the
structure of a database. The core verbs for DML are SELECT, INSERT, DELETE,
UPDATE, COMMIT and ROLLBACK.
What is the significance of DataBaseMetaData.tableIndexStatistics? How to
obtain and use it?
To answer the second question first, the tableIndexStatistic constant in the TYPE
column will identify one of the rows in the ResultSet returned when
DatabaseMetaData.getIndexInfo() is invoked. If you analyze the wordy API, a
tableIndexStatistic row will contain the number of rows in the table in the
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CARDINALITY column and the number of pages used for the table in the PAGES
column.
What types of DataSource objects are specified in the Optional Package?
* Basic - Provides a standard Connection object.
* Pooled - Provides a Connection pool and returns a Connection that is controlled by
the pool.
* Distributed - Provides a Connection that can participate in distributed transactions (
more than one DBMS is involved). It is anticipated, but not enforced, that a distributed
DataSource will also provide pooling.
However, there are no standard methods available in the DataSource class to
determine if one has obtained a pooled and/or distributed Connection.
What is a JDBC 2.0 DataSource?
The DataSource class was introduced in the JDBC 2.0 Optional Package as an easier,
more generic means of obtaining a Connection. The actual driver providing services is
defined to the DataSource outside the application ( Of course, a production quality app
can and should provide this information outside the app anyway, usually with
properties files or ResourceBundles ). The documentation expresses the view that
DataSource will replace the common DriverManager method.
Does the database server have to be running Java or have Java support in order
for my remote JDBC client app to access the database?
The answer should always be no. The two critical requirements are LAN/internet
connectivity and an appropriate JDBC driver. Connectivity is usually via TCP/IP, but
other communication protocols are possible. Unspoken, but assumed here is that the
DBMS has been started to listen on a communications port. It is the JDBC driver's job
to convert the SQL statements and JDBC calls to the DBMS' native protocol. From the
server's point of view, it's just another data request coming into the port, the
programming language used to send the data is irrelevant at that point.
Which Java and java.sql data types map to my specific database types?
JDBC is, of necessity, reliant on the driver and underlying DBMS. These do not
always adhere to standards as closely as we would like, including differing names for
standard Java types. To deal with this, first, there are a number of tables available in
the JDK JDBC documentation dealing with types.
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Are the code examples from the JDBC API Tutorial and Reference, Second
Edition available online?
Yes.
When an SQL select statement doesn't return any rows, is an SQLException
thrown?
No. If you want to throw an exception, you could wrap your SQL related code in a
custom class and throw something like ObjectNotFoundException when the returned
ResultSet is empty.
Why should I consider optimistic versus pessimistic approaches to database
updates?
In a modern database, possibly the two most important issues are data integrity and
concurrency ( multiple users have access to and can update the data ). Either approach
can be appropriate, depending on the application, but it is important to be aware of
possible consequences to avoid being blindsided.
A pessimistic approach, with locks, is usually seen as good for data integrity, although
it can be bad for concurrency, especially the longer a lock is held. In particular, it
guarantees against 'lost updates' - defined as an update performed by one process
between the time of access and update by another process, which overwrites the
interim update. However, other users are blocked from updating the data and possibly
reading it as well if the read access also tries to acquire a lock. A notorious problem
can arise when a user accesses data for update and then doesn't act on it for a period of
time. Another situation that occurred with one of my clients is that a batch ( noninteractive ) process may need to update data while an interactive user has an update
lock on the same data. In that case, data integrity goes out the window and, depending
on how the application is written, more problems may be introduced. ( No, we did not
write the interactive update program and yes, we had recovery procedures in place. )
An optimstic approach can alleviate lock concurrency problems, but requires more
code and care for integrity. The "optimistic" definition usually says that expectations
of update clashes are rare, but I view them as normal occurrances in a heavily used
database. The basics are that any changes between time of access and time of update
must be detected and taken into account. This is often done by comparing timestamps,
but one must be sure that the timestamp is always changed for an update and, of
course, that the table contains a timestamp column. A more involved, but more
complete method involves saving the original columns and using them in the 'Where'
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clause of the Update statement. If the update fails, the data has changed and the latest
data should be reaccessed.
What is optimistic concurrency?
An optimistic approach dispenses with locks ( except during the actual update ) and
usually involves comparison of timestamps, or generations of data to ensure that data
hasn't changed between access and update times. It's generally explained that the term
optimistic is used because the expectation is that a clash between multiple updates to
the same data will seldom occur.
What is pessimistic concurrency?
With a pessimistic approach, locks are used to ensure that no users, other than the one
who holds the lock, can update data. It's generally explained that the term pessimistic
is used because the expectation is that many users will try to update the same data, so
one is pessimistic that an update will be able to complete properly. Locks may be
acquired, depending on the DBMS vendor, automatically via the selected Isolation
Level. Some vendors also implement 'Select... for Update', which explicitly acquires a
lock.
Can I get information about a ResultSet's associated Statement and Connection
in a method without having or adding specific arguments for the Statement and
Connection?
Yes. Use ResultSet.getStatement(). From the resulting Statement you can use
Statement.getConnection().
How can I tell if my JDBC driver normalizes java.sql.Date and java.sql.Time
objects?
To actually determine the values, the objects must be converted to a java.util.Date and
examined. See What does normalization mean for java.sql.Date and java.sql.Time? for
the definition of normalization. Notice that even a debugger will not show whether
these objects have been normalized, since the getXXX methods in java.sql.Date for
time elements and in java.sql.Time for date elements throw an exception.
So, while a java.sql.Date may show 2001-07-26, it's normalized only if the
java.util.Date value is:
Thu Jul 26 00:00:00 EDT 2001
and while a java.sql.Time may show 14:01:00, it's normalized only if the java.util.Date
value is:
Thu Jan 01 14:01:00 EST 1970
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What is the most efficient method of replicating data between databases using
JDBC?
Within Java, the most efficient method would be, opening connections using the JDBC
and inserting or updating the records from one database to the other database, but it
depends upon the databases being replicated. If you are using Oracle databases, it has
standard methods for replication, and you do not need the JDBC for the replication.
Use snapshots like updateable and read-only.
There are different kind of replication. Let us consider the most widely used ones:
A) One Master - One slave
I) If there is not a significant difference between the structure of the database tables,
the following method would be useful.
FromDatabase=A; ToDatabase=B
1) Open JDBC connections between the databases A and B.
2) Read a record (RA ) from A using an SQL query.
3) Store the values in the local variables in the Java program.
4) Insert the record in B if PK does not exist for the record RA in B.
5) If the PK exists in B, update the record in B.
6) Repeat the steps 2-5 'til all the records are read by the query.
7) If there are multiple tables to be replicated, repeat steps 2-7 using the different
queries.
II)If there is significant difference between the structure of the database tables, the
following method would be useful.
FromDatabase=A; ToDatabase=B
1) Open the JDBC connections to the databases A.
2) Read a record ( RA ) from A using an SQL query.
3) Write the output to an XML file-XMLA, according to the DTD for the records for
the database A structure.
4) Repeat steps 2 & 3 'til all the records are written to XMLA.
5) If there are more queries, repeat steps repeat steps from 2-4 and write the records to
the different entities in the XML file.
6) Transform the XMLA file using the XSL and XSLT to the format useful for the
database B and write to the XML file-XMLB.
7) Open the second JDBC connection to the Database B.
8) Read the XMLB file, one record at a time.
9) Insert the record in B if PK does not exist for the record RA in B.
10) If the PK exists in B, update the record in B.
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B) One Master - Multiple slaves
The difference here is to open multiple JDBC connections to write to the different
databases one record at a time.
C) Multiple Masters:
For multiple masters, use timestamps to compare the times of the records to find out
which is the latest record when a record is found in all the master databases.
Alternatively, create a column to store the time and date a record is inserted or
updated. When records are deleted, record the event in a log file along with the PK.
Prepared statements and batch updates should be used wherever possible in this
scenario.
What is the difference between setMaxRows(int) and SetFetchSize(int)? Can
either reduce processing time?
setFetchSize(int) defines the number of rows that will be read from the database when
the ResultSet needs more rows. The method in the java.sql.Statement interface will set
the 'default' value for all the ResultSet derived from that Statement; the method in the
java.sql.ResultSet interface will override that value for a specific ResultSet. Since
database fetches can be expensive in a networked environment, fetch size has an
impact on performance.
setMaxRows(int) sets the limit of the maximum nuber of rows in a ResultSet object. If
this limit is exceeded, the excess rows are "silently dropped". That's all the API says,
so the setMaxRows method may not help performance at all other than to decrease
memory usage. A value of 0 (default) means no limit.
Since we're talking about interfaces, be careful because the implementation of drivers
is often different from database to database and, in some cases, may not be
implemented or have a null implementation. Always refer to the driver documentation.
What is JDO?
JDO provides for the transparent persistence of data in a data store agnostic manner,
supporting object, hierarchical, as well as relational stores.
When I intersperse table creation or other DDL statements with DML
statements, I have a problem with a transaction being commited before I want it
to be. Everything ( commit and rollback ) works fine as long as I don't create
another table. How can I resolve the issue?
While the questioner found a partially workable method for his particular DBMS, as
mentioned in the section on transactions in my JDBC 2.0 Fundamentals Short Course:
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DDL statements in a transaction may be ignored or may cause a commit to occur. The
behavior is DBMS dependent and can be discovered by use of
DatabaseMetaData.dataDefinitionCausesTransactionCommit() and
DatabaseMetaData.dataDefinitionIgnoredInTransactions(). One way to avoid
unexpected results is to separate DML and DDL transactions.
The only generally effective way to "rollback" table creation is to delete the table.
What's the best way, in terms of performance, to do multiple insert/update
statements, a PreparedStatement or Batch Updates?
Because PreparedStatement objects are precompiled, their execution can be faster than
that of Statement objects. Consequently, an SQL statement that is executed many
times is often created as a PreparedStatement object to increase efficiency.
A CallableStatement object provides a way to call stored procedures in a standard
manner for all DBMSes. Their execution can be faster than that of PreparedStatement
object.
Batch updates are used when you want to execute multiple statements together.
Actually, there is no conflict here. While it depends on the driver/DBMS engine as to
whether or not you will get an actual performance benefit from batch updates,
Statement, PreparedStatement, and CallableStatement can all execute the addBatch()
method.
I need to have result set on a page where the user can sort on the column headers.
Any ideas?
One possibility: Have an optional field in your form or GET url called (appropriately)
ORDER with a default value of either "no order" or whatever you want your default
ordering to be (i.e. timestamp, username, whatever). When you get your request, see
what the value of the ORDER element is. If it's null or blank, use the default. Use that
value to build your SQL query, and display the results to the page. If you're caching
data in your servlet, you can use the Collection framework to sort your data (see
java.util.Collections) if you can get it into a List format. Then, you can create a
Collator which can impose a total ordering on your results.
What are the components of the JDBC URL for Oracle's "thin" driver and how
do I use them?
Briefly: jdbc:oracle:thin:@hostname:port:oracle-sid
1. in green the Oracle sub-protocol (can be oracle:oci7:@, oracle:oci8:@,
racle:thin:@, etc...) is related on the driver you are unsign and the protocol to
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communicate with server.
2. in red the network machine name, or its ip address, to locate the server where oracle
is running.
3. in blue the port (it is complementary to the address to select the specific oracle
service)
4. in magenta the sid, select on which database you want to connect.
example:
jdbc:oracle:thin:@MyOracleHost:1521:MyDB
IHere's an example:
jdbc:oracle:thin:scott/tiger@MyOracleHost:1521:MyDB
where user=scott and pass=tiger.
Why doesn't JDBC accept URLs instead of a URL string?
In order for something to be a java.net.URL, a protocol handler needs to be installed.
Since there is no one universal protocol for databases behind JDBC, the URLs are
treated as strings. In Java 1.4, these URL strings have a class called java.net.URI.
However, you still can't use a URI to load a JDBC driver, without converting it to a
string.
What JDBC objects generate SQLWarnings?
Connections, Statements and ResultSets all have a getWarnings method that allows
retrieval. Keep in mind that prior ResultSet warnings are cleared on each new read and
prior Statement warnings are cleared with each new execution. getWarnings() itself
does not clear existing warnings, but each object has a clearWarnings method.
What's the fastest way to normalize a Time object?
Of the two recommended ways when using a Calendar( see How do I create a
java.sql.Time object? ), in my tests, this code ( where c is a Calendar and t is a Time ):
c.set( Calendar.YEAR, 1970 );
c.set( Calendar.MONTH, Calendar.JANUARY );
c.set( Calendar.DATE, 1 );
c.set( Calendar.MILLISECOND, 0 );
t = new java.sql.Time( c.getTime().getTime() );
was always at least twice as fast as:
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t = java.sql.Time.valueOf(
c.get(Calendar.HOUR_OF_DAY) + ":" +
c.get(Calendar.MINUTE) + ":" +
c.get(Calendar.SECOND) );
When the argument sent to valueOf() was hardcoded ( i.e. valueOf( "13:50:10" ), the
time difference over 1000 iterations was negligible.
What does normalization mean for java.sql.Date and java.sql.Time?
These classes are thin wrappers extending java.util.Date, which has both date and time
components. java.sql.Date should carry only date information and a normalized
instance has the time information set to zeros. java.sql.Time should carry only time
information and a normalized instance has the date set to the Java epoch ( January 1,
1970 ) and the milliseconds portion set to zero.
How do I create a java.sql.Date object?
java.sql.Date descends from java.util.Date, but uses only the year, month and day
values. There are two methods to create a Date object. The first uses a Calendar object,
setting the year, month and day portions to the desired values. The hour, minute,
second and millisecond values must be set to zero. At that point,
Calendar.getTime().getTime() is invoked to get the java.util.Date milliseconds. That
value is then passed to a java.sql.Date constructor:
Calendar cal = Calendar.getInstance();
// set Date portion to January 1, 1970
cal.set( cal.YEAR, 1970 );
cal.set( cal.MONTH, cal.JANUARY );
cal.set( cal.DATE, 1 );
cal.set( cal.HOUR_OF_DAY, 0 );
cal.set( cal.MINUTE, 0 );
cal.set( cal.SECOND, 0 );
cal.set( cal.MILLISECOND, 0 );
java.sql.Date jsqlD =
new java.sql.Date( cal.getTime().getTime() );
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The second method is java.sql.Date's valueOf method. valueOf() accepts a String,
which must be the date in JDBC time escape format - "yyyy-mm-dd". For example,
java.sql.Date jsqlD = java.sql.Date.valueOf( "2010-01-31" );
creates a Date object representing January 31, 2010. To use this method with a
Calendar object, use:
java.sql.Date jsqlD = java.sql.Date.valueOf(
cal.get(cal.YEAR) + ":" +
cal.get(cal.MONTH) + ":" +
cal.get(cal.DATE) );
which produces a Date object with the same value as the first example.
How do I create a java.sql.Time object?
java.sql.Time descends from java.util.Date, but uses only the hour, minute and second
values. There are two methods to create a Time object. The first uses a Calendar
object, setting the year, month and day portions to January 1, 1970, which is Java's
zero epoch. The millisecond value must also be set to zero. At that point,
Calendar.getTime().getTime() is invoked to get the time in milliseconds. That value is
then passed to a Time constructor:
Calendar cal = Calendar.getInstance();
// set Date portion to January 1, 1970
cal.set( cal.YEAR, 1970 );
cal.set( cal.MONTH, cal.JANUARY );
cal.set( cal.DATE, 1 );
cal.set( cal.MILLISECOND, 0 );
java.sql.Time jsqlT =
new java.sql.Time( cal.getTime().getTime() );
The second method is Time's valueOf method. valueOf() accepts a String, which must
be the time in JDBC time escape format - "hh:mm:ss". For example,
java.sql.Time jsqlT = java.sql.Time.valueOf( "18:05:00" );
creates a Time object representing 6:05 p.m. To use this method with a Calendar
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object, use:
java.sql.Time jsqlT = java.sql.Time.valueOf(
cal.get(cal.HOUR_OF_DAY) + ":" +
cal.get(cal.MINUTE) + ":" +
cal.get(cal.SECOND) );
which produces a Time object with the same value as the first example.
What scalar functions can I expect to be supported by JDBC?
JDBC supports numeric, string, time, date, system, and conversion functions on scalar
values. For a list of those supported and additional information, see section A.1.4
Support Scalar Functions in the JDBC Data Access API For Driver Writers. Note that
drivers are only expected to support those scalar functions that are supported by the
underlying DB engine.
What does setFetchSize() really do?
The API documentation explains it pretty well, but a number of programmers seem to
have a misconception of its functionality. The first thing to note is that it may do
nothing at all; it is only a hint, even to a JDBC Compliant driver. setFetchSize() is
really a request for a certain sized blocking factor, that is, how much data to send at a
time.
Because trips to the server are expensive, sending a larger number of rows can be
more efficient. It may be more efficient on the server side as well, depending on the
particular SQL statement and the DB engine. That would be true if the data could be
read straight off an index and the DB engine paid attention to the fetch size. In that
case, the DB engine could return only enough data per request to match the fetch size.
Don't count on that behavior. In general, the fetch size will be transparent to your
program and only determines how often requests are sent to the server as you traverse
the data.
Also, both Statement and ResultSet have setFetchSize methods. If used with a
Statement, all ResultSets returned by that Statement will have the same fetch size. The
method can be used at any time to change the fetch size for a given ResultSet. To
determine the current or default size, use the getFetchSize methods.
Is there a practical limit for the number of SQL statements that can be added to
an instance of a Statement object
While the specification makes no mention of any size limitation for
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Statement.addBatch(), this seems to be dependent, as usual, on the driver. Among
other things, it depends on the type of container/collection used. I know of at least one
driver that uses a Vector and grows as needed. I've seen questions about another driver
that appears to peak somewhere between 500 and 1000 statements. Unfortunately,
there doesn't appear to be any metadata information regarding possible limits. Of
course, in a production quality driver, one would expect an exception from an
addBatch() invocation that went beyond the command list's limits.
How can I determine whether a Statement and its ResultSet will be closed on a
commit or rollback?
Use the DatabaseMetaData methods supportsOpenStatementsAcrossCommit() and
supportsOpenStatementsAcrossRollback().
How do I get runtime information about the JDBC Driver?
Use the following DatabaseMetaData methods:
getDriverMajorVersion()
getDriverMinorVersion()
getDriverName()
getDriverVersion()
How do I create an updatable ResultSet?
Just as is required with a scrollable ResultSet, the Statement must be capable of
returning an updatable ResultSet. This is accomplished by asking the Connection to
return the appropriate type of Statement using Connection.createStatement(int
resultSetType, int resultSetConcurrency). The resultSetConcurrency parameter must
be ResultSet.CONCUR_UPDATABLE. The actual code would look like this:
Statement stmt = con.createStatement( ResultSet.TYPE_SCROLL_SENSITIVE,
ResultSet.CONCUR_UPDATABLE );
Note that the spec allows a driver to return a different type of Statement/ResultSet than
that requested, depending on capabilities and circumstances, so the actual type
returned should be checked with ResultSet.getConcurrency().
How can I connect to an Oracle database not on the web server from an
untrusted applet?
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You can use the thin ORACLE JDBC driver in an applet (with some extra parameters
on the JDBC URL). Then, if you have NET8, you can use the connection manager of
NET8 on the web server to proxy the connection request to the database server.
How can I insert multiple rows into a database in a single transaction?
//turn off the implicit commit
Connection.setAutoCommit(false);
//..your insert/update/delete goes here
Connection.Commit();
a new transaction is implicitly started.
JDBC 2.0 provides a set of methods for executing a batch of database commands.
Specifically, the java.sql.Statement interface provides three methods: addBatch(),
clearBatch() and executeBatch(). Their documentation is pretty straight forward.
The implementation of these methods is optional, so be sure that your driver supports
these.
How do I display and parse a date?
The Java I18N way is to use a DateFormat. While SimpleDateFormat, which is
generally returned, creates a large number of objects, it is locale aware and will handle
most of your needs. The following sample code initially creates a java.sql.Date object
and formats it for the default locale. An initial actionPerformed call additionally
formats/displays it for a German locale and also displays the resulting java.sql.Date in
standard escape format. Other dates can be entered and parsed after the initial display.
// JDFDP.java - Display and Parse java.sql.Date
import java.sql.*;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.text.*;
import java.util.*;
public class JDFDP extends JFrame
implements ActionListener,
WindowListener
{
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// create a java.sql.Date
java.sql.Date jsqlDate = new java.sql.Date(
System.currentTimeMillis() );
DateFormat dfLocal = DateFormat.getDateInstance(
DateFormat.SHORT );
DateFormat dfGermany = DateFormat.getDateInstance(
DateFormat.SHORT, Locale.GERMANY );
JButton jb = new JButton( "Go" );
JLabel jlI = new JLabel("Input a Date:"),
jlD = new JLabel("Display German:"),
jlP = new JLabel("Parsed:");
JPanel jp = new JPanel();
JTextField jtI = new JTextField( 10 ),
jtD = new JTextField( 10 ),
jtP = new JTextField( 10 );
public JDFDP()
{
super( "JDFDP" );
addWindowListener( this );
jb.addActionListener( this );
jp.add(jlI);
jp.add(jtI);
jp.add(jb);
jp.add(jlD);
jp.add(jtD);
jp.add(jlP);
jp.add(jtP);
getContentPane().add( jp, BorderLayout.CENTER );
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pack();
// set text by sending dummy event
jtI.setText( dfLocal.format( jsqlDate ) );
actionPerformed(
new ActionEvent( this, 12, "12" ) );
show();
} // end constructor
// ActionListener Implementation
public void actionPerformed(ActionEvent e)
{
jtD.setText( "" );
jtP.setText( "" );
try
{
java.util.Date d = dfLocal.parse(
jtI.getText() );
jtI.setText( dfLocal.format( d ) );
jtD.setText( dfGermany.format( d ) );
d = dfGermany.parse( jtD.getText() );
// get new java.sql.Date
jsqlDate = new java.sql.Date( d.getTime() );
jtP.setText( jsqlDate.toString() );
}
catch( ParseException pe ) { jtI.setText( "" ); }
} // End actionPerformed
// Window Listener Implementation
public void windowOpened(WindowEvent e) {}
public void windowClosing(WindowEvent e)
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{
dispose();
System.exit(0);
}
public void windowClosed(WindowEvent e) {}
public void windowIconified(WindowEvent e) {}
public void windowDeiconified(WindowEvent e) {}
public void windowActivated(WindowEvent e) {}
public void windowDeactivated(WindowEvent e) {}
// End Window Listener Implementation
public static void main(String[] args)
{
new JDFDP();
}
} // end class JDFDP
How can I retrieve string data from a database in Unicode format?
The data is already in Unicode when it arrives
in your program. Conversion from and to the
encoding/charset/CCSID in the database from/to
Unicode in the program is part of the JDBC driver's job.
If, for some reason, you want to see the data in
'\uHHHH' format ( where 'H' is the hex value ),
the following code, while not very efficient,
should give you some ideas:
public class UniFormat
{
public static void main( String[] args )
{
char[] ac = args[0].toCharArray();
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int iValue;
String s = null;
StringBuffer sb = new StringBuffer();
for( int ndx = 0; ndx < ac.length; ndx++ )
{
iValue = ac[ndx];
if( iValue < 0x10 )
{
s = "\\u000";
}
else
if( iValue < 0x100 )
{
s = "\\u00";
}
else
if( iValue < 0x1000 )
{
s = "\\u0";
}
sb.append( s + Integer.toHexString( iValue ) );
} // end for
System.out.println("The Unicode format of " +
args[0] + " is " + sb );
} // end main
} // end class UniFormat
Can ResultSets be passed between methods of a class? Are there any special
usage
Yes. There is no reason that a ResultSet can't be used as a method parameter just like
any other object reference. You must ensure that access to the ResultSet is
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synchronized. This should not be a problem is the ResultSet is a method variable
passed as a method parameter - the ResultSet will have method scope and multi-thread
access would not be an issue.
As an example, say you have several methods that obtain a ResultSet from the same
table(s) and same columns, but use different queries. If you want these ResultSets to
be processed the same way, you would have another method for that. This could look
something like:
public List getStudentsByLastName(String lastName) {
ResultSet rs = ... (JDBC code to retrieve students by last name);
return processResultSet(rs);
}
public List getStudentsByFirstName(String firstName) {
ResultSet rs = ... (JDBC code to retrieve students by first name);
return processResultSet(rs);
}
private List processResultSet(ResultSet rs) {
List l = ... (code that iterates through ResultSet to build a List of Student objects);
return l;
}
Since the ResultSet always has method scope - sychronization is never an issue.
1. There is only one ResultSet. Dont assume that the ResultSet is at the start (or in any
good state...) just because you received it as a parameter. Previous operations
involving the ResultSet will have had the side-effect of changing its state.
2. You will need to be careful about the order in which you close the ResultSet and
CallableStatement/PreparedStatement/etc
From my own experience using the Oracle JDBC drivers and CallableStatements the
following statements are true:
* If you close the CallableStatement the ResultSet retrieved from that
CallableStatement immediately goes out-of-scope.
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* If you close the ResultSet without reading it fully, you must close the
CallableStatement or risk leaking a cursor on the database server.
* If you close the CallableStatement without reading it's associated ResultSet fully,
you risk leaking a cursor on the database server.
No doubt, these observations are valid only for Oracle drivers. Perhaps only for some
versions of Oracle drivers.
The recommended sequence seems to be:
* Open the statement
* Retrieve the ResultSet from the statement
* Read what you need from the ResultSet
* Close the ResultSet
* Close the Statement
How can I convert a java array to a java.sql.Array?
A Java array is a first class object and all of the references basically use
PreparedStatement.setObject() or ResultSet.updateObject() methods for putting the
array to an ARRAY in the database. Here's a basic example:
String[] as = { "One", "Two", "Three" };
...
PreparedStatement ps = con.prepareStatement(
"UPDATE MYTABLE SET ArrayNums = ? WHERE MyKey = ?" );
...
ps.setObject( 1, as );
Could we get sample code for retrieving more than one parameter from a stored
procedure?
Assume we have a stored procedure with this signature:
MultiSP (IN I1 INTEGER, OUT O1 INTEGER, INOUT IO1 INTEGER)
The code snippet to retrieve the OUT and INOUT parameters follows:
CallableStatement cs = connection.prepareCall( "(CALL MultiSP(?, ?, ?))" );
cs.setInt(1, 1); // set the IN parm I1 to 1
cs.setInt(3, 3); // set the INOUT parm IO1 to 3
cs.registerOutParameter(2, Types.INTEGER); // register the OUT parm O1
cs.registerOutParameter(3, Types.INTEGER); // register the INOUT parm IO1
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cs.execute();
int iParm2 = cs.getInt(2);
int iParm3 = cs.getInt(3);
cs.close();
The code really is just additive; be sure that for each IN parameter that setXXX() is
called and that for each INOUT and OUT parameter that registerOutParameter() is
called.
What is the difference between client and server database cursors?
What you see on the client side is the current row of the cursor which called a Result
(ODBC) or ResultSet (JDBC). The cursor is a server-side entity only and remains on
the server side.
How can I pool my database connections so I don't have to keep reconnecting to
the database?
There are plenty of connection pool implementations described in books or availalble
on the net. Most of them implement the same model. The process is always the same :
* you gets a reference to the pool
* you gets a free connection from the pool
* you performs your different tasks
* you frees the connection to the pool
Since your application retrieves a pooled connection, you don't consume your time to
connect / disconnect from your data source. You can find some implementation of
pooled connection over the net, for example:
* Db Connection Broker (http://www.javaexchange.com/), a package quite stable ( I
used it in the past to pool an ORACLE database on VMS system)
You can look at the JDBC 2.0 standard extension API specification from SUN which
defines a number of additional concepts.
How can I connect to an Excel spreadsheet file using jdbc?
Let's say you have created the following Excel spreadsheet in a worksheet called
Sheet1 (the default sheet name). And you've saved the file in c:\users.xls.
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USERID FIRST_NAME LAST_NAME
pkua Peter Kua
jlsmith John Smith
gh2312 Everett Johnson
chimera Faiz Abdullah
roy6943 Roy Sudirman
Since Excel comes with an ODBC driver, we'll use the JDBC-ODBC bridge driver
that comes packaged with Sun's JDK to connect to our spreadsheet.
In Excel, the name of the worksheet is the equivalent of the database table name, while
the header names found on the first row of the worksheet is the equivalent of the table
field names. Therefore, when accessing Excel via jdbc, it is very important to place
your data with the headers starting at row 1.
1. Create a new ODBC Data Source using the Microsoft Excel Driver. Name the DSN
"excel", and have it point to c:\users.xls.
2. Type in the following code:
package classes;
import java.sql.*;
public class TestServer
{
static
{
try {
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
}
catch (Exception e) {
System.err.println(e);
}
}
public static void main(String args[]) {
Connection conn=null;
Statement stmt=null;
String sql="";
ResultSet rs=null;
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try {
conn=DriverManager.getConnection("jdbc:odbc:excel","","");
stmt=conn.createStatement();
sql="select * from [Sheet1$]";
rs=stmt.executeQuery(sql);
while(rs.next()){
System.out.println(rs.getString("USERID")+
" "+ rs.getString("FIRST_NAME")+" "+
rs.getString("LAST_NAME"));
}
}
catch (Exception e){
System.err.println(e);
}
finally {
try{
rs.close();
stmt.close();
conn.close();
rs=null;
stmt=null;
conn=null;
}
catch(Exception e){}
}
}
}
Notice that we have connected to the Excel ODBC Data Source the same way we
would connect to any normal database server.
The only significant difference is in the SELECT statement. Although your data is
residing in the worksheet called "Sheet1", you'll have to refer to the sheet as Sheet1$
in your SQL statements. And because the dollar sign symbol is a reserved character in
SQL, you'll have to encapsulate the word Sheet1$ in brackets, as shown in the code.
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How do I execute stored procedures?
Here is an example on how to execute a stored procedure with JDBC (to use this in a
servlet is the same the only thing is that you create the connection and callable
statement in the init() of the servlet):
package DBTest;
import java.sql.*;
public class JdbcTest {
private String msDbUrl = "jdbc:odbc:ms";
private String msJdbcClass = "sun.jdbc.odbc.JdbcOdbcDriver";
private Connection mcDbAccess;
private CallableStatement msProcedure;
public JdbcTest() {
try {
Class.forName( msDbUrl ).newInstance();
mcDbAccess = DriverManager.getConnection( msJdbcClass, "milestone", "milestone"
);
msProcedure = mcDbAccess.prepareCall(
"{? = call sp_sav_Bom_Header( ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ? ) }"
);
msProcedure.registerOutParameter( 1, java.sql.Types.VARCHAR );
msProcedure.setInt( 2, -1 );
msProcedure.setInt( 3, 39 );
msProcedure.setString( 4, "format" );
long ltTest = new java.util.Date().getTime();
System.out.println( "Today: " + ltTest );
msProcedure.setTimestamp( 5, new Timestamp( ltTest ) );
msProcedure.setString( 6, "type" );
msProcedure.setString( 7, "submitter" );
msProcedure.setString( 8, "email" );
msProcedure.setString( 9, "phone" );
msProcedure.setString( 10, "comments" );
msProcedure.setString( 11, "label" );
msProcedure.setInt( 12, 52 );
msProcedure.setBoolean( 13, true );
msProcedure.setBoolean( 14, false );
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msProcedure.setInt( 15, 53 );
msProcedure.setString( 16, "runtime" );
msProcedure.setString( 17, "configuration" );
msProcedure.setBoolean( 18, true );
msProcedure.setBoolean( 19, false );
msProcedure.setString( 20, "special instructions" );
msProcedure.setInt( 21, 54 );
ResultSet lrsReturn = null;
System.out.println( "Execute: " + (lrsReturn = msProcedure.executeQuery() ) );
while( lrsReturn.next() ) {
System.out.println( "Got from result set: " + lrsReturn.getInt( 1 ) );
}
System.out.println( "Got from stored procedure: " + msProcedure.getString( 1 ) );
} catch( Throwable e ) {
e.printStackTrace();
}
}
public static void main(String[] args) {
new JdbcTest();
}
}
I also tried it by using a native JDBC driver (i-net) and it also works fine. The only
problem we encounter with JDBC-ODBC bridge is that a stored procedure pads spaces
to the full length of a VARCHAR but the native JDBC behaves right. Therefore I
suggest to use JDBC native drivers.
The above example uses the MS SQL Server.
How can I get data from multiple ResultSets?
With certain database systems, a stored procedure can return multiple result sets,
multiple update counts, or some combination of both. Also, if you are providing a user
with the ability to enter any SQL statement, you don't know if you are going to get a
ResultSet or an update count back from each statement, without analyzing the
contents. The Statement.execute() method helps in these cases.
Method Statement.execute() returns a boolean to tell you the type of response:
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* true indicates next result is a ResultSet
Use Statement.getResultSet to get the ResultSet
* false indicates next result is an update count
Use Statement.getUpdateCount to get the update count
* false also indicates no more results
Update count is -1 when no more results (usually 0 or positive)
After processing each response, you use Statement.getMoreResults to check for more
results, again returning a boolean. The following demonstrates the processing of
multiple result sets:
boolean result = stmt.execute(" ... ");
int updateCount = stmt.getUpdateCount();
while (result || (updateCount != -1)) {
if(result) {
ResultSet r = stmt.getResultSet();
// process result set
} else if(updateCount != -1) {
// process update count
}
result = stmt.getMoreResults();
updateCount = stmt.getUpdateCount();
}
How can resultset records be restricted to certain rows?
The easy answer is "Use a JDBC 2.0 compliant driver".
With a 2.0 driver, you can use the setFetchSize() method within a Statement or a
ResultSet object.
For example,
Statement stmt = con.createStatement();
stmt.setFetchSize(400);
ResultSet rs = stmt.executeQuery("select * from customers");
will change the default fetch size to 400.
You can also control the direction in which the rows are processed. For instance:
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stmt.setFetchDirection(ResultSet.FETCH_REVERSE)
will process the rows from bottom up.
The driver manager usually defaults to the most efficient fetch size...so you may try
experimenting with different value for optimal performance.
How do I insert an image file (or other raw data) into a database?
All raw data types (including binary documents or images) should be read and
uploaded to the database as an array of bytes, byte[]. Originating from a binary file,
1. Read all data from the file using a FileInputStream.
2. Create a byte array from the read data.
3. Use method setBytes(int index, byte[] data); of java.sql.PreparedStatement to
upload the data.
How can I connect from an applet to a database on the server?
There are two ways of connecting to a database on the server side.
1. The hard way. Untrusted applets cannot touch the hard disk of a computer. Thus,
your applet cannot use native or other local files (such as JDBC database drivers) on
your hard drive. The first alternative solution is to create a digitally signed applet
which may use locally installed JDBC drivers, able to connect directly to the database
on the server side.
2. The easy way. Untrusted applets may only open a network connection to the server
from which they were downloaded. Thus, you must place a database listener (either
the database itself, or a middleware server) on the server node from which the applet
was downloaded. The applet would open a socket connection to the middleware
server, located on the same computer node as the webserver from which the applet
was downloaded. The middleware server is used as a mediator, connecting to and
extract data from the database.
Can I use the JDBC-ODBC bridge driver in an applet?
Short answer: No.
Longer answer: You may create a digitally signed applet using a Certicate to
circumvent the security sandbox of the browser.
Which is the preferred collection class to use for storing database result sets?
When retrieving database results, the best collection implementation to use is the
LinkedList. The benefits include:
* Retains the original retrieval order
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* Has quick insertion at the head/tail
* Doesn't have an internal size limitation like a Vector where when the size is
exceeded a new internal structure is created (or you have to find out size beforehand to
size properly)
* Permits user-controlled synchronization unlike the pre-Collections Vector which is
always synchronized
Basically:
ResultSet result = stmt.executeQuery("...");
List list = new LinkedList();
while(result.next()) {
list.add(result.getString("col"));
}
If there are multiple columns in the result set, you'll have to combine them into their
own data structure for each row. Arrays work well for that as you know the size,
though a custom class might be best so you can convert the contents to the proper type
when extracting from databse, instead of later.
The java.sql package contains mostly interfaces. When and how are these
interfaces implemented while connecting to database?
The implementation of these interfaces is all part of the driver. A JDBC driver is not
just one class - it is a complete set of database-specific implementations for the
interfaces defined by the JDBC.
These driver classes come into being through a bootstrap process. This is best shown
by stepping through the process of using JDBC to connect to a database, using
Oracle's type 4 JDBC driver as an example:
* First, the main driver class must be loaded into the VM:
Class.forName("oracle.jdbc.driver.OracleDriver");
The specified driver must implement the Driver interface. A class initializer (static
code block) within the OracleDriver class registers the driver with the DriverManager.
* Next, we need to obtain a connection to the database:
String jdbcURL = "jdbc:oracle:thin:@www.jguru.com:1521:ORCL";
Connection connection = DriverManager.getConnection(jdbcURL);
DriverManager determines which registered driver to use by invoking the
acceptsURL(String url) method of each driver, passing each the JDBC URL. The first
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driver to return "true" in response will be used for this connection. In this example,
OracleDriver will return "true", so DriverManager then invokes the connect() method
of OracleDriver to obtain an instance of OracleConnection. It is this database-specific
connection instance implementing the Connection interface that is passed back from
the DriverManager.getConnection() call.
* The bootstrap process continues when you create a statement:
Statement statement = connection.createStatement();
The connection reference points to an instance of OracleConnection. This databasespecific implementation of Connection returns a database-specific implementation of
Statement, namely OracleStatement
* Invoking the execute() method of this statement object will execute the databasespecific code necessary to issue an SQL statement and retrieve the results:
ResultSet result = statement.executeQuery("SELECT * FROM TABLE");
Again, what is actually returned is an instance of OracleResultSet, which is an Oraclespecific implementation of the ResultSet interface.
So the purpose of a JDBC driver is to provide these implementations that hide all the
database-specific details behind standard Java interfaces.
How can I manage special characters (for example: " _ ' % ) when I execute an
INSERT query? If I don't filter the quoting marks or the apostrophe, for
example, the SQL string will cause an error.
In JDBC, strings containing SQL commands are just normal strings - the SQL is not
parsed or interpreted by the Java compiler. So there is no special mechanism for
dealing with special characters; if you need to use a quote (") within a Java string, you
must escape it.
The Java programming language supports all the standard C escapes, such as \n for
newline, \t for tab, etc. In this case, you would use \" to represent a quote within a
string literal:
String stringWithQuote =
"\"No,\" he replied, \"I did not like that salted licorice.\"";
This only takes care of one part of the problem: letting us control the exact string that
is passed on to the database. If you want tell the database to interpret characters like a
single quote (') literally (and not as string delimiters, for instance), you need to use a
different method. JDBC allows you to specify a separate, SQL escape character that
causes the character following to be interpreted literally, rather than as a special
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character.
An example of this is if you want to issue the following SQL command:
SELECT * FROM BIRDS
WHERE SPECIES='Williamson's Sapsucker'
In this case, the apostrophe in "Williamson's" is going to cause a problem for the
database because SQL will interpret it as a string delimiter. It is not good enough to
use the C-style escape \', because that substitution would be made by the Java compiler
before the string is sent to the database.
Different flavors of SQL provide different methods to deal with this situation. JDBC
abstracts these methods and provides a solution that works for all databases. With
JDBC you could write the SQL as follows:
Statement statement = // obtain reference to a Statement
statement.executeQuery(
"SELECT * FROM BIRDS WHERE SPECIES='Williamson/'s Sapsucker' {escape
'/'}");
The clause in curly braces, namely {escape '/'}, is special syntax used to inform JDBC
drivers what character the programmer has chosen as an escape character. The forward
slash used as the SQL escape has no special meaning to the Java compiler; this escape
sequence is interpreted by the JDBC driver and translated into database-specific SQL
before the SQL command is issued to the database.
Escape characters are also important when using the SQL LIKE clause. This usage is
explicitly addressed in section 11.5 of the JDBC specification:
The characters "%" and "_" have special meaning in SQL LIKE clauses (to match zero
or more characters, or exactly one character, respectively). In order to interpret them
literally, they can be preceded with a special escape character in strings, e.g. "\". In
order to specify the escape character used to quote these characters, include the
following syntax on the end of the query:
{escape 'escape-character'}
For example, the query
SELECT NAME FROM IDENTIFIERS WHERE ID LIKE '\_%' {escape '\'}
finds identifier names that begin with an underbar.
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How can I make batch updates using JDBC?
One of the more advanced features of JDBC 2.0 is the ability to submit multiple
update statements to the database for processing as a single unit. This batch updating
can be significantly more efficient compared to JDBC 1.0, where each update
statement has to be executed separately.
Consider the following code segment demonstrating a batch update:
try {
dbCon.setAutoCommit(false);
Statement stmt= dbCon.createStatement();
stmt.addBatch("INSERT INTO bugs "+
"VALUES (1007, 'Server stack overflow', 1,2,{d '1999-01-01'})");
stmt.addBatch("INSERT INTO bugs "+
"VALUES (1008,'Cannot load DLL', 3,1,{d '1999-01-01'})");
stmt.addBatch("INSERT INTO bugs "+
"VALUES (1009,'Applet locks up',2,2,{d '1999-01-01'})");
int[] updCnt = stmt.executeBatch();
dbCon.commit();
} catch (BatchUpdateException be) {
//handle batch update exception
int[] counts = be.getUpdateCounts();
for (int i=0; I counts.length; i++) {
System.out.println("Statement["+i+"] :"+counts[i]);
}
dbCon.rollback();
}
catch (SQLException e) {
//handle SQL exception
dbCon.rollback();
}
Before carrying out a batch update, it is important to disable the auto-commit mode by
calling setAutoCommit(false). This way, you will be able to rollback the batch
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transaction in case one of the updates fail for any reason. When the Statement object is
created, it is automatically associated a "command list", which is initially empty. We
then add our SQL update statements to this command list, by making successive calls
to the addBatch() method. On calling executeBatch(), the entire command list is sent
over to the database, and are then executed in the order they were added to the list. If
all the commands in the list are executed successfully, their corresponding update
counts are returned as an array of integers. Please note that you always have to clear
the existing batch by calling clearBatch() before creating a new one.
If any of the updates fail to execute within the database, a BatchUpdateException is
thrown in response to it. In case there is a problem in returning the update counts of
each SQL statement, a SQLException will be thrown to indicate the error.
How do I extract SQL table column type information?
Use the getColumns method of the java.sql.DatabaseMetaData interface to investigate
the column type information of a particular table. Note that most arguments to the
getColumns method (pinpointing the column in question) may be null, to broaden the
search criteria. A code sample can be seen below:
public static void main(String[] args) throws Exception
{
// Load the database driver - in this case, we
// use the Jdbc/Odbc bridge driver.
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
// Open a connection to the database
Connection conn = DriverManager.getConnection("[jdbcURL]",
"[login]", "[passwd]");
// Get DatabaseMetaData
DatabaseMetaData dbmd = conn.getMetaData();
// Get all column types for the table "sysforeignkeys", in schema
// "dbo" and catalog "test".
ResultSet rs = dbmd.getColumns("test", "dbo", "sysforeignkeys", "%");
// Printout table data
while(rs.next())
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{
// Get dbObject metadata
String dbObjectCatalog = rs.getString(1);
String dbObjectSchema = rs.getString(2);
String dbObjectName = rs.getString(3);
String dbColumnName = rs.getString(4);
String dbColumnTypeName = rs.getString(6);
int dbColumnSize = rs.getInt(7);
int dbDecimalDigits = rs.getInt(9);
String dbColumnDefault = rs.getString(13);
int dbOrdinalPosition = rs.getInt(17);
String dbColumnIsNullable = rs.getString(18);
// Printout
System.out.println("Col(" + dbOrdinalPosition + "): " + dbColumnName
+ " (" + dbColumnTypeName +")");
System.out.println(" Nullable: " + dbColumnIsNullable +
", Size: " + dbColumnSize);
System.out.println(" Position in table: " + dbOrdinalPosition
+ ", Decimal digits: " + dbDecimalDigits);
}
// Free database resources
rs.close();
conn.close();
}
How can I investigate the parameters to send into and receive from a database
stored procedure?
Use the method getProcedureColumns in interface DatabaseMetaData to probe a
stored procedure for metadata. The exact usage is described in the code below.
NOTE! This method can only discover parameter values. For databases where a
returning ResultSet is created simply by executing a SELECT statement within a
stored procedure (thus not sending the return ResultSet to the java application via a
declared parameter), the real return value of the stored procedure cannot be detected.
This is a weakness for the JDBC metadata mining which is especially present when
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handling Transact-SQL databases such as those produced by SyBase and Microsoft.
public static void main(String[] args) throws Exception
{
// Load the database driver - in this case, we
// use the Jdbc/Odbc bridge driver.
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver")
;
// Open a connection to the database
Connection conn = DriverManager.getConnection("[jdbcURL]",
"[login]", "[passwd]");
// Get DatabaseMetaData
DatabaseMetaData dbmd = conn.getMetaData();
// Get all column definitions for procedure "getFoodsEaten" in
// schema "testlogin" and catalog "dbo".
System.out.println("Procedures are called '" + dbmd.getProcedureTerm() +"' in the
DBMS.");
ResultSet rs = dbmd.getProcedureColumns("test", "dbo", "getFoodsEaten", "%");
// Printout table data
while(rs.next())
{
// Get procedure metadata
String dbProcedureCatalog = rs.getString(1);
String dbProcedureSchema = rs.getString(2);
String dbProcedureName = rs.getString(3);
String dbColumnName = rs.getString(4);
short dbColumnReturn = rs.getShort(5);
String dbColumnReturnTypeName = rs.getString(7);
int dbColumnPrecision = rs.getInt(8);
int dbColumnByteLength = rs.getInt(9);
short dbColumnScale = rs.getShort(10);
short dbColumnRadix = rs.getShort(11);
String dbColumnRemarks = rs.getString(13);
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// Interpret the return type (readable for humans)
String procReturn = null;
switch(dbColumnReturn)
{
case DatabaseMetaData.procedureColumnIn:
procReturn = "In";
break;
case DatabaseMetaData.procedureColumnOut:
procReturn = "Out";
break;
case DatabaseMetaData.procedureColumnInOut:
procReturn = "In/Out";
break;
case DatabaseMetaData.procedureColumnReturn:
procReturn = "return value";
break;
case DatabaseMetaData.procedureColumnResult:
procReturn = "return ResultSet";
default:
procReturn = "Unknown";
}
// Printout
System.out.println("Procedure: " + dbProcedureCatalog + "." + dbProcedureSchema
+ "." + dbProcedureName);
System.out.println(" ColumnName [ColumnType(ColumnPrecision)]: " +
dbColumnName
+ " [" + dbColumnReturnTypeName + "(" + dbColumnPrecision + ")]");
System.out.println(" ColumnReturns: " + procReturn + "(" +
dbColumnReturnTypeName + ")");
System.out.println(" Radix: " + dbColumnRadix + ", Scale: " + dbColumnScale);
System.out.println(" Remarks: " + dbColumnRemarks);
}
// Close database resources
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rs.close();
conn.close();
}
How do I check what table-like database objects (table, view, temporary table,
alias) are present in a particular database?
Use java.sql.DatabaseMetaData to probe the database for metadata. Use the getTables
method to retrieve information about all database objects (i.e. tables, views, system
tables, temporary global or local tables or aliases). The exact usage is described in the
code below.
NOTE! Certain JDBC drivers throw IllegalCursorStateExceptions when you try to
access fields in the ResultSet in the wrong order (i.e. not consecutively). Thus, you
should not change the order in which you retrieve the metadata from the ResultSet.
public static void main(String[] args) throws Exception
{
// Load the database driver - in this case, we
// use the Jdbc/Odbc bridge driver.
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
// Open a connection to the database
Connection conn = DriverManager.getConnection("[jdbcURL]",
"[login]", "[passwd]");
// Get DatabaseMetaData
DatabaseMetaData dbmd = conn.getMetaData();
// Get all dbObjects. Replace the last argument in the getTables
// method with objectCategories below to obtain only database
// tables. (Sending in null retrievs all dbObjects).
String[] objectCategories = {"TABLE"};
ResultSet rs = dbmd.getTables(null, null, "%", null);
// Printout table data
while(rs.next())
{
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// Get dbObject metadata
String dbObjectCatalog = rs.getString(1);
String dbObjectSchema = rs.getString(2);
String dbObjectName = rs.getString(3);
String dbObjectType = rs.getString(4);
// Printout
System.out.println("" + dbObjectType + ": " + dbObjectName);
System.out.println(" Catalog: " + dbObjectCatalog);
System.out.println(" Schema: " + dbObjectSchema);
}
// Close database resources
rs.close();
conn.close();
}
What does ResultSet actually contain? Is it the actual data of the result or some
links to databases? If it is the actual data then why can't we access it after
connection is closed?
A ResultSet is an interface. Its implementation depends on the driver and hence ,what
it "contains" depends partially on the driver and what the query returns.
For example with the Odbc bridge what the underlying implementation layer contains
is an ODBC result set. A Type 4 driver executing a stored procedure that returns a
cursor - on an oracle database it actually returns a cursor in the databse. The oracle
cursor can however be processed like a ResultSet would be from the client. Closing a
connection closes all interaction with the database and releases any locks that might
have been obtained in the process.
How do I extract the SQL statements required to move all tables and views from
an existing database to another database?
The operation is performed in 9 steps:
1. Open a connection to the source database. Use the DriverManager class.
2. Find the entire physical layout of the current database. Use the DatabaseMetaData
interface.
3. Create DDL SQL statements for re-creating the current database structure. Use the
DatabaseMetaData interface.
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4. Build a dependency tree, to determine the order in which tables must be setup. Use
the DatabaseMetaData interface.
5. Open a connection to the target database. Use the DriverManager class.
6. Execute all DDL SQL statements from (3) in the order given by (4) in the target
database to setup the table and view structure. Use the PreparedStatement interface.
7. If (6) threw exceptions, abort the entire process.
8. Loop over all tables in the physical structure to generate DML SQL statements for
re-creating the data inside the table. Use the ResultSetMetaData interface.
9. Execute all DML SQL statements from (8) in the target database.
How do I check what table types exist in a database?
Use the getTableTypes method of interface java.sql.DatabaseMetaData to probe the
database for table types. The exact usage is described in the code below.
public static void main(String[] args) throws Exception
{
// Load the database driver - in this case, we
// use the Jdbc/Odbc bridge driver.
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
// Open a connection to the database
Connection conn = DriverManager.getConnection("[jdbcURL]",
"[login]", "[passwd]");
// Get DatabaseMetaData
DatabaseMetaData dbmd = conn.getMetaData();
// Get all table types.
ResultSet rs = dbmd.getTableTypes();
// Printout table data
while(rs.next())
{
// Printout
System.out.println("Type: " + rs.getString(1));
}
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// Close database resources
rs.close();
conn.close();
}
What is the advantage of using a PreparedStatement?
For SQL statements that are executed repeatedly, using a PreparedStatement object
would almost always be faster than using a Statement object. This is because creating
a PreparedStatement object by explicitly giving the SQL statement causes the
statement to be precompiled within the database immediately. Thus, when the
PreparedStatement is later executed, the DBMS does not have to recompile the SQL
statement and prepared an execution plan - it simply runs the statement.
Typically, PreparedStatement objects are used for SQL statements that take
parameters. However, they can also be used with repeatedly executed SQL statements
that do not accept parameters.
How do I find all database stored procedures in a database?
Use the getProcedures method of interface java.sql.DatabaseMetaData to probe the
database for stored procedures. The exact usage is described in the code below.
public static void main(String[] args) throws Exception
{
// Load the database driver - in this case, we
// use the Jdbc/Odbc bridge driver.
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
// Open a connection to the database
Connection conn = DriverManager.getConnection("[jdbcURL]",
"[login]", "[passwd]");
// Get DatabaseMetaData
DatabaseMetaData dbmd = conn.getMetaData();
// Get all procedures.
System.out.println("Procedures are called '"
+ dbmd.getProcedureTerm() +"' in the DBMS.");
ResultSet rs = dbmd.getProcedures(null, null, "%");
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// Printout table data
while(rs.next())
{
// Get procedure metadata
String dbProcedureCatalog = rs.getString(1);
String dbProcedureSchema = rs.getString(2);
String dbProcedureName = rs.getString(3);
String dbProcedureRemarks = rs.getString(7);
short dbProcedureType = rs.getShort(8);
// Make result readable for humans
String procReturn = (dbProcedureType == DatabaseMetaData.procedureNoResult
? "No Result" : "Result");
// Printout
System.out.println("Procedure: " + dbProcedureName
+ ", returns: " + procReturn);
System.out.println(" [Catalog | Schema]: [" + dbProcedureCatalog
+ " | " + dbProcedureSchema + "]");
System.out.println(" Comments: " + dbProcedureRemarks);
}
// Close database resources
rs.close();
conn.close();
}
How can I investigate the physical structure of a database?
The JDBC view of a database internal structure can be seen in the image below.
* Several database objects (tables, views, procedures etc.) are contained within a
Schema.
* Several schema (user namespaces) are contained within a catalog.
* Several catalogs (database partitions; databases) are contained within a DB server
(such as Oracle, MS SQL
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The DatabaseMetaData interface has methods for discovering all the Catalogs,
Schemas, Tables and Stored Procedures in the database server. The methods are pretty
intuitive, returning a ResultSet with a single String column; use them as indicated in
the code below:
public static void main(String[] args) throws Exception
{
// Load the database driver - in this case, we
// use the Jdbc/Odbc bridge driver.
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
// Open a connection to the database
Connection conn = DriverManager.getConnection("[jdbcURL]",
"[login]", "[passwd]");
// Get DatabaseMetaData
DatabaseMetaData dbmd = conn.getMetaData();
// Get all Catalogs
System.out.println("\nCatalogs are called '" + dbmd.getCatalogTerm()
+ "' in this RDBMS.");
processResultSet(dbmd.getCatalogTerm(), dbmd.getCatalogs());
// Get all Schemas
System.out.println("\nSchemas are called '" + dbmd.getSchemaTerm()
+ "' in this RDBMS.");
processResultSet(dbmd.getSchemaTerm(), dbmd.getSchemas());
// Get all Table-like types
System.out.println("\nAll table types supported in this RDBMS:");
processResultSet("Table type", dbmd.getTableTypes());
// Close the Connection
conn.close();
}
public static void processResultSet(String preamble, ResultSet rs)
throws SQLException
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{
// Printout table data
while(rs.next())
{
// Printout
System.out.println(preamble + ": " + rs.getString(1));
}
// Close database resources
rs.close();
}
How does the Java Database Connectivity (JDBC) work?
The JDBC is used whenever a Java application should communicate with a relational
database for which a JDBC driver exists. JDBC is part of the Java platform standard;
all visible classes used in the Java/database communication are placed in package
java.sql.
Main JDBC classes:
* DriverManager. Manages a list of database drivers. Matches connection requests
from the java application with the proper database driver using communication
subprotocol. The first driver that recognizes a certain subprotocol under jdbc (such as
odbc or dbAnywhere/dbaw) will be used to establish a database Connection.
* Driver. The database communications link, handling all communication with the
database. Normally, once the driver is loaded, the developer need not call it explicitly.
* Connection. Interface with all methods for contacting a database
* Statement. Encapsulates an SQL statement which is passed to the database to be
parsed, compiled, planned and executed.
* ResultSet. The answer/result from a statement. A ResultSet is a fancy 2D list which
encapsulates all outgoing results from a given SQL query.
What is Metadata and why should I use it?
Metadata ('data about data') is information about one of two things:
1. Database information (java.sql.DatabaseMetaData), or
2. Information about a specific ResultSet (java.sql.ResultSetMetaData).
Use DatabaseMetaData to find information about your database, such as its
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capabilities and structure. Use ResultSetMetaData to find information about the results
of an SQL query, such as size and types of columns.
How do I create a database connection?
The database connection is created in 3 steps:
1. Find a proper database URL (see FAQ on JDBC URL)
2. Load the database driver
3. Ask the Java DriverManager class to open a connection to your database
In java code, the steps are realized in code as follows:
1. Create a properly formatted JDBR URL for your database. (See FAQ on JDBC
URL for more information). A JDBC URL has the form
jdbc:someSubProtocol://myDatabaseServer/theDatabaseName
2.
try {
Class.forName("my.database.driver");
}
catch(Exception ex)
{
System.err.println("Could not load database driver: " + ex);
}
3. Connection conn = DriverManager.getConnection("a.JDBC.URL",
"databaseLogin", "databasePassword");
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Servlet
Interview Questions &
Answers
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Servlet Interview Questions and Answers
What is Servlet?
A servlet is a Java technology-based Web component, managed by a container called
servlet container or servlet engine, that generates dynamic content and interacts with
web clients via a request\/response paradigm.
Why is Servlet so popular?
Because servlets are platform-independent Java classes that are compiled to platformneutral byte code that can be loaded dynamically into and run by a Java technologyenabled Web server.
What is servlet container?
The servlet container is a part of a Web server or application server that provides the
network services over which requests and responses are sent, decodes MIME-based
requests, and formats MIME-based responses. A servlet container also contains and
manages servlets through their lifecycle.
When a client request is sent to the servlet container, how does the container
choose which servlet to invoke?
The servlet container determines which servlet to invoke based on the configuration of
its servlets, and calls it with objects representing the request and response.
If a servlet is not properly initialized, what exception may be thrown?
During initialization or service of a request, the servlet instance can throw an
UnavailableException or a ServletException.
Given the request path below, which are context path, servlet path and path
info?
/bookstore/education/index.html
context path: /bookstore
servlet path: /education
path info: /index.html
What is filter? Can filter be used as request or response?
A filter is a reusable piece of code that can transform the content of HTTP
requests,responses, and header information. Filters do not generally create a response
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229
or respond to a request as servlets do, rather they modify or adapt the requests for a
resource, and modify or adapt responses from a resource.
When using servlets to build the HTML, you build a DOCTYPE line, why do you
do that?
I know all major browsers ignore it even though the HTML 3.2 and 4.0 specifications
require it. But building a DOCTYPE line tells HTML validators which version of
HTML you are using so they know which specification to check your document
against. These validators are valuable debugging services, helping you catch HTML
syntax errors.
What is new in ServletRequest interface ? (Servlet 2.4)
The following methods have been added to ServletRequest 2.4 version:
public int getRemotePort()
public java.lang.String getLocalName()
public java.lang.String getLocalAddr()
public int getLocalPort()
Request parameter How to find whether a parameter exists in the request
object?
1.boolean hasFoo = !(request.getParameter("foo") == null ||
request.getParameter("foo").equals(""));
2. boolean hasParameter = request.getParameterMap().contains(theParameter);
(which works in Servlet 2.3+)
How can I send user authentication information while making URL Connection?
You'll want to use HttpURLConnection.setRequestProperty and set all the appropriate
headers to HTTP authorization.
Can we use the constructor, instead of init(), to initialize servlet?
Yes , of course you can use the constructor instead of init(). There's nothing to stop
you. But you shouldn't. The original reason for init() was that ancient versions of Java
couldn't dynamically invoke constructors with arguments, so there was no way to give
the constructur a ServletConfig. That no longer applies, but servlet containers still will
only call your no-arg constructor. So you won't have access to a ServletConfig or
ServletContext.
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How can a servlet refresh automatically if some new data has entered the
database?
You can use a client-side Refresh or Server Push
The code in a finally clause will never fail to execute, right?
Using System.exit(1); in try block will not allow finally code to execute.
What mechanisms are used by a Servlet Container to maintain session
information?
Cookies, URL rewriting, and HTTPS protocol information are used to maintain
session information
Difference between GET and POST
In GET your entire form submission can be encapsulated in one URL, like a hyperlink.
query length is limited to 260 characters, not secure, faster, quick and easy.
In POST Your name/value pairs inside the body of the HTTP request, which makes
for a cleaner URL and imposes no size limitations on the form's output. It is used to
send a chunk of data to the server to be processed, more versatile, most secure.
What is session?
The session is an object used by a servlet to track a user's interaction with a Web
application across multiple HTTP requests.
What is servlet mapping?
The servlet mapping defines an association between a URL pattern and a servlet. The
mapping is used to map requests to servlets.
What is servlet context ?
The servlet context is an object that contains a servlet's view of the Web application
within which the servlet is running. Using the context, a servlet can log events, obtain
URL references to resources, and set and store attributes that other servlets in the
context can use. (answer supplied by Sun's tutorial).
Which interface must be implemented by all servlets?
Servlet interface.
Explain the life cycle of Servlet.
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Loaded(by the container for first request or on start up if config file suggests load-onstartup), initialized( using init()), service(service() or doGet() or doPost()..),
destroy(destroy()) and unloaded.
When is the servlet instance created in the life cycle of servlet? What is the
importance of configuring a servlet?
An instance of servlet is created when the servlet is loaded for the first time in the
container. Init() method is used to configure this servlet instance. This method is
called only once in the life time of a servlet, hence it makes sense to write all those
configuration details about a servlet which are required for the whole life of a servlet
in this method.
Why don't we write a constructor in a servlet?
Container writes a no argument constructor for our servlet.
When we don't write any constructor for the servlet, how does container create
an instance of servlet?
Container creates instance of servlet by calling
Class.forName(className).newInstance().
Once the destroy() method is called by the container, will the servlet be
immediately destroyed? What happens to the tasks(threads) that the servlet
might be executing at that time?
Yes, but Before calling the destroy() method, the servlet container waits for the
remaining threads that are executing the servlet‘s service() method to finish.
What is the difference between callling a RequestDispatcher using
ServletRequest and ServletContext?
We can give relative URL when we use ServletRequest and not while using
ServletContext.
Why is it that we can't give relative URL's when using
ServletContext.getRequestDispatcher() when we can use the same while calling
ServletRequest.getRequestDispatcher()?
Since ServletRequest has the current request path to evaluae the relative path while
ServletContext does not.
==============================================
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Data Structures
Interview Questions &
Answers
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Data Structures Interview Questions and Answers
What is data structure?
A data structure is a way of organizing data that considers not only the items stored,
but also their relationship to each other. Advance knowledge about the relationship
between data items allows designing of efficient algorithms for the manipulation of
data.
List out the areas in which data structures are applied extensively?
Compiler Design, Operating System, Database Management System, Statistical
analysis package, Numerical Analysis, Graphics, Artificial Intelligence, Simulation
If you are using C language to implement the heterogeneous linked list, what
pointer type will you use?
The heterogeneous linked list contains different data types in its nodes and we need a
link, pointer to connect them. It is not possible to use ordinary pointers for this. So we
go for void pointer. Void pointer is capable of storing pointer to any type as it is a
generic pointer type.
What is the data structures used to perform recursion?
Stack. Because of its LIFO (Last In First Out) property it remembers its caller, so
knows whom to return when the function has to return. Recursion makes use of system
stack for storing the return addresses of the function calls. Every recursive function
has its equivalent iterative (non-recursive) function. Even when such equivalent
iterative procedures are written, explicit stack is to be used.
What are the methods available in storing sequential files ?
Straight merging, Natural merging, Polyphase sort, Distribution of Initial runs.
List out few of the Application of tree data-structure?
The manipulation of Arithmetic expression, Symbol Table construction, Syntax
analysis.
In RDBMS, what is the efficient data structure used in the internal storage
representation?
B+ tree. Because in B+ tree, all the data is stored only in leaf nodes, that makes
searching easier. This corresponds to the records that shall be stored in leaf nodes.
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What is a spanning Tree?
A spanning tree is a tree associated with a network. All the nodes of the graph appear
on the tree once. A minimum spanning tree is a spanning tree organized so that the
total edge weight between nodes is minimized.
Does the minimum spanning tree of a graph give the shortest distance between
any 2 specified nodes?
Minimal spanning tree assures that the total weight of the tree is kept at its minimum.
But it doesn't mean that the distance between any two nodes involved in the minimumspanning tree is minimum.
Whether Linked List is linear or Non-linear data structure?
According to Access strategies Linked list is a linear one. According to Storage
Linked List is a Non-linear one.
What is the quickest sorting method to use?
The answer depends on what you mean by quickest. For most sorting problems, it just
doesn't matter how quick the sort is because it is done infrequently or other operations
take significantly more time anyway. Even in cases in which sorting speed is of the
essence, there is no one answer. It depends on not only the size and nature of the data,
but also the likely order. No algorithm is best in all cases. There are three sorting
methods in this author's toolbox that are all very fast and that are useful in different
situations. Those methods are quick sort, merge sort, and radix sort.
The Quick Sort
The quick sort algorithm is of the divide and conquer type. That means it works by
reducing a sorting problem into several easier sorting problems and solving each of
them. A dividing value is chosen from the input data, and the data is partitioned into
three sets: elements that belong before the dividing value, the value itself, and
elements that come after the dividing value. The partitioning is performed by
exchanging elements that are in the first set but belong in the third with elements that
are in the third set but belong in the first Elements that are equal to the dividing
element can be put in any of the three sets the algorithm will still work properly.
The Merge Sort
The merge sort is a divide and conquer sort as well. It works by considering the data to
be sorted as a sequence of already-sorted lists (in the worst case, each list is one
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element long). Adjacent sorted lists are merged into larger sorted lists until there is a
single sorted list containing all the elements. The merge sort is good at sorting lists and
other data structures that are not in arrays, and it can be used to sort things that don't fit
into memory. It also can be implemented as a stable sort.
The Radix Sort
The radix sort takes a list of integers and puts each element on a smaller list,
depending on the value of its least significant byte. Then the small lists are
concatenated, and the process is repeated for each more significant byte until the list is
sorted. The radix sort is simpler to implement on fixed-length data such as ints.
How can I search for data in a linked list?
Unfortunately, the only way to search a linked list is with a linear search, because the
only way a linked list's members can be accessed is sequentially. Sometimes it is
quicker to take the data from a linked list and store it in a different data structure so
that searches can be more efficient.
What is the heap?
The heap is where malloc(), calloc(), and realloc() get memory.
Getting memory from the heap is much slower than getting it from the stack. On the
other hand, the heap is much more flexible than the stack. Memory can be allocated at
any time and deallocated in any order. Such memory isn't deallocated automatically;
you have to call free().
Recursive data structures are almost always implemented with memory from the heap.
Strings often come from there too, especially strings that could be very long at
runtime. If you can keep data in a local variable (and allocate it from the stack), your
code will run faster than if you put the data on the heap. Sometimes you can use a
better algorithm if you use the heap faster, or more robust, or more flexible. Its a
tradeoff.
If memory is allocated from the heap, its available until the program ends. That's great
if you remember to deallocate it when you're done. If you forget, it's a problem. A
�memory leak is some allocated memory that's no longer needed but isn't
deallocated. If you have a memory leak inside a loop, you can use up all the memory
on the heap and not be able to get any more. (When that happens, the allocation
functions return a null pointer.) In some environments, if a program doesn't deallocate
everything it allocated, memory stays unavailable even after the program ends.
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What is the easiest sorting method to use?
The answer is the standard library function qsort(). It's the easiest sort by far for
several reasons:
It is already written.
It is already debugged.
It has been optimized as much as possible (usually).
Void qsort(void *buf, size_t num, size_t size, int (*comp)(const void *ele1, const void
*ele2));
What is the bucket size, when the overlapping and collision occur at same time?
One. If there is only one entry possible in the bucket, when the collision occurs, there
is no way to accommodate the colliding value. This results in the overlapping of
values.
In an AVL tree, at what condition the balancing is to be done?
If the pivotal value (or the Height factor) is greater than 1 or less than 1.
Minimum number of queues needed to implement the priority queue?
Two. One queue is used for actual storing of data and another for storing priorities.
How many different trees are possible with 10 nodes ?
1014 - For example, consider a tree with 3 nodes(n=3), it will have the maximum
combination of 5 different (ie, 23 - 3 =? 5) trees.
What is a node class?
A node class is a class that, relies on the base class for services and implementation,
provides a wider interface to users than its base class, relies primarily on virtual
functions in its public interface depends on all its direct and indirect base class can be
understood only in the context of the base class can be used as base for further
derivation
can be used to create objects. A node class is a class that has added new services or
functionality beyond the services inherited from its base class.
When can you tell that a memory leak will occur?
A memory leak occurs when a program loses the ability to free a block of dynamically
allocated memory.
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What is placement new?
When you want to call a constructor directly, you use the placement new. Sometimes
you have some raw memory that‘s already been allocated, and you need to construct
an object in the memory you have. Operator new‘s special version placement new
allows you to do it.
class Widget
{
public :
Widget(int widgetsize);
…
Widget* Construct_widget_int_buffer(void *buffer,int widgetsize)
{
return new(buffer) Widget(widgetsize);
}
};
This function returns a pointer to a Widget object that‘s constructed within the buffer
passed to the function. Such a function might be useful for applications using shared
memory or memory-mapped I/O, because objects in such applications must be placed
at specific addresses or in memory allocated by special routines.
List out the areas in which data structures are applied extensively ?
Compiler Design, Operating System, Database Management System, Statistical
analysis package, Numerical Analysis, Graphics, Artificial Intelligence, Simulation
If you are using C language to implement the heterogeneous linked list, what
pointer type will you use?
The heterogeneous linked list contains different data types in its nodes and we need a
link, pointer to connect them. It is not possible to use ordinary pointers for this. So we
go for void pointer. Void pointer is capable of storing pointer to any type as it is a
generic pointer type.
What is the data structures used to perform recursion?
Stack. Because of its LIFO (Last In First Out) property it remembers its caller so
knows whom to return when the function has to return. Recursion makes use of system
stack for storing the return addresses of the function calls. Every recursive function
has its equivalent iterative (non-recursive) function. Even when such equivalent
iterative procedures are written, explicit stack is to be used.
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Whether Linked List is linear or Non-linear data structure?
According to Access strategies Linked list is a linear one.
According to Storage Linked List is a Non-linear one
Tell how to check whether a linked list is circular ?
Create two pointers, each set to the start of the list. Update each as follows:
while (pointer1)
{
pointer1 = pointer1->next;
pointer2 = pointer2->next; if (pointer2) pointer2=pointer2->next;
if (pointer1 == pointer2)
??????{
print (\‖circular\n\‖);
}
}
What is the difference between ARRAY and STACK?
STACK follows LIFO. Thus the item that is first entered would be the last removed.
In array the items can be entered or removed in any order. Basically each member
access is done using index. No strict order is to be followed here to remove a
particular element.
What is the difference between NULL AND VOID pointer?
NULL can be value for pointer type variables.
VOID is a type identifier which has not size.
NULL and void are not same. Example: void* ptr = NULL;
What is precision?
Precision refers the accuracy of the decimal portion of a value. Precision is the number
of digits allowed after the decimal point.
What is impact of signed numbers on the memory?
Sign of the number is the first bit of the storage allocated for that number. So you get
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one bit less for storing the number. For example if you are storing an 8-bit number,
without sign, the range is 0-255. If you decide to store sign you get 7 bits for the
number plus one bit for the sign. So the range is -128 to +127.
How memory is reserved using a declaration statement ?
Memory is reserved using data type in the variable declaration. A programming
language implementation has predefined sizes for its data types.
For example, in C# the declaration int i; will reserve 32 bits for variable i.
A pointer declaration reserves memory for the address or the pointer variable, but not
for the data that it will point to. The memory for the data pointed by a pointer has to be
allocated at runtime.
The memory reserved by the compiler for simple variables and for storing pointer
address is allocated on the stack, while the memory allocated for pointer referenced
data at runtime is allocated on the heap.
How many parts are there in a declaration statement?
There are two main parts, variable identifier and data type and the third type is
optional which is type qualifier like signed/unsigned.
Is Pointer a variable?
Yes, a pointer is a variable and can be used as an element of a structure and as an
attribute of a class in some programming languages such as C++, but not Java.
However, the contents of a pointer is a memory address of another location of
memory, which is usually the memory address of another variable, element of a
structure, or attribute of a class.
What is Data Structure?
A data structure is a group of data elements grouped together under one name. These
data elements, known as members, can have different types and different lengths.
Some are used to store the data of same type and some are used to store different types
of data.
What is significance of ” * ” ?
The symbol ―*‖ tells the computer that you are declaring a pointer.
Actually it depends on context.
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In a statement like int *ptr; the ‗*‘ tells that you are declaring a pointer.
In a statement like int i = *ptr; it tells that you want to assign value pointed to by ptr to
variable i.
The symbol ―*‖ is also called as Indirection Operator/ Dereferencing Operator.
Why do we Use a Multidimensional Array?
A multidimensional array can be useful to organize subgroups of data within an array.
In addition to organizing data stored in elements of an array, a multidimensional array
can store memory addresses of data in a pointer array and an array of pointers.
Multidimensional arrays are used to store information in a matrix form.
e.g. a railway timetable, schedule cannot be stored as a single dimensional array.
One can use a 3-D array for storing height, width and length of each room on each
floor of a building.
How do you assign an address to an element of a pointer array ?
We can assign a memory address to an element of a pointer array by using the address
operator, which is the ampersand (&), in an assignment statement such as
ptemployee[0] = &projects[2];
Run Time Memory Allocation is known as ?
Allocating memory at runtime is called a dynamically allocating memory. In this, you
dynamically allocate memory by using the new operator when declaring the array, for
example : int grades[] = new int[10];
What method is used to place a value onto the top of a stack?
push() method, Push is the direction that data is being added to the stack. push()
member method places a value onto the top of a stack.
What method removes the value from the top of a stack?
The pop() member method removes the value from the top of a stack, which is then
returned by the pop() member method to the statement that calls the pop() member
method.
What does isEmpty() member method determines?
isEmpty() checks if the stack has at least one element. This method is called by Pop()
before retrieving and returning the top element.
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What is a queue ?
A Queue is a sequential organization of data. A queue is a first in first out type of data
structure. An element is inserted at the last position and an element is always taken out
from the first position.
What is the relationship between a queue and its underlying array?
Data stored in a queue is actually stored in an array. Two indexes, front and end will
be used to identify the start and end of the queue.
When an element is removed front will be incremented by 1. In case it reaches past the
last index available it will be reset to 0. Then it will be checked with end. If it is
greater than end queue is empty.
When an element is added end will be incremented by 1. In case it reaches past the last
index available it will be reset to 0. After incrementing it will be checked with front. If
they are equal queue is full.
Which process places data at the back of the queue?
Enqueue is the process that places data at the back of the queue.
Why is the isEmpty() member method called?
The isEmpty() member method is called within the dequeue process to determine if
there is an item in the queue to be removed i.e. isEmpty() is called to decide whether
the queue has at least one element. This method is called by the dequeue() method
before returning the front element.
How is the front of the queue calculated ?
The front of the queue is calculated by front = (front+1) % size.
What does each entry in the Link List called?
Each entry in a linked list is called a node. Think of a node as an entry that has three
sub entries. One sub entry contains the data, which may be one attribute or many
attributes. Another points to the previous node, and the last points to the next node.
When you enter a new item on a linked list, you allocate the new node and then set the
pointers to previous and next nodes.
What is Linked List ?
Linked List is one of the fundamental data structures. It consists of a sequence of?
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nodes, each containing arbitrary data fields and one or two (‖links‖) pointing to the
next and/or previous nodes. A linked list is a self-referential datatype because it
contains a pointer or link to another data of the same type. Linked lists permit insertion
and removal of nodes at any point in the list in constant time, but do not allow random
access.
What member function places a new node at the end of the linked list?
The appendNode() member function places a new node at the end of the linked list.
The appendNode() requires an integer representing the current data of the node.
How is any Data Structure application is classified among files?
A linked list application can be organized into a header file, source file and main
application file. The first file is the header file that contains the definition of the
NODE structure and the LinkedList class definition. The second file is a source code
file containing the implementation of member functions of the LinkedList class. The
last file is the application file that contains code that creates and uses the LinkedList
class.
Which file contains the definition of member functions?
Definitions of member functions for the Linked List class are contained in the
LinkedList.cpp file.
What are the major data structures used in the following areas : RDBMS,
Network data model & Hierarchical data model.
1. RDBMS Array (i.e. Array of structures)
2. Network data model Graph
3. Hierarchical data model Trees.
Difference between calloc and malloc ?
malloc: allocate n bytes
calloc: allocate m times n bytes initialized to 0
=====================================================
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Networking
Interview Questions &
Answers
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Networking Interview Questions and Answers
What is an Object server?
With an object server, the Client/Server application is written as a set of
communicating objects. Client object communicate with server objects using an
Object Request Broker (ORB). The client invokes a method on a remote object. The
ORB locates an instance of that object server class, invokes the requested method and
returns the results to the client object. Server objects must provide support for
concurrency and sharing. The ORB brings it all together.
What is a Transaction server?
With a transaction server, the client invokes remote procedures that reside on the
server with an SQL database engine. These remote procedures on the server execute a
group of SQL statements. The network exchange consists of a single request/reply
message. The SQL statements either all succeed or fail as a unit.
What is a Database Server?
With a database server, the client passes SQL requests as messages to the database
server. The results of each SQL command are returned over the network. The server
uses its own processing power to find the request data instead of passing all the
records back to the client and then getting it find its own data. The result is a much
more efficient use of distributed processing power. It is also known as SQL engine.
What are the most typical functional units of the Client/Server applications?
User interface
Business Logic and
Shared data.
What are all the Extended services provided by the OS?
Ubiquitous communications
Network OS extension
Binary large objects (BLOBs)
Global directories and Network yellow pages
Authentication and Authorization services
System management
Network time
Database and transaction services
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Internet services
Object- oriented services
What are Triggers and Rules?
Triggers are special user defined actions usually in the form of stored procedures, that
are automatically invoked by the server based on data related events. It can perform
complex actions and can use the full power of procedural languages.
A rule is a special type of trigger that is used to perform simple checks on data.
What is meant by Transparency?
Transparency really means hiding the network and its servers from the users and even
the application programmers.
What are TP-Lite and TP-Heavy Monitors?
TP-Lite is simply the integration of TP Monitor functions in the database engines. TPHeavy are TP Monitors which supports the Client/Server architecture and allow PC to
initiate some very complex multiserver transaction from the desktop.
What are the two types of OLTP?
TP lite, based on stored procedures. TP heavy, based on the TP monitors.
What is a Web server?
This new model of Client/Server consists of thin, protable, "universal" clients that talk
to superfat servers. In the simplet form, a web server returns documents when clients
ask for them by name. The clients and server communicate using an RPC-like protocol
called HTTP.
What are Super servers?
These are fully-loaded machines which includes multiprocessors, high-speed disk
arrays for intervive I/O and fault tolerant features.
What is a TP Monitor?
There is no commonly accepted definition for a TP monitor. According to Jeri
Edwards' a TP Monitor is "an OS for transaction processing".
TP Monitor does mainly two things extremely well. They are Process
management and Transaction management.?
They were originally introduced to run classes of applications that could service
hundreds and sometimes thousands of clients. TP Monitors provide an OS - on top of
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existing OS - that connects in real time these thousands of humans with a pool of
shared server processes.
What is meant by Asymmetrical protocols?
There is a many-to-one relationship between clients and server. Clients always initiate
the dialog by requesting a service. Servers are passively awaiting for requests from
clients.
What are the types of Transparencies?
The types of transparencies the NOS middleware is expected to provide are:Location transparency
Namespace transparency
Logon transparency
Replication transparency
Local/Remote access transparency
Distributed time transparency
Failure transparency and
Administration transparency.
What is the difference between trigger and rule?
The triggers are called implicitly by database generated events, while stored
procedures are called explicitly by client applications.
What are called Transactions?
The grouped SQL statements are called Transactions (or) A transaction is a collection
of actions embused with ACID properties.
What are the building blocks of Client/Server?
The client
The server and
Middleware.
Explain the building blocks of Client/Server?
The client side building block runs the client side of the application.
The server side building block runs the server side of the application.
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Transport stack
Network OS
Service-specific middleware.
What are all the Base services provided by the OS?
Task preemption
Task priority
Semaphores
Interprocess communications (IPC)
Local/Remote Interprocess communication
Threads
Intertask protection
Multiuser
High performance file system
Efficient memory management and
Dynamically linked Run-time extensions.
What are the roles of SQL?
SQL is an interactive query language for ad hoc database queries.
SQL is a database programming language.
SQL is a data definition and data administration language.
SQL is the language of networked database servers
SQL helps protect the data in a multi-user networked environment.
Because of these multifacted roles it plays, physicists might call SQL as "The grand
unified theory of database".
What are the characteristics of Client/Server?
Service
Shared resources
Asymmentrical protocols
Transparency of location
Mix-and-match
Message based exchanges
Encapsulation of services
Scalability
Integrity
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and-match components of almost any level. Clients and servers are loosely coupled
systems that interact through a message-passing mechanism.
What is Structured Query Langauge (SQL)?
SQL is a powerful set-oriented language which was developed by IBM research for
the databases that adhere to the relational model. It consists of a short list of powerful,
yet highly flexible, commands that can be used to manipulate information collected in
tables. Through SQL, we can manipulate and control sets of records at a time.
What is Remote Procedure Call (RPC)?
RPC hides the intricacies of the network by using the ordinary procedure call
mechanism familiar to every programmer. A client process calls a function on a
remote server and suspends itself until it gets back the results. Parameters are passed
like in any ordinary procedure. The RPC, like an ordinary procedure, is synchoronous.
The process that issues the call waits until it gets the results.
Under the covers, the RPC run-time software collects values for the parameters, forms
a message, and sends it to the remote server. The server receives the request, unpack
the parameters, calls the procedures, and sends the reply back to the client. It is a
telephone-like metaphor.
What are the main components of Transaction-based Systems?
Resource Manager
Transaction Manager and
Application Program.
What are the three types of SQL database server architecture?
Process-per-client Architecture. (Example: Oracle 6, Informix )
Multithreaded Architecture. (Example: Sybase, SQL server)
Hybrid Architecture
What are the Classification of clients?
Non-GUI clients - Two types are:Non-GUI clients that do not need multi-tasking
(Example: Automatic Teller Machines (ATM), Cell phone)
Non-GUI clients that need multi-tasking
(Example: ROBOTs)
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GUI clients
OOUI clients
What are called Non-GUI clients, GUI Clients and OOUI Clients?
Non-GUI Client: These are applications, generate server requests with a minimal
amount of human interaction.
GUI Clients: These are applicatoins, where occassional requests to the server result
from a human interacting with a GUI
(Example: Windows 3.x, NT 3.5)
OOUI clients : These are applications, which are highly-iconic, object-oriented user
interface that provides seamless access to information in very visual formats.
(Example: MAC OS, Windows 95, NT 4.0)
What is Message Oriented Middleware (MOM)?
MOM allows general purpose messages to be exchanged in a Client/Server system
using message queues. Applications communicate over networks by simply putting
messages in the queues and getting messages from queues. It typically provides a very
simple high level APIs to its services.
MOM's messaging and queuing allow clients and servers to communicate across a
network without being linked by a private, dedicated, logical connection. The clients
and server can run at different times. It is a post-office like metaphor.
What is meant by Middleware?
Middleware is a distributed software needed to support interaction between clients and
servers. In short, it is the software that is in the middle of the Client/Server systems
and it acts as a bridge between the clients and servers. It starts with the API set on the
client side that is used to invoke a service and it covers the transmission of the request
over the network and the resulting response.
It neither includes the software that provides the actual service - that is in the servers
domain nor the user interface or the application login - that's in clients domain.
What are the functions of the typical server program?
It waits for client-initiated requests. Executes many requests at the same time. Takes
care of VIP clients first. Initiates and runs background task activity. Keeps running.
Grown bigger and faster.
What is meant by Symmentric Multiprocessing (SMP)?
It treats all processors as equal. Any processor can do the work of any other processor.
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Applications are divided into threads that can run concurrently on any available
processor. Any processor in the pool can run the OS kernel and execute user-written
threads.
What are General Middleware?
It includes the communication stacks, distributed directories, authentication services,
network time, RPC, Queuing services along with the network OS extensions such as
the distributed file and print services.
What are Service-specific middleware?
It is needed to accomplish a particular Client/Server type of services which includes:Database specific middleware
OLTP specific middleware
Groupware specific middleware
Object specific middleware
Internet specific middleware and
System management specific middleware.
What is meant by Asymmetric Multiprocessing (AMP)?
It imposses hierarchy and a division of labour among processors. Only one designated
processor, the master, controls (in a tightly coupled arrangement) slave processors
dedicated to specific functions.
What is OLTP?
In the transaction server, the client component usually includes GUI and the server
components usually consists of SQL transactions against a database. These
applications are called OLTP (Online Transaction Processing) OLTP Applications
typically,
Receive a fixed set of inputs from remote clients. Perform multiple pre-compiled SQL
comments against a local database. Commit the work and Return a fixed set of results.
What is meant by 3-Tier architecture?
In 3-tier Client/Server systems, the application logic (or process) lives in the middle
tier and it is separated from the data and the user interface. In theory, the 3-tier
Client/Server systems are more scalable, robust and flexible.
Example: TP monitor, Web.
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What is meant by 2-Tier architecture?
In 2-tier Client/Server systems, the application logic is either burried inside the user
interface on the client or within the database on the server.
Example: File servers and Database servers with stored procedures.
What is Load balancing?
If the number of incoming clients requests exceeds the number of processes in a server
class, the TP Monitor may dynamically start new ones and this is called Load
balancing.
What are called Fat clients and Fat servers?
If the bulk of the application runs on the Client side, then it is Fat clients. It is used for
decision support and personal software.
If the bulk of the application runs on the Server side, then it is Fat servers. It tries to
minimize network interchanges by creating more abstract levels of services.
What is meant by Horizontal scaling and Vertical scaling?
Horizontal scaling means adding or removing client workstations with only a slight
performance impact. Vertical scaling means migrating to a larger and faster server
machine or multiservers.
What is Groupware server?
Groupware addresses the management of semi-structured information such as text,
image, mail, bulletin boards and the flow of work. These Client/Server systems have
people in direct contact with other people.
What are the two broad classes of middleware?
General middleware
Service-specific middleware.
What are the types of Servers?
File servers
Database servers Transaction servers Groupware servers Object servers Web servers.
What is a File server?
File servers are useful for sharing files across a network. With a file server, the client
passes requests for file records over nerwork to file server.
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What are the five major technologies that can be used to create Client/Server
applications?
Database Servers
TP Monitors
Groupware
Distributed Objects
Intranets.
What is Client/Server?
Clients and Servers are separate logical entities that work together over a network to
accomplish a task. Many systems with very different architectures that are connected
together are also called Client/Server.
List out the benefits obtained by using the Client/Server oriented TP Monitors?
Client/Server applications development framework.
Firewalls of protection.
High availability.
Load balancing.
MOM integration.
Scalability of functions.
Reduced system cost.
What are the services provided by the Operating System?
Extended services - These are add-on modular software components that are layered
on top of base service.
What is ACID property?
ACID is a term coined by Andrew Reuter in 1983, which stands for Atomicity,
Consistence, Isolation and Durability.
What are Stored procedures?
A stored procedure i s named collection of SQL statements and procedural logic that is
compiled, verified and stored in a server database. It is typically treated like any other
database object. Stored procedures accept input parameters so that a single procedure
can be used over the network by multiple clients using different input data. A single
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remote message triggers the execution of a collection of stored SQL statements. The
results is a reduction of network traffic and better performance.
What is wide-mouth frog?
Wide-mouth frog is the simplest known key distribution center (KDC) authentication
protocol.
What is passive topology?
When the computers on the network simply listen and receive the signal, they are
referred to as passive because they don‘t amplify the signal in any way.
Example for passive topology - linear bus.
What is region?
When hierarchical routing is used, the routers are divided into what we call regions,
with each router knowing all the details about how to route packets to destinations
within its own region, but knowing nothing about the internal structure of other
regions.
What is virtual channel?
Virtual channel is normally a connection from one source to one destination, although
multicast connections are also permitted. The other name for virtual channel is virtual
circuit.
Difference between the communication and transmission?
Transmission is a physical movement of information and concern issues like bit
polarity, synchronization, clock etc.
Communication means the meaning full exchange of information between two
communication media.
What is the difference between TFTP and FTP application layer protocols?
The Trivial File Transfer Protocol (TFTP) allows a local host to obtain files from a
remote host but does not provide reliability or security. It uses the fundamental packet
delivery services offered by UDP.
The File Transfer Protocol (FTP) is the standard mechanism provided by TCP / IP for
copying a file from one host to another. It uses the services offered by TCP and so is
reliable and secure. It establishes two connections (virtual circuits) between the hosts,
one for data transfer and another for control information.
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What are the advantages and disadvantages of the three types of routing tables?
The three types of routing tables are fixed, dynamic, and fixed central. The fixed table
must be manually modified every time there is a change. A dynamic table changes its
information based on network traffic, reducing the amount of manual maintenance. A
fixed central table lets a manager modify only one table, which is then read by other
devices. The fixed central table reduces the need to update each machine's table, as
with the fixed table. Usually a dynamic table causes the fewest problems for a network
administrator, although the table's contents can change without the administrator being
aware of the change.
What is Beaconing?
The process that allows a network to self-repair networks problems. The stations on
the network notify the other stations on the ring when they are not receiving the
transmissions. Beaconing is used in Token ring and FDDI networks.
What does the Mount protocol do ?
The Mount protocol returns a file handle and the name of the file system in which a
requested file resides. The message is sent to the client from the server after reception
of a client's request.
What are Digrams and Trigrams?
The most common two letter combinations are called as digrams. e.g. th, in, er, re and
an. The most common three letter combinations are called as trigrams. e.g. the, ing,
and, and ion.
What is the HELLO protocol used for?
The HELLO protocol uses time instead of distance to determine optimal routing. It is
an alternative to the Routing Information Protocol.
What is the minimum and maximum length of the header in the TCP segment
and IP datagram?
The header should have a minimum length of 20 bytes and can have a maximum
length of 60 bytes.
What do you meant by "triple X" in Networks?
The function of PAD (Packet Assembler Disassembler) is described in a document
known as X.3. The standard protocol has been defined between the terminal and the
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PAD, called X.28; another standard protocol exists between the PAD and the network,
called X.29. Together, these three recommendations are often called "triple X".
What is attenuation?
The degeneration of a signal over distance on a network cable is called attenuation.
What is Protocol Data Unit?
The data unit in the LLC level is called the protocol data unit (PDU). The PDU
contains of four fields a destination service access point (DSAP), a source service
access point (SSAP), a control field and an information field. DSAP, SSAP are
addresses used by the LLC to identify the protocol stacks on the receiving and sending
machines that are generating and using the data. The control field specifies whether
the PDU frame is a information frame (I - frame) or a supervisory frame (S - frame) or
a unnumbered frame (U - frame).
What are the data units at different layers of the TCP / IP protocol suite?
The data unit created at the application layer is called a message, at the transport layer
the data unit created is called either a segment or an user datagram, at the network
layer the data unit created is called the datagram, at the data link layer the datagram is
encapsulated in to a frame and finally transmitted as signals along the transmission
media.
What is difference between ARP and RARP?
The address resolution protocol (ARP) is used to associate the 32 bit IP address with
the 48 bit physical address, used by a host or a router to find the physical address of
another host on its network by sending a ARP query packet that includes the IP
address of the receiver.
The reverse address resolution protocol (RARP) allows a host to discover its Internet
address when it knows only its physical address.
What is MAC address?
The address for a device as it is identified at the Media Access Control (MAC) layer in
the network architecture. MAC address is usually stored in ROM on the network
adapter card and is unique.
What is terminal emulation, in which layer it comes?
Telnet is also called as terminal emulation. It belongs to application layer.
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What are the types of Transmission media?
Signals are usually transmitted over some transmission media that are broadly
classified in to two categories:Guided Media:
These are those that provide a conduit from one device to another that include twistedpair, coaxial cable and fiber-optic cable. A signal traveling along any of these media is
directed and is contained by the physical limits of the medium. Twisted-pair and
coaxial cable use metallic that accept and transport signals in the form of electrical
current. Optical fiber is a glass or plastic cable that accepts and transports signals in
the form of light.
Unguided Media:
This is the wireless media that transport electromagnetic waves without using a
physical conductor. Signals are broadcast either through air. This is done through radio
communication, satellite communication and cellular telephony.
What are major types of networks and explain?
Server-based network.
Peer-to-peer network.
Peer-to-peer network, computers can act as both servers sharing resources and as
clients using the resources.
Server-based networks provide centralized control of network resources and rely on
server computers to provide security and network administration.
What is SAP?
Series of interface points that allow other computers to communicate with the other
layers of network protocol stack.
What is multicast routing?
Sending a message to a group is called multicasting, and its routing algorithm is called
multicast routing.
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What is the difference between routable and non- routable protocols?
Routable protocols can work with a router and can be used to build large networks.
Non-Routable protocols are designed to work on small, local networks and cannot be
used with a router.
What is REX?
Request to Exit (REX) - A signal that informs the controller that someone has
requested to exit from a secure area.
What are the different type of networking / internetworking devices?
Repeater:
Also called a regenerator, it is an electronic device that operates only at physical layer.
It receives the signal in the network before it becomes weak, regenerates the original
bit pattern and puts the refreshed copy back in to the link.
Bridges:
These operate both in the physical and data link layers of LANs of same type. They
divide a larger network in to smaller segments. They contain logic that allow them to
keep the traffic for each segment separate and thus are repeaters that relay a frame
only the side of the segment containing the intended recipent and control congestion.
Routers:
They relay packets among multiple interconnected networks (i.e. LANs of different
type). They operate in the physical, data link and network layers. They contain
software that enable them to determine which of the several possible paths is the best
for a particular transmission. Gateways: They relay packets among networks that have
different protocols (e.g. between a LAN and a WAN). They accept a packet formatted
for one protocol and convert it to a packet formatted for another protocol before
forwarding it. They operate in all seven layers of the OSI model.
What is redirector?
Redirector is software that intercepts file or prints I/O requests and translates them into
network requests. This comes under presentation layer.
What is packet filter?
Packet filter is a standard router equipped with some extra functionality. The extra
functionality allows every incoming or outgoing packet to be inspected. Packets
meeting some criterion are forwarded normally. Those that fail the test are dropped.
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What is logical link control?
One of two sublayers of the data link layer of OSI reference model, as defined by the
IEEE 802 standard. This sublayer is responsible for maintaining the link between
computers when they are sending data across the physical network connection.
What is traffic shaping?
One of the main causes of congestion is that traffic is often busy. If hosts could be
made to transmit at a uniform rate, congestion would be less common. Another open
loop method to help manage congestion is forcing the packet to be transmitted at a
more predictable rate. This is called traffic shaping.
What is NETBIOS and NETBEUI?
NETBIOS is a programming interface that allows I/O requests to be sent to and
received from a remote computer and it hides the networking hardware from
applications.
NETBEUI is NetBIOS extended user interface. A transport protocol designed by
microsoft and IBM for the use on small subnets.
Why should you care about the OSI Reference Model?
It provides a framework for discussing network operations and design.
What is Proxy ARP?
is using a router to answer ARP requests. This will be done when the originating host
believes that a destination is local, when in fact is lies beyond router.
What is EGP (Exterior Gateway Protocol)?
It is the protocol the routers in neighboring autonomous systems use to identify the set
of networks that can be reached within or via each autonomous system.
What is IGP (Interior Gateway Protocol)?
It is any routing protocol used within an autonomous system.
What is OSPF?
It is an Internet routing protocol that scales well, can route traffic along multiple paths,
and uses knowledge of an Internet's topology to make accurate routing decisions.
What is Kerberos?
It is an authentication service developed at the Massachusetts Institute of Technology.
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Kerberos uses encryption to prevent intruders from discovering passwords and gaining
unauthorized access to files.
What is SLIP (Serial Line Interface Protocol)?
It is a very simple protocol used for transmission of IP datagrams across a serial line.
What is Mail Gateway?
It is a system that performs a protocol translation between different electronic mail
delivery protocols.
What is RIP (Routing Information Protocol)?
It is a simple protocol used to exchange information between the routers.
What is NVT (Network Virtual Terminal)?
It is a set of rules defining a very simple virtual terminal interaction. The NVT is used
in the start of a Telnet session.
What is source route?
It is a sequence of IP addresses identifying the route a datagram must follow. A source
route may optionally be included in an IP datagram header.
What is BGP (Border Gateway Protocol)?
It is a protocol used to advertise the set of networks that can be reached with in an
autonomous system. BGP enables this information to be shared with the autonomous
system. This is newer than EGP (Exterior Gateway Protocol).
What is Gateway-to-Gateway protocol?
It is a protocol formerly used to exchange routing information between Internet core
routers.
What is Project 802?
It is a project started by IEEE to set standards that enable intercommunication between
equipment from a variety of manufacturers. It is a way for specifying functions of the
physical layer, the data link layer and to some extent the network layer to allow for
interconnectivity of major LAN protocols.
It consists of the following:
802.1 is an internetworking standard for compatibility of different LANs and MANs
across protocols.
802.2 Logical link control (LLC) is the upper sublayer of the data link layer which is
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non-architecture-specific, that is remains the same for all IEEE-defined LANs. Media
access control (MAC) is the lower sublayer of the data link layer that contains some
distinct modules each carrying proprietary information specific to the LAN product
being used. The modules are
Ethernet LAN (802.3), Token ring LAN (802.4), Token bus LAN (802.5).
802.6 is distributed queue dual bus (DQDB) designed to be used in MANs.
What is silly window syndrome?
It is a problem that can ruin TCP performance. This problem occurs when data are
passed to the sending TCP entity in large blocks, but an interactive application on the
receiving side reads 1 byte at a time.
What is a Multi-homed Host?
It is a host that has a multiple network interfaces and that requires multiple IP
addresses is called as a Multi-homed Host.
What is autonomous system?
It is a collection of routers under the control of a single administrative authority and
that uses a common Interior Gateway Protocol.
What is the difference between interior and exterior neighbor gateways?
Interior gateways connect LANs of one organization, whereas exterior gateways
connect the organization to the outside world.
What is MAU?
In token Ring , hub is called Multistation Access Unit(MAU).
Explain 5-4-3 rule.?
In a Ethernet network, between any two points on the network, there can be no more
than five network segments or four repeaters, and of those five segments only three of
segments can be populated.
What is difference between baseband and broadband transmission?
In a baseband transmission, the entire bandwidth of the cable is consumed by a single
signal. In broadband transmission, signals are sent on multiple frequencies, allowing
multiple signals to be sent simultaneously.
What is ICMP?
ICMP is Internet Control Message Protocol, a network layer protocol of the TCP/IP
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suite used by hosts and gateways to send notification of datagram problems back to the
sender. It uses the echo test / reply to test whether a destination is reachable and
responding. It also handles both control and error messages.
What is Brouter?
Hybrid devices that combine the features of both bridges and routers.
What is frame relay, in which layer it comes?
Frame relay is a packet switching technology. It will operate in the data link layer.
What is External Data Representation?
External Data Representation is a method of encoding data within an RPC message,
used to ensure that the data is not system-dependent.
What is Bandwidth?
Every line has an upper limit and a lower limit on the frequency of signals it can carry.
This limited range is called the bandwidth.
What protocol is used by DNS name servers?
DNS uses UDP for communication between servers. It is a better choice than TCP
because of the improved speed a connectionless protocol offers. Of course,
transmission reliability suffers with UDP.
What is the range of addresses in the classes of internet addresses?
Class A 0.0.0.0 - 127.255.255.255
Class B 128.0.0.0 - 191.255.255.255
Class C 192.0.0.0 - 223.255.255.255
Class D 224.0.0.0 - 239.255.255.255
Class E 240.0.0.0 - 247.255.255.255
What are the important topologies for networks?
BUS topology:
In this each computer is directly connected to primary network cable in a single line.
Advantages:
Inexpensive, easy to install, simple to understand, easy to extend.
STAR topology:
In this all computers are connected using a central hub.
Advantages:
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Can be inexpensive, easy to install and reconfigure and easy to trouble shoot physical
problems.
RING topology:
In this all computers are connected in loop.
Advantages:
All computers have equal access to network media, installation can be simple, and
signal does not degrade as much as in other topologies because each computer
regenerates it.
Difference between bit rate and baud rate?
Bit rate is the number of bits transmitted during one second whereas baud rate refers to
the number of signal units per second that are required to represent those bits.
baud rate = bit rate / N
where N is no-of-bits represented by each signal shift.
What is anonymous FTP and why would you use it?
Anonymous FTP enables users to connect to a host without using a valid login and
password. Usually, anonymous FTP uses a login called anonymous or guest, with the
password usually requesting the user's ID for tracking purposes only. Anonymous FTP
is used to enable a large number of users to access files on the host without having to
go to the trouble of setting up logins for them all. Anonymous FTP systems usually
have strict controls over the areas an anonymous user can access.
What is the difference between an unspecified passive open and a fully specified
passive open?
An unspecified passive open has the server waiting for a connection request from a
client. A fully specified passive open has the server waiting for a connection from a
specific client.
What is virtual path?
Along any transmission path from a given source to a given destination, a group of
virtual circuits can be grouped together into what is called path.
Explain the function of Transmission Control Block?
A TCB is a complex data structure that contains a considerable amount of information
about each connection.
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What is a DNS resource record?
A resource record is an entry in a name server's database. There are several types of
resource records used, including name-to-address resolution information. Resource
records are maintained as ASCII files.
What is a pseudo tty?
A pseudo tty or false terminal enables external machines to connect through Telnet or
rlogin. Without a pseudo tty, no connection can take place.
What is the Network Time Protocol?
A protocol that assures accurate local timekeeping with reference to radio and atomic
clocks located on the Internet. This protocol is capable of synchronising distributed
clocks within milliseconds over long time periods. It is defined in STD 12, RFC 1119.
What is mesh network?
A network in which there are multiple network links between computers to provide
multiple paths for data to travel.
What is RAID?
A method for providing fault tolerance by using multiple hard disk drives.
What is a Management Information Base (MIB)?
A Management Information Base is part of every SNMP-managed device. Each
SNMP agent has the MIB database that contains information about the device's status,
its performance, connections, and configuration. The MIB is queried by SNMP.
What is cladding?
A layer of a glass surrounding the center fiber of glass inside a fiber-optic cable.
What is subnet?
A generic term for section of a large networks usually separated by a bridge or
router. A gateway operates at the upper levels of the OSI model and translates
information between two completely different network architectures or data formats.
What is point-to-point protocol?
A communications protocol used to connect computers to remote networking services
including Internet service providers.
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What are 10Base2, 10Base5 and 10BaseT Ethernet LANs ?
10Base2—An Ethernet term meaning a maximum transfer rate of 10 Megabits per
second that uses baseband signaling, with a contiguous cable segment length of 100
meters and a maximum of 2 segments
10Base5—An Ethernet term meaning a maximum transfer rate of 10 Megabits per
second that uses baseband signaling, with 5 continuous segments not exceeding 100
meters per segment.
10BaseT—An Ethernet term meaning a maximum transfer rate of 10 Megabits per
second that uses baseband signaling and twisted pair cabling.
What are the possible ways of data exchange?
(i) Simplex
(ii) Half-duplex
(iii) Full-duplex.
What are the two types of transmission technology available?
(i) Broadcast
(ii) point-to-point.
How do I convert a numeric IP address like 192.18.97.39 into a hostname like
java.sun.com?
String hostname = InetAddress.getByName("192.18.97.39").getHostName();
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Operating System
Interview Questions
&
Answers
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Operating System Interview Questions and Answers
What's OPERATING SYSTEM?
An Operating System, or OS, is a software program that enables the computer
hardware to communicate and operate with the computer software. Without a
computer Operating System, a computer would be useless.
OPERATING SYSTEM TYPES
As computers have progressed and developed so have the types of operating systems.
Below is a basic list of the different types of operating systems and a few examples of
Operating Systems that fall into each of the categories. Many computer Operating
Systems will fall into more than one of the below categories.
GUI - Short for Graphical User Interface, a GUI Operating System contains graphics
and icons and is commonly navigated by using a computer mouse. See our GUI
dictionary definition for a complete definition. Below are some examples of GUI
Operating Systems.
System 7.x
Windows 98
Windows CE
Multi-user - A multi-user Operating System allows for multiple users to use the same
computer at the same time and/or different times. See our multi-user dictionary
definition for a complete definition for a complete definition. Below are some
examples of multi-user Operating Systems.
Linux
Unix
Windows 2000
Windows XP
Mac OS X
Multiprocessing - An Operating System capable of supporting and utilizing more
than one computer processor. Below are some examples of multiprocessing Operating
Systems.
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Linux
Unix
Windows 2000
Windows XP
Mac OS X
Multitasking - An Operating system that is capable of allowing multiple software
processes to run at the same time. Below are some examples of multitasking Operating
Systems.
Unix
Windows 2000
Windows XP
Mac OS X
Multithreading - Operating systems that allow different parts of a software program
to run concurrently. Operating systems that would fall into this category are:
Linux
Unix
Windows 2000
Windows XP
Mac OS X
What are the basic functions of an operating system?
- Operating system controls and coordinates the use of the hardware among the
various applications programs for various uses. Operating system acts as resource
allocator and manager. Since there are many possibly conflicting requests for
resources the operating system must decide which requests are allocated resources to
operating the computer system efficiently and fairly. Also operating system is control
program which controls the user programs to prevent errors and improper use of the
computer. It is especially concerned with the operation and control of I/O devices.
Why paging is used?
-Paging is solution to external fragmentation problem which is to permit the logical
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address space of a process to be noncontiguous, thus allowing a process to be
allocating physical memory wherever the latter is available.
While running DOS on a PC, which command would be used to duplicate the
entire diskette?
diskcopy
What resources are used when a thread created? How do they differ from those
when a process is created?
When a thread is created the threads does not require any new resources to execute the
thread shares the resources like memory of the process to which they belong to. The
benefit of code sharing is that it allows an application to have several different threads
of activity all within the same address space. Whereas if a new process creation is very
heavyweight because it always requires new address space to be created and even if
they share the memory then the inter process communication is expensive when
compared to the communication between the threads.
What is virtual memory?
Virtual memory is hardware technique where the system appears to have more
memory that it actually does. This is done by time-sharing, the physical memory and
storage parts of the memory one disk when they are not actively being used.
What is Throughput, Turnaround time, waiting time and Response time?
Throughput – number of processes that complete their execution per time unit.
Turnaround time – amount of time to execute a particular process. Waiting time –
amount of time a process has been waiting in the ready queue. Response time –
amount of time it takes from when a request was submitted until the first response is
produced, not output (for time-sharing environment).
What is the state of the processor, when a process is waiting for some event to
occur?
Waiting state
What is the important aspect of a real-time system or Mission Critical Systems?
A real time operating system has well defined fixed time constraints. Process must be
done within the defined constraints or the system will fail. An example is the operating
system for a flight control computer or an advanced jet airplane. Often used as a
control device in a dedicated application such as controlling scientific experiments,
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medical imaging systems, industrial control systems, and some display systems. RealTime systems may be either hard or soft real-time. Hard real-time: Secondary storage
limited or absent, data stored in short term memory, or read-only memory (ROM),
Conflicts with time-sharing systems, not supported by general-purpose operating
systems. Soft real-time: Limited utility in industrial control of robotics, Useful in
applications (multimedia, virtual reality) requiring advanced operating-system
features.
What is the difference between Hard and Soft real-time systems?
- A hard real-time system guarantees that critical tasks complete on time. This goal
requires that all delays in the system be bounded from the retrieval of the stored data
to the time that it takes the operating system to finish any request made of it. A soft
real time system where a critical real-time task gets priority over other tasks and
retains that priority until it completes. As in hard real time systems kernel delays need
to be bounded
What is the cause of thrashing? How does the system detect thrashing?
Once it detects thrashing, what can the system do to eliminate this problem? Thrashing is caused by under allocation of the minimum number of pages required by
a process, forcing it to continuously page fault. The system can detect thrashing by
evaluating the level of CPU utilization as compared to the level of multiprogramming.
It can be eliminated by reducing the level of multiprogramming.
What is multi tasking, multi programming, multi threading?
Multi programming: Multiprogramming is the technique of running several programs
at a time using timesharing. It allows a computer to do several things at the same time.
Multiprogramming creates logical parallelism. The concept of multiprogramming is
that the operating system keeps several jobs in memory simultaneously. The operating
system selects a job from the job pool and starts executing a job, when that job needs
to wait for any i/o operations the CPU is switched to another job. So the main idea
here is that the CPU is never idle. Multi tasking: Multitasking is the logical extension
of multiprogramming .The concept of multitasking is quite similar to
multiprogramming but difference is that the switching between jobs occurs so
frequently that the users can interact with each program while it is running. This
concept is also known as time-sharing systems. A time-shared operating system uses
CPU scheduling and multiprogramming to provide each user with a small portion of
time-shared system. Multi threading: An application typically is implemented as a
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separate process with several threads of control. In some situations a single application
may be required to perform several similar tasks for example a web server accepts
client requests for web pages, images, sound, and so forth. A busy web server may
have several of clients concurrently accessing it. If the web server ran as a traditional
single-threaded process, it would be able to service only one client at a time. The
amount of time that a client might have to wait for its request to be serviced could be
enormous. So it is efficient to have one process that contains multiple threads to serve
the same purpose. This approach would multithread the web-server process, the server
would create a separate thread that would listen for client requests when a request was
made rather than creating another process it would create another thread to service the
request. To get the advantages like responsiveness, Resource sharing economy and
utilization of multiprocessor architectures multithreading concept can be used.
What is hard disk and what is its purpose?
Hard disk is the secondary storage device, which holds the data in bulk, and it holds
the data on the magnetic medium of the disk. Hard disks have a hard platter that holds
the magnetic medium, the magnetic medium can be easily erased and rewritten, and a
typical desktop machine will have a hard disk with a capacity of between 10 and 40
gigabytes. Data is stored onto the disk in the form of files.
What is fragmentation? Different types of fragmentation?
- Fragmentation occurs in a dynamic memory allocation system when many of the free
blocks are too small to satisfy any request. External Fragmentation: External
Fragmentation happens when a dynamic memory allocation algorithm allocates some
memory and a small piece is left over that cannot be effectively used. If too much
external fragmentation occurs, the amount of usable memory is drastically reduced.
Total memory space exists to satisfy a request, but it is not contiguous. Internal
Fragmentation: Internal fragmentation is the space wasted inside of allocated memory
blocks because of restriction on the allowed sizes of allocated blocks. Allocated
memory may be slightly larger than requested memory; this size difference is memory
internal to a partition, but not being used
What is DRAM? In which form does it store data?
- DRAM is not the best, but it‘s cheap, does the job, and is available almost
everywhere you look. DRAM data resides in a cell made of a capacitor and a
transistor. The capacitor tends to lose data unless it‘s recharged every couple of
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milliseconds, and this recharging tends to slow down the performance of DRAM
compared to speedier RAM types.
What is Dispatcher?
- Dispatcher module gives control of the CPU to the process selected by the short-term
scheduler; this involves: Switching context, Switching to user mode, Jumping to the
proper location in the user program to restart that program, dispatch latency – time it
takes for the dispatcher to stop one process and start another running.
What is CPU Scheduler?
- Selects from among the processes in memory that are ready to execute, and allocates
the CPU to one of them. CPU scheduling decisions may take place when a process:
1.Switches from running to waiting state. 2.Switches from running to ready state.
3.Switches from waiting to ready. 4.Terminates. Scheduling under 1 and 4 is nonpreemptive. All other scheduling is preemptive.
What is Context Switch?
- Switching the CPU to another process requires saving the state of the old process and
loading the saved state for the new process. This task is known as a context switch.
Context-switch time is pure overhead, because the system does no useful work while
switching. Its speed varies from machine to machine, depending on the memory speed,
the number of registers which must be copied, the existed of special instructions(such
as a single instruction to load or store all registers).
What is cache memory?
- Cache memory is random access memory (RAM) that a computer microprocessor
can access more quickly than it can access regular RAM. As the microprocessor
processes data, it looks first in the cache memory and if it finds the data there (from a
previous reading of data), it does not have to do the more time-consuming reading of
data from larger memory.
What is a Safe State and what is its use in deadlock avoidance?
- When a process requests an available resource, system must decide if immediate
allocation leaves the system in a safe state. System is in safe state if there exists a safe
sequence of all processes. Deadlock Avoidance: ensure that a system will never enter
an unsafe state.
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What is a Real-Time System?
- A real time process is a process that must respond to the events within a certain time
period. A real time operating system is an operating system that can run real time
processes successfully
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Linux
Interview Questions
&
Answers
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Linux Interview Questions and Answers
You need to see the last fifteen lines of the files dog, cat and horse. What
command should you use?
Ans :tail -15 dog cat horse
Explanation : The tail utility displays the end of a file. The -15 tells tail to display the
last fifteen lines of each specified file.
Who owns the data dictionary?
The SYS user owns the data dictionary. The SYS and SYSTEM users are created
when the database is created.
You routinely compress old log files. You now need to examine a log from two
months ago. In order to view its contents without first having to decompress it,
use the _________ utility.
Ans : zcat
Explanation : The zcat utility allows you to examine the contents of a compressed
file much the same way that cat displays a file.
You suspect that you have two commands with the same name as the command is
not producing the expected results. What command can you use to determine the
location of the command being run?
Ans :which
Explanation : The which command searches your path until it finds a command that
matches the command you are looking for and displays its full path.
You locate a command in the /bin directory but do not know what it does. What
command can you use to determine its purpose.
Ans : whatis
Explanation : The whatis command displays a summary line from the man page for
the specified command.
You wish to create a link to the /data directory in bob's home directory so you
issue the command ln /data /home/bob/datalink but the command fails. What
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option should you use in this command line to be successful.
Ans :Use the -F option
Explanation : In order to create a link to a directory you must use the -F option.
When you issue the command ls -l, the first character of the resulting display
represents the file's ___________.
Ans : type
Explanation : The first character of the permission block designates the type of file
that is being displayed.
What utility can you use to show a dynamic listing of running
processes? __________
Ans : top
Explanation : The top utility shows a listing of all running processes that is
dynamically updated.
Where is standard output usually directed?
Ans : to the screen or display
Explanation : By default, your shell directs standard output to your screen or display.
You wish to restore the file memo.ben which was backed up in the tarfile
MyBackup.tar. What command should you type?
Ans : tar xf MyBackup.tar memo.ben
Explanation : This command uses the x switch to extract a file. Here the file
memo.ben will be restored from the tarfile MyBackup.tar.
You need to view the contents of the tarfile called MyBackup.tar. What command
would you use?
Ans : tar tf MyBackup.tar
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Explanation : The t switch tells tar to display the contents and the f modifier specifies
which file to examine.
You want to create a compressed backup of the users' home directories. What
utility should you use?
Ans :tar
Explanation : You can use the z modifier with tar to compress your archive at the
same time as creating it.
What daemon is responsible for tracking events on your system?
Ans :syslogd
The syslogd daemon is responsible for tracking system information and saving it to
specified log files.
You have a file called phonenos that is almost 4,000 lines long. What text filter
can you use to split it into four pieces each 1,000 lines long?
Ans :split
The split text filter will divide files into equally sized pieces. The default length of
each piece is 1,000 lines.
You would like to temporarily change your command line editor to be vi. What
command should you type to change it?
Ans : set -o vi
The set command is used to assign environment variables. In this case, you are
instructing your shell to assign vi as your command line editor. However, once you log
off and log back in you will return to the previously defined command line editor.
What account is created when you install Linux?
Ans : root
Whenever you install Linux, only one user account is created. This is the superuser
account also known as root.
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What command should you use to check the number of files and disk space used
and each user's defined quotas?
Ans : repquota
The repquota command is used to get a report on the status of the quotas you have set
including the amount of allocated space and amount of used space.
In order to run fsck on the root partition, the root partition must be mounted as?
Ans : readonly
You cannot run fsck on a partition that is mounted as read-write.
In order to improve your system's security you decide to implement shadow
passwords. What command should you use?
Ans : pwconv
The pwconv command creates the file /etc/shadow and changes all passwords to 'x' in
the /etc/passwd file.
Bob Armstrong, who has a username of boba, calls to tell you he forgot his
password. What command should you use to reset his command?
passwd boba
The passwd command is used to change your password. If you do not specify a
username, your password will be changed.
The top utility can be used to change the priority of a running process? Another
utility that can also be used to change priority is ___________?
nice
Both the top and nice utilities provide the capability to change the priority of a running
process.
What command should you type to see all the files with an extension of 'mem'
listed in reverse alphabetical order in the /home/ben/memos directory.
ls -r /home/ben/memos/*.mem
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The -c option used with ls results in the files being listed in chronological order. You
can use wildcards with the ls command to specify a pattern of filenames.
What file defines the levels of messages written to system log files?
kernel.h
To determine the various levels of messages that are defined on your system, examine
the kernel.h file.
What command is used to remove the password assigned to a group?
gpasswd -r
The gpasswd command is used to change the password assigned to a group. Use the -r
option to remove the password from the group.
What command would you type to use the cpio to create a backup called
backup.cpio of all the users' home directories?
find /home | cpio -o > backup.cpio
The find command is used to create a list of the files and directories contained in
home. This list is then piped to the cpio utility as a list of files to include and the
output is saved to a file called backup.cpio.
What can you type at a command line to determine which shell you are using?
echo $SHELL
The name and path to the shell you are using is saved to the SHELL environment
variable. You can then use the echo command to print out the value of any variable by
preceding the variable's name with $. Therefore, typing echo $SHELL will display the
name of your shell.
What type of local file server can you use to provide the distribution installation
materials to the new machine during a network installation?
A) Inetd
B) FSSTND
C) DNS
D) NNTP
E) NFS
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E - You can use an NFS server to provide the distribution installation materials to the
machine on which you are performing the installation. Answers a, b, c, and d are all
valid items but none of them are file servers. Inetd is the superdaemon which controls
all intermittently used network services. The FSSTND is the Linux File System
Standard. DNS provides domain name resolution, and NNTP is the transfer protocol
for usenet news.
If you type the command cat dog & > cat what would you see on your
display? Choose one:
a. Any error messages only.
b. The contents of the file dog.
c. The contents of the file dog and any error messages.
d. Nothing as all output is saved to the file cat.
d
When you use & > for redirection, it redirects both the standard output and standard
error. The output would be saved to the file cat.
You are covering for another system administrator and one of the users asks you
to restore a file for him. You locate the correct tarfile by checking the backup log
but do not know how the directory structure was stored. What command can you
use to determine this?
Choose one:
a. tar fx tarfile dirname
b. tar tvf tarfile filename
c. tar ctf tarfile
d. tar tvf tarfile
d
The t switch will list the files contained in the tarfile. Using the v modifier will display
the stored directory structure.
You have the /var directory on its own partition. You have run out of space.
What should you do? Choose one:
a. Reconfigure your system to not write to the log files.
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b. Use fips to enlarge the partition.
c. Delete all the log files.
d. Delete the partition and recreate it with a larger size.
d
The only way to enlarge a partition is to delete it and recreate it. You will then have to
restore the necessary files from backup.
You have a new application on a CD-ROM that you wish to install. What should
your first step be?
Choose one:
a. Read the installation instructions on the CD-ROM.
b. Use the mount command to mount your CD-ROM as read-write.
c. Use the umount command to access your CD-ROM.
d. Use the mount command to mount your CD-ROM as read-only.
d
Before you can read any of the files contained on the CD-ROM, you must first mount
the CD-ROM.
When you create a new partition, you need to designate its size by defining the
starting and ending _____________.
cylinders
When creating a new partition you must first specify its starting cylinder. You can
then either specify its size or the ending cylinder.
What key combination can you press to suspend a running job and place it in the
background?
ctrl-z
Using ctrl-z will suspend a job and put it in the background.
The easiest, most basic form of backing up a file is to _____ it to another
location.
copy
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The easiest most basic form of backing up a file is to make a copy of that file to
another location such as a floppy disk.
What type of server is used to remotely assign IP addresses to machines during
the installation process?
A) SMB
B) NFS
C) DHCP
D) FT
E) HTTP
C - You can use a DHCP server to assign IP addresses to individual machines during
the installation process. Answers a, b, d, and e list legitimate Linux servers, but these
servers do not provide IP addresses. The SMB, or Samba, tool is used for file and print
sharing across multi-OS networks. An NFS server is for file sharing across Linux networks. FTP is a file storage server that allows people to browse and retrieve
information by logging in to it, and HTTP is for the Web.
Which password package should you install to ensure that the central password
file couldn't be stolen easily?
A) PAM
B) tcp_wrappers
C) shadow
D) securepass
E) ssh
C - The shadow password package moves the central password file to a more secure
location. Answers a, b, and e all point to valid packages, but none of these places the
password file in a more secure location. Answer d points to an invalid package.
When using useradd to create a new user account, which of the following tasks is
not done automatically.
Choose one:
a. Assign a UID.
b. Assign a default shell.
c. Create the user's home directory.
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d. Define the user's home directory.
c
The useradd command will use the system default for the user's home directory. The
home directory is not created, however, unless you use the -m option.
You want to enter a series of commands from the command-line. What would be
the quickest way to do this?
Choose One
a. Press enter after entering each command and its arguments
b. Put them in a script and execute the script
c. Separate each command with a semi-colon (;) and press enter after the last
command
d. Separate each command with a / and press enter after the last command
c
The semi-colon may be used to tell the shell that you are entering multiple commands
that should be executed serially. If these were commands that you would frequently
want to run, then a script might be more efficient. However, to run these commands
only once, enter the commands directly at the command line.
You attempt to use shadow passwords but are unsuccessful. What characteristic
of the /etc/passwd file may cause this?
Choose one:
a. The login command is missing.
b. The username is too long.
c. The password field is blank.
d. The password field is prefaced by an asterisk.
c
The password field must not be blank before converting to shadow passwords.
When you install a new application, documentation on that application is also
usually installed. Where would you look for the documentation after installing an
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application called MyApp?
Choose one:
a. /usr/MyApp
b. /lib/doc/MyApp
c. /usr/doc/MyApp
d. In the same directory where the application is installed.
c
The default location for application documentation is in a directory named for the
application in the /usr/doc directory.
What file would you edit in your home directory to change which window
manager you want to use?
A) Xinit
B) .xinitrc
C) XF86Setup
D) xstart
E) xf86init
Answer: B - The ~/.xinitrc file allows you to set which window man-ager you want to
use when logging in to X from that account.
Answers a, d, and e are all invalid files. Answer c is the main X server configuration
file.
What command allows you to set a processor-intensive job to use less CPU time?
A) ps
B) nice
C) chps
D) less
E) more
Answer: B - The nice command is used to change a job's priority level, so that it runs
slower or faster. Answers a, d, and e are valid commands but are not used to change
process information. Answer c is an invalid command.
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While logged on as a regular user, your boss calls up and wants you to create a
new user account immediately. How can you do this without first having to close
your work, log off and logon as root?
Choose one:
a. Issue the command rootlog.
b. Issue the command su and type exit when finished.
c. Issue the command su and type logoff when finished.
d. Issue the command logon root and type exit when finished.
Answer: b
You can use the su command to imitate any user including root. You will be prompted
for the password for the root account. Once you have provided it you are logged in as
root and can do any administrative duties.
There are seven fields in the /etc/passwd file. Which of the following lists all the
fields in the correct order?
Choose one:
a. username, UID, GID, home directory, command, comment
b. username, UID, GID, comment, home directory, command
c. UID, username, GID, home directory, comment, command
d. username, UID, group name, GID, home directory, comment
Answer: b
The seven fields required for each line in the /etc/passwd file are username, UID, GID,
comment, home directory, command. Each of these fields must be separated by a
colon even if they are empty.
Which of the following commands will show a list of the files in your home
directory including hidden files and the contents of all subdirectories?
Choose one:
a. ls -c home
b. ls -aR /home/username
c. ls -aF /home/username
d. ls -l /home/username
Answer: b
The ls command is used to display a listing of files. The -a option will cause hidden
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files to be displayed as well. The -R option causes ls to recurse down the directory
tree. All of this starts at your home directory.
In order to prevent a user from logging in, you can add a(n) ________at the
beginning of the password field.
Answer: asterick
If you add an asterick at the beginning of the password field in the /etc/passwd file,
that user will not be able to log in.
You have a directory called /home/ben/memos and want to move it to
/home/bob/memos so you issue the command mv /home/ben/memos /home/bob.
What is the results of this action?
Choose one:
a. The files contained in /home/ben/memos are moved to the directory
/home/bob/memos/memos.
b. The files contained in /home/ben/memos are moved to the directory
/home/bob/memos.
c. The files contained in /home/ben/memos are moved to the directory
/home/bob/.
d. The command fails since a directory called memos already exists in the target
directory.
Answer: a
When using the mv command to move a directory, if a directory of the same name
exists then a subdirectory is created for the files to be moved.
Which of the following tasks is not necessary when creating a new user by editing
the /etc/passwd file?
Choose one:
a. Create a link from the user's home directory to the shell the user will use.
b. Create the user's home directory
c. Use the passwd command to assign a password to the account.
d. Add the user to the specified group.
Answer: a
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There is no need to link the user's home directory to the shell command. Rather, the
specified shell must be present on your system.
You issue the following command useradd -m bobm But the user cannot logon.
What is the problem?
Choose one:
a. You need to assign a password to bobm's account using the passwd command.
b. You need to create bobm's home directory and set the appropriate
permissions.
c. You need to edit the /etc/passwd file and assign a shell for bobm's account.
d. The username must be at least five characters long.
Answer: a
The useradd command does not assign a password to newly created accounts. You will
still need to use the passwd command to assign a password.
You wish to print the file vacations with 60 lines to a page. Which of the following
commands will accomplish this? Choose one:
a. pr -l60 vacations | lpr
b. pr -f vacations | lpr
c. pr -m vacations | lpr
d. pr -l vacations | lpr
Answer: a
The default page length when using pr is 66 lines. The -l option is used to specify a
different length.
Which file defines all users on your system?
Choose one:
a. /etc/passwd
b. /etc/users
c. /etc/password
d. /etc/user.conf
Answer: a
The /etc/passwd file contains all the information on users who may log into your
system. If a user account is not contained in this file, then the user cannot log in.
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Which two commands can you use to delete directories?
A) rm
B) rm -rf
C) rmdir
D) rd
E) rd -rf
Answer(s): B, C - You can use rmdir or rm -rf to delete a directory. Answer a is
incorrect, because the rm command without any specific flags will not delete a
directory, it will only delete files. Answers d and e point to a non-existent command.
Which partitioning tool is available in all distributions?
A) Disk Druid
B) fdisk
C) Partition Magic
D) FAT32
E) System Commander
Answer(s): B - The fdisk partitioning tool is available in all Linux distributions.
Answers a, c, and e all handle partitioning, but do not come with all distributions. Disk
Druid is made by Red Hat and used in its distribution along with some derivatives.
Partition Magic and System Commander are tools made by third-party companies.
Answer d is not a tool, but a file system type. Specifically, FAT32 is the file system
type used in Windows 98.
Which partitions might you create on the mail server's hard drive(s) other than
the root, swap, and boot partitions?
[Choose all correct answers]
A) /var/spool
B) /tmp
C) /proc
D) /bin
E) /home
Answer(s): A, B, E - Separating /var/spool onto its own partition helps to ensure that if
something goes wrong with the mail server or spool, the output cannot overrun the file
system. Putting /tmp on its own partition prevents either software or user items in the
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/tmp directory from overrunning the file system. Placing /home off on its own is
mostly useful for system re-installs or upgrades, allowing you to not have to wipe the
/home hierarchy along with other areas. Answers c and d are not possible, as the /proc
portion of the file system is virtual-held in RAM-not placed on the hard drives, and the
/bin hierarchy is necessary for basic system functionality and, therefore, not one that
you can place on a different partition.
When planning your backup strategy you need to consider how often you will
perform a backup, how much time the backup takes and what media you will use.
What other factor must you consider when planning your backup
strategy? _________
what to backup
Choosing which files to backup is the first step in planning your backup strategy.
What utility can you use to automate rotation of logs?
Answer: logrotate
The logrotate command can be used to automate the rotation of various logs.
In order to display the last five commands you have entered using the history
command, you would type ___________ .
Answer: history 5
The history command displays the commands you have previously entered. By passing
it an argument of 5, only the last five commands will be displayed.
What command can you use to review boot messages?
Answer: dmesg
The dmesg command displays the system messages contained in the kernel ring
buffer. By using this command immediately after booting your computer, you will see
the boot messages.
What is the minimum number of partitions you need to install Linux?
Answer: 2
Linux can be installed on two partitions, one as / which will contain all files and a
swap partition.
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What is the name and path of the main system log?
Answer: /var/log/messages
By default, the main system log is /var/log/messages.
Of the following technologies, which is considered a client-side script?
A) JavaScript
B) Java
C) ASP
D) C++
Answer: A - JavaScript is the only client-side script listed. Java and C++ are complete
programming languages. Active Server Pages are parsed on the server with the results
being sent to the client in HTML
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Unix
Interview Questions &
Answers
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Unix Interview Questions and Answers
How are devices represented in UNIX?
All devices are represented by files called special files that are located in/dev
directory. Thus, device files and other files are named and accessed in the same way.
A 'regular file' is just an ordinary data file in the disk. A 'block special file' represents a
device with characteristics similar to a disk (data transfer in terms of blocks). A
'character special file' represents a device with characteristics similar to a keyboard
(data transfer is by stream of bits in sequential order).
What is 'inode'?
All UNIX files have its description stored in a structure called 'inode'. The inode
contains info about the file-size, its location, time of last access, time of last
modification, permission and so on. Directories are also represented as files and have
an associated inode. In addition to descriptions about the file, the inode contains
pointers to the data blocks of the file. If the file is large, inode has indirect pointer to a
block of pointers to additional data blocks (this further aggregates for larger files). A
block is typically 8k.
Inode consists of the following fields:
File owner identifier
File type
File access permissions
File access times
Number of links
File size
Location of the file data
Brief about the directory representation in UNIX
A Unix directory is a file containing a correspondence between filenames and inodes.
A directory is a special file that the kernel maintains. Only kernel modifies directories,
but processes can read directories. The contents of a directory are a list of filename
and inode number pairs. When new directories are created, kernel makes two entries
named '.' (refers to the directory itself) and '..' (refers to parent directory).
System call for creating directory is mkdir (pathname, mode).
What are the Unix system calls for I/O?
open(pathname,flag,mode) - open file
creat(pathname,mode) - create file
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close(filedes) - close an open file
read(filedes,buffer,bytes) - read data from an open file
write(filedes,buffer,bytes) - write data to an open file
lseek(filedes,offset,from) - position an open file
dup(filedes) - duplicate an existing file descriptor
dup2(oldfd,newfd) - duplicate to a desired file descriptor
fcntl(filedes,cmd,arg) - change properties of an open file
ioctl(filedes,request,arg) - change the behaviour of an open file
The difference between fcntl anf ioctl is that the former is intended for any open file,
while the latter is for device-specific operations.
How do you change File Access Permissions?
Every file has following attributes:
owner's user ID ( 16 bit integer )
owner's group ID ( 16 bit integer )
File access mode word
'r w x -r w x- r w x'
(user permission-group permission-others permission)
r-read, w-write, x-execute
To change the access mode, we use chmod(filename,mode).
Example 1:
To change mode of myfile to 'rw-rw-r--' (ie. read, write permission for user read,write permission for group - only read permission for others) we give the args as:
chmod(myfile,0664) .
Each operation is represented by discrete values
'r' is 4
'w' is 2
'x' is 1
Therefore, for 'rw' the value is 6(4+2).
Example 2:
To change mode of myfile to 'rwxr--r--' we give the args as:
chmod(myfile,0744).
What are links and symbolic links in UNIX file system?
A link is a second name (not a file) for a file. Links can be used to assign more than
one name to a file, but cannot be used to assign a directory more than one name or link
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filenames on different computers.
Symbolic link 'is' a file that only contains the name of another file.Operation on the
symbolic link is directed to the file pointed by the it.Both the limitations of links are
eliminated in symbolic links.
Commands for linking files are:
Link ln filename1 filename2
Symbolic link ln -s filename1 filename2
What is a FIFO?
FIFO are otherwise called as 'named pipes'. FIFO (first-in-first-out) is a special file
which is said to be data transient. Once data is read from named pipe, it cannot be read
again. Also, data can be read only in the order written. It is used in interprocess
communication where a process writes to one end of the pipe (producer) and the other
reads from the other end (consumer).
How do you create special files like named pipes and device files?
The system call mknod creates special files in the following sequence.
1. kernel assigns new inode,
2. sets the file type to indicate that the file is a pipe, directory or special file,
3. If it is a device file, it makes the other entries like major, minor device numbers.
For example:
If the device is a disk, major device number refers to the disk controller and minor
device number is the disk.
Discuss the mount and unmount system calls
The privileged mount system call is used to attach a file system to a directory of
another file system; the unmount system call detaches a file system. When you mount
another file system on to your directory, you are essentially splicing one directory tree
onto a branch in another directory tree. The first argument to mount call is the mount
point, that is , a directory in the current file naming system. The second argument is
the file system to mount to that point. When you insert a cdrom to your unix system's
drive, the file system in the cdrom automatically mounts to /dev/cdrom in your system.
How does the inode map to data block of a file?
Inode has 13 block addresses. The first 10 are direct block addresses of the first 10
data blocks in the file. The 11th address points to a one-level index block. The 12th
address points to a two-level (double in-direction) index block. The 13th address
points to a three-level(triple in-direction)index block. This provides a very large
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maximum file size with efficient access to large files, but also small files are accessed
directly in one disk read.
What is a shell?
A shell is an interactive user interface to an operating system services that allows an
user to enter commands as character strings or through a graphical user interface. The
shell converts them to system calls to the OS or forks off a process to execute the
command. System call results and other information from the OS are presented to the
user through an interactive interface. Commonly used shells are sh,csh,ks etc.
Brief about the initial process sequence while the system boots up.
While booting, special process called the 'swapper' or 'scheduler' is created with
Process-ID 0. The swapper manages memory allocation for processes and influences
CPU allocation. The swapper inturn creates 3 children:
the process dispatcher,
vhand and
dbflush
with IDs 1,2 and 3 respectively.
This is done by executing the file /etc/init. Process dispatcher gives birth to the shell.
Unix keeps track of all the processes in an internal data structure called the Process
Table (listing command is ps -el).
What are various IDs associated with a process?
Unix identifies each process with a unique integer called ProcessID. The process that
executes the request for creation of a process is called the 'parent process' whose PID
is 'Parent Process ID'. Every process is associated with a particular user called the
'owner' who has privileges over the process. The identification for the user is 'UserID'.
Owner is the user who executes the process. Process also has 'Effective User ID' which
determines the access privileges for accessing resources like files.
getpid() -process id
getppid() -parent process id
getuid() -user id
geteuid() -effective user id
Explain fork() system call.
The `fork()' used to create a new process from an existing process. The new process is
called the child process, and the existing process is called the parent. We can tell
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which is which by checking the return value from `fork()'. The parent gets the child's
pid returned to him, but the child gets 0 returned to him.
Predict the output of the following program code
main()
{
fork();
printf("Hello World!");
}
Answer:
Hello World!Hello World!
Explanation:
The fork creates a child that is a duplicate of the parent process. The child begins from
the fork().All the statements after the call to fork() will be executed twice.(once by the
parent process and other by child). The statement before fork() is executed only by the
parent process.
Predict the output of the following program code
main()
{
fork(); fork(); fork();
printf("Hello World!");
}
Answer:
"Hello World" will be printed 8 times.
Explanation:
2^n times where n is the number of calls to fork()
List the system calls used for process management:
System calls Description
fork() To create a new process
exec() To execute a new program in a process
wait() To wait until a created process completes its execution
exit() To exit from a process execution
getpid() To get a process identifier of the current process
getppid() To get parent process identifier
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nice() To bias the existing priority of a process
brk() To increase/decrease the data segment size of a process
How can you get/set an environment variable from a program?:
Getting the value of an environment variable is done by using `getenv()'. Setting the
value of an environment variable is done by using `putenv()'.
How can a parent and child process communicate?
A parent and child can communicate through any of the normal inter-process
communication schemes (pipes, sockets, message queues, shared memory), but also
have some special ways to communicate that take advantage of their relationship as a
parent and child. One of the most obvious is that the parent can get the exit status of
the child.
What is a zombie?
When a program forks and the child finishes before the parent, the kernel still keeps
some of its information about the child in case the parent might need it - for example,
the parent may need to check the child's exit status. To be able to get this information,
the parent calls `wait()'; In the interval between the child terminating and the parent
calling `wait()', the child is said to be a `zombie' (If you do `ps', the child will have a
`Z' in its status field to indicate this.)
What are the process states in Unix?
As a process executes it changes state according to its circumstances. Unix processes
have the following states:
Running : The process is either running or it is ready to run .
Waiting : The process is waiting for an event or for a resource.
Stopped : The process has been stopped, usually by receiving a signal.
Zombie : The process is dead but have not been removed from the process table.
What Happens when you execute a program?
When you execute a program on your UNIX system, the system creates a special
environment for that program. This environment contains everything needed for the
system to run the program as if no other program were running on the system. Each
process has process context, which is everything that is unique about the state of the
program you are currently running. Every time you execute a program the UNIX
system does a fork, which performs a series of operations to create a process context
and then execute your program in that context. The steps include the following:
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Allocate a slot in the process table, a list of currently running programs kept by UNIX.
Assign a unique process identifier (PID) to the process.
iCopy the context of the parent, the process that requested the spawning of the new
process.
Return the new PID to the parent process. This enables the parent process to examine
or control the process directly. After the fork is complete, UNIX runs your program.
What Happens when you execute a command?
When you enter 'ls' command to look at the contents of your current working
directory, UNIX does a series of things to create an environment for ls and the run it:
The shell has UNIX perform a fork. This creates a new process that the shell will use
to run the ls program. The shell has UNIX perform an exec of the ls program. This
replaces the shell program and data with the program and data for ls and then starts
running that new program. The ls program is loaded into the new process context,
replacing the text and data of the shell. The ls program performs its task, listing the
contents of the current directory.
What is a Daemon?
A daemon is a process that detaches itself from the terminal and runs, disconnected, in
the background, waiting for requests and responding to them. It can also be defined as
the background process that does not belong to a terminal session. Many system
functions are commonly performed by daemons, including the sendmail daemon,
which handles mail, and the NNTP daemon, which handles USENET news. Many
other daemons may exist. Some of the most common daemons are:
init: Takes over the basic running of the system when the kernel has finished the boot
process.
inetd: Responsible for starting network services that do not have their own stand-alone
daemons. For example, inetd usually takes care of incoming rlogin, telnet, and ftp
connections.
cron: Responsible for running repetitive tasks on a regular schedule.
What is 'ps' command for?
The ps command prints the process status for some or all of the running processes. The
information given are the process identification number (PID),the amount of time that
the process has taken to execute so far etc.
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How would you kill a process?
The kill command takes the PID as one argument; this identifies which process to
terminate. The PID of a process can be got using 'ps' command.
What is an advantage of executing a process in background?
The most common reason to put a process in the background is to allow you to do
something else interactively without waiting for the process to complete. At the end of
the command you add the special background symbol, &. This symbol tells your shell
to execute the given command in the background.
Example: cp *.* ../backup& (cp is for copy)
How do you execute one program from within another?
The system calls used for low-level process creation are execlp() and execvp(). The
execlp call overlays the existing program with the new one , runs that and exits. The
original program gets back control only when an error occurs.
execlp(path,file_name,arguments..); //last argument must be NULL A variant of
execlp called execvp is used when the number of arguments is not known in advance.
execvp(path,argument_array); //argument array should be terminated by NULL
What is IPC? What are the various schemes available?
The term IPC (Inter-Process Communication) describes various ways by which
different process running on some operating system communicate between each other.
Various schemes available are as follows: Pipes:
One-way communication scheme through which different process can communicate.
The problem is that the two processes should have a common ancestor (parent-child
relationship). However this problem was fixed with the introduction of named-pipes
(FIFO).
Message Queues :
Message queues can be used between related and unrelated processes running on a
machine.
Shared Memory:
This is the fastest of all IPC schemes. The memory to be shared is mapped into the
address space of the processes (that are sharing). The speed achieved is attributed to
the fact that there is no kernel involvement. But this scheme needs synchronization.
Various forms of synchronisation are mutexes, condition-variables, read-write locks,
record-locks, and semaphores.
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What is the difference between Swapping and Paging?
Swapping: Whole process is moved from the swap device to the main memory for
execution. Process size must be less than or equal to the available main memory. It is
easier to implementation and overhead to the system. Swapping systems does not
handle the memory more flexibly as compared to the paging systems.
Paging:
Only the required memory pages are moved to main memory from the swap device for
execution. Process size does not matter. Gives the concept of the virtual memory.
It provides greater flexibility in mapping the virtual address space into the physical
memory of the machine. Allows more number of processes to fit in the main memory
simultaneously. Allows the greater process size than the available physical memory.
Demand paging systems handle the memory more flexibly.
What is major difference between the Historic Unix and the new BSD release of
Unix System V in terms of Memory Management?
Historic Unix uses Swapping – entire process is transferred to the main memory from
the swap device, whereas the Unix System V uses Demand Paging – only the part of
the process is moved to the main memory. Historic Unix uses one Swap Device and
Unix System V allow multiple Swap Devices.
What is the main goal of the Memory Management?
It decides which process should reside in the main memory, Manages the parts of the
virtual address space of a process which is non-core resident, Monitors the available
main memory and periodically write the processes into the swap device to provide
more processes fit in the main memory simultaneously.
What is a Map?
A Map is an Array, which contains the addresses of the free space in the swap device
that are allocatable resources, and the number of the resource units available there.
This allows First-Fit allocation of contiguous blocks of a resource. Initially the Map
contains one entry – address (block offset from the starting of the swap area) and the
total number of resources. Kernel treats each unit of Map as a group of disk blocks. On
the allocation and freeing of the resources Kernel updates the Map for accurate
information.
What scheme does the Kernel in Unix System V follow while choosing a swap
device among the multiple swap devices?
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Kernel follows Round Robin scheme choosing a swap device among the multiple
swap devices in Unix System V.
What is a Region?
A Region is a continuous area of a process‘s address space (such as text, data and
stack). The kernel in a ‗Region Table‘ that is local to the process maintains region.
Regions are sharable among the process.
What are the events done by the Kernel after a process is being swapped out
from the main memory?
When Kernel swaps the process out of the primary memory, it performs the following:
Kernel decrements the Reference Count of each region of the process. If the reference
count becomes zero, swaps the region out of the main memory,
Kernel allocates the space for the swapping process in the swap device,
Kernel locks the other swapping process while the current swapping operation is going
on,
The Kernel saves the swap address of the region in the region table.
Is the Process before and after the swap are the same? Give reason.
Process before swapping is residing in the primary memory in its original form. The
regions (text, data and stack) may not be occupied fully by the process, there may be
few empty slots in any of the regions and while swapping Kernel do not bother about
the empty slots while swapping the process out. After swapping the process resides in
the swap (secondary memory) device. The regions swapped out will be present but
only the occupied region slots but not the empty slots that were present before
assigning. While swapping the process once again into the main memory, the Kernel
referring to the Process Memory Map, it assigns the main memory accordingly taking
care of the empty slots in the regions.
What do you mean by u-area (user area) or u-block?
This contains the private data that is manipulated only by the Kernel. This is local to
the Process, i.e. each process is allocated a u-area.
What are the entities that are swapped out of the main memory while swapping
the process out of the main memory?
All memory space occupied by the process, process‘s u-area, and Kernel stack are
swapped out, theoretically. Practically, if the process‘s u-area contains the Address
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Translation Tables for the process then Kernel implementations do not swap the uarea.
What is Fork swap?
fork() is a system call to create a child process. When the parent process calls fork()
system call, the child process is created and if there is short of memory then the child
process is sent to the read-to-run state in the swap device, and return to the user state
without swapping the parent process. When the memory will be available the child
process will be swapped into the main memory.
What is Expansion swap?
At the time when any process requires more memory than it is currently allocated, the
Kernel performs Expansion swap. To do this Kernel reserves enough space in the
swap device. Then the address translation mapping is adjusted for the new virtual
address space but the physical memory is not allocated. At last Kernel swaps the
process into the assigned space in the swap device. Later when the Kernel swaps the
process into the main memory this assigns memory according to the new address
translation mapping.
How the Swapper works?
The swapper is the only process that swaps the processes. The Swapper operates only
in the Kernel mode and it does not uses System calls instead it uses internal Kernel
functions for swapping. It is the archetype of all kernel process.
What are the processes that are not bothered by the swapper? Give Reason.
Zombie process: They do not take any up physical memory.
Processes locked in memories that are updating the region of the process.
Kernel swaps only the sleeping processes rather than the ‗ready-to-run‘ processes, as
they have the higher probability of being scheduled than the Sleeping processes.
What are the requirements for a swapper to work?
The swapper works on the highest scheduling priority. Firstly it will look for any
sleeping process, if not found then it will look for the ready-to-run process for
swapping. But the major requirement for the swapper to work the ready-to-run process
must be core-resident for at least 2 seconds before swapping out. And for swapping in
the process must have been resided in the swap device for at least 2 seconds. If the
requirement is not satisfied then the swapper will go into the wait state on that event
and it is awaken once in a second by the Kernel.
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What are the criteria for choosing a process for swapping into memory from the
swap device?
The resident time of the processes in the swap device, the priority of the processes and
the amount of time the processes had been swapped out.
What are the criteria for choosing a process for swapping out of the memory to
the swap device?
The process‘s memory resident time,
Priority of the process and
The nice value.
What do you mean by nice value?
Nice value is the value that controls {increments or decrements} the priority of the
process. This value that is returned by the nice () system call. The equation for using
nice value is: Priority = (―recent CPU usage‖/constant) + (base- priority) + (nice
value) Only the administrator can supply the nice value. The nice () system call works
for the running process only. Nice value of one process cannot affect the nice value of
the other process.
What are conditions on which deadlock can occur while swapping the processes?
All processes in the main memory are asleep.
All ‗ready-to-run‘ processes are swapped out.
There is no space in the swap device for the new incoming process that are swapped
out of the main memory.
There is no space in the main memory for the new incoming process.
What are conditions for a machine to support Demand Paging?
Memory architecture must based on Pages,
The machine must support the ‗restartable‘ instructions.
What is „the principle of locality‟?
It‘s the nature of the processes that they refer only to the small subset of the total data
space of the process. i.e. the process frequently calls the same subroutines or executes
the loop instructions.
What is the working set of a process?
The set of pages that are referred by the process in the last ‗n‘, references, where ‗n‘ is
called the window of the working set of the process.
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What is the window of the working set of a process?
The window of the working set of a process is the total number in which the process
had referred the set of pages in the working set of the process.
What is called a page fault?
Page fault is referred to the situation when the process addresses a page in the working
set of the process but the process fails to locate the page in the working set. And on a
page fault the kernel updates the working set by reading the page from the secondary
device.
What are data structures that are used for Demand Paging?
Kernel contains 4 data structures for Demand paging. They are,
Page table entries,
Disk block descriptors,
Page frame data table (pfdata),
Swap-use table.
What are the bits that support the demand paging?
Valid, Reference, Modify, Copy on write, Age. These bits are the part of the page
table entry, which includes physical address of the page and protection bits.
Page address
Age
Copy on write
Modify
Reference
Valid
Protection
How the Kernel handles the fork() system call in traditional Unix and in the
System V Unix, while swapping?
Kernel in traditional Unix, makes the duplicate copy of the parent‘s address space and
attaches it to the child‘s process, while swapping. Kernel in System V Unix,
manipulates the region tables, page table, and pfdata table entries, by incrementing the
reference count of the region table of shared regions.
Difference between the fork() and vfork() system call?
During the fork() system call the Kernel makes a copy of the parent process‘s address
space and attaches it to the child process. But the vfork() system call do not makes any
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copy of the parent‘s address space, so it is faster than the fork() system call. The child
process as a result of the vfork() system call executes exec() system call. The child
process from vfork() system call executes in the parent‘s address space (this can
overwrite the parent‘s data and stack ) which suspends the parent process until the
child process exits.
What is BSS(Block Started by Symbol)?
A data representation at the machine level, that has initial values when a program
starts and tells about how much space the kernel allocates for the un-initialized data.
Kernel initializes it to zero at run-time.
What is Page-Stealer process?
This is the Kernel process that makes rooms for the incoming pages, by swapping the
memory pages that are not the part of the working set of a process. Page-Stealer is
created by the Kernel at the system initialization and invokes it throughout the lifetime
of the system. Kernel locks a region when a process faults on a page in the region, so
that page stealer cannot steal the page, which is being faulted in.
Name two paging states for a page in memory?
The two paging states are:
The page is aging and is not yet eligible for swapping,
The page is eligible for swapping but not yet eligible for reassignment to other virtual
address space.
What are the phases of swapping a page from the memory?
Page stealer finds the page eligible for swapping and places the page number in the list
of pages to be swapped. Kernel copies the page to a swap device when necessary and
clears the valid bit in the page table entry, decrements the pfdata reference count, and
places the pfdata table entry at the end of the free list if its reference count is 0.
What is page fault? Its types?
Page fault refers to the situation of not having a page in the main memory when any
process references it. There are two types of page fault :
Validity fault,
Protection fault.
In what way the Fault Handlers and the Interrupt handlers are different?
Fault handlers are also an interrupt handler with an exception that the interrupt
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handlers cannot sleep. Fault handlers sleep in the context of the process that caused the
memory fault. The fault refers to the running process and no arbitrary processes are
put to sleep.
What is validity fault?
If a process referring a page in the main memory whose valid bit is not set, it results in
validity fault. The valid bit is not set for those pages:
that are outside the virtual address space of a process,
that are the part of the virtual address space of the process but no physical address is
assigned to it.
What does the swapping system do if it identifies the illegal page for swapping?
If the disk block descriptor does not contain any record of the faulted page, then this
causes the attempted memory reference is invalid and the kernel sends a
―Segmentation violation‖ signal to the offending process. This happens when the
swapping system identifies any invalid memory reference.
What are states that the page can be in, after causing a page fault?
On a swap device and not in memory,
On the free page list in the main memory,
In an executable file,
Marked ―demand zero‖,
Marked ―demand fill‖.
In what way the validity fault handler concludes?
It sets the valid bit of the page by clearing the modify bit.
It recalculates the process priority.
At what mode the fault handler executes?
At the Kernel Mode.
What do you mean by the protection fault?
Protection fault refers to the process accessing the pages, which do not have the access
permission. A process also incur the protection fault when it attempts to write a page
whose copy on write bit was set during the fork() system call.
How the Kernel handles the copy on write bit of a page, when the bit is set?
In situations like, where the copy on write bit of a page is set and that page is shared
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by more than one process, the Kernel allocates new page and copies the content to the
new page and the other processes retain their references to the old page. After copying
the Kernel updates the page table entry with the new page number. Then Kernel
decrements the reference count of the old pfdata table entry. In cases like, where the
copy on write bit is set and no processes are sharing the page, the Kernel allows the
physical page to be reused by the processes. By doing so, it clears the copy on write
bit and disassociates the page from its disk copy (if one exists), because other process
may share the disk copy. Then it removes the pfdata table entry from the page-queue
as the new copy of the virtual page is not on the swap device. It decrements the swapuse count for the page and if count drops to 0, frees the swap space.
For which kind of fault the page is checked first?
The page is first checked for the validity fault, as soon as it is found that the page is
invalid (valid bit is clear), the validity fault handler returns immediately, and the
process incur the validity page fault. Kernel handles the validity fault and the process
will incur the protection fault if any one is present.
In what way the protection fault handler concludes?
After finishing the execution of the fault handler, it sets the modify and protection bits
and clears the copy on write bit. It recalculates the process-priority and checks for
signals.
How the Kernel handles both the page stealer and the fault handler?
The page stealer and the fault handler thrash because of the shortage of the memory. If
the sum of the working sets of all processes is greater that the physical memory then
the fault handler will usually sleep because it cannot allocate pages for a process. This
results in the reduction of the system throughput because Kernel spends too much time
in overhead, rearranging the memory in the frantic pace.
Explain different types of Unix systems.
The most widely used are: 1. System V (AT&T) 2. AIX (IBM) 3. BSD (Berkeley) 4.
Solaris (Sun) 5. Xenix ( A PC version of Unix)
Explain kernal and shell.
Kernal: It carries out basic operating system functions such as allocating memory,
accessing files and handling communications. Shell:A shell provides the user interface
to the kernal.There are 3 major shells : C-shell, Bourne shell , Korn shell
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What is ex and vi ?
ex is Unix line editor and vi is the standard Unix screen editor.
Which are typical system directories below the root directory?
(1)/bin: contains many programs which will be executed by users (2)/etc : files used by
administrator (3)/dev: hardware devices (4)/lib: system libraries (5)/usr: application
software (6)/home: home directories for different systems.
Construct pipes to execute the following jobs.
1. Output of who should be displayed on the screen with value of total number of users
who have logged in displayed at the bottom of the list.
2. Output of ls should be displayed on the screen and from this output the lines
containing the word ‗poem‘ should be counted and the count should be stored in a file.
3. Contents of file1 and file2 should be displayed on the screen and this output should
be appended in a file
.
From output of ls the lines containing ‗poem‘ should be displayed on the screen along
with the count.
4. Name of cities should be accepted from the keyboard . This list should be combined
with the list present in a file. This combined list should be sorted and the sorted list
should be stored in a file ‗newcity‘.
5. All files present in a directory dir1 should be deleted any error while deleting should
be stored in a file ‗errorlog‘.
Explain the following commands.
$ ls > file1
$ banner hi-fi > message
$ cat par.3 par.4 par.5 >> report
$ cat file1>file1
$ date ; who
$ date ; who > logfile
$ (date ; who) > logfile
What is the significance of the “tee” command?
It reads the standard input and sends it to the standard output while redirecting a copy
of what it has read to the file specified by the user.
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What does the command “ $who | sort –logfile > newfile” do?
The input from a pipe can be combined with the input from a file . The trick is to use
the special symbol ―-― (a hyphen) for those commands that recognize the hyphen as
std input.
In the above command the output from who becomes the std input to sort , meanwhile
sort opens the file logfile, the contents of this file is sorted together with the output of
who (rep by the hyphen) and the sorted output is redirected to the file newfile.
What does the command “$ls | wc –l > file1” do?
ls becomes the input to wc which counts the number of lines it receives as input and
instead of displaying this count , the value is stored in file1.
Which of the following commands is not a filter man , (b) cat , (c) pg , (d) head
man A filter is a program which can receive a flow of data from std input, process (or
filter) it and send the result to the std output.
How is the command “$cat file2 “ different from “$cat >file2 and >> redirection
operators ?
is the output redirection operator when used it overwrites while >> operator appends
into the file.
Explain the steps that a shell follows while processing a command.
After the command line is terminated by the key, the shell goes ahead with processing
the command line in one or more passes. The sequence is well defined and assumes
the following order.
Parsing: The shell first breaks up the command line into words, using spaces and the
delimiters, unless quoted. All consecutive occurrences of a space or tab are replaced
here with a single space.
Variable evaluation: All words preceded by a $ are valuated as variables, unless
quoted or escaped.
Command substitution: Any command surrounded by back quotes is executed by the
shell which then replaces the standard output of the command into the command line.
Wild-card interpretation: The shell finally scans the command line for wild-cards (the
characters *, ?, [, ]).
Any word containing a wild-card is replaced by a sorted list of
filenames that match the pattern. The list of these filenames then forms the arguments
to the command.
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PATH evaluation: It finally looks for the PATH variable to determine the sequence of
directories it has to search in order to hunt for the command.
What difference between cmp and diff commands?
cmp - Compares two files byte by byte and displays the first mismatch diff - tells the
changes to be made to make the files identical
What is the use of „grep‟ command?
‗grep‘ is a pattern search command. It searches for the pattern, specified in the
command line with appropriate option, in a file(s).
Syntax : grep
Example : grep 99mx mcafile
What is the difference between cat and more command?
Cat displays file contents. If the file is large the contents scroll off the screen before
we view it. So command 'more' is like a pager which displays the contents page by
page.
Write a command to kill the last background job?
Kill $!
Which command is used to delete all files in the current directory and all its subdirectories?
rm -r *
Write a command to display a file‟s contents in various formats?
$od -cbd file_name
c - character, b - binary (octal), d-decimal, od=Octal Dump.
What will the following command do?
$ echo *
It is similar to 'ls' command and displays all the files in the current directory.
Is it possible to create new a file system in UNIX?
Yes, ‗mkfs‘ is used to create a new file system.
Is it possible to restrict incoming message?
Yes, using the ‗mesg‘ command.
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What is the use of the command "ls -x chapter[1-5]"
ls stands for list; so it displays the list of the files that starts with 'chapter' with suffix
'1' to '5', chapter1, chapter2, and so on.
Is „du‟ a command? If so, what is its use?
Yes, it stands for ‗disk usage‘. With the help of this command you can find the disk
capacity and free space of the disk.
Is it possible to count number char, line in a file; if so, How?
Yes, wc-stands for word count.
wc -c for counting number of characters in a file.
wc -l for counting lines in a file.
Name the data structure used to maintain file identification?
‗inode‘, each file has a separate inode and a unique inode number.
How many prompts are available in a UNIX system?
Two prompts, PS1 (Primary Prompt), PS2 (Secondary Prompt).
How does the kernel differentiate device files and ordinary files?
Kernel checks 'type' field in the file's inode structure.
How to switch to a super user status to gain privileges?
Use ‗su‘ command. The system asks for password and when valid entry is made the
user gains super user (admin) privileges.
What are shell variables?
Shell variables are special variables, a name-value pair created and maintained by the
shell.
Example: PATH, HOME, MAIL and TERM
What is redirection?
Directing the flow of data to the file or from the file for input or output.
Example : ls > wc
How to terminate a process which is running and the specialty on command kill
0?
With the help of kill command we can terminate the process.
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Syntax: kill pid
Kill 0 - kills all processes in your system except the login shell.
What is a pipe and give an example?
A pipe is two or more commands separated by pipe char '|'. That tells the shell to
arrange for the output of the preceding command to be passed as input to the following
command.
Example : ls -l | pr
The output for a command ls is the standard input of pr.
When a sequence of commands are combined using pipe, then it is called pipeline.
Explain kill() and its possible return values.
There are four possible results from this call:
‗kill()‘ returns 0. This implies that a process exists with the given PID, and the system
would allow you to send signals to it. It is system-dependent whether the process
could be a zombie.
‗kill()‘ returns -1, ‗errno == ESRCH‘ either no process exists with the given PID, or
security enhancements are causing the system to deny its existence. (On some systems,
the process could be a zombie.)
‗kill()‘ returns -1, ‗errno == EPERM‘ the system would not allow you to kill the
specified process. This means that either the process exists (again, it could be a
zombie) or draconian security enhancements are present (e.g. your process is not
allowed to send signals to *anybody*).
‗kill()‘ returns -1, with some other value of ‗errno‘ you are in trouble! The most-used
technique is to assume that success or failure with ‗EPERM‘ implies that the process
exists, and any other error implies that it doesn't.
An alternative exists, if you are writing specifically for a system (or all those systems)
that provide a ‗/proc‘ filesystem: checking for the existence of ‗/proc/PID‘ may work.
=====================================================================
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Software Testing
Interview Questions &
Answers
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Manual Testing Interview Questions and Answers
What makes a good test engineer?
A good test engineer has a 'test to break' attitude, an ability to take the point of view of
the customer, a strong desire for quality, and an attention to detail. Tact and diplomacy
are useful in maintaining a cooperative relationship with developers, and an ability to
communicate with both technical (developers) and non-technical (customers,
management) people is useful. Previous software development experience can be
helpful as it provides a deeper understanding of the software development process,
gives the tester an appreciation for the developers' point of view, and reduce the
learning curve in automated test tool programming. Judgment skills are needed to
assess high-risk areas of an application on which to focus testing efforts when time is
limited.
What makes a good Software QA engineer?
The same qualities a good tester has are useful for a QA engineer. Additionally, they
must be able to understand the entire software development process and how it can fit
into the business approach and goals of the organization. Communication skills and
the ability to understand various sides of issues are important. In organizations in the
early stages of implementing QA processes, patience and diplomacy are especially
needed. An ability to find problems as well as to see 'what's missing' is important for
inspections and reviews.
What makes a good QA or Test manager?
A good QA, test, or QA/Test(combined) manager should:
• be familiar with the software development process
• be able to maintain enthusiasm of their team and promote a positive atmosphere,
despite
• what is a somewhat 'negative' process (e.g., looking for or preventing problems)
• be able to promote teamwork to increase productivity
• be able to promote cooperation between software, test, and QA engineers
• have the diplomatic skills needed to promote improvements in QA processes
• have the ability to withstand pressures and say 'no' to other managers when quality is
insufficient or QA processes are not being adhered to
• have people judgement skills for hiring and keeping skilled personnel
• be able to communicate with technical and non-technical people, engineers,
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managers, and customers.
• be able to run meetings and keep them focused
What's the role of documentation in QA?
Critical. (Note that documentation can be electronic, not necessarily paper.) QA
practices should be documented such that they are repeatable. Specifications, designs,
business rules, inspection reports, configurations, code changes, test plans, test cases,
bug reports, user manuals, etc. should all be documented. There should ideally be a
system for easily finding and obtaining documents and determining what
documentation will have a particular piece of information. Change management for
documentation should be used if possible.
What's the big deal about 'requirements'?
One of the most reliable methods of insuring problems, or failure, in a complex
software project is to have poorly documented requirements specifications.
Requirements are the details describing an application's externally-perceived
functionality and properties. Requirements should be clear, complete, reasonably
detailed, cohesive, attainable, and testable. A non-testable requirement would be, for
example, 'user-friendly' (too subjective). A testable requirement would be something
like 'the user must enter their previously-assigned password to access the application'.
Determining and organizing requirements details in a useful and efficient way can be a
difficult effort; different methods are available depending on the particular project.
Many books are available that describe various approaches to this task. (See the
Bookstore section's 'Software Requirements Engineering' category for books on
Software Requirements.)
Care should be taken to involve ALL of a project's significant 'customers' in the
requirements process. 'Customers' could be in-house personnel or out, and could
include end-users, customer acceptance testers, customer contract officers, customer
management, future software maintenance engineers, salespeople, etc. Anyone who
could later derail the project if their expectations aren't met should be included if
possible.
Organizations vary considerably in their handling of requirements specifications.
Ideally, the requirements are spelled out in a document with statements such as 'The
product shall.....'. 'Design' specifications should not be confused with 'requirements';
design specifications should be traceable back to the requirements.
In some organizations requirements may end up in high level project plans, functional
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specification documents, in design documents, or in other documents at various levels
of detail. No matter what they are called, some type of documentation with detailed
requirements will be needed by testers in order to properly plan and execute tests.
Without such documentation, there will be no clear-cut way to determine if a software
application is performing correctly.
'Agile' methods such as XP use methods requiring close interaction and cooperation
between programmers and customers/end-users to iteratively develop requirements.
The programmer uses 'Test first' development to first create automated unit testing
code, which essentially embodies the requirements.
What steps are needed to develop and run software tests?
The following are some of the steps to consider:
• Obtain requirements, functional design, and internal design specifications and other
necessary documents
• Obtain budget and schedule requirements
• Determine project-related personnel and their responsibilities, reporting
requirements, required standards and processes (such as release processes, change
processes, etc.)
• Identify application's higher-risk aspects, set priorities, and determine scope and
limitations of tests
• Determine test approaches and methods - unit, integration, functional, system, load,
usability tests, etc.
• Determine test environment requirements (hardware, software, communications,
etc.)
• Determine testware requirements (record/playback tools, coverage analyzers, test
tracking, problem/bug tracking, etc.)
• Determine test input data requirements
• Identify tasks, those responsible for tasks, and labor requirements
• Set schedule estimates, timelines, milestones
• Determine input equivalence classes, boundary value analyses, error classes
• Prepare test plan document and have needed reviews/approvals
• Write test cases
• Have needed reviews/inspections/approvals of test cases
• Prepare test environment and testware, obtain needed user manuals/reference
documents/configuration guides/installation guides, set up test tracking processes, set
up logging and archiving processes, set up or obtain test input data
• Obtain and install software releases
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• Perform tests
• Evaluate and report results
• Track problems/bugs and fixes
• Retest as needed
• Maintain and update test plans, test cases, test environment, and testware through life
cycle
What's a 'test plan'?
A software project test plan is a document that describes the objectives, scope,
approach, and focus of a software testing effort. The process of preparing a test plan is
a useful way to think through the efforts needed to validate the acceptability of a
software product. The completed document will help people outside the test group
understand the 'why' and 'how' of product validation. It should be thorough enough to
be useful but not so thorough that no one outside the test group will read it. The
following are some of the items that might be included in a test plan, depending on the
particular project:
• Title
• Identification of software including version/release numbers
• Revision history of document including authors, dates, approvals
• Table of Contents
• Purpose of document, intended audience
• Objective of testing effort
• Software product overview
• Relevant related document list, such as requirements, design documents, other test
plans, etc.
• Relevant standards or legal requirements
• Traceability requirements
• Relevant naming conventions and identifier conventions
• Overall software project organization and personnel/contact-info/responsibilties
• Test organization and personnel/contact-info/responsibilities
• Assumptions and dependencies
• Project risk analysis
• Testing priorities and focus
• Scope and limitations of testing
• Test outline - a decomposition of the test approach by test type, feature,
functionality, process, system, module, etc. as applicable
• Outline of data input equivalence classes, boundary value analysis, error classes
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• Test environment - hardware, operating systems, other required software, data
configurations, interfaces to other systems
• Test environment validity analysis - differences between the test and production
systems and their impact on test validity.
• Test environment setup and configuration issues
• Software migration processes
• Software CM processes
• Test data setup requirements
• Database setup requirements
• Outline of system-logging/error-logging/other capabilities, and tools such as screen
capture software, that will be used to help describe and report bugs
• Discussion of any specialized software or hardware tools that will be used by testers
to help track the cause or source of bugs
• Test automation - justification and overview
• Test tools to be used, including versions, patches, etc.
• Test script/test code maintenance processes and version control
• Problem tracking and resolution - tools and processes
• Project test metrics to be used
• Reporting requirements and testing deliverables
• Software entrance and exit criteria
• Initial sanity testing period and criteria
• Test suspension and restart criteria
• Personnel allocation
• Personnel pre-training needs
• Test site/location
• Outside test organizations to be utilized and their purpose, responsibilties,
deliverables, contact persons, and coordination issues
• Relevant proprietary, classified, security, and licensing issues.
• Open issues
• Appendix - glossary, acronyms, etc.
What's a 'test case'?
• A test case is a document that describes an input, action, or event and an expected
response, to determine if a feature of an application is working correctly. A test case
should contain particulars such as test case identifier, test case name, objective, test
conditions/setup, input data requirements, steps, and expected results.
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• Note that the process of developing test cases can help find problems in the
requirements or design of an application, since it requires completely thinking through
the operation of the application. For this reason, it's useful to prepare test cases early in
the development cycle if possible.
What should be done after a bug is found?
The bug needs to be communicated and assigned to developers that can fix it. After the
problem is resolved, fixes should be re-tested, and determinations made regarding
requirements for regression testing to check that fixes didn't create problems
elsewhere. If a problem-tracking system is in place, it should encapsulate these
processes. A variety of commercial problem-tracking/management software tools are
available (see the 'Tools' section for web resources with listings of such tools). The
following are items to consider in the tracking process:
• Complete information such that developers can understand the bug, get an idea of it's
severity, and reproduce it if necessary.
• Bug identifier (number, ID, etc.)
• Current bug status (e.g., 'Released for Retest', 'New', etc.)
• The application name or identifier and version
• The function, module, feature, object, screen, etc. where the bug occurred
• Environment specifics, system, platform, relevant hardware specifics
• Test case name/number/identifier
• One-line bug description
• Full bug description
• Description of steps needed to reproduce the bug if not covered by a test case or if
the developer doesn't have easy access to the test case/test script/test tool
• Names and/or descriptions of file/data/messages/etc. used in test
• File excerpts/error messages/log file excerpts/screen shots/test tool logs that would
be helpful in finding the cause of the problem
• Severity estimate (a 5-level range such as 1-5 or 'critical'-to-'low' is common)
• Was the bug reproducible?
• Tester name
• Test date
• Bug reporting date
• Name of developer/group/organization the problem is assigned to
• Description of problem cause
• Description of fix
• Code section/file/module/class/method that was fixed
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• Date of fix
• Application version that contains the fix
• Tester responsible for retest
• Retest date
• Retest results
• Regression testing requirements
• Tester responsible for regression tests
• Regression testing results
A reporting or tracking process should enable notification of appropriate personnel at
various stages. For instance, testers need to know when retesting is needed, developers
need to know when bugs are found and how to get the needed information, and
reporting/summary capabilities are needed for managers.
What is 'configuration management'?
Configuration management covers the processes used to control, coordinate, and track:
code, requirements, documentation, problems, change requests, designs,
tools/compilers/libraries/patches, changes made to them, and who makes the changes.
(See the 'Tools' section for web resources with listings of configuration management
tools. Also see the Bookstore section's 'Configuration Management' category for useful
books with more information.)
What if the software is so buggy it can't really be tested at all?
The best bet in this situation is for the testers to go through the process of reporting
whatever bugs or blocking-type problems initially show up, with the focus being on
critical bugs. Since this type of problem can severely affect schedules, and indicates
deeper problems in the software development process (such as insufficient unit testing
or insufficient integration testing, poor design, improper build or release procedures,
etc.) managers should be notified, and provided with some documentation as evidence
of the problem.
How can it be known when to stop testing?
This can be difficult to determine. Many modern software applications are so complex,
and run in such an interdependent environment, that complete testing can never be
done. Common factors in deciding when to stop are:
• Deadlines (release deadlines, testing deadlines, etc.)
• Test cases completed with certain percentage passed
• Test budget depleted
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• Coverage of code/functionality/requirements reaches a specified point
• Bug rate falls below a certain level
• Beta or alpha testing period ends
What if there isn't enough time for thorough testing?
Use risk analysis to determine where testing should be focused.
Since it's rarely possible to test every possible aspect of an application, every possible
combination of events, every dependency, or everything that could go wrong, risk
analysis is appropriate to most software development projects. This requires
judgement skills, common sense, and experience. (If warranted, formal methods are
also available.) Considerations can include:
• Which functionality is most important to the project's intended purpose?
• Which functionality is most visible to the user?
• Which functionality has the largest safety impact?
• Which functionality has the largest financial impact on users?
• Which aspects of the application are most important to the customer?
• Which aspects of the application can be tested early in the development cycle?
• Which parts of the code are most complex, and thus most subject to errors?
• Which parts of the application were developed in rush or panic mode?
• Which aspects of similar/related previous projects caused problems?
• Which aspects of similar/related previous projects had large maintenance expenses?
• Which parts of the requirements and design are unclear or poorly thought out?
• What do the developers think are the highest-risk aspects of the application?
• What kinds of problems would cause the worst publicity?
• What kinds of problems would cause the most customer service complaints?
• What kinds of tests could easily cover multiple functionalities?
• Which tests will have the best high-risk-coverage to time-required ratio?
What if the project isn't big enough to justify extensive testing?
Consider the impact of project errors, not the size of the project. However, if extensive
testing is still not justified, risk analysis is again needed and the same considerations
as described previously in 'What if there isn't enough time for thorough testing?' apply.
The tester might then do ad hoc testing, or write up a limited test plan based on the
risk analysis.
What can be done if requirements are changing continuously?
A common problem and a major headache.
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• Work with the project's stakeholders early on to understand how requirements might
change so that alternate test plans and strategies can be worked out in advance, if
possible.
• It's helpful if the application's initial design allows for some adaptability so that later
changes do not require redoing the application from scratch.
• If the code is well-commented and well-documented this makes changes easier for
the developers.
• Use rapid prototyping whenever possible to help customers feel sure of their
requirements and minimize changes.
• The project's initial schedule should allow for some extra time commensurate with
the possibility of changes.
• Try to move new requirements to a 'Phase 2' version of an application, while using
the original requirements for the 'Phase 1' version.
• Negotiate to allow only easily-implemented new requirements into the project, while
moving more difficult new requirements into future versions of the application.
• Be sure that customers and management understand the scheduling impacts, inherent
risks, and costs of significant requirements changes. Then let management or the
customers (not the developers or testers) decide if the changes are warranted - after all,
that's their job.
• Balance the effort put into setting up automated testing with the expected effort
required to re-do them to deal with changes.
• Try to design some flexibility into automated test scripts.
• Focus initial automated testing on application aspects that are most likely to remain
unchanged.
• Devote appropriate effort to risk analysis of changes to minimize regression testing
needs.
• Design some flexibility into test cases (this is not easily done; the best bet might be
to minimize the detail in the test cases, or set up only higher-level generic-type test
plans)
• Focus less on detailed test plans and test cases and more on ad hoc testing (with an
understanding of the added risk that this entails).
What if the application has functionality that wasn't in the requirements?
It may take serious effort to determine if an application has significant unexpected or
hidden functionality, and it would indicate deeper problems in the software
development process. If the functionality isn't necessary to the purpose of the
application, it should be removed, as it may have unknown impacts or dependencies
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that were not taken into account by the designer or the customer. If not removed,
design information will be needed to determine added testing needs or regression
testing needs. Management should be made aware of any significant added risks as a
result of the unexpected functionality. If the functionality only effects areas such as
minor improvements in the user interface, for example, it may not be a significant risk.
How can Software QA processes be implemented without stifling productivity?
By implementing QA processes slowly over time, using consensus to reach agreement
on processes, and adjusting and experimenting as an organization grows and matures,
productivity will be improved instead of stifled. Problem prevention will lessen the
need for problem detection, panics and burn-out will decrease, and there will be
improved focus and less wasted effort. At the same time, attempts should be made to
keep processes simple and efficient, minimize paperwork, promote computer-based
processes and automated tracking and reporting, minimize time required in meetings,
and promote training as part of the QA process. However, no one - especially talented
technical types - likes rules or bureacracy, and in the short run things may slow down
a bit. A typical scenario would be that more days of planning and development will be
needed, but less time will be required for late-night bug-fixing and calming of irate
customers.
What if an organization is growing so fast that fixed QA processes are
impossible?
This is a common problem in the software industry, especially in new technology
areas. There is no easy solution in this situation, other than:
• Hire good people
• Management should 'ruthlessly prioritize' quality issues and maintain focus on the
customer
• Everyone in the organization should be clear on what 'quality' means to the customer
How does a client/server environment affect testing?
Client/server applications can be quite complex due to the multiple dependencies
among clients, data communications, hardware, and servers. Thus testing requirements
can be extensive. When time is limited (as it usually is) the focus should be on
integration and system testing. Additionally, load/stress/performance testing may be
useful in determining client/server application limitations and capabilities. There are
commercial tools to assist with such testing. (See the 'Tools' section for web resources
with listings that include these kinds of test tools.)
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How can World Wide Web sites be tested?
Web sites are essentially client/server applications - with web servers and 'browser'
clients. Consideration should be given to the interactions between html pages, TCP/IP
communications, Internet connections, firewalls, applications that run in web pages
(such as applets, javascript, plug-in applications), and applications that run on the
server side (such as cgi scripts, database interfaces, logging applications, dynamic
page generators, asp, etc.). Additionally, there are a wide variety of servers and
browsers, various versions of each, small but sometimes significant differences
between them, variations in connection speeds, rapidly changing technologies, and
multiple standards and protocols. The end result is that testing for web sites can
become a major ongoing effort. Other considerations might include:
• What are the expected loads on the server (e.g., number of hits per unit time?), and
what kind of performance is required under such loads (such as web server response
time, database query response times). What kinds of tools will be needed for
performance testing (such as web load testing tools, other tools already in house that
can be adapted, web robot downloading tools, etc.)?
• Who is the target audience? What kind of browsers will they be using? What kind of
connection speeds will they by using? Are they intra- organization (thus with likely
high connection speeds and similar browsers) or Internet-wide (thus with a wide
variety of connection speeds and browser types)?
• What kind of performance is expected on the client side (e.g., how fast should pages
appear, how fast should animations, applets, etc. load and run)?
• Will down time for server and content maintenance/upgrades be allowed? how
much?
• What kinds of security (firewalls, encryptions, passwords, etc.) will be required and
what is it expected to do? How can it be tested?
• How reliable are the site's Internet connections required to be? And how does that
affect backup system or redundant connection requirements and testing?
• What processes will be required to manage updates to the web site's content, and
what are the requirements for maintaining, tracking, and controlling page content,
graphics, links, etc.?
• Which HTML specification will be adhered to? How strictly? What variations will
be allowed for targeted browsers?
• Will there be any standards or requirements for page appearance and/or graphics
throughout a site or parts of a site??
• How will internal and external links be validated and updated? how often?
• Can testing be done on the production system, or will a separate test system be
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required? How are browser caching, variations in browser option settings, dial-up
connection variabilities, and real-world internet 'traffic congestion' problems to be
accounted for in testing?
• How extensive or customized are the server logging and reporting requirements; are
they considered an integral part of the system and do they require testing?
• How are cgi programs, applets, javascripts, ActiveX components, etc. to be
maintained, tracked, controlled, and tested?
Some sources of site security information include the Usenet newsgroup
'comp.security.announce' and links concerning web site security in the 'Other
Resources' section.
Some usability guidelines to consider - these are subjective and may or may not apply
to a given situation (Note: more information on usability testing issues can be found in
articles about web site usability in the 'Other Resources' section):
• Pages should be 3-5 screens max unless content is tightly focused on a single topic.
If larger, provide internal links within the page.
• The page layouts and design elements should be consistent throughout a site, so that
it's clear to the user that they're still within a site.
• Pages should be as browser-independent as possible, or pages should be provided or
generated based on the browser-type.
• All pages should have links external to the page; there should be no dead-end pages.
• The page owner, revision date, and a link to a contact person or organization should
be included on each page.
Many new web site test tools have appeared in the recent years and more than 280 of
them are listed in the 'Web Test Tools' section.
How is testing affected by object-oriented designs?
Well-engineered object-oriented design can make it easier to trace from code to
internal design to functional design to requirements. While there will be little affect on
black box testing (where an understanding of the internal design of the application is
unnecessary), white-box testing can be oriented to the application's objects. If the
application was well-designed this can simplify test design.
What is Extreme Programming and what's it got to do with testing?
Extreme Programming (XP) is a software development approach for small teams on
risk-prone projects with unstable requirements. It was created by Kent Beck who
described the approach in his book 'Extreme Programming Explained' (See the
Softwareqatest.com Books page.). Testing ('extreme testing') is a core aspect of
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Extreme Programming. Programmers are expected to write unit and functional test
code first - before the application is developed. Test code is under source control along
with the rest of the code. Customers are expected to be an integral part of the project
team and to help develope scenarios for acceptance/black box testing. Acceptance tests
are preferably automated, and are modified and rerun for each of the frequent
development iterations. QA and test personnel are also required to be an integral part
of the project team. Detailed requirements documentation is not used, and frequent rescheduling, re-estimating, and re-prioritizing is expected. For more info see the XPrelated listings in the Softwareqatest.com 'Other Resources' section.
What is 'Software Quality Assurance'?
Software QA involves the entire software development PROCESS - monitoring and
improving the process, making sure that any agreed-upon standards and procedures are
followed, and ensuring that problems are found and dealt with. It is oriented to
'prevention'. (See the Bookstore section's 'Software QA' category for a list of useful
books on Software Quality Assurance.)
What is 'Software Testing'?
Testing involves operation of a system or application under controlled conditions and
evaluating the results (eg, 'if the user is in interface A of the application while using
hardware B, and does C, then D should happen'). The controlled conditions should
include both normal and abnormal conditions. Testing should intentionally attempt to
make things go wrong to determine if things happen when they shouldn't or things
don't happen when they should. It is oriented to 'detection'. (See the Bookstore
section's 'Software Testing' category for a list of useful books on Software Testing.)
• Organizations vary considerably in how they assign responsibility for QA and
testing. Sometimes they're the combined responsibility of one group or individual.
Also common are project teams that include a mix of testers and developers who work
closely together, with overall QA processes monitored by project managers. It will
depend on what best fits an organization's size and business structure.
What are some recent major computer system failures caused by software bugs?
• A major U.S. retailer was reportedly hit with a large government fine in October of
2003 due to web site errors that enabled customers to view one anothers' online
orders.
• News stories in the fall of 2003 stated that a manufacturing company recalled all
their transportation products in order to fix a software problem causing instability in
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certain circumstances. The company found and reported the bug itself and initiated the
recall procedure in which a software upgrade fixed the problems.
• In August of 2003 a U.S. court ruled that a lawsuit against a large online brokerage
company could proceed; the lawsuit reportedly involved claims that the company was
not fixing system problems that sometimes resulted in failed stock trades, based on the
experiences of 4 plaintiffs during an 8-month period. A previous lower court's ruling
that "...six miscues out of more than 400 trades does not indicate negligence." was
invalidated.
• In April of 2003 it was announced that the largest student loan company in the U.S.
made a software error in calculating the monthly payments on 800,000 loans.
Although borrowers were to be notified of an increase in their required payments, the
company will still reportedly lose $8 million in interest. The error was uncovered
when borrowers began reporting inconsistencies in their bills.
• News reports in February of 2003 revealed that the U.S. Treasury Department mailed
50,000 Social Security checks without any beneficiary names. A spokesperson
indicated that the missing names were due to an error in a software change.
Replacement checks were subsequently mailed out with the problem corrected, and
recipients were then able to cash their Social Security checks.
• In March of 2002 it was reported that software bugs in Britain's national tax system
resulted in more than 100,000 erroneous tax overcharges. The problem was partly
attibuted to the difficulty of testing the integration of multiple systems.
• A newspaper columnist reported in July 2001 that a serious flaw was found in offthe-shelf software that had long been used in systems for tracking certain U.S. nuclear
materials. The same software had been recently donated to another country to be used
in tracking their own nuclear materials, and it was not until scientists in that country
discovered the problem, and shared the information, that U.S. officials became aware
of the problems.
• According to newspaper stories in mid-2001, a major systems development
contractor was fired and sued over problems with a large retirement plan management
system. According to the reports, the client claimed that system deliveries were late,
the software had excessive defects, and it caused other systems to crash.
• In January of 2001 newspapers reported that a major European railroad was hit by
the aftereffects of the Y2K bug. The company found that many of their newer trains
would not run due to their inability to recognize the date '31/12/2000'; the trains were
started by altering the control system's date settings.
• News reports in September of 2000 told of a software vendor settling a lawsuit with
a large mortgage lender; the vendor had reportedly delivered an online mortgage
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processing system that did not meet specifications, was delivered late, and didn't
work.
• In early 2000, major problems were reported with a new computer system in a large
suburban U.S. public school district with 100,000+ students; problems included
10,000 erroneous report cards and students left stranded by failed class registration
systems; the district's CIO was fired. The school district decided to reinstate it's
original 25-year old system for at least a year until the bugs were worked out of the
new system by the software vendors.
• In October of 1999 the $125 million NASA Mars Climate Orbiter spacecraft was
believed to be lost in space due to a simple data conversion error. It was determined
that spacecraft software used certain data in English units that should have been in
metric units. Among other tasks, the orbiter was to serve as a communications relay
for the Mars Polar Lander mission, which failed for unknown reasons in December
1999. Several investigating panels were convened to determine the process failures
that allowed the error to go undetected.
• Bugs in software supporting a large commercial high-speed data network affected
70,000 business customers over a period of 8 days in August of 1999. Among those
affected was the electronic trading system of the largest U.S. futures exchange, which
was shut down for most of a week as a result of the outages.
• In April of 1999 a software bug caused the failure of a $1.2 billion U.S. military
satellite launch, the costliest unmanned accident in the history of Cape Canaveral
launches. The failure was the latest in a string of launch failures, triggering a complete
military and industry review of U.S. space launch programs, including software
integration and testing processes. Congressional oversight hearings were requested.
• A small town in Illinois in the U.S. received an unusually large monthly electric bill
of $7 million in March of 1999. This was about 700 times larger than its normal bill. It
turned out to be due to bugs in new software that had been purchased by the local
power company to deal with Y2K software issues.
• In early 1999 a major computer game company recalled all copies of a popular new
product due to software problems. The company made a public apology for releasing a
product before it was ready.
Why is it often hard for management to get serious about quality assurance?
Solving problems is a high-visibility process; preventing problems is low-visibility.
This is illustrated by an old parable:
In ancient China there was a family of healers, one of whom was known throughout
the land and employed as a physician to a great lord. The physician was asked which
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of his family was the most skillful healer. He replied,
"I tend to the sick and dying with drastic and dramatic treatments, and on occasion
someone is cured and my name gets out among the lords."
"My elder brother cures sickness when it just begins to take root, and his skills are
known among the local peasants and neighbors."
"My eldest brother is able to sense the spirit of sickness and eradicate it before it takes
form. His name is unknown outside our home."
Why does software have bugs?
• miscommunication or no communication - as to specifics of what an application
should or shouldn't do (the application's requirements).
• software complexity - the complexity of current software applications can be difficult
to comprehend for anyone without experience in modern-day software development.
Windows-type interfaces, client-server and distributed applications, data
communications, enormous relational databases, and sheer size of applications have all
contributed to the exponential growth in software/system complexity. And the use of
object-oriented techniques can complicate instead of simplify a project unless it is
well-engineered.
• programming errors - programmers, like anyone else, can make mistakes.
• changing requirements (whether documented or undocumented) - the customer may
not understand the effects of changes, or may understand and request them anyway redesign, rescheduling of engineers, effects on other projects, work already completed
that may have to be redone or thrown out, hardware requirements that may be affected,
etc. If there are many minor changes or any major changes, known and unknown
dependencies among parts of the project are likely to interact and cause problems, and
the complexity of coordinating changes may result in errors. Enthusiasm of
engineering staff may be affected. In some fast-changing business environments,
continuously modified requirements may be a fact of life. In this case, management
must understand the resulting risks, and QA and test engineers must adapt and plan for
continuous extensive testing to keep the inevitable bugs from running out of control see 'What can be done if requirements are changing continuously?' in Part 2 of the
FAQ.
• time pressures - scheduling of software projects is difficult at best, often requiring a
lot of guesswork. When deadlines loom and the crunch comes, mistakes will be made.
• egos - people prefer to say things like:
'no problem'
'piece of cake'
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'I can whip that out in a few hours'
'it should be easy to update that old code'
instead of:
'that adds a lot of complexity and we could end up
making a lot of mistakes'
'we have no idea if we can do that; we'll wing it'
'I can't estimate how long it will take, until I
take a close look at it'
'we can't figure out what that old spaghetti code
did in the first place'
If there are too many unrealistic 'no problem's', the result is bugs.
• poorly documented code - it's tough to maintain and modify code that is badly
written or poorly documented; the result is bugs. In many organizations management
provides no incentive for programmers to document their code or write clear,
understandable, maintainable code. In fact, it's usually the opposite: they get points
mostly for quickly turning out code, and there's job security if nobody else can
understand it ('if it was hard to write, it should be hard to read').
• software development tools - visual tools, class libraries, compilers, scripting tools,
etc. often introduce their own bugs or are poorly documented, resulting in added bugs.
How can new Software QA processes be introduced in an existing organization?
• A lot depends on the size of the organization and the risks involved. For large
organizations with high-risk (in terms of lives or property) projects, serious
management buy-in is required and a formalized QA process is necessary.
• Where the risk is lower, management and organizational buy-in and QA
implementation may be a slower, step-at-a-time process. QA processes should be
balanced with productivity so as to keep bureaucracy from getting out of hand.
• For small groups or projects, a more ad-hoc process may be appropriate, depending
on the type of customers and projects. A lot will depend on team leads or managers,
feedback to developers, and ensuring adequate communications among customers,
managers, developers, and testers.
• The most value for effort will be in (a) requirements management processes, with a
goal of clear, complete, testable requirement specifications embodied in requirements
or design documentation and (b) design inspections and code inspections.
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What is verification? validation?
Verification typically involves reviews and meetings to evaluate documents, plans,
code, requirements, and specifications. This can be done with checklists, issues lists,
walkthroughs, and inspection meetings. Validation typically involves actual testing
and takes place after verifications are completed. The term 'IV & V' refers to
Independent Verification and Validation.
What is a 'walkthrough'?
A 'walkthrough' is an informal meeting for evaluation or informational purposes. Little
or no preparation is usually required.
What's an 'inspection'?
An inspection is more formalized than a 'walkthrough', typically with 3-8 people
including a moderator, reader, and a recorder to take notes. The subject of the
inspection is typically a document such as a requirements spec or a test plan, and the
purpose is to find problems and see what's missing, not to fix anything. Attendees
should prepare for this type of meeting by reading thru the document; most problems
will be found during this preparation. The result of the inspection meeting should be a
written report. Thorough preparation for inspections is difficult, painstaking work, but
is one of the most cost effective methods of ensuring quality. Employees who are most
skilled at inspections are like the 'eldest brother' in the parable in 'Why is it often hard
for management to get serious about quality assurance?'. Their skill may have low
visibility but they are extremely valuable to any software development organization,
since bug prevention is far more cost-effective than bug detection.
What kinds of testing should be considered?
• Black box testing - not based on any knowledge of internal design or code. Tests are
based on requirements and functionality.
• White box testing - based on knowledge of the internal logic of an application's code.
Tests are based on coverage of code statements, branches, paths, conditions.
• unit testing - the most 'micro' scale of testing; to test particular functions or code
modules. Typically done by the programmer and not by testers, as it requires detailed
knowledge of the internal program design and code. Not always easily done unless the
application has a well-designed architecture with tight code; may require developing
test driver modules or test harnesses.
• incremental integration testing - continuous testing of an application as new
functionality is added; requires that various aspects of an application's functionality be
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independent enough to work separately before all parts of the program are completed,
or that test drivers be developed as needed; done by programmers or by testers.
• integration testing - testing of combined parts of an application to determine if they
function together correctly. The 'parts' can be code modules, individual applications,
client and server applications on a network, etc. This type of testing is especially
relevant to client/server and distributed systems.
• functional testing - black-box type testing geared to functional requirements of an
application; this type of testing should be done by testers. This doesn't mean that the
programmers shouldn't check that their code works before releasing it (which of
course applies to any stage of testing.)
• system testing - black-box type testing that is based on overall requirements
specifications; covers all combined parts of a system.
• end-to-end testing - similar to system testing; the 'macro' end of the test scale;
involves testing of a complete application environment in a situation that mimics realworld use, such as interacting with a database, using network communications, or
interacting with other hardware, applications, or systems if appropriate.
• sanity testing or smoke testing - typically an initial testing effort to determine if a
new software version is performing well enough to accept it for a major testing effort.
For example, if the new software is crashing systems every 5 minutes, bogging down
systems to a crawl, or corrupting databases, the software may not be in a 'sane' enough
condition to warrant further testing in its current state.
• regression testing - re-testing after fixes or modifications of the software or its
environment. It can be difficult to determine how much re-testing is needed, especially
near the end of the development cycle. Automated testing tools can be especially
useful for this type of testing.
• acceptance testing - final testing based on specifications of the end-user or customer,
or based on use by end-users/customers over some limited period of time.
• load testing - testing an application under heavy loads, such as testing of a web site
under a range of loads to determine at what point the system's response time degrades
or fails.
• stress testing - term often used interchangeably with 'load' and 'performance' testing.
Also used to describe such tests as system functional testing while under unusually
heavy loads, heavy repetition of certain actions or inputs, input of large numerical
values, large complex queries to a database system, etc.
• performance testing - term often used interchangeably with 'stress' and 'load' testing.
Ideally 'performance' testing (and any other 'type' of testing) is defined in requirements
documentation or QA or Test Plans.
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• usability testing - testing for 'user-friendliness'. Clearly this is subjective, and will
depend on the targeted end-user or customer. User interviews, surveys, video
recording of user sessions, and other techniques can be used. Programmers and testers
are usually not appropriate as usability testers.
• install/uninstall testing - testing of full, partial, or upgrade install/uninstall processes.
• recovery testing - testing how well a system recovers from crashes, hardware
failures, or other catastrophic problems.
• security testing - testing how well the system protects against unauthorized internal
or external access, willful damage, etc; may require sophisticated testing techniques.
• compatability testing - testing how well software performs in a particular
hardware/software/operating system/network/etc. environment.
• exploratory testing - often taken to mean a creative, informal software test that is not
based on formal test plans or test cases; testers may be learning the software as they
test it.
• ad-hoc testing - similar to exploratory testing, but often taken to mean that the testers
have significant understanding of the software before testing it.
• user acceptance testing - determining if software is satisfactory to an end-user or
customer.
• comparison testing - comparing software weaknesses and strengths to competing
products.
• alpha testing - testing of an application when development is nearing completion;
minor design changes may still be made as a result of such testing. Typically done by
end-users or others, not by programmers or testers.
• beta testing - testing when development and testing are essentially completed and
final bugs and problems need to be found before final release. Typically done by endusers or others, not by programmers or testers.
• mutation testing - a method for determining if a set of test data or test cases is useful,
by deliberately introducing various code changes ('bugs') and retesting with the
original test data/cases to determine if the 'bugs' are detected. Proper implementation
requires large computational resources.
What are 5 common problems in the software development process?
• poor requirements - if requirements are unclear, incomplete, too general, or not
testable, there will be problems.
• unrealistic schedule - if too much work is crammed in too little time, problems are
inevitable.
• inadequate testing - no one will know whether or not the program is any good until
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the customer complains or systems crash.
• featuritis - requests to pile on new features after development is underway; extremely
common.
• miscommunication - if developers don't know what's needed or customer's have
erroneous expectations, problems are guaranteed.
What are 5 common solutions to software development problems?
• solid requirements - clear, complete, detailed, cohesive, attainable, testable
requirements that are agreed to by all players. Use prototypes to help nail down
requirements.
• realistic schedules - allow adequate time for planning, design, testing, bug fixing, retesting, changes, and documentation; personnel should be able to complete the project
without burning out.
• adequate testing - start testing early on, re-test after fixes or changes, plan for
adequate time for testing and bug-fixing.
• stick to initial requirements as much as possible - be prepared to defend against
changes and additions once development has begun, and be prepared to explain
consequences. If changes are necessary, they should be adequately reflected in related
schedule changes. If possible, use rapid prototyping during the design phase so that
customers can see what to expect. This will provide them a higher comfort level with
their requirements decisions and minimize changes later on.
• communication - require walkthroughs and inspections when appropriate; make
extensive use of group communication tools - e-mail, groupware, networked bugtracking tools and change management tools, intranet capabilities, etc.; insure that
documentation is available and up-to-date - preferably electronic, not paper; promote
teamwork and cooperation; use protoypes early on so that customers' expectations are
clarified.
What is software 'quality'?
Quality software is reasonably bug-free, delivered on time and within budget, meets
requirements and/or expectations, and is maintainable. However, quality is obviously a
subjective term. It will depend on who the 'customer' is and their overall influence in
the scheme of things. A wide-angle view of the 'customers' of a software development
project might include end-users, customer acceptance testers, customer contract
officers, customer management, the development organization's
management/accountants/testers/salespeople, future software maintenance engineers,
stockholders, magazine columnists, etc. Each type of 'customer' will have their own
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slant on 'quality' - the accounting department might define quality in terms of profits
while an end-user might define quality as user-friendly and bug-free.
What is 'good code'?
'Good code' is code that works, is bug free, and is readable and maintainable. Some
organizations have coding 'standards' that all developers are supposed to adhere to, but
everyone has different ideas about what's best, or what is too many or too few rules.
There are also various theories and metrics, such as McCabe Complexity metrics. It
should be kept in mind that excessive use of standards and rules can stifle productivity
and creativity. 'Peer reviews', 'buddy checks' code analysis tools, etc. can be used to
check for problems and enforce standards.
For C and C++ coding, here are some typical ideas to consider in setting
rules/standards; these may or may not apply to a particular situation:
• minimize or eliminate use of global variables.
• use descriptive function and method names - use both upper and lower case, avoid
abbreviations, use as many characters as necessary to be adequately descriptive (use of
more than 20 characters is not out of line); be consistent in naming conventions.
• use descriptive variable names - use both upper and lower case, avoid abbreviations,
use as many characters as necessary to be adequately descriptive (use of more than 20
characters is not out of line); be consistent in naming conventions.
• function and method sizes should be minimized; less than 100 lines of code is good,
less than 50 lines is preferable.
• function descriptions should be clearly spelled out in comments preceding a
function's code.
• organize code for readability.
• use whitespace generously - vertically and horizontally
• each line of code should contain 70 characters max.
• one code statement per line.
• coding style should be consistent throught a program (eg, use of brackets,
indentations, naming conventions, etc.)
• in adding comments, err on the side of too many rather than too few comments; a
common rule of thumb is that there should be at least as many lines of comments
(including header blocks) as lines of code.
• no matter how small, an application should include documentaion of the overall
program function and flow (even a few paragraphs is better than nothing); or if
possible a separate flow chart and detailed program documentation.
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• make extensive use of error handling procedures and status and error logging.
• for C++, to minimize complexity and increase maintainability, avoid too many levels
of inheritance in class heirarchies (relative to the size and complexity of the
application). Minimize use of multiple inheritance, and minimize use of operator
overloading (note that the Java programming language eliminates multiple inheritance
and operator overloading.)
• for C++, keep class methods small, less than 50 lines of code per method is
preferable.
• for C++, make liberal use of exception handlers
What is 'good design'?
'Design' could refer to many things, but often refers to 'functional design' or 'internal
design'. Good internal design is indicated by software code whose overall structure is
clear, understandable, easily modifiable, and maintainable; is robust with sufficient
error-handling and status logging capability; and works correctly when implemented.
Good functional design is indicated by an application whose functionality can be
traced back to customer and end-user requirements. (See further discussion of
functional and internal design in 'What's the big deal about requirements?' in FAQ #2.)
For programs that have a user interface, it's often a good idea to assume that the end
user will have little computer knowledge and may not read a user manual or even the
on-line help; some common rules-of-thumb include:
• the program should act in a way that least surprises the user
• it should always be evident to the user what can be done next and how to exit
• the program shouldn't let the users do something stupid without warning them.
What is SEI? CMM? ISO? IEEE? ANSI? Will it help?
• SEI = 'Software Engineering Institute' at Carnegie-Mellon University; initiated by
the U.S. Defense Department to help improve software development processes.
• CMM = 'Capability Maturity Model', developed by the SEI. It's a model of 5 levels
of organizational 'maturity' that determine effectiveness in delivering quality software.
It is geared to large organizations such as large U.S. Defense Department contractors.
However, many of the QA processes involved are appropriate to any organization, and
if reasonably applied can be helpful. Organizations can receive CMM ratings by
undergoing assessments by qualified auditors.
Level 1 - characterized by chaos, periodic panics, and heroic efforts required by
individuals to successfully complete projects. Few if any processes in place; successes
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may not be repeatable.
Level 2 - software project tracking, requirements management, realistic planning, and
configuration management processes are in place; successful practices can be
repeated.
Level 3 - standard software development and maintenance processes are integrated
throughout an organization; a Software Engineering Process Group is is in place to
oversee software processes, and training programs are used to ensure understanding
and compliance.
Level 4 - metrics are used to track productivity, processes, and products. Project
performance is predictable, and quality is consistently high.
Level 5 - the focus is on continouous process improvement. The impact of new
processes and technologies can be predicted and effectively implemented when
required.
Perspective on CMM ratings: During 1997-2001, 1018 organizations were assessed.
Of those, 27% were rated at Level 1, 39% at 2, 23% at 3, 6% at 4, and 5% at 5. (For
ratings during the period 1992-96, 62% were at Level 1, 23% at 2, 13% at 3, 2% at 4,
and
0.4% at 5.) The median size of organizations was 100 software
engineering/maintenance personnel; 32% of organizations were U.S. federal
contractors or agencies. For those rated at
Level 1, the most problematical key process area was in Software Quality Assurance.
• ISO = 'International Organisation for Standardization' - The ISO 9001:2000 standard
(which replaces the previous standard of 1994) concerns quality systems that are
assessed by outside auditors, and it applies to many kinds of production and
manufacturing organizations, not just software. It covers documentation, design,
development, production, testing, installation, servicing, and other processes. The full
set of standards consists of: (a)Q9001-2000 - Quality Management Systems:
Requirements; (b)Q9000-2000 - Quality Management Systems: Fundamentals and
Vocabulary; (c)Q9004-2000 - Quality Management Systems: Guidelines for
Performance Improvements. To be ISO 9001 certified, a third-party auditor assesses
an organization, and certification is typically good for about 3 years, after which a
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complete reassessment is required. Note that ISO certification does not necessarily
indicate quality products - it indicates only that documented processes are followed.
Also see http://www.iso.ch/ for the latest information. In the U.S. the standards can be
purchased via the ASQ web site at http://e-standards.asq.org/
• IEEE = 'Institute of Electrical and Electronics Engineers' - among other things,
creates standards such as 'IEEE Standard for Software Test Documentation'
(IEEE/ANSI Standard 829), 'IEEE Standard of Software Unit Testing (IEEE/ANSI
Standard 1008), 'IEEE Standard for Software Quality Assurance Plans' (IEEE/ANSI
Standard 730), and others.
• ANSI = 'American National Standards Institute', the primary industrial standards
body in the U.S.; publishes some software-related standards in conjunction with the
IEEE and ASQ (American Society for Quality).
• Other software development process assessment methods besides CMM and ISO
9000 include SPICE, Trillium, TickIT. and Bootstrap.
What is the 'software life cycle'?
The life cycle begins when an application is first conceived and ends when it is no
longer in use. It includes aspects such as initial concept, requirements analysis,
functional design, internal design, documentation planning, test planning, coding,
document preparation, integration, testing, maintenance, updates, retesting, phase-out,
and other aspects.
Will automated testing tools make testing easier?
• Possibly. For small projects, the time needed to learn and implement them may not
be worth it. For larger projects, or on-going long-term projects they can be valuable.
• A common type of automated tool is the 'record/playback' type. For example, a tester
could click through all combinations of menu choices, dialog box choices, buttons, etc.
in an application GUI and have them 'recorded' and the results logged by a tool. The
'recording' is typically in the form of text based on a scripting language that is
interpretable by the testing tool. If new buttons are added, or some underlying code in
the application is changed, etc. the application might then be retested by just 'playing
back' the 'recorded' actions, and comparing the logging results to check effects of the
changes. The problem with such tools is that if there are continual changes to the
system being tested, the 'recordings' may have to be changed so much that it becomes
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very time-consuming to continuously update the scripts. Additionally, interpretation
and analysis of results (screens, data, logs, etc.) can be a difficult task. Note that there
are record/playback tools for text-based interfaces also, and for all types of platforms.
• Other automated tools can include:
code analyzers - monitor code complexity, adherence to standards, etc.
coverage analyzers - these tools check which parts of the code have been exercised by
a test, and may be oriented to code statement coverage, condition coverage, path
coverage, etc.
memory analyzers - such as bounds-checkers and leak detectors.
load/performance test tools - for testing client/server and web applications under
various load
levels.
web test tools - to check that links are valid, HTML code usage is correct, client-side
and server-side programs work, a web site's interactions are secure.
other tools - for test case management, documentation management, bug reporting,
and configuration management.
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Microprocessor
Interview Questions &
Answers
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Microprocessor Interview Questions and Answers
What is a Microprocessor?
Microprocessor is a program-controlled device, which fetches the instructions from
memory, decodes and executes the instructions. Most Micro Processor are single- chip
devices.
What are the flags in 8086?
In 8086 Carry flag, Parity flag, Auxiliary carry flag, Zero flag, Overflow flag, Trace
flag, Interrupt flag, Direction flag, and Sign flag.
Why crystal is a preferred clock source?
Because of high stability, large Q (Quality Factor) & the frequency that doesn‘t drift
with aging. Crystal is used as a clock source most of the times.
In 8085 which is called as High order / Low order Register?
Flag is called as Low order register & Accumulator is called as High order Register.
What is Tri-state logic?
Three Logic Levels are used and they are High, Low, High impedance state. The high
and low are normal logic levels & high impedance state is electrical open circuit
conditions. Tri-state logic has a third line called enable line.
What happens when HLT instruction is executed in processor?
The Micro Processor enters into Halt-State and the buses are tri-stated.
Which Stack is used in 8085?
LIFO (Last In First Out) stack is used in 8085.In this type of Stack the last stored
information can be retrieved first
What is Program counter?
Program counter holds the address of either the first byte of the next instruction to be
fetched for execution or the address of the next byte of a multi byte instruction, which
has not been completely fetched. In both the cases it gets incremented automatically
one by one as the instruction bytes get fetched. Also Program register keeps the
address of the next instruction.
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What are the various registers in 8085?
Accumulator register, Temporary register, Instruction register, Stack Pointer, Program
Counter are the various registers in 8085
What is 1st / 2nd / 3rd / 4th generation processor?
The processor made of PMOS / NMOS / HMOS / HCMOS technology is called 1st /
2nd / 3rd / 4th generation processor, and it is made up of 4 / 8 / 16 / 32 bits.
Name the processor lines of two major manufacturers?
High-end: Intel - Pentium (II, III, 4), AMD - Athlon. Low-end: Intel - Celeron, AMD Duron. 64-bit: Intel - Itanium 2, AMD - Opteron.
What‟s the speed and device maximum specs for Firewire?
IEEE 1394 (Firewire) supports the maximum of 63 connected devices with speeds up
to 400 Mbps. Where‘s MBR located on the disk? Main Boot Record is located in
sector 0, track 0, head 0, cylinder 0 of the primary active partition.
Where does CPU Enhanced mode originate from?
Intel‘s 80386 was the first 32-bit processor, and since the company had to backwardsupport the 8086. All the modern Intel-based processors run in the Enhanced mode,
capable of switching between Real mode (just like the real 8086) and Protected mode,
which is the current mode of operation.
How many bit combinations are there in a byte?
Byte contains 8 combinations of bits.
Have you studied buses? What types?
There are three types of buses.
Address bus: This is used to carry the Address to the memory to fetch either
Instruction or Data.
Data bus : This is used to carry the Data from the memory.
Control bus : This is used to carry the Control signals like RD/WR, Select etc.
What is the Maximum clock frequency in 8086?
5 Mhz is the Maximum clock frequency in 8086.
What is meant by Maskable interrupts?
An interrupt that can be turned off by the programmer is known as Maskable interrupt.
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What is Non-Maskable interrupts?
An interrupt which can be never be turned off (ie. disabled) is known as NonMaskable interrupt
What are the different functional units in 8086?
Bus Interface Unit and Execution unit, are the two different functional units in 8086.
What are the various segment registers in 8086?
Code, Data, Stack, Extra Segment registers in 8086.
What does EU do?
Execution Unit receives program instruction codes and data from BIU, executes these
instructions and store the result in general registers.
Which Stack is used in 8086? k is used in 8086?
FIFO (First In First Out) stack is used in 8086.In this type of Stack the first stored
information is retrieved first.
What are the flags in 8086?
In 8086 Carry flag, Parity flag, Auxiliary carry flag, Zero flag, Overflow flag, Trace
flag, Interrupt flag, Direction flag, and Sign flag.
What is SIM and RIM instructions?
SIM is Set Interrupt Mask. Used to mask the hardware interrupts.
RIM is Read Interrupt Mask. Used to check whether the interrupt is Masked or not.
What is the difference between 8086 and 8088?
The BIU in 8088 is 8-bit data bus & 16- bit in 8086.Instruction queue is 4 byte long in
8088and 6 byte in 8086.
Give example for Non-Maskable interrupts?
Trap is known as Non-Maskable interrupts, which is used in emergency condition.
Give examples for Micro controller?
Z80, Intel MSC51 &96, Motorola are the best examples of Microcontroller.
What is clock frequency for 8085?
3 MHz is the maximum clock frequency for 8085.
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Give an example of one address microprocessor?
8085 is a one address microprocessor.
Give examples for 8 / 16 / 32 bit Microprocessor?
8-bit Processor - 8085 / Z80 / 6800; 16-bit Processor - 8086 / 68000 / Z8000; 32-bit
Processor - 80386 / 80486
What is meant by a bus?
A bus is a group of conducting lines that carriers data, address, & control signals.
What are the various registers in 8085?
Accumulator register, Temporary register, Instruction register, Stack Pointer, Program
Counter are the various registers in 8085
Why crystal is a preferred clock source?
Because of high stability, large Q (Quality Factor) & the frequency that doesn‘t drift
with aging. Crystal is used as a clock source most of the times.
In 8085 which is called as High order / Low order Register?
Flag is called as Low order register & Accumulator is called as High order Register.
Name 5 different addressing modes?
Immediate, Direct, Register, Register indirect, Implied addressing modes
In what way interrupts are classified in 8085?
In 8085 the interrupts are classified as Hardware and Software interrupts.
What is the difference between primary & secondary storage device?
In primary storage device the storage capacity is limited. It has a volatile memory. In
secondary storage device the storage capacity is larger. It is a nonvolatile memory.
Primary devices are: RAM / ROM. Secondary devices are: Floppy disc / Hard disk.
Which Stack is used in 8085?
LIFO (Last In First Out) stack is used in 8085.In this type of Stack the last stored
information can be retrieved first.
What is Program counter?
Program counter holds the address of either the first byte of the next instruction to be
fetched for execution or the address of the next byte of a multi byte instruction, which
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has not been completely fetched. In both the cases it gets incremented automatically
one by one as the instruction bytes get fetched. Also Program register keeps the
address of the next instruction.
What is the RST for the TRAP?
RST 4.5 is called as TRAP.
What are level-triggering interrupt?
RST 6.5 & RST 5.5 are level-triggering interrupts.
Which interrupt is not level-sensitive in 8085?
RST 7.5 is a raising edge-triggering interrupt.
What are Software interrupts?
RST0, RST1, RST2, RST3, RST4, RST5, RST6, RST7.
What are the various flags used in 8085?
Sign flag, Zero flag, Auxiliary flag, Parity flag, Carry flag.
In 8085 name the 16 bit registers?
Stack pointer and Program counter all have 16 bits.
What is Stack Pointer?
Stack pointer is a special purpose 16-bit register in the Microprocessor, which holds
the address of the top of the stack.
What happens when HLT instruction is executed in processor?
The Micro Processor enters into Halt-State and the buses are tri-stated.
What does Quality factor mean?
The Quality factor is also defined, as Q. So it is a number, which reflects the lossness
of a circuit. Higher the Q, the lower are the losses.
How many interrupts are there in 8085?
There are 12 interrupts in 8085.
What is Tri-state logic?
Three Logic Levels are used and they are High, Low, High impedance state. The high
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and low are normal logic levels & high impedance state is electrical open circuit
conditions. Tri-state logic has a third line called enable line.
Which interrupt has the highest priority?
TRAP has the highest priority
What are Hardware interrupts?
TRAP, RST7.5, RST6.5, RST5.5, INTR
Can an RC circuit be used as clock source for 8085?
Yes, it can be used, if an accurate clock frequency is not required. Also, the
component cost is low compared to LC or Crystal
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Oracle
Interview Questions
&
Answers
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347
Oracle Interview Questions and Answers
What are the components of physical database structure of Oracle database?
Oracle database is comprised of three types of files. One or more datafiles, two are
more redo log files, and one or more control files.
What are the components of logical database structure of Oracle database?
There are tablespaces and database's schema objects.
What is a tablespace?
A database is divided into Logical Storage Unit called tablespaces. A tablespace is
used to grouped related logical structures together.
What is SYSTEM tablespace and when is it created?
Every Oracle database contains a tablespace named SYSTEM, which is automatically
created when the database is created. The SYSTEM tablespace always contains the
data dictionary tables for the entire database.
Explain the relationship among database, tablespace and data file ?
Each databases logically divided into one or more tablespaces one or more data files
are explicitly created for each tablespace.
What is schema?
A schema is collection of database objects of a user.
What are Schema Objects?
Schema objects are the logical structures that directly refer to the database's data.
Schema objects include tables, views, sequences, synonyms, indexes, clusters,
database triggers, procedures, functions packages and database links.
Can objects of the same schema reside in different tablespaces?
Yes.
Can a tablespace hold objects from different schemes?
Yes.
What is Oracle table?
A table is the basic unit of data storage in an Oracle database. The tables of a database
hold all of the user accessible data. Table data is stored in rows and columns.
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What is an Oracle view?
A view is a virtual table. Every view has a query attached to it. (The query is a
SELECT statement that identifies the columns and rows of the table(s) the view uses.)
What is Partial Backup ?
A Partial Backup is any operating system backup short of a full backup, taken while
the database is open or shut down.
What is Mirrored on-line Redo Log ?
A mirrored on-line redo log consists of copies of on-line redo log files physically
located on separate disks, changes made to one member of the group are made to all
members.
What is Full Backup ?
A full backup is an operating system backup of all data files, on-line redo log files and
control file that constitute ORACLE database and the parameter.
Can a View based on another View ?
Yes.
Can a Tablespace hold objects from different Schemes ?
Yes.
Can objects of the same Schema reside in different tablespace ?
Yes.
What is the use of Control File ?
When an instance of an ORACLE database is started, its control file is used to identify
the database and redo log files that must be opened for database operation to proceed.
It is also used in database recovery.
Do View contain Data ?
Views do not contain or store data.
What are the Referential actions supported by FOREIGN KEY integrity
constraint ?
UPDATE and DELETE Restrict - A referential integrity rule that disallows the update
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or deletion of referenced data. DELETE Cascade - When a referenced row is deleted
all associated dependent rows are deleted.
What are the type of Synonyms?
There are two types of Synonyms Private and Public.
What is a Redo Log ?
The set of Redo Log files YSDATE,UID,USER or USERENV SQL functions, or the
pseudo columns LEVEL or ROWNUM.
What is an Index Segment ?
Each Index has an Index segment that stores all of its data.
Explain the relationship among Database, Tablespace and Data file?
Each databases logically divided into one or more tablespaces one or more data files
are explicitly created for each tablespace
What are the different type of Segments ?
Data Segment, Index Segment, Rollback Segment and Temporary Segment.
What are Clusters ?
Clusters are groups of one or more tables physically stores together to share common
columns and are often used together.
What is an Integrity Constrains ?
An integrity constraint is a declarative way to define a business rule for a column of a
table.
What is an Index ?
An Index is an optional structure associated with a table to have direct access to rows,
which can be created to increase the performance of data retrieval. Index can be
created on one or more columns of a table.
What is an Extent ?
An Extent is a specific number of contiguous data blocks, obtained in a single
allocation, and used to store a specific type of information.
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What is a View ?
A view is a virtual table. Every view has a Query attached to it. (The Query is a
SELECT statement that identifies the columns and rows of the table(s) the view uses.)
What is Table ?
A table is the basic unit of data storage in an ORACLE database. The tables of a
database hold all of the user accessible data. Table data is stored in rows and columns.
Can a view based on another view?
Yes.
What are the advantages of views?
- Provide an additional level of table security, by restricting access to a predetermined
set of rows and columns of a table.
- Hide data complexity.
- Simplify commands for the user.
- Present the data in a different perspective from that of the base table.
- Store complex queries.
What is an Oracle sequence?
A sequence generates a serial list of unique numbers for numerical columns of a
database's tables.
What is a synonym?
A synonym is an alias for a table, view, sequence or program unit.
What are the types of synonyms?
There are two types of synonyms private and public.
What is a private synonym?
Only its owner can access a private synonym.
What is a public synonym?
Any database user can access a public synonym.
What are synonyms used for?
- Mask the real name and owner of an object.
- Provide public access to an object
- Provide location transparency for tables, views or program units of a remote
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database.
- Simplify the SQL statements for database users.
What is an Oracle index?
An index is an optional structure associated with a table to have direct access to rows,
which can be created to increase the performance of data retrieval. Index can be
created on one or more columns of a table.
How are the index updates?
Indexes are automatically maintained and used by Oracle. Changes to table data are
automatically incorporated into all relevant indexes.
What is a Tablespace?
A database is divided into Logical Storage Unit called tablespace. A tablespace is used
to grouped related logical structures together
What is Rollback Segment ?
A Database contains one or more Rollback Segments to temporarily store "undo"
information.
What are the Characteristics of Data Files ?
A data file can be associated with only one database. Once created a data file can't
change size. One or more data files form a logical unit of database storage called a
tablespace.
How to define Data Block size ?
A data block size is specified for each ORACLE database when the database is
created. A database users and allocated free database space in ORACLE data blocks.
Block size is specified in INIT.ORA file and can‘t be changed latter.
What does a Control file Contain ?
A Control file records the physical structure of the database. It contains the following
information.
Database Name
Names and locations of a database's files and redolog files.
Time stamp of database creation.
What is difference between UNIQUE constraint and PRIMARY KEY constraint
?
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A column defined as UNIQUE can contain Nulls while a column defined as
PRIMARY KEY can't contain Nulls.
What is Index Cluster ?
A Cluster with an index on the Cluster Key
When does a Transaction end ?
When it is committed or Rollbacked.
What is the effect of setting the value "ALL_ROWS" for OPTIMIZER_GOAL
parameter of the ALTER SESSION command ? What are the factors that affect
OPTIMIZER in choosing an Optimization approach ?
Answer The OPTIMIZER_MODE initialization parameter Statistics in the Data
Dictionary the OPTIMIZER_GOAL parameter of the ALTER SESSION command
hints in the statement.
What is the effect of setting the value "CHOOSE" for OPTIMIZER_GOAL,
parameter of the ALTER SESSION Command ?
The Optimizer chooses Cost_based approach and optimizes with the goal of best
throughput if statistics for atleast one of the tables accessed by the SQL statement exist
in the data dictionary. Otherwise the OPTIMIZER chooses RULE_based approach.
How does one create a new database? (for DBA)
One can create and modify Oracle databases using the Oracle "dbca" (Database
Configuration Assistant) utility. The dbca utility is located in the
$ORACLE_HOME/bin directory. The Oracle Universal Installer (oui) normally starts
it after installing the database server software.
One can also create databases manually using scripts. This option, however, is falling
out of fashion, as it is quite involved and error prone. Look at this example for creating
and Oracle 9i database:
CONNECT SYS AS SYSDBA
ALTER SYSTEM SET DB_CREATE_FILE_DEST='/u01/oradata/';
ALTER SYSTEM SET DB_CREATE_ONLINE_LOG_DEST_1='/u02/oradata/';
ALTER SYSTEM SET DB_CREATE_ONLINE_LOG_DEST_2='/u03/oradata/';
CREATE DATABASE;
What database block size should I use? (for DBA)
Oracle recommends that your database block size match, or be multiples of your
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operating system block size. One can use smaller block sizes, but the performance cost
is significant. Your choice should depend on the type of application you are running. If
you have many small transactions as with OLTP, use a smaller block size. With fewer
but larger transactions, as with a DSS application, use a larger block size. If you are
using a volume manager, consider your "operating system block size" to be 8K. This is
because volume manager products use 8K blocks (and this is not configurable).
What are the different approaches used by Optimizer in choosing an execution
plan ?
Rule-based and Cost-based.
What does ROLLBACK do ?
ROLLBACK retracts any of the changes resulting from the SQL statements in the
transaction.
How does one coalesce free space ? (for DBA)
SMON coalesces free space (extents) into larger, contiguous extents every 2 hours and
even then, only for a short period of time.
SMON will not coalesce free space if a tablespace's default storage parameter
"pctincrease" is set to 0. With Oracle 7.3 one can manually coalesce a tablespace using
the ALTER TABLESPACE ... COALESCE; command, until then use:
SQL> alter session set events 'immediate trace name coalesce level n';
Where 'n' is the tablespace number you get from SELECT TS#, NAME FROM
SYS.TS$;
You can get status information about this process by selecting from the
SYS.DBA_FREE_SPACE_COALESCED dictionary view.
How does one prevent tablespace fragmentation? (for DBA)
Always set PCTINCREASE to 0 or 100.
Bizarre values for PCTINCREASE will contribute to fragmentation. For example if
you set PCTINCREASE to 1 you will see that your extents are going to have weird
and wacky sizes: 100K, 100K, 101K, 102K, etc. Such extents of bizarre size are rarely
re-used in their entirety. PCTINCREASE of 0 or 100 gives you nice round extent sizes
that can easily be reused. E.g.. 100K, 100K, 200K, 400K, etc.
Use the same extent size for all the segments in a given tablespace. Locally Managed
tablespaces (available from 8i onwards) with uniform extent sizes virtually eliminates
any tablespace fragmentation. Note that the number of extents per segment does not
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cause any performance issue anymore, unless they run into thousands and thousands
where additional I/O may be required to fetch the additional blocks where extent maps
of the segment are stored.
Where can one find the high water mark for a table? (for DBA)
There is no single system table, which contains the high water mark (HWM) for a
table. A table's HWM can be calculated using the results from the following SQL
statements:
SELECT BLOCKS
FROM DBA_SEGMENTS
WHERE OWNER=UPPER(owner) AND SEGMENT_NAME = UPPER(table);
ANALYZE TABLE owner.table ESTIMATE STATISTICS;
SELECT EMPTY_BLOCKS
FROM DBA_TABLES
WHERE OWNER=UPPER(owner) AND SEGMENT_NAME = UPPER(table);
Thus, the tables' HWM = (query result 1) - (query result 2) - 1
NOTE: You can also use the DBMS_SPACE package and calculate the HWM =
TOTAL_BLOCKS - UNUSED_BLOCKS - 1.
What is COST-based approach to optimization ?
Considering available access paths and determining the most efficient execution plan
based on statistics in the data dictionary for the tables accessed by the statement and
their associated clusters and indexes.
What does COMMIT do ?
COMMIT makes permanent the changes resulting from all SQL statements in the
transaction. The changes made by the SQL statements of a transaction become visible
to other user sessions transactions that start only after transaction is committed.
How are extents allocated to a segment? (for DBA)
Oracle8 and above rounds off extents to a multiple of 5 blocks when more than 5
blocks are requested. If one requests 16K or 2 blocks (assuming a 8K block size),
Oracle doesn't round it up to 5 blocks, but it allocates 2 blocks or 16K as requested. If
one asks for 8 blocks, Oracle will round it up to 10 blocks.
Space allocation also depends upon the size of contiguous free space available. If one
asks for 8 blocks and Oracle finds a contiguous free space that is exactly 8 blocks, it
would give it you. If it were 9 blocks, Oracle would also give it to you. Clearly Oracle
doesn't always round extents to a multiple of 5 blocks.
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The exception to this rule is locally managed tablespaces. If a tablespace is created
with local extent management and the extent size is 64K, then Oracle allocates 64K or
8 blocks assuming 8K-block size. Oracle doesn't round it up to the multiple of 5 when
a tablespace is locally managed.
Can one rename a database user (schema)? (for DBA)
No, this is listed as Enhancement Request 158508. Workaround:
Do a user-level export of user A
create new user B
Import system/manager fromuser=A touser=B
Drop user A
Define Transaction ?
A Transaction is a logical unit of work that comprises one or more SQL statements
executed by a single user.
What is Read-Only Transaction ?
A Read-Only transaction ensures that the results of each query executed in the
transaction are consistant with respect to the same point in time.
What is a deadlock ? Explain .
Two processes wating to update the rows of a table which are locked by the other
process then deadlock arises. In a database environment this will often happen because
of not issuing proper row lock commands. Poor design of front-end application may
cause this situation and the performance of server will reduce drastically.
These locks will be released automatically when a commit/rollback operation
performed or any one of this processes being killed externally.
What is a Schema ?
The set of objects owned by user account is called the schema.
What is a cluster Key ?
The related columns of the tables are called the cluster key. The cluster key is indexed
using a cluster index and its value is stored only once for multiple tables in the cluster.
What is Parallel Server ?
Multiple instances accessing the same database (Only In Multi-CPU environments)
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What are the basic element of Base configuration of an oracle Database ?
It consists of
one or more data files.
one or more control files.
two or more redo log files.
The Database contains
multiple users/schemas
one or more rollback segments
one or more tablespaces
Data dictionary tables
User objects (table,indexes,views etc.,)
The server that access the database consists of
SGA (Database buffer, Dictionary Cache Buffers, Redo log buffers, Shared SQL pool)
SMON (System MONito)
PMON (Process MONitor)
LGWR (LoG Write)
DBWR (Data Base Write)
ARCH (ARCHiver)
CKPT (Check Point)
RECO
Dispatcher
User Process with associated PGS
What is clusters ?
Group of tables physically stored together because they share common columns and
are often used together is called Cluster.
What is an Index ? How it is implemented in Oracle Database ?
An index is a database structure used by the server to have direct access of a row in a
table. An index is automatically created when a unique of primary key constraint
clause is specified in create table comman (Ver 7.0)
What is a Database instance ? Explain
A database instance (Server) is a set of memory structure and background processes
that access a set of database files.
The process can be shared by all users. The memory structure that are used to store
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most queried data from database. This helps up to improve database performance by
decreasing the amount of I/O performed against data file.
What is the use of ANALYZE command ?
To perform one of these function on an index, table, or cluster:
- To collect statistics about object used by the optimizer and store them in the data
dictionary.
- To delete statistics about the object used by object from the data dictionary.
- To validate the structure of the object.
- To identify migrated and chained rows of the table or cluster.
What is default tablespace ?
The Tablespace to contain schema objects created without specifying a tablespace
name.
What are the system resources that can be controlled through Profile ?
The number of concurrent sessions the user can establish the CPU processing time
available to the user's session the CPU processing time available to a single call to
ORACLE made by a SQL statement the amount of logical I/O available to the user's
session the amout of logical I/O available to a single call to ORACLE made by a SQL
statement the allowed amount of idle time for the user's session the allowed amount of
connect time for the user's session.
What is Tablespace Quota ?
The collective amount of disk space available to the objects in a schema on a
particular tablespace.
What are the different Levels of Auditing ?
Statement Auditing, Privilege Auditing and Object Auditing.
What is Statement Auditing ?
Statement auditing is the auditing of the powerful system privileges without regard to
specifically named objects.
What are the database administrators utilities available ?
SQL * DBA - This allows DBA to monitor and control an ORACLE database. SQL *
Loader - It loads data from standard operating system files (Flat files) into ORACLE
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database tables. Export (EXP) and Import (imp) utilities allow you to move existing
data in ORACLE format to and from ORACLE database.
How can you enable automatic archiving ?
Shut the database
Backup the database
Modify/Include LOG_ARCHIVE_START_TRUE in init.ora file.
Start up the database.
What are roles? How can we implement roles ?
Roles are the easiest way to grant and manage common privileges needed by different
groups of database users. Creating roles and assigning provides to roles. Assign each
role to group of users. This will simplify the job of assigning privileges to individual
users.
What are Roles ?
Roles are named groups of related privileges that are granted to users or other roles.
What are the use of Roles ?
REDUCED GRANTING OF PRIVILEGES - Rather than explicitly granting the same
set of privileges to many users a database administrator can grant the privileges for a
group of related users granted to a role and then grant only the role to each member of
the group.
DYNAMIC PRIVILEGE MANAGEMENT - When the privileges of a group must
change, only the privileges of the role need to be modified. The security domains of all
users granted the group's role automatically reflect the changes made to the role.
SELECTIVE AVAILABILITY OF PRIVILEGES - The roles granted to a user can be
selectively enable (available for use) or disabled (not available for use). This allows
specific control of a user's privileges in any given situation.
APPLICATION AWARENESS - A database application can be designed to
automatically enable and disable selective roles when a user attempts to use the
application.
What is Privilege Auditing ?
Privilege auditing is the auditing of the use of powerful system privileges without
regard to specifically named objects.
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What is Object Auditing ?
Object auditing is the auditing of accesses to specific schema objects without regard to
user.
What is Auditing ?
Monitoring of user access to aid in the investigation of database use.
How does one see the uptime for a database? (for DBA )
Look at the following SQL query:
SELECT to_char (startup_time,'DD-MON-YYYY HH24: MI: SS') "DB Startup Time"
FROM sys.v_$instance;
Marco Bergman provided the following alternative solution:
SELECT to_char (logon_time,'Dy dd Mon HH24: MI: SS') "DB Startup Time"
FROM sys.v_$session
WHERE Sid=1 /* this is pmon */
/
Users still running on Oracle 7 can try one of the following queries:
Column STARTED format a18 head 'STARTUP TIME'
Select C.INSTANCE,
to_date (JUL.VALUE, 'J')
|| to_char (floor (SEC.VALUE/3600), '09')
|| ':'
-- || Substr (to_char (mod (SEC.VALUE/60, 60), '09'), 2, 2)
|| Substr (to_char (floor (mod (SEC.VALUE/60, 60)), '09'), 2, 2)
|| '.'
|| Substr (to_char (mod (SEC.VALUE, 60), '09'), 2, 2) STARTED
from SYS.V_$INSTANCE JUL,
SYS.V_$INSTANCE SEC,
SYS.V_$THREAD C
Where JUL.KEY like '%JULIAN%'
and SEC.KEY like '%SECOND%';
Select to_date (JUL.VALUE, 'J')
|| to_char (to_date (SEC.VALUE, 'SSSSS'), ' HH24:MI:SS') STARTED
from SYS.V_$INSTANCE JUL,
SYS.V_$INSTANCE SEC
where JUL.KEY like '%JULIAN%'
and SEC.KEY like '%SECOND%';
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select to_char (to_date (JUL.VALUE, 'J') + (SEC.VALUE/86400), -Return a DATE
'DD-MON-YY HH24:MI:SS') STARTED
from V$INSTANCE JUL,
V$INSTANCE SEC
where JUL.KEY like '%JULIAN%'
and SEC.KEY like '%SECOND%';
Where are my TEMPFILES, I don't see them in V$DATAFILE or
DBA_DATA_FILE? (for DBA )
Tempfiles, unlike normal datafiles, are not listed in v$datafile or dba_data_files.
Instead query v$tempfile or dba_temp_files:
SELECT * FROM v$tempfile;
SELECT * FROM dba_temp_files;
How do I find used/free space in a TEMPORARY tablespace? (for DBA )
Unlike normal tablespaces, true temporary tablespace information is not listed in
DBA_FREE_SPACE. Instead use the V$TEMP_SPACE_HEADER view:
SELECT tablespace_name, SUM (bytes used), SUM (bytes free)
FROM V$temp_space_header
GROUP BY tablespace_name;
What is a profile ?
Each database user is assigned a Profile that specifies limitations on various system
resources available to the user.
How will you enforce security using stored procedures?
Don't grant user access directly to tables within the application. Instead grant the
ability to access the procedures that access the tables. When procedure executed it will
execute the privilege of procedures owner. Users cannot access tables except via the
procedure.
How can one see who is using a temporary segment? (for DBA )
For every user using temporary space, there is an entry in SYS.V$_LOCK with type
'TS'.
All temporary segments are named 'ffff.bbbb' where 'ffff' is the file it is in and 'bbbb' is
first block of the segment. If your temporary tablespace is set to TEMPORARY, all
sorts are done in one large temporary segment. For usage stats, see
SYS.V_$SORT_SEGMENT
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From Oracle 8.0, one can just query SYS.v$sort_usage. Look at these examples:
select s.username, u."USER", u.tablespace, u.contents, u.extents, u.blocks
from sys.v_$session s, sys.v_$sort_usage u
where s.addr = u.session_addr
/
select s.osuser, s.process, s.username, s.serial#,
Sum (u.blocks)*vp.value/1024 sort_size
from sys.v_$session s, sys.v_$sort_usage u, sys.v_$parameter VP
where s.saddr = u.session_addr
and vp.name = 'db_block_size'
and s.osuser like '&1'
group by s.osuser, s.process, s.username, s.serial#, vp.value
/
How does one get the view definition of fixed views/tables?
Query v$fixed_view_definition. Example: SELECT * FROM
v$fixed_view_definition WHERE view_name='V$SESSION';
What are the dictionary tables used to monitor a database spaces ?
DBA_FREE_SPACE
DBA_SEGMENTS
DBA_DATA_FILES.
How can we specify the Archived log file name format and destination?
By setting the following values in init.ora file. LOG_ARCHIVE_FORMAT = arch
%S/s/T/tarc (%S - Log sequence number and is zero left paded, %s - Log sequence
number not padded. %T - Thread number lef-zero-paded and %t - Thread number not
padded). The file name created is arch 0001 are if %S is used.
LOG_ARCHIVE_DEST = path.
What is user Account in Oracle database?
An user account is not a physical structure in Database but it is having important
relationship to the objects in the database and will be having certain privileges.
When will the data in the snapshot log be used?
We must be able to create a after row trigger on table (i.e., it should be not be already
available) After giving table privileges. We cannot specify snapshot log name because
oracle uses the name of the master table in the name of the database objects that
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support its snapshot log. The master table name should be less than or equal to 23
characters. (The table name created will be MLOGS_tablename, and trigger name will
be TLOGS name).
What dynamic data replication?
Updating or Inserting records in remote database through database triggers. It may fail
if remote database is having any problem.
What is Two-Phase Commit ?
Two-phase commit is mechanism that guarantees a distributed transaction either
commits on all involved nodes or rolls back on all involved nodes to maintain data
consistency across the global distributed database. It has two phase, a Prepare Phase
and a Commit Phase.
How can you Enforce Referential Integrity in snapshots ?
Time the references to occur when master tables are not in use. Peform the reference
the manually immdiately locking the master tables. We can join tables in snopshots by
creating a complex snapshots that will based on the master tables.
What is a SQL * NET?
SQL *NET is ORACLE's mechanism for interfacing with the communication
protocols used by the networks that facilitate distributed processing and distributed
databases. It is used in Clint-Server and Server-Server communications.
What is a SNAPSHOT ?
Snapshots are read-only copies of a master table located on a remote node which is
periodically refreshed to reflect changes made to the master table.
What is the mechanism provided by ORACLE for table replication ?
Snapshots and SNAPSHOT LOGs
What is snapshots?
Snapshot is an object used to dynamically replicate data between distribute database at
specified time intervals. In ver 7.0 they are read only.
What are the various type of snapshots?
Simple and Complex.
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Describe two phases of Two-phase commit ?
Prepare phase - The global coordinator (initiating node) ask a participants to prepare
(to promise to commit or rollback the transaction, even if there is a failure) Commit Phase - If all participants respond to the coordinator that they are prepared, the
coordinator asks all nodes to commit the transaction, if all participants cannot prepare,
the coordinator asks all nodes to roll back the transaction.
What is snapshot log ?
It is a table that maintains a record of modifications to the master table in a snapshot. It
is stored in the same database as master table and is only available for simple
snapshots. It should be created before creating snapshots.
What are the benefits of distributed options in databases?
Database on other servers can be updated and those transactions can be grouped
together with others in a logical unit.
Database uses a two phase commit.
What are the options available to refresh snapshots ?
COMPLETE - Tables are completely regenerated using the snapshots query and the
master tables every time the snapshot referenced.
FAST - If simple snapshot used then a snapshot log can be used to send the changes to
the snapshot tables.
FORCE - Default value. If possible it performs a FAST refresh; Otherwise it will
perform a complete refresh.
What is a SNAPSHOT LOG ?
A snapshot log is a table in the master database that is associated with the master table.
ORACLE uses a snapshot log to track the rows that have been updated in the master
table. Snapshot logs are used in updating the snapshots based on the master table.
What is Distributed database ?
A distributed database is a network of databases managed by multiple database servers
that appears to a user as single logical database. The data of all databases in the
distributed database can be simultaneously accessed and modified.
How can we reduce the network traffic?
- Replication of data in distributed environment.
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- Using snapshots to replicate data.
- Using remote procedure calls.
Differentiate simple and complex, snapshots ?
- A simple snapshot is based on a query that does not contains GROUP BY clauses,
CONNECT BY clauses, JOINs, sub-query or snashot of operations.
- A complex snapshots contain atleast any one of the above.
What are the Built-ins used for sending Parameters to forms?
You can pass parameter values to a form when an application executes the call_form,
New_form, Open_form or Run_product.
Can you have more than one content canvas view attached with a window?
Yes. Each window you create must have atleast one content canvas view assigned to
it. You can also create a window that has manipulated content canvas view. At run
time only one of the content canvas views assign to a window is displayed at a time.
Is the After report trigger fired if the report execution fails?
Yes.
Does a Before form trigger fire when the parameter form is suppressed?
Yes.
What is SGA?
The System Global Area in an Oracle database is the area in memory to facilitate the
transfer of information between users. It holds the most recently requested structural
information between users. It holds the most recently requested structural information
about the database. The structure is database buffers, dictionary cache, redo log buffer
and shared pool area.
What is a shared pool?
The data dictionary cache is stored in an area in SGA called the shared pool. This will
allow sharing of parsed SQL statements among concurrent users.
What is mean by Program Global Area (PGA)?
It is area in memory that is used by a single Oracle user process.
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What is a data segment?
Data segment are the physical areas within a database block in which the data
associated with tables and clusters are stored.
What are the factors causing the reparsing of SQL statements in SGA?
Due to insufficient shared pool size.
Monitor the ratio of the reloads takes place while executing SQL statements. If the
ratio is greater than 1 then increase the SHARED_POOL_SIZE.
What are clusters?
Clusters are groups of one or more tables physically stores together to share common
columns and are often used together.
What is cluster key?
The related columns of the tables in a cluster are called the cluster key.
Do a view contain data?
Views do not contain or store data.
What is user Account in Oracle database?
A user account is not a physical structure in database but it is having important
relationship to the objects in the database and will be having certain privileges.
How will you enforce security using stored procedures?
Don't grant user access directly to tables within the application. Instead grant the
ability to access the procedures that access the tables. When procedure executed it will
execute the privilege of procedures owner. Users cannot access tables except via the
procedure.
What are the dictionary tables used to monitor a database space?
DBA_FREE_SPACE
DBA_SEGMENTS
DBA_DATA_FILES.
Can a property clause itself be based on a property clause?
Yes
If a parameter is used in a query without being previously defined, what diff.
exist between. report 2.0 and 2.5 when the query is applied?
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While both reports 2.0 and 2.5 create the parameter, report 2.5 gives a message that a
bind parameter has been created.
What are the sql clauses supported in the link property sheet?
Where start with having.
What is trigger associated with the timer?
When-timer-expired.
What are the trigger associated with image items?
When-image-activated fires when the operators double clicks on an image itemwhenimage-pressed fires when an operator clicks or double clicks on an image item
What are the different windows events activated at runtimes?
When_window_activated
When_window_closed
When_window_deactivated
When_window_resized
Within this triggers, you can examine the built in system variable system.
event_window to determine the name of the window for which the trigger fired.
When do you use data parameter type?
When the value of a data parameter being passed to a called product is always the
name of the record group defined in the current form. Data parameters are used to pass
data to products invoked with the run_product built-in subprogram.
What is difference between open_form and call_form?
when one form invokes another form by executing open_form the first form remains
displayed, and operators can navigate between the forms as desired. when one form
invokes another form by executing call_form, the called form is modal with respect to
the calling form. That is, any windows that belong to the calling form are disabled, and
operators cannot navigate to them until they first exit the called form.
What is new_form built-in?
When one form invokes another form by executing new_form oracle form exits the
first form and releases its memory before loading the new form calling new form
completely replace the first with the second. If there are changes pending in the first
form, the operator will be prompted to save them before the new form is loaded.
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What is the "LOV of Validation" Property of an item? What is the use of it?
When LOV for Validation is set to True, Oracle Forms compares the current value of
the text item to the values in the first column displayed in the LOV. Whenever the
validation event occurs. If the value in the text item matches one of the values in the
first column of the LOV, validation succeeds, the LOV is not displayed, and
processing continues normally. If the value in the text item does not match one of the
values in the first column of the LOV, Oracle Forms displays the LOV and uses the
text item value as the search criteria to automatically reduce the list.
What is the diff. when Flex mode is mode on and when it is off?
When flex mode is on, reports automatically resizes the parent when the child is
resized.
What is the diff. when confine mode is on and when it is off?
When confine mode is on, an object cannot be moved outside its parent in the layout.
What are visual attributes?
Visual attributes are the font, color, pattern proprieties that you set for form and menu
objects that appear in your application interface.
Which of the two views should objects according to possession?
view by structure.
What are the two types of views available in the object navigator(specific to
report 2.5)?
View by structure and view by type .
What are the vbx controls?
Vbx control provide a simple method of building and enhancing user interfaces. The
controls can use to obtain user inputs and display program outputs.vbx control where
originally develop as extensions for the ms visual basic environments and include such
items as sliders, rides and knobs.
What is the use of transactional triggers?
Using transactional triggers we can control or modify the default functionality of the
oracle forms.
How do you create a new session while open a new form?
Using open_form built-in setting the session option Ex. Open_form('Stocks ',active,
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session). when invoke the multiple forms with open form and call_form in the same
application, state whether the following are true/False
What are the ways to monitor the performance of the report?
Use reports profile executable statement. Use SQL trace facility.
If two groups are not linked in the data model editor, What is the hierarchy
between them?
Two group that is above are the left most rank higher than the group that is to right or
below it.
An open form can not be execute the call_form procedure if you chain of called
forms has been initiated by another open form?
True
Explain about horizontal, Vertical tool bar canvas views?
Tool bar canvas views are used to create tool bars for individual windows. Horizontal
tool bars are display at the top of a window, just under its menu bar. Vertical Tool bars
are displayed along the left side of a window
What is the purpose of the product order option in the column property sheet?
To specify the order of individual group evaluation in a cross products.
What is the use of image_zoom built-in?
To manipulate images in image items.
How do you reference a parameter indirectly?
To indirectly reference a parameter use the NAME IN, COPY 'built-ins to indirectly
set and reference the parameters value' Example name_in ('capital parameter my
param'), Copy ('SURESH','Parameter my_param')
What is a timer?
Timer is an "internal time clock" that you can programmatically create to perform an
action each time the times.
What are the two phases of block coordination?
There are two phases of block coordination: the clear phase and the population phase.
During, the clear phase, Oracle Forms navigates internally to the detail block and
flushes the obsolete detail records. During the population phase, Oracle Forms issues a
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SELECT statement to repopulate the detail block with detail records associated with
the new master record. These operations are accomplished through the execution of
triggers.
What are Most Common types of Complex master-detail relationships?
There are three most common types of complex master-detail relationships:
master with dependent details
master with independent details
detail with two masters
What is a text list?
The text list style list item appears as a rectangular box which displays the fixed
number of values. When the text list contains values that can not be displayed, a
vertical scroll bar appears, allowing the operator to view and select undisplayed
values.
What is term?
The term is terminal definition file that describes the terminal form which you are
using r20run.
What is use of term?
The term file which key is correspond to which oracle report functions.
What is pop list?
The pop list style list item appears initially as a single field (similar to a text item
field). When the operator selects the list icon, a list of available choices appears.
What is the maximum no of chars the parameter can store?
The maximum no of chars the parameter can store is only valid for char parameters,
which can be upto 64K. No parameters default to 23Bytes and Date parameter default
to 7Bytes.
What are the default extensions of the files created by library module?
The default file extensions indicate the library module type and storage format .pll pl/sql library module binary
What are the Coordination Properties in a Master-Detail relationship?
The coordination properties are
Deferred
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Auto-Query
These Properties determine when the population phase of block
coordination should occur.
How do you display console on a window ?
The console includes the status line and message line, and is displayed at the bottom of
the window to which it is assigned.To specify that the console should be displayed, set
the console window form property to the name of any window in the form. To include
the console, set console window to Null.
What are the different Parameter types?
Text ParametersData Parameters
State any three mouse events system variables?
System.mouse_button_pressedSystem.mouse_button_shift
What are the types of calculated columns available?
Summary, Formula, Placeholder column.
Explain about stacked canvas views?
Stacked canvas view is displayed in a window on top of, or "stacked" on the content
canvas view assigned to that same window. Stacked canvas views obscure some part
of the underlying content canvas view, and or often shown and hidden
programmatically.
How does one do off-line database backups? (for DBA )
Shut down the database from sqlplus or server manager. Backup all files to secondary
storage (eg. tapes). Ensure that you backup all data files, all control files and all log
files. When completed, restart your database.
Do the following queries to get a list of all files that needs to be backed up:
select name from sys.v_$datafile;
select member from sys.v_$logfile;
select name from sys.v_$controlfile;
Sometimes Oracle takes forever to shutdown with the "immediate" option. As
workaround to this problem, shutdown using these commands:
alter system checkpoint;
shutdown abort
startup restrict
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shutdown immediate
Note that if you database is in ARCHIVELOG mode, one can still use archived log
files to roll forward from an off-line backup. If you cannot take your database down
for a cold (off-line) backup at a convenient time, switch your database into
ARCHIVELOG mode and perform hot (on-line) backups.
What is the difference between SHOW_EDITOR and EDIT_TEXTITEM?
Show editor is the generic built-in which accepts any editor name and takes some
input string and returns modified output string. Whereas the edit_textitem built-in
needs the input focus to be in the text item before the built-in is executed.
What are the built-ins that are used to Attach an LOV programmatically to an
item?
set_item_property
get_item_property
(by setting the LOV_NAME property)
How does one do on-line database backups? (for DBA )
Each tablespace that needs to be backed-up must be switched into backup mode before
copying the files out to secondary storage (tapes). Look at this simple example.
ALTER TABLESPACE xyz BEGIN BACKUP;
! cp xyfFile1 /backupDir/
ALTER TABLESPACE xyz END BACKUP;
It is better to backup tablespace for tablespace than to put all tablespaces in backup
mode. Backing them up separately incurs less overhead. When done, remember to
backup your control files. Look at this example:
ALTER SYSTEM SWITCH LOGFILE; -- Force log switch to update control file
headers
ALTER DATABASE BACKUP CONTROLFILE TO '/backupDir/control.dbf';
NOTE: Do not run on-line backups during peak processing periods. Oracle will write
complete database blocks instead of the normal deltas to redo log files while in backup
mode. This will lead to excessive database archiving and even database freezes.
How does one backup a database using RMAN? (for DBA )
The biggest advantage of RMAN is that it only backup used space in the database.
Rman doesn't put tablespaces in backup mode, saving on redo generation overhead.
RMAN will re-read database blocks until it gets a consistent image of it. Look at this
simple backup example.
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rman target sys/*** nocatalog
run {
allocate channel t1 type disk;
backup
format '/app/oracle/db_backup/%d_t%t_s%s_p%p'
( database );
release channel t1;
}
Example RMAN restore:
rman target sys/*** nocatalog
run {
allocate channel t1 type disk;
# set until time 'Aug 07 2000 :51';
restore tablespace users;
recover tablespace users;
release channel t1;
}
The examples above are extremely simplistic and only useful for illustrating basic
concepts. By default Oracle uses the database controlfiles to store information about
backups. Normally one would rather setup a RMAN catalog database to store RMAN
metadata in. Read the Oracle Backup and Recovery Guide before implementing any
RMAN backups.
Note: RMAN cannot write image copies directly to tape. One needs to use a thirdparty media manager that integrates with RMAN to backup directly to tape.
Alternatively one can backup to disk and then manually copy the backups to tape.
What are the different file extensions that are created by oracle reports?
Rep file and Rdf file.
What is strip sources generate options?
Removes the source code from the library file and generates a library files that
contains only pcode. The resulting file can be used for final deployment, but can not
be subsequently edited in the designer.ex. f45gen module=old_lib.pll
userid=scott/tiger strip_source YES output_file
How does one put a database into ARCHIVELOG mode? (for DBA )
The main reason for running in archivelog mode is that one can provide 24-hour
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availability and guarantee complete data recoverability. It is also necessary to enable
ARCHIVELOG mode before one can start to use on-line database backups. To enable
ARCHIVELOG mode, simply change your database startup command script, and
bounce the database:
SQLPLUS> connect sys as sysdba
SQLPLUS> startup mount exclusive;
SQLPLUS> alter database archivelog;
SQLPLUS> archive log start;
SQLPLUS> alter database open;
NOTE1: Remember to take a baseline database backup right after enabling archivelog
mode. Without it one would not be able to recover. Also, implement an archivelog
backup to prevent the archive log directory from filling-up.
NOTE2: ARCHIVELOG mode was introduced with Oracle V6, and is essential for
database point-in-time recovery. Archiving can be used in combination with on-line
and off-line database backups.
NOTE3: You may want to set the following INIT.ORA parameters when enabling
ARCHIVELOG mode: log_archive_start=TRUE, log_archive_dest=... and
log_archive_format=...
NOTE4: You can change the archive log destination of a database on-line with the
ARCHIVE LOG START TO 'directory'; statement. This statement is often used to
switch archiving between a set of directories.
NOTE5: When running Oracle Real Application Server (RAC), you need to shut down
all nodes before changing the database to ARCHIVELOG mode.
What is the basic data structure that is required for creating an LOV?
Record Group.
How does one backup archived log files? (for DBA )
One can backup archived log files using RMAN or any operating system backup
utility. Remember to delete files after backing them up to prevent the archive log
directory from filling up. If the archive log directory becomes full, your database will
hang! Look at this simple RMAN backup script:
RMAN> run {
2> allocate channel dev1 type disk;
3> backup
4> format '/app/oracle/arch_backup/log_t%t_s%s_p%p'
5> (archivelog all delete input);
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6> release channel dev1;
7> }
Does Oracle write to data files in begin/hot backup mode? (for DBA )
Oracle will stop updating file headers, but will continue to write data to the database
files even if a tablespace is in backup mode.
In backup mode, Oracle will write out complete changed blocks to the redo log files.
Normally only deltas (changes) are logged to the redo logs. This is done to enable
reconstruction of a block if only half of it was backed up (split blocks). Because of
this, one should notice increased log activity and archiving during on-line backups.
What is the Maximum allowed length of Record group Column?
Record group column names cannot exceed 30 characters.
Which parameter can be used to set read level consistency across multiple
queries?
Read only
What are the different types of Record Groups?
Query Record Groups
NonQuery Record Groups
State Record Groups
From which designation is it preferred to send the output to the printed?
Previewer
What are difference between post database commit and post-form commit?
Post-form commit fires once during the post and commit transactions process, after the
database commit occurs. The post-form-commit trigger fires after inserts, updates and
deletes have been posted to the database but before the transactions have been
finalized in the issuing the command. The post-database-commit trigger fires after
oracle forms issues the commit to finalized transactions.
What are the different display styles of list items?
Pop_listText_listCombo box
Which of the above methods is the faster method?
performing the calculation in the query is faster.
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With which function of summary item is the compute at options required?
percentage of total functions.
What are parameters?
Parameters provide a simple mechanism for defining and setting the valuesof inputs
that are required by a form at startup. Form parameters are variables of type
char,number,date that you define at design time.
What are the three types of user exits available ?
Oracle Precompiler exits, Oracle call interface, NonOracle user exits.
How many windows in a form can have console?
Only one window in a form can display the console, and you cannot change the
console assignment at runtime.
What is an administrative (privileged) user? (for DBA )
Oracle DBAs and operators typically use administrative accounts to manage the
database and database instance. An administrative account is a user that is granted
SYSOPER or SYSDBA privileges. SYSDBA and SYSOPER allow access to a
database instance even if it is not running. Control of these privileges is managed
outside of the database via password files and special operating system groups. This
password file is created with the orapwd utility.
What are the two repeating frame always associated with matrix object?
One down repeating frame below one across repeating frame.
What are the master-detail triggers?
On-Check_delete_masterOn_clear_detailsOn_populate_details
How does one connect to an administrative user? (for DBA )
If an administrative user belongs to the "dba" group on Unix, or the "ORA_DBA"
(ORA_sid_DBA) group on NT, he/she can connect like this:
connect / as sysdba
No password is required. This is equivalent to the desupported "connect internal"
method.
A password is required for "non-secure" administrative access. These passwords are
stored in password files. Remote connections via Net8 are classified as non-secure.
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Look at this example:
connect sys/password as sysdba
How does one create a password file? (for DBA )
The Oracle Password File ($ORACLE_HOME/dbs/orapw or orapwSID) stores
passwords for users with administrative privileges. One needs to create a password
files before remote administrators (like OEM) will be allowed to connect.
Follow this procedure to create a new password file:
. Log in as the Oracle software owner
. Runcommand: orapwd file=$ORACLE_HOME/dbs/orapw$ORACLE_SID
password=mypasswd
. Shutdown the database (SQLPLUS> SHUTDOWN IMMEDIATE)
. Edit the INIT.ORA file and ensure REMOTE_LOGIN_PASSWORDFILE=exclusive
is set.
. Startup the database (SQLPLUS> STARTUP)
NOTE: The orapwd utility presents a security risk in that it receives a password from
the command line. This password is visible in the process table of many systems.
Administrators needs to be aware of this!
Is it possible to modify an external query in a report which contains it?
No.
Does a grouping done for objects in the layout editor affect the grouping done in
the data model editor?
No.
How does one add users to a password file? (for DBA )
One can select from the SYS.V_$PWFILE_USERS view to see which users are listed
in the password file. New users can be added to the password file by granting them
SYSDBA or SYSOPER privileges, or by using the orapwd utility. GRANT SYSDBA
TO scott;
If a break order is set on a column would it affect columns which are under the
column?
No
Why are OPS$ accounts a security risk in a client/server environment? (for DBA)
If you allow people to log in with OPS$ accounts from Windows Workstations, you
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cannot be sure who they really are. With terminals, you can rely on operating system
passwords, with Windows, you cannot.
If you set REMOTE_OS_AUTHENT=TRUE in your init.ora file, Oracle assumes that
the remote OS has authenticated the user. If REMOTE_OS_AUTHENT is set to
FALSE (recommended), remote users will be unable to connect without a password.
IDENTIFIED EXTERNALLY will only be in effect from the local host. Also, if you
are using "OPS$" as your prefix, you will be able to log on locally with or without a
password, regardless of whether you have identified your ID with a password or
defined it to be IDENTIFIED EXTERNALLY.
Do user parameters appear in the data modal editor in 2.5?
No
Can you pass data parameters to forms?
No
Is it possible to link two groups inside a cross products after the cross products
group has been created?
no
What are the different modals of windows?
Modalless windows
Modal windows
What are modal windows?
Modal windows are usually used as dialogs, and have restricted functionality
compared to modelless windows. On some platforms for example operators cannot
resize, scroll or iconify a modal window.
What are the different default triggers created when Master Deletes Property is
set to Non-isolated?
Master Deletes Property Resulting Triggers
---------------------------------------------------Non-Isolated(the default) On-Check-Delete-Master
On-Clear-Details
On-Populate-Details
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What are the different default triggers created when Master Deletes Property is
set to isolated?
Master Deletes Property Resulting Triggers
--------------------------------------------------Isolated On-Clear-Details
On-Populate-Details
What are the different default triggers created when Master Deletes Property is
set to Cascade?
Master Deletes Property Resulting Triggers
--------------------------------------------------Cascading On-Clear-Details
On-Populate-Details
Pre-delete
What is the diff. bet. setting up of parameters in reports 2.0 reports2.5?
LOVs can be attached to parameters in the reports 2.5 parameter form.
What are the difference between lov & list item?
Lov is a property where as list item is an item. A list item can have only one column,
lov can have one or more columns.
What is the advantage of the library?
Libraries provide a convenient means of storing client-side program units and sharing
them among multiple applications. Once you create a library, you can attach it to any
other form, menu, or library modules. When you can call library program units from
triggers menu items commands and user named routine, you write in the modules to
which you have attach the library. When a library attaches another library, program
units in the first library can reference program units in the attached library. Library
support dynamic loading-that is library program units are loaded into an application
only when needed. This can significantly reduce the run-time memory requirements of
applications.
What is lexical reference? How can it be created?
Lexical reference is place_holder for text that can be embedded in a sql statements. A
lexical reference can be created using & before the column or parameter name.
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What is system.coordination_operation?
It represents the coordination causing event that occur on the master block in masterdetail relation.
What is synchronize?
It is a terminal screen with the internal state of the form. It updates the screen display
to reflect the information that oracle forms has in its internal representation of the
screen.
What use of command line parameter cmd file?
It is a command line argument that allows you to specify a file that contain a set of
arguments for r20run.
What is a Text_io Package?
It allows you to read and write information to a file in the file system.
What is forms_DDL?
Issues dynamic Sql statements at run time, including server side pl/SQl and DDL
How is link tool operation different bet. reports 2 & 2.5?
In Reports 2.0 the link tool has to be selected and then two fields to be linked are
selected and the link is automatically created. In 2.5 the first field is selected and the
link tool is then used to link the first field to the second field.
What are the different styles of activation of ole Objects?
In place activationExternal activation
How do you reference a Parameter?
In Pl/Sql, You can reference and set the values of form parameters using bind
variables syntax. Ex. PARAMETER name = '' or :block.item = PARAMETER
Parameter name
What is the difference between object embedding & linking in Oracle forms?
In Oracle forms, Embedded objects become part of the form module, and linked
objects are references from a form module to a linked source file.
Name of the functions used to get/set canvas properties?
Get_view_property, Set_view_property
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What are the built-ins that are used for setting the LOV properties at runtime?
get_lov_property
set_lov_property
What are the built-ins used for processing rows?
Get_group_row_count(function)
Get_group_selection_count(function)
Get_group_selection(function)
Reset_group_selection(procedure)
Set_group_selection(procedure)
Unset_group_selection(procedure)
What are built-ins used for Processing rows?
GET_GROUP_ROW_COUNT(function)
GET_GROUP_SELECTION_COUNT(function)
GET_GROUP_SELECTION(function)
RESET_GROUP_SELECTION(procedure)
SET_GROUP_SELECTION(procedure)
UNSET_GROUP_SELECTION(procedure)
What are the built-in used for getting cell values?
Get_group_char_cell(function)
Get_groupcell(function)
Get_group_number_cell(function)
What are the built-ins used for Getting cell values?
GET_GROUP_CHAR_CELL (function)
GET_GROUPCELL(function)
GET_GROUP_NUMBET_CELL(function)
Atleast how many set of data must a data model have before a data model can be
base on it?
Four
To execute row from being displayed that still use column in the row which
property can be used?
Format trigger.
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What are different types of modules available in oracle form?
Form module - a collection of objects and code routines Menu modules - a collection
of menus and menu item commands that together make up an application menu library
module - a collection of user named procedures, functions and packages that can be
called from other modules in the application
What is the remove on exit property?
For a modelless window, it determines whether oracle forms hides the window
automatically when the operators navigates to an item in the another window.
What is WHEN-Database-record trigger?
Fires when oracle forms first marks a record as an insert or an update. The trigger fires
as soon as oracle forms determines through validation that the record should be
processed by the next post or commit as an insert or update. c generally occurs only
when the operators modifies the first item in the record, and after the operator attempts
to navigate out of the item.
What is a difference between pre-select and pre-query?
Fires during the execute query and count query processing after oracle forms
constructs the select statement to be issued, but before the statement is actually issued.
The pre-query trigger fires just before oracle forms issues the select statement to the
database after the operator as define the example records by entering the query criteria
in enter query mode.Pre-query trigger fires before pre-select trigger.
What are built-ins associated with timers?
find_timercreate_timerdelete_timer
What are the built-ins used for finding object ID functions?
Find_group(function)
Find_column(function)
What are the built-ins used for finding Object ID function?
FIND_GROUP(function)
FIND_COLUMN(function)
Any attempt to navigate programmatically to disabled form in a call_form stack
is allowed?
False
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Use the Add_group_row procedure to add a row to a static record group 1. true
or false?
False
What third party tools can be used with Oracle EBU/ RMAN? (for DBA)
The following Media Management Software Vendors have integrated their media
management software packages with Oracle Recovery Manager and Oracle7
Enterprise Backup Utility. The Media Management Vendors will provide first line
technical support for the integrated backup/recover solutions.
Veritas NetBackup
EMC Data Manager (EDM)
HP OMNIBack II
IBM's Tivoli Storage Manager - formerly ADSM
Legato Networker
ManageIT Backup and Recovery
Sterling Software's SAMS:Alexandria - formerly from Spectralogic
Sun Solstice Backup
Why and when should one tune? (for DBA)
One of the biggest responsibilities of a DBA is to ensure that the Oracle database is
tuned properly. The Oracle RDBMS is highly tunable and allows the database to be
monitored and adjusted to increase its performance. One should do performance
tuning for the following reasons:
The speed of computing might be wasting valuable human time (users waiting for
response); Enable your system to keep-up with the speed business is conducted; and
Optimize hardware usage to save money (companies are spending millions on
hardware). Although this FAQ is not overly concerned with hardware issues, one
needs to remember than you cannot tune a Buick into a Ferrari.
How can a break order be created on a column in an existing group? What are
the various sub events a mouse double click event involves?
By dragging the column outside the group.
What is the use of place holder column? What are the various sub events a mouse
double click event involves?
A placeholder column is used to hold calculated values at a specified place rather than
allowing is to appear in the actual row where it has to appear.
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What is the use of hidden column? What are the various sub events a mouse
double click event involves?
A hidden column is used to when a column has to embed into boilerplate text.
What database aspects should be monitored? (for DBA)
One should implement a monitoring system to constantly monitor the following
aspects of a database. Writing custom scripts, implementing Oracle's Enterprise
Manager, or buying a third-party monitoring product can achieve this. If an alarm is
triggered, the system should automatically notify the DBA (e-mail, page, etc.) to take
appropriate action.
Infrastructure availability:
. Is the database up and responding to requests
. Are the listeners up and responding to requests
. Are the Oracle Names and LDAP Servers up and responding to requests
. Are the Web Listeners up and responding to requests
Things that can cause service outages:
. Is the archive log destination filling up?
. Objects getting close to their max extents
. User and process limits reached
Things that can cause bad performance:
See question "What tuning indicators can one use?".
Where should the tuning effort be directed? (for DBA)
Consider the following areas for tuning. The order in which steps are listed needs to be
maintained to prevent tuning side effects. For example, it is no good increasing the
buffer cache if you can reduce I/O by rewriting a SQL statement. Database Design (if
it's not too late):
Poor system performance usually results from a poor database design. One should
generally normalize to the 3NF. Selective denormalization can provide valuable
performance improvements. When designing, always keep the "data access path" in
mind. Also look at proper data partitioning, data replication, aggregation tables for
decision support systems, etc.
Application Tuning:
Experience showed that approximately 80% of all Oracle system performance
problems are resolved by coding optimal SQL. Also consider proper scheduling of
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batch tasks after peak working hours.
Memory Tuning:
Properly size your database buffers (shared pool, buffer cache, log buffer, etc) by
looking at your buffer hit ratios. Pin large objects into memory to prevent frequent
reloads.
Disk I/O Tuning:
Database files needs to be properly sized and placed to provide maximum disk
subsystem throughput. Also look for frequent disk sorts, full table scans, missing
indexes, row chaining, data fragmentation, etc
Eliminate Database Contention:
Study database locks, latches and wait events carefully and eliminate where possible.
Tune the Operating System:
Monitor and tune operating system CPU, I/O and memory utilization. For more
information, read the related Oracle FAQ dealing with your specific operating system.
What are the various sub events a mouse double click event involves? What are
the various sub events a mouse double click event involves?
Double clicking the mouse consists of the mouse down, mouse up, mouse click, mouse
down & mouse up events.
What are the default parameter that appear at run time in the parameter screen?
What are the various sub events a mouse double click event involves?
Destype and Desname.
What are the built-ins used for Creating and deleting groups?
CREATE-GROUP (function)
CREATE_GROUP_FROM_QUERY(function)
DELETE_GROUP(procedure)
What are different types of canvas views?
Content canvas views
Stacked canvas views
Horizontal toolbar
vertical toolbar.
What are the different types of Delete details we can establish in Master-Details?
Cascade
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Isolate
Non-isolate
What is relation between the window and canvas views?
Canvas views are the back ground objects on which you place the interface items
(Text items), check boxes, radio groups etc.,) and boilerplate objects (boxes, lines,
images etc.,) that operators interact with us they run your form . Each canvas views
displayed in a window.
What is a User_exit?
Calls the user exit named in the user_exit_string. Invokes a 3Gl program by name
which has been properly linked into your current oracle forms executable.
How is it possible to select generate a select set for the query in the query
property sheet?
By using the tables/columns button and then specifying the table and the column
names.
How can values be passed bet. precompiler exits & Oracle call interface?
By using the statement EXECIAFGET & EXECIAFPUT.
How can a square be drawn in the layout editor of the report writer?
By using the rectangle tool while pressing the (Constraint) key.
How can a text file be attached to a report while creating in the report writer?
By using the link file property in the layout boiler plate property sheet.
How can I message to passed to the user from reports?
By using SRW.MESSAGE function.
Does one need to drop/ truncate objects before importing? (for DBA)
Before one import rows into already populated tables, one needs to truncate or drop
these tables to get rid of the old data. If not, the new data will be appended to the
existing tables. One must always DROP existing Sequences before re-importing. If the
sequences are not dropped, they will generate numbers inconsistent with the rest of the
database. Note: It is also advisable to drop indexes before importing to speed up the
import process. Indexes can easily be recreated after the data was successfully
imported.
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How can a button be used in a report to give a drill down facility?
By setting the action associated with button to Execute pl/sql option and using the
SRW.Run_report function.
Can one import/export between different versions of Oracle? (for DBA)
Different versions of the import utility is upwards compatible. This means that one can
take an export file created from an old export version, and import it using a later
version of the import utility. This is quite an effective way of upgrading a database
from one release of Oracle to the next.
Oracle also ships some previous catexpX.sql scripts that can be executed as user SYS
enabling older imp/exp versions to work (for backwards compatibility). For example,
one can run $ORACLE_HOME/rdbms/admin/catexp7.sql on an Oracle 8 database to
allow the Oracle 7.3 exp/imp utilities to run against an Oracle 8 database.
What are different types of images?
Boiler plate imagesImage Items
Can one export to multiple files?/ Can one beat the Unix 2 Gig limit? (for DBA)
From Oracle8i, the export utility supports multiple output files. This feature enables
large exports to be divided into files whose sizes will not exceed any operating system
limits (FILESIZE= parameter). When importing from multi-file export you must
provide the same filenames in the same sequence in the FILE= parameter. Look at this
example:
exp SCOTT/TIGER FILE=D:\F1.dmp,E:\F2.dmp FILESIZE=10m LOG=scott.log
Use the following technique if you use an Oracle version prior to 8i:
Create a compressed export on the fly. Depending on the type of data, you probably
can export up to 10 gigabytes to a single file. This example uses gzip. It offers the best
compression I know of, but you can also substitute it with zip, compress or whatever.
# create a named pipe
mknod exp.pipe p
# read the pipe - output to zip file in the background
gzip < exp.pipe > scott.exp.gz &
# feed the pipe
exp userid=scott/tiger file=exp.pipe ...
What is bind reference and how can it be created?
Bind reference are used to replace the single value in sql, pl/sql statements a bind
reference can be created using a (:) before a column or a parameter name.
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How can one improve Import/ Export performance? (for DBA)
EXPORT:
. Set the BUFFER parameter to a high value (e.g. 2M)
. Set the RECORDLENGTH parameter to a high value (e.g. 64K)
. Stop unnecessary applications to free-up resources for your job.
. If you run multiple export sessions, ensure they write to different physical disks.
. DO NOT export to an NFS mounted filesystem. It will take forever.
IMPORT:
. Create an indexfile so that you can create indexes AFTER you have imported data.
Do this by setting INDEXFILE to a filename and then import. No data will be
imported but a file containing index definitions will be created. You must edit this file
afterwards and supply the passwords for the schemas on all CONNECT statements.
. Place the file to be imported on a separate physical disk from the oracle data files
. Increase DB_CACHE_SIZE (DB_BLOCK_BUFFERS prior to 9i) considerably in
the init$SID.ora file
. Set the LOG_BUFFER to a big value and restart oracle.
. Stop redo log archiving if it is running (ALTER DATABASE NOARCHIVELOG;)
. Create a BIG tablespace with a BIG rollback segment inside. Set all other rollback
segments offline (except the SYSTEM rollback segment of course). The rollback
segment must be as big as your biggest table (I think?)
. Use COMMIT=N in the import parameter file if you can afford it
. Use ANALYZE=N in the import parameter file to avoid time consuming ANALYZE
statements
. Remember to run the indexfile previously created
Give the sequence of execution of the various report triggers?
Before form , After form , Before report, Between page, After report.
What are the common Import/ Export problems? (for DBA )
ORA-00001: Unique constraint (...) violated - You are importing duplicate rows. Use
IGNORE=NO to skip tables that already exist (imp will give an error if the object is
re-created).
ORA-01555: Snapshot too old - Ask your users to STOP working while you are
exporting or use parameter CONSISTENT=NO
ORA-01562: Failed to extend rollback segment - Create bigger rollback segments or
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set parameter COMMIT=Y while importing
IMP-00015: Statement failed ... object already exists... - Use the IGNORE=Y import
parameter to ignore these errors, but be careful as you might end up with duplicate
rows.
Why is it preferable to create a fewer no. of queries in the data model?
Because for each query, report has to open a separate cursor and has to rebind, execute
and fetch data.
Where is the external query executed at the client or the server?
At the server.
Where is a procedure return in an external pl/sql library executed at the client or
at the server?
At the client.
What is coordination Event?
Any event that makes a different record in the master block the current record is a
coordination causing event.
What is the difference between OLE Server & Ole Container?
An Ole server application creates ole Objects that are embedded or linked in ole
Containers ex. Ole servers are ms_word & ms_excel. OLE containers provide a place
to store, display and manipulate objects that are created by ole server applications. Ex.
oracle forms is an example of an ole Container.
What is an object group?
An object group is a container for a group of objects; you define an object group when
you want to package related objects, so that you copy or reference them in other
modules.
What is an LOV?
An LOV is a scrollable popup window that provides the operator with either a single
or multi column selection list.
At what point of report execution is the before Report trigger fired?
After the query is executed but before the report is executed and the records are
displayed.
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What are the built -ins used for Modifying a groups structure?
ADD-GROUP_COLUMN (function)
ADD_GROUP_ROW (procedure)
DELETE_GROUP_ROW(procedure)
What is an user exit used for?
A way in which to pass control (and possibly arguments ) form Oracle report to
another Oracle products of 3 GL and then return control ( and ) back to Oracle reports.
What is the User-Named Editor?
A user named editor has the same text editing functionality as the default editor, but,
because it is a named object, you can specify editor attributes such as windows display
size, position, and title.
My database was terminated while in BACKUP MODE, do I need to recover?
(for DBA)
If a database was terminated while one of its tablespaces was in BACKUP MODE
(ALTER TABLESPACE xyz BEGIN BACKUP;), it will tell you that media recovery
is required when you try to restart the database. The DBA is then required to recover
the database and apply all archived logs to the database. However, from Oracle7.2,
you can simply take the individual datafiles out of backup mode and restart the
database.
ALTER DATABASE DATAFILE '/path/filename' END BACKUP;
One can select from V$BACKUP to see which datafiles are in backup mode. This
normally saves a significant amount of database down time.
Thiru Vadivelu contributed the following:
From Oracle9i onwards, the following command can be used to take all of the
datafiles out of hot backup mode:
ALTER DATABASE END BACKUP;
The above commands need to be issued when the database is mounted.
What is a Static Record Group?
A static record group is not associated with a query, rather, you define its structure and
row values at design time, and they remain fixed at runtime.
What is a record group?
A record group is an internal Oracle Forms that structure that has a column/row
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framework similar to a database table. However, unlike database tables, record groups
are separate objects that belong to the form module which they are defined.
My database is down and I cannot restore. What now? (for DBA )
Recovery without any backup is normally not supported, however, Oracle Consulting
can sometimes extract data from an offline database using a utility called DUL (Disk
UnLoad). This utility reads data in the data files and unloads it into SQL*Loader or
export dump files. DUL does not care about rollback segments, corrupted blocks, etc,
and can thus not guarantee that the data is not logically corrupt. It is intended as an
absolute last resort and will most likely cost your company a lot of money!!!
I've lost my REDOLOG files, how can I get my DB back? (for DBA)
The following INIT.ORA parameter may be required if your current redo logs are
corrupted or blown away. Caution is advised when enabling this parameter as you
might end-up losing your entire database. Please contact Oracle Support before using
it. _allow_resetlogs_corruption = true
What is a property clause?
A property clause is a named object that contains a list of properties and their settings.
Once you create a property clause you can base other object on it. An object based on
a property can inherit the setting of any property in the clause that makes sense for that
object.
What is a physical page ? & What is a logical page ?
A physical page is a size of a page. That is output by the printer. The logical page is
the size of one page of the actual report as seen in the Previewer.
I've lost some Rollback Segments, how can I get my DB back? (for DBA)
Re-start your database with the following INIT.ORA parameter if one of your rollback
segments is corrupted. You can then drop the corrupted rollback segments and create it
from scratch.
Caution is advised when enabling this parameter, as uncommitted transactions will be
marked as committed. One can very well end up with lost or inconsistent data!!!
Please contact Oracle Support before using it. _Corrupted_rollback_segments =
(rbs01, rbs01, rbs03, rbs04)
What are the differences between EBU and RMAN? (for DBA)
Enterprise Backup Utility (EBU) is a functionally rich, high performance interface for
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backing up Oracle7 databases. It is sometimes referred to as OEBU for Oracle
Enterprise Backup Utility. The Oracle Recovery Manager (RMAN) utility that ships
with Oracle8 and above is similar to Oracle7's EBU utility. However, there is no direct
upgrade path from EBU to RMAN.
How does one create a RMAN recovery catalog? (for DBA)
Start by creating a database schema (usually called rman). Assign an appropriate
tablespace to it and grant it the recovery_catalog_owner role. Look at this example:
sqlplus sys
SQL>create user rman identified by rman;
SQL> alter user rman default tablespace tools temporary tablespace temp;
SQL> alter user rman quota unlimited on tools;
SQL> grant connect, resource, recovery_catalog_owner to rman;
SQL> exit;
Next, log in to rman and create the catalog schema. Prior to Oracle 8i this was done by
running the catrman.sql script. rman catalog rman/rman
RMAN>create catalog tablespace tools;
RMAN> exit;
You can now continue by registering your databases in the catalog. Look at this
example:
rman catalog rman/rman target backdba/backdba
RMAN> register database;
How can a group in a cross products be visually distinguished from a group that
does not form a cross product?
A group that forms part of a cross product will have a thicker border.
What is the frame & repeating frame?
A frame is a holder for a group of fields. A repeating frame is used to display a set of
records when the no. of records that are to displayed is not known before.
What is a combo box?
A combo box style list item combines the features found in list and text item. Unlike
the pop list or the text list style list items, the combo box style list item will both
display fixed values and accept one operator entered value.
What are three panes that appear in the run time pl/sql interpreter?
1. Source pane.
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2. interpreter pane.
3. Navigator pane.
What are the two panes that Appear in the design time pl/sql interpreter?
1. Source pane.
2. Interpreter pane
What are the two ways by which data can be generated for a parameters list of
values?
1. Using static values.
2. Writing select statement.
What are the various methods of performing a calculation in a report ?
1. Perform the calculation in the SQL statements itself.
2. Use a calculated / summary column in the data model.
What are the default extensions of the files created by menu module?
.mmb,
.mmx
What are the default extensions of the files created by forms modules?
.fmb - form module binary
.fmx - form module executable
To display the page no. for each page on a report what would be the source &
logical page no. or & of physical page no.?
& physical page no.
It is possible to use raw devices as data files and what is the advantages over file.
system files ?
Yes. The advantages over file system files. I/O will be improved because Oracle is
bye-passing the kernnel which writing into disk. Disk Corruption will be very less.
What are disadvantages of having raw devices ?
We should depend on export/import utility for backup/recovery (fully reliable) The tar
command cannot be used for physical file backup, instead we can use dd command
which is less flexible and has limited recoveries.
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What is the significance of having storage clause ?
We can plan the storage for a table as how much initial extents are required, how
much can be extended next, how much % should leave free for managing row
updations etc.,
What is the use of INCTYPE option in EXP command ?
Type export should be performed COMPLETE,CUMULATIVE,INCREMENTAL.
List the sequence of events when a large transaction that exceeds beyond its optimal
value when an entry wraps and causes the rollback segment toexpand into anotion
Completes. e. will be written.
What is the use of FILE option in IMP command ?
The name of the file from which import should be performed.
What is a Shared SQL pool?
The data dictionary cache is stored in an area in SGA called the Shared SQL Pool.
This will allow sharing of parsed SQL statements among concurrent users.
What is hot backup and how it can be taken?
Taking backup of archive log files when database is open. For this the ARCHIVELOG
mode should be enabled. The following files need to be backed up. All data files. All
Archive log, redo log files. All control files.
List the Optional Flexible Architecture (OFA) of Oracle database? or How can
we organize the tablespaces in Oracle database to have maximum performance ?
SYSTEM - Data dictionary tables.
DATA - Standard operational tables.
DATA2- Static tables used for standard operations
INDEXES - Indexes for Standard operational tables.
INDEXES1 - Indexes of static tables used for standard operations.
TOOLS - Tools table.
TOOLS1 - Indexes for tools table.
RBS - Standard Operations Rollback Segments,
RBS1,RBS2 - Additional/Special Rollback segments.
TEMP - Temporary purpose tablespace
TEMP_USER - Temporary tablespace for users.
USERS - User tablespace.
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How to implement the multiple control files for an existing database ?
Shutdown the database Copy one of the existing control file to new location Edit
Config ora file by adding new control file. name Restart the database.
What is advantage of having disk shadowing/ Mirroring ?
Shadow set of disks save as a backup in the event of disk failure. In most Operating
System if any disk failure occurs it automatically switchover to place of failed disk.
Improved performance because most OS support volume shadowing can direct file I/O
request to use the shadow set of files instead of the main set of files. This reduces I/O
load on the main set of disks.
How will you force database to use particular rollback segment ?
SET TRANSACTION USE ROLLBACK SEGMENT rbs_name.
Why query fails sometimes ?
Rollback segment dynamically extent to handle larger transactions entry loads. A
single transaction may wipeout all available free space in the Rollback Segment
Tablespace. This prevents other user using Rollback segments.
What is the use of RECORD LENGTH option in EXP command ?
Record length in bytes.
How will you monitor rollback segment status ?
Querying the DBA_ROLLBACK_SEGS view
IN USE - Rollback Segment is on-line.
AVAILABLE - Rollback Segment available but not on-line.
OFF-LINE - Rollback Segment off-line
INVALID - Rollback Segment Dropped.
NEEDS RECOVERY - Contains data but need recovery or corupted.
PARTLY AVAILABLE - Contains data from an unresolved transaction involving a
distributed database.
What is meant by Redo Log file mirroring ? How it can be achieved?
Process of having a copy of redo log files is called mirroring. This can be achieved by
creating group of log files together, so that LGWR will automatically writes them to
all the members of the current on-line redo log group. If any one group fails then
database automatically switch over to next group. It degrades performance.
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Which parameter in Storage clause will reduce no. of rows per block?
PCTFREE parameter
Row size also reduces no of rows per block.
What is meant by recursive hints ?
Number of times processes repeatedly query the dictionary table is called recursive
hints. It is due to the data dictionary cache is too small. By increasing the
SHARED_POOL_SIZE parameter we can optimize the size of Data Dictionary Cache.
What is the use of PARFILE option in EXP command ?
Name of the parameter file to be passed for export.
What is the difference between locks, latches, enqueues and semaphores? (for
DBA)
A latch is an internal Oracle mechanism used to protect data structures in the SGA
from simultaneous access. Atomic hardware instructions like TEST-AND-SET is used
to implement latches. Latches are more restrictive than locks in that they are always
exclusive. Latches are never queued, but will spin or sleep until they obtain a resource,
or time out.
Enqueues and locks are different names for the same thing. Both support queuing and
concurrency. They are queued and serviced in a first-in-first-out (FIFO) order.
Semaphores are an operating system facility used to control waiting. Semaphores are
controlled by the following Unix parameters: semmni, semmns and semmsl. Typical
settings are:
semmns = sum of the "processes" parameter for each instance
(see init<instance>.ora for each instance)
semmni = number of instances running simultaneously;
semmsl = semmns
What is a logical backup?
Logical backup involves reading a set of database records and writing them into a file.
Export utility is used for taking backup and Import utility is used to recover from
backup.
Where can one get a list of all hidden Oracle parameters? (for DBA)
Oracle initialization or INIT.ORA parameters with an underscore in front are hidden
or unsupported parameters. One can get a list of all hidden parameters by executing
this query:
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select *
from SYS.X$KSPPI
where substr(KSPPINM,1,1) = '_';
The following query displays parameter names with their current value:
select a.ksppinm "Parameter", b.ksppstvl "Session Value", c.ksppstvl "Instance Value"
from x$ksppi a, x$ksppcv b, x$ksppsv c
where a.indx = b.indx and a.indx = c.indx
and substr(ksppinm,1,1)='_'
order by a.ksppinm;
Remember: Thou shall not play with undocumented parameters!
What is a database EVENT and how does one set it? (for DBA)
Oracle trace events are useful for debugging the Oracle database server. The following
two examples are simply to demonstrate syntax. Refer to later notes on this page for an
explanation of what these particular events do.
Either adding them to the INIT.ORA parameter file can activate events. E.g.
event='1401 trace name errorstack, level 12'
... or, by issuing an ALTER SESSION SET EVENTS command: E.g.
alter session set events '10046 trace name context forever, level 4';
The alter session method only affects the user's current session, whereas changes to the
INIT.ORA file will affect all sessions once the database has been restarted.
What is a Rollback segment entry ?
It is the set of before image data blocks that contain rows that are modified by a
transaction. Each Rollback Segment entry must be completed within one rollback
segment. A single rollback segment can have multiple rollback segment entries.
What database events can be set? (for DBA)
The following events are frequently used by DBAs and Oracle Support to diagnose
problems:
" 10046 trace name context forever, level 4 Trace SQL statements and show bind
variables in trace output.
" 10046 trace name context forever, level 8 This shows wait events in the SQL trace
files
" 10046 trace name context forever, level 12 This shows both bind variable names and
wait events in the SQL trace files
" 1401 trace name errorstack, level 12 1401 trace name errorstack, level 4 1401 trace
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name processstate Dumps out trace information if an ORA-1401 "inserted value too
large for column" error occurs. The 1401 can be replaced by any other Oracle Server
error code that you want to trace.
" 60 trace name errorstack level 10 Show where in the code Oracle gets a deadlock
(ORA-60), and may help to diagnose the problem.
The following lists of events are examples only. They might be version specific, so
please call Oracle before using them:
" 10210 trace name context forever, level 10 10211 trace name context forever, level
10 10231 trace name context forever, level 10 These events prevent database block
corruptions
" 10049 trace name context forever, level 2 Memory protect cursor
" 10210 trace name context forever, level 2 Data block check
" 10211 trace name context forever, level 2 Index block check
" 10235 trace name context forever, level 1 Memory heap check
" 10262 trace name context forever, level 300 Allow 300 bytes memory leak for
connections
Note: You can use the Unix oerr command to get the description of an event. On Unix,
you can type "oerr ora 10053" from the command prompt to get event details.
How can one dump internal database structures? (for DBA)
The following (mostly undocumented) commands can be used to obtain information
about internal database structures.
o Dump control file contents
alter session set events 'immediate trace name CONTROLF level 10'
/
o Dump file headers
alter session set events 'immediate trace name FILE_HDRS level 10'
/
o Dump redo log headers
alter session set events 'immediate trace name REDOHDR level 10'
/
o Dump the system state
NOTE: Take 3 successive SYSTEMSTATE dumps, with 10-minute intervals alter
session set events 'immediate trace name SYSTEMSTATE level 10'
/
o Dump the process state
alter session set events 'immediate trace name PROCESSSTATE level 10'
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/
o Dump Library Cache details
alter session set events 'immediate trace name library cache level 10'
/
o Dump optimizer statistics whenever a SQL statement is parsed (hint: change
statement or flush pool) alter session set events '10053 trace name context forever,
level 1'
/
o Dump a database block (File/ Block must be converted to DBA address) Convert file
and block number to a DBA (database block address).
Eg: variable x varchar2;
exec :x := dbms_utility.make_data_block_address(1,12);
print x
alter session set events 'immediate trace name blockdump level 50360894'
/
What are the different kind of export backups?
Full back - Complete database
Incremental - Only affected tables from last incremental date/full backup date.
Cumulative backup - Only affected table from the last cumulative date/full backup
date.
How free extents are managed in Ver 6.0 and Ver 7.0 ?
Free extents cannot be merged together in Ver 6.0.
Free extents are periodically coalesces with the neighboring free extent in Ver 7.0
What is the use of RECORD option in EXP command?
For Incremental exports, the flag indirects whether a record will be stores data
dictionary tables recording the export.
What is the use of ROWS option in EXP command ?
Flag to indicate whether table rows should be exported. If 'N' only DDL statements for
the database objects will be created.
What is the use of COMPRESS option in EXP command ?
Flag to indicate whether export should compress fragmented segments into single
extents.
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How will you swap objects into a different table space for an existing database ?
Export the user
Perform import using the command imp system/manager file=export.dmp
indexfile=newrite.sql.
This will create all definitions into newfile.sql. Drop necessary objects.
Run the script newfile.sql after altering the tablespaces.
Import from the backup for the necessary objects.
How does Space allocation table place within a block ?
Each block contains entries as follows
Fixed block header
Variable block header
Row Header,row date (multiple rows may exists)
PCTEREE (% of free space for row updation in future)
What are the factors causing the reparsing of SQL statements in SGA?
Due to insufficient Shared SQL pool size. Monitor the ratio of the reloads takes place
while executing SQL statements. If the ratio is greater than 1 then increase the
SHARED_POOL_SIZE. LOGICAL & PHYSICAL ARCHITECTURE OF
DATABASE.
What is dictionary cache ?
Dictionary cache is information about the databse objects stored in a data dictionary
table.
What is a Control file ?
Database overall physical architecture is maintained in a file called control file. It will
be used to maintain internal consistency and guide recovery operations. Multiple
copies of control files are advisable.
What is Database Buffers ?
Database buffers are cache in the SGA used to hold the data blocks that are read from
the data segments in the database such as tables, indexes and clusters
DB_BLOCK_BUFFERS parameter in INIT.ORA decides the size.
How will you create multiple rollback segments in a database ?
Create a database which implicitly creates a SYSTEM Rollback Segment in a
SYSTEM tablespace. Create a Second Rollback Segment name R0 in the SYSTEM
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tablespace. Make new rollback segment available (After shutdown, modify init.ora file
and Start database) Create other tablespaces (RBS) for rollback segments. Deactivate
Rollback Segment R0 and activate the newly created rollback segments.
What is cold backup? What are the elements of it?
Cold backup is taking backup of all physical files after normal shutdown of database.
We need to take.
- All Data files.
- All Control files.
- All on-line redo log files.
- The init.ora file (Optional)
What is meant by redo log buffer ?
Changes made to entries are written to the on-line redo log files. So that they can be
used in roll forward operations during database recoveries. Before writing them into
the redo log files, they will first brought to redo log buffers in SGA and LGWR will
write into files frequently. LOG_BUFFER parameter will decide the size.
How will you estimate the space required by a non-clustered tables?
Calculate the total header size
Calculate the available dataspace per data block
Calculate the combined column lengths of the average row
Calculate the total average row size.
Calculate the average number rows that can fit in a block
Calculate the number of blocks and bytes required for the table.
After arriving the calculation, add 10 % additional space to calculate the initial extent
size for a working table.
How will you monitor the space allocation ?
By querying DBA_SEGMENT table/view.
What is meant by free extent ?
A free extent is a collection of continuous free blocks in tablespace. When a segment
is dropped its extents are reallocated and are marked as free.
What is the use of IGNORE option in IMP command ?
A flag to indicate whether the import should ignore errors encounter when issuing
CREATE commands.
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What is the use of ANALYSE ( Ver 7) option in EXP command ?
A flag to indicate whether statistical information about the exported objects should be
written to export dump file.
What is the use of ROWS option in IMP command ?
A flag to indicate whether rows should be imported. If this is set to 'N' then only DDL
for database objects will be executed.
What is the use of INDEXES option in EXP command ?
A flag to indicate whether indexes on tables will be exported.
What is the use of INDEXES option in IMP command ?
A flag to indicate whether import should import index on tables or not.
What is the use of GRANT option in EXP command?
A flag to indicate whether grants on databse objects will be exported or not. Value is
'Y' or 'N'.
What is the use of GRANT option in IMP command ?
A flag to indicate whether grants on database objects will be imported.
What is the use of FULL option in EXP command ?
A flag to indicate whether full databse export should be performed.
What is the use of SHOW option in IMP command ?
A flag to indicate whether file content should be displayed or not.
What is the use of CONSTRAINTS option in EXP command ?
A flag to indicate whether constraints on table need to be exported.
What is the use of CONSISTENT (Ver 7) option in EXP command ?
A flag to indicate whether a read consistent version of all the exported objects should
be maintained.
What are the different methods of backing up oracle database ?
- Logical Backups
- Cold Backups
- Hot Backups (Archive log)
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What is the difference between ON-VALIDATE-FIELD trigger and a POSTCHANGE trigger ?
When you changes the Existing value to null, the On-validate field trigger will fire
post change trigger will not fire. At the time of execute-query post-change trigger will
fire, on-validate field trigger will not fire.
When is PRE-QUERY trigger executed ?
When Execute-query or count-query Package procedures are invoked.
How do you trap the error in forms 3.0 ?
using On-Message or On-Error triggers.
How many pages you can in a single form ?
Unlimited
While specifying master/detail relationship between two blocks specifying the
join condition is a must ?
True or False. ?
True
EXIT_FORM is a restricted package procedure ?
a. True b. False
True
What is the usage of an ON-INSERT,ON-DELETE and ON-UPDATE
TRIGGERS ?
These triggers are executes when inserting, deleting and updating operations are
performed and can be used to change the default function of insert, delete or update
respectively. For Eg, instead of inserting a row in a table an existing row can be
updated in the same table.
What are the types of Pop-up window ?
the pop-up field editor
pop-up list of values
pop-up pages.
Alert :
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What is an SQL *FORMS ?
SQL *forms is 4GL tool for developing and executing; Oracle based interactive
application.
How do you control the constraints in forms ?
Select the use constraint property is ON Block definition screen.
BLOCK
What is the difference between restricted and unrestricted package procedure ?
Restricted package procedure that affects the basic functions of SQL * Forms. It
cannot used in all triggers except key triggers. Unrestricted package procedure that
does not interfere with the basic functions of SQL * Forms it can be used in any
triggers.
A query fetched 10 records How many times does a PRE-QUERY Trigger and
POST-QUERY Trigger will get executed ?
PRE-QUERY fires once.
POST-QUERY fires 10 times.
Give the sequence in which triggers fired during insert operations, when the
following 3 triggers are defined at the same block level ?
a. ON-INSERT b. POST-INSERT c. PRE-INSERT
State the order in which these triggers are executed ?
POST-FIELD,ON-VALIDATE-FIELD,POST-CHANGE and KEY-NEXTFLD. KEYNEXTFLD,POST-CHANGE, ON-VALIDATE-FIELD, POST-FIELD. g.
What the PAUSE package procedure does ?
Pause suspends processing until the operator presses a function key
What do you mean by a page ?
Pages are collection of display information, such as constant text and graphics
What are the type of User Exits ?
ORACLE Precompliers user exits
OCI (ORACLE Call Interface)
Non-ORACEL user exits.
Page :
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What is the difference between an ON-VALIDATE-FIELD trigger and a trigger
?
On-validate-field trigger fires, when the field Validation status New or changed. Postfield-trigger whenever the control leaving form the field, it will fire.
Can we use a restricted package procedure in ON-VALIDATE-FIELD Trigger ?
No
Is a Key startup trigger fires as result of a operator pressing a key explicitly ?
No
Can we use GO-BLOCK package in a pre-field trigger ?
No
Can we create two blocks with the same name in form 3.0 ?
No
What does an on-clear-block Trigger fire?
It fires just before SQL * forms the current block.
Name the two files that are created when you generate the form give the filex
extension ?
INP (Source File)
FRM (Executable File)
What package procedure used for invoke sql *plus from sql *forms ?
Host (E.g. Host (sqlplus))
What is the significance of PAGE 0 in forms 3.0 ?
Hide the fields for internal calculation.
What are the different types of key triggers ?
Function Key
Key-function
Key-others
Key-startup
What is the difference between a Function Key Trigger and Key Function
Trigger ?
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Function key triggers are associated with individual SQL*FORMS function keys You
can attach Key function triggers to 10 keys or key sequences that normally do not
perform any SQL * FORMS operations. These keys referred as key F0 through key
F9.
Committed block sometimes refer to a BASE TABLE ?
False
Error_Code is a package proecdure ?
a. True b. false
False
When is cost based optimization triggered? (for DBA)
It's important to have statistics on all tables for the CBO (Cost Based Optimizer) to
work correctly. If one table involved in a statement does not have statistics, Oracle has
to revert to rule-based optimization for that statement. So you really want for all tables
to have statistics right away; it won't help much to just have the larger tables analyzed.
Generally, the CBO can change the execution plan when you:
1. Change statistics of objects by doing an ANALYZE;
2. Change some initialization parameters (for example: hash_join_enabled,
sort_area_size, db_file_multiblock_read_count).
How can one optimize %XYZ% queries? (for DBA)
It is possible to improve %XYZ% queries by forcing the optimizer to scan all the
entries from the index instead of the table. This can be done by specifying hints. If the
index is physically smaller than the table (which is usually the case) it will take less
time to scan the entire index than to scan the entire table.
What Enter package procedure does ?
Enter Validate-data in the current validation unit.
Where can one find I/O statistics per table? (for DBA)
The UTLESTAT report shows I/O per tablespace but one cannot see what tables in the
tablespace has the most I/O. The $ORACLE_HOME/rdbms/admin/catio.sql script
creates a sample_io procedure and table to gather the required information. After
executing the procedure, one can do a simple SELECT * FROM io_per_object; to
extract the required information. For more details, look at the header comments in the
$ORACLE_HOME/rdbms/admin/catio.sql script.
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My query was fine last week and now it is slow. Why? (for DBA)
The likely cause of this is because the execution plan has changed. Generate a current
explain plan of the offending query and compare it to a previous one that was taken
when the query was performing well. Usually the previous plan is not available.
Some factors that can cause a plan to change are:
. Which tables are currently analyzed? Were they previously analyzed? (ie. Was the
query using RBO and now CBO?)
. Has OPTIMIZER_MODE been changed in INIT.ORA?
. Has the DEGREE of parallelism been defined/changed on any table?
. Have the tables been re-analyzed? Were the tables analyzed using estimate or
compute? If estimate, what percentage was used?
. Have the statistics changed?
. Has the INIT.ORA parameter DB_FILE_MULTIBLOCK_READ_COUNT been
changed?
. Has the INIT.ORA parameter SORT_AREA_SIZE been changed?
. Have any other INIT.ORA parameters been changed?
. What do you think the plan should be? Run the query with hints to see if this
produces the required performance.
Why is Oracle not using the damn index? (for DBA)
This problem normally only arises when the query plan is being generated by the Cost
Based Optimizer. The usual cause is because the CBO calculates that executing a Full
Table Scan would be faster than accessing the table via the index.
Fundamental things that can be checked are:
. USER_TAB_COLUMNS.NUM_DISTINCT - This column defines the number of
distinct values the column holds.
. USER_TABLES.NUM_ROWS - If NUM_DISTINCT = NUM_ROWS then using an
index would be preferable to doing a FULL TABLE SCAN. As the NUM_DISTINCT
decreases, the cost of using an index increase thereby is making the index less
desirable.
. USER_INDEXES.CLUSTERING_FACTOR - This defines how ordered the rows
are in the index. If CLUSTERING_FACTOR approaches the number of blocks in the
table, the rows are ordered. If it approaches the number of rows in the table, the rows
are randomly ordered. In such a case, it is unlikely that index entries in the same leaf
block will point to rows in the same data blocks.
. Decrease the INIT.ORA parameter DB_FILE_MULTIBLOCK_READ_COUNT - A
higher value will make the cost of a FULL TABLE SCAN cheaper.
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. Remember that you MUST supply the leading column of an index, for the index to be
used (unless you use a FAST FULL SCAN or SKIP SCANNING).
. There are many other factors that affect the cost, but sometimes the above can help to
show why an index is not being used by the CBO. If from checking the above you still
feel that the query should be using an index, try specifying an index hint. Obtain an
explain plan of the query either using TKPROF with TIMED_STATISTICS, so that
one can see the CPU utilization, or with AUTOTRACE to see the statistics. Compare
this to the explain plan when not using an index.
When should one rebuild an index? (for DBA)
You can run the 'ANALYZE INDEX VALIDATE STRUCTURE' command on the
affected indexes - each invocation of this command creates a single row in the
INDEX_STATS view. This row is overwritten by the next ANALYZE INDEX
command, so copy the contents of the view into a local table after each ANALYZE.
The 'badness' of the index can then be judged by the ratio of 'DEL_LF_ROWS' to
'LF_ROWS'.
What are the unrestricted procedures used to change the popup screen position
during run time ?
Anchor-view
Resize -View
Move-View.
What is an Alert ?
An alert is window that appears in the middle of the screen overlaying a portion of the
current display.
Deleting a page removes information about all the fields in that page ?
a. True. b. False
a. True.
Two popup pages can appear on the screen at a time ?Two popup pages can
appear on the screen at a time ?
a. True. b. False?
a. True.
Classify the restricted and unrestricted procedure from the following.
a. Call
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b. User-Exit
c. Call-Query
d. Up
e. Execute-Query
f. Message
g. Exit-From
h. Post
i. Break?
a. Call - unrestricted
b. User Exit - Unrestricted
c. Call_query - Unrestricted
d. Up - Restricted
e. Execute Query - Restricted
f. Message - Restricted
g. Exit_form - Restricted
h. Post - Restricted
i. Break - Unrestricted.
What is an User Exits ?
A user exit is a subroutine which are written in programming languages using pro*C
pro *Cobol , etc., that link into the SQL * forms executable.
What is a Trigger ?
A piece of logic that is executed at or triggered by a SQL *forms event.
What is a Package Procedure ?
A Package procedure is built in PL/SQL procedure.
What is the maximum size of a form ?
255 character width and 255 characters Length.
What is the difference between system.current_field and system.cursor_field ?
1. System.current_field gives name of the field.
2. System.cursor_field gives name of the field with block name.
List the system variables related in Block and Field?
1. System.block_status
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2. System.current_block
3. System.current_field
4. System.current_value
5. System.cursor_block
6. System.cursor_field
7. System.field_status.
What are the different types of Package Procedure ?
1. Restricted package procedure.
2. Unrestricted package procedure.
What are the types of TRIGGERS ?
1. Navigational Triggers.
2. Transaction Triggers.
Identify package function from the following ?
1. Error-Code
2. Break
3. Call
4. Error-text
5. Form-failure
6. Form-fatal
7. Execute-query
8. Anchor View
9. Message_code?
1. Error_Code
2. Error_Text
3. Form_Failure
4. Form_Fatal
5. Message_Code
Can you attach an lov to a field at run-time? if yes, give the build-in name.?
Yes. Set_item_proprety
Is it possible to attach same library to more than one form?
Yes
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Can you attach an lov to a field at design time?
Yes
List the windows event triggers available in Forms 4.0?
When-window-activated,
when-window-closed,
when-window-deactivated,
when-window-resized
What are the triggers associated with the image item?
When-Image-activated(Fires when the operator double clicks on an image Items)
When-image-pressed(fires when the operator selects or deselects the image item)
What is a visual attribute?
Visual Attributes are the font, color and pattern characteristics of objects that operators
see and intract with in our application.
How many maximum number of radio buttons can you assign to a radio group?
Unlimited no of radio buttons can be assigned to a radio group
How do you pass the parameters from one form to another form?
To pass one or more parameters to a called form, the calling form must perform the
following steps in a trigger or user named routine execute the create_parameter_list
built-in function to programmatically. Create a parameter list to execute the add
parameter built-in procedure to add one or more parameters list. Execute the
call_form, New_form or run_product built_in procedure and include the name or id of
the parameter list to be passed to the called form.
What is a Layout Editor?
The Layout Editor is a graphical design facility for creating and arranging items and
boilerplate text and graphics objects in your application's interface.
List the Types of Items?
Text item.
Chart item.
Check box.
Display item.
Image item.
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List item.
Radio Group.
User Area item.
List system variables available in forms 4.0, and not available in forms 3.0?
System.cordination_operation
System Date_threshold
System.effective_Date
System.event_window
System.suppress_working
What are the display styles of an alert?
Stop, Caution, note
What built-in is used for showing the alert during run-time?
Show_alert.
What built-in is used for changing the properties of the window dynamically?
Set_window_property
Canvas-View
What are the different types of windows?
Root window, secondary window.
What is a predefined exception available in forms 4.0?
Raise form_trigger_failure
What is a radio Group?
Radio groups display a fixed no of options that are mutually Exclusive. User can select
one out of n number of options.
What are the different type of a record group?
Query record group
Static record group
Non query record group
What are the menu items that oracle forms 4.0 supports?
Plain, Check,Radio, Separator, Magic
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Give the equivalent term in forms 4.0 for the following. Page, Page 0?
Page - Canvas-View
Page 0 - Canvas-view null.
What triggers are associated with the radio group?
Only when-radio-changed trigger associated with radio group
Visual Attributes.
What are the triggers associated with a check box?
Only When-checkbox-activated Trigger associated with a Check box.
Can you attach an alert to a field?
No
Can a root window be made modal?
No
What is a list item?
It is a list of text elements.
List some built-in routines used to manipulate images in image_item?
Image_add
Image_and
Image_subtract
Image_xor
Image_zoom
Can you change the alert messages at run-time?
If yes, give the name of the built-in to change the alert messages at run-time. Yes.
Set_alert_property.
What is the built-in used to get and set lov properties during run-time?
Get_lov_property
Set_lov_property
Record Group
What is the built-in routine used to count the no of rows in a group?
Get_group _row_count
System Variables
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Give the Types of modules in a form?
Form
Menu
Library
Write the Abbreviation for the following File Extension 1. FMB 2. MMB 3.
PLL?
FMB ----- Form Module Binary.
MMB ----- Menu Module Binary.
PLL ------ PL/SQL Library Module Binary.
List the built-in routine for controlling window during run-time?
Find_window,
get_window_property,
hide_window,
move_window,
resize_window,
set_window_property,
show_View
List the built-in routine for controlling window during run-time?
Find_canvas
Get-Canvas_property
Get_view_property
Hide_View
Replace_content_view
Scroll_view
Set_canvas_property
Set_view_property
Show_view
Alert
What is the built-in function used for finding the alert?
Find_alert
Editors
List the editors availables in forms 4.0?
Default editor
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User_defined editors
system editors.
What buil-in routines are used to display editor dynamically?
Edit_text item
show_editor
LOV
What is an Lov?
A list of values is a single or multi column selection list displayed in a pop-up window
What is a record Group?
A record group is an internal oracle forms data structure that has a similar column/row
frame work to a database table
Give built-in routine related to a record groups?
Create_group (Function)
Create_group_from_query(Function)
Delete_group(Procedure)
Add_group_column(Function)
Add_group_row(Procedure)
Delete_group_row(Procedure)
Populate_group(Function)
Populate_group_with_query(Function)
Set_group_Char_cell(procedure)
List the built-in routines for the controlling canvas views during run-time?
Find_canvas
Get-Canvas_property
Get_view_property
Hide_View
Replace_content_view
Scroll_view
Set_canvas_property
Set_view_property
Show_view
Alert
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System.effective_date system variable is read only True/False?
False
What are the built_in used to trapping errors in forms 4?
Error_type return character
Error_code return number
Error_text return char
Dbms_error_code return no.
Dbms_error_text return char
What is Oracle Financials? (for DBA)
Oracle Financials products provide organizations with solutions to a wide range of
long- and short-term accounting system issues. Regardless of the size of the business,
Oracle Financials can meet accounting management demands with:
Oracle Assets: Ensures that an organization's property and equipment investment is
accurate and that the correct asset tax accounting strategies are chosen.
Oracle General Ledger: Offers a complete solution to journal entry, budgeting,
allocations, consolidation, and financial reporting needs.
Oracle Inventory: Helps an organization make better inventory decisions by
minimizing stock and maximizing cash flow.
Oracle Order Entry: Provides organizations with a sophisticated order entry system for
managing customer commitments.
Oracle Payables: Lets an organization process more invoices with fewer staff members
and tighter controls. Helps save money through maximum discounts, bank float, and
prevention of duplicate payment.
Oracle Personnel: Improves the management of employee- related issues by retaining
and making available every form of personnel data.
Oracle Purchasing: Improves buying power, helps negotiate bigger discounts,
eliminates paper flow, increases financial controls, and increases productivity.
Oracle Receivables:. Improves cash flow by letting an organization process more
payments faster, without off-line research. Helps correctly account for cash, reduce
outstanding receivables, and improve collection effectiveness.
Oracle Revenue Accounting Gives an organization timely and accurate revenue and
flexible commissions reporting.
Oracle Sales Analysis: Allows for better forecasting, planning. and reporting of sales
information.
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What are the design facilities available in forms 4.0?
Default Block facility.
Layout Editor.
Menu Editor.
Object Lists.
Property Sheets.
PL/SQL Editor.
Tables Columns Browser.
Built-ins Browser.
What is the most important module in Oracle Financials? (for DBA)
The General Ledger (GL) module is the basis for all other Oracle Financial modules.
All other modules provide information to it. If you implement Oracle Financials, you
should switch your current GL system first.GL is relatively easy to implement. You
should go live with it first to give your implementation team a chance to be familiar
with Oracle Financials.
What are the types of canvas-views?
Content View, Stacked View.
What is the MultiOrg and what is it used for? (for DBA)
MultiOrg or Multiple Organizations Architecture allows multiple operating units and
their relationships to be defined within a single installation of Oracle Applications.
This keeps each operating unit's transaction data separate and secure.
Use the following query to determine if MuliOrg is intalled:
select multi_org_flag from fnd_product_groups;
What is the difference between Fields and FlexFields? (for DBA)
A field is a position on a form that one uses to enter, view, update, or delete
information. A field prompt describes each field by telling what kind of information
appears in the field, or alternatively, what kind of information should be entered in the
field.
A flexfield is an Oracle Applications field made up of segments. Each segment has an
assigned name and a set of valid values. Oracle Applications uses flexfields to capture
information about your organization. There are two types of flexfields: key flexfields
and descriptive flexfields.
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Explain types of Block in forms4.0?
Base table Blocks.
Control Blocks.
1. A base table block is one that is associated with a specific database table or view.
2. A control block is a block that is not associated with a database table. ITEMS
What is an Alert?
An alert is a modal window that displays a message notifies the operator of some
application condition
What are the built-in routines is available in forms 4.0 to create and manipulate a
parameter list?
Add_parameter
Create_Parameter_list
Delete_parameter
Destroy_parameter_list
Get_parameter_attr
Get_parameter_list
set_parameter_attr
What is a record Group?
A record group is an internal oracle forms data structure that has a similar column/row
frame work to a database table
What is a Navigable item?
A navigable item is one that operators can navigate to with the keyboard during
default navigation, or that Oracle forms can navigate to by executing a navigational
built-in procedure.
What is a library in Forms 4.0?
A library is a collection of Pl/SQL program units, including user named procedures,
functions & packages
How image_items can be populate to field in forms 4.0?
A fetch from a long raw database column PL/Sql assignment to executing the
read_image_file built_in procedure to get an image from the file system.
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What is the content view and stacked view?
A content view is the "Base" view that occupies the entire content pane of the window
in which it is displayed. A stacked view differs from a content canvas view in that it is
not the base view for the window to which it is assigned
What is a Check Box?
A Check Box is a two state control that indicates whether a certain condition or value
is on or off, true or false. The display state of a check box is always either "checked"
or "unchecked".
What is a canvas-view?
A canvas-view is the background object on which you layout the interface items (textitems, check boxes, radio groups, and so on.) and boilerplate objects that operators see
and interact with as they run your form. At run-time, operators can see only those
items that have been assigned to a specific canvas. Each canvas, in term, must be
displayed in a specific window.
Explain the following file extension related to library?
.pll,.lib,.pld
The library pll files is a portable design file comparable to an fmb form file
The library lib file is a plat form specific, generated library file comparable to a fmx
form file
The pld file is Txt format file and can be used for source controlling your library files
Parameter
Explain the usage of WHERE CURRENT OF clause in cursors ?
WHERE CURRENT OF clause in an UPDATE,DELETE statement refers to the latest
row fetched from a cursor. Database Triggers
Name the tables where characteristics of Package, procedure and functions are
stored ?
User_objects, User_Source and User_error.
Explain the two type of Cursors ?
There are two types of cursors, Implicit Cursor and Explicit Cursor. PL/SQL uses
Implicit Cursors for queries. User defined cursors are called Explicit Cursors. They
can be declared and used.
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What are two parts of package ?
The two parts of package are PACKAGE SPECIFICATION & PACKAGE BODY.
Package Specification contains declarations that are global to the packages and local to
the schema. Package Body contains actual procedures and local declaration of the
procedures and cursor declarations.
What are two virtual tables available during database trigger execution ?
The table columns are referred as OLD.column_name and NEW.column_name.
For triggers related to INSERT only NEW.column_name values only available.
For triggers related to UPDATE only OLD.column_name NEW.column_name values
only available.
For triggers related to DELETE only OLD.column_name values only available.
What is Fine Grained Auditing? (for DBA)
Fine Grained Auditing (DBMS_FGA) allows auditing records to be generated when
certain rows are selected from a table. A list of defined policies can be obtained from
DBA_AUDIT_POLICIES. Audit records are stored in DBA_FGA_AUDIT_TRAIL.
Look at this example:
o Add policy on table with autiting condition...
execute dbms_fga.add_policy('HR', 'EMP', 'policy1', 'deptno > 10');
o Must ANALYZE, this feature works with CBO (Cost Based Optimizer)
analyze table EMP compute statistics;
select * from EMP where c1 = 11; -- Will trigger auditing
select * from EMP where c1 = 09; -- No auditing
o Now we can see the statments that triggered the auditing condition...
select sqltext from sys.fga_log$;
delete from sys.fga_log$;
What is a package ? What are the advantages of packages ? What is Pragma
EXECPTION_INIT ? Explain the usage ?
The PRAGMA EXECPTION_INIT tells the complier to associate an exception with
an oracle error. To get an error message of a specific oracle error. e.g. PRAGMA
EXCEPTION_INIT (exception name, oracle error number)
What is a Virtual Private Database? (for DBA)
Oracle 8i introduced the notion of a Virtual Private Database (VPD). A VPD offers
Fine-Grained Access Control (FGAC) for secure separation of data. This ensures that
users only have access to data that pertains to them. Using this option, one could even
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store multiple companies' data within the same schema, without them knowing about
it. VPD configuration is done via the DBMS_RLS (Row Level Security) package.
Select from SYS.V$VPD_POLICY to see existing VPD configuration.
What is Raise_application_error ?
Raise_application_error is a procedure of package DBMS_STANDARD which allows
to issue an user_defined error messages from stored sub-program or database trigger.
What is Oracle Label Security? (for DBA)
Oracle Label Security (formerly called Trusted Oracle MLS RDBMS) uses the VPD
(Virtual Private Database) feature of Oracle8i to implement row level security. Access
to rows are restricted according to a user's security sensitivity tag or label. Oracle
Label Security is configured, controlled and managed from the Policy Manager, an
Enterprise Manager-based GUI utility.
Give the structure of the procedure ?
PROCEDURE name (parameter list.....)
is
local variable declarations
BEGIN
Executable statements.
Exception.
exception handlers
end;
What is OEM (Oracle Enterprise Manager)? (for DBA)
OEM is a set of systems management tools provided by Oracle Corporation for
managing the Oracle environment. It provides tools to monitor the Oracle environment
and automate tasks (both one-time and repetitive in nature) to take database
administration a step closer to "Lights Out" management.
Question What is PL/SQL ?
PL/SQL is a procedural language that has both interactive SQL and procedural
programming language constructs such as iteration, conditional branching.
What are the components of OEM? (for DBA)
Oracle Enterprise Manager (OEM) has the following components:
. Management Server (OMS): Middle tier server that handles communication with the
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intelligent agents. The OEM Console connects to the management server to monitor
and configure the Oracle enterprise.
. Console: This is a graphical interface from where one can schedule jobs, events, and
monitor the database. The console can be opened from a Windows workstation, Unix
XTerm (oemapp command) or Web browser session (oem_webstage).
. Intelligent Agent (OIA): The OIA runs on the target database and takes care of the
execution of jobs and events scheduled through the Console.
What happens if a procedure that updates a column of table X is called in a
database trigger of the same table ?
Mutation of table occurs.
Is it possible to use Transaction control Statements such a ROLLBACK or
COMMIT in Database Trigger ? Why ?
It is not possible. As triggers are defined for each table, if you use COMMIT of
ROLLBACK in a trigger, it affects logical transaction processing.
How many types of database triggers can be specified on a table ? What are they
?
Insert Update Delete
Before Row o.k. o.k. o.k.
After Row o.k. o.k. o.k.
Before Statement o.k. o.k. o.k.
After Statement o.k. o.k. o.k.
If FOR EACH ROW clause is specified, then the trigger for each Row affected by the
statement.
If WHEN clause is specified, the trigger fires according to the returned Boolean value.
What are the modes of parameters that can be passed to a procedure ?
IN,OUT,IN-OUT parameters.
Where the Pre_defined_exceptions are stored ?
In the standard package.
Procedures, Functions & Packages ;
Write the order of precedence for validation of a column in a table ?
I. done using Database triggers.
ii. done using Integarity Constraints.?
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I & ii.
Give the structure of the function ?
FUNCTION name (argument list .....) Return datatype is
local variable declarations
Begin
executable statements
Exception
execution handlers
End;
Explain how procedures and functions are called in a PL/SQL block ?
Function is called as part of an expression.
sal := calculate_sal ('a822');
procedure is called as a PL/SQL statement
calculate_bonus ('A822');
What are advantages fo Stored Procedures?
Extensibility,Modularity, Reusability, Maintainability and one time compilation.
What is an Exception ? What are types of Exception ?
Exception is the error handling part of PL/SQL block. The types are Predefined and
user defined. Some of Predefined exceptions are.
CURSOR_ALREADY_OPEN
DUP_VAL_ON_INDEX
NO_DATA_FOUND
TOO_MANY_ROWS
INVALID_CURSOR
INVALID_NUMBER
LOGON_DENIED
NOT_LOGGED_ON
PROGRAM-ERROR
STORAGE_ERROR
TIMEOUT_ON_RESOURCE
VALUE_ERROR
ZERO_DIVIDE
OTHERS.
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What are the PL/SQL Statements used in cursor processing ?
DECLARE CURSOR name, OPEN cursor name, FETCH cursor name INTO or
Record types, CLOSE cursor name.
What are the components of a PL/SQL Block ?
Declarative part, Executable part and Exception part.
Datatypes PL/SQL
What is a database trigger ? Name some usages of database trigger ?
Database trigger is stored PL/SQL program unit associated with a specific database
table. Usages are Audit data modifications, Log events transparently, Enforce complex
business rules Derive column values automatically, Implement complex security
authorizations. Maintain replicate tables.
What is a cursor ? Why Cursor is required ?
Cursor is a named private SQL area from where information can be accessed. Cursors
are required to process rows individually for queries returning multiple rows.
What is a cursor for loop ?
Cursor for loop implicitly declares %ROWTYPE as loop index, opens a cursor,
fetches rows of values from active set into fields in the record and closes when all the
records have been processed.
e.g.. FOR emp_rec IN C1 LOOP
salary_total := salary_total +emp_rec sal;
END LOOP;
What will happen after commit statement ?
Cursor C1 is
Select empno,
ename from emp;
Begin
open C1; loop
Fetch C1 into
eno.ename;
Exit When
C1 %notfound;----commit;
end loop;
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end;
The cursor having query as SELECT .... FOR UPDATE gets closed after
COMMIT/ROLLBACK.
The cursor having query as SELECT.... does not get closed even after
COMMIT/ROLLBACK.
How packaged procedures and functions are called from the following?
a. Stored procedure or anonymous block
b. an application program such a PRC *C, PRO* COBOL
c. SQL *PLUS??
a. PACKAGE NAME.PROCEDURE NAME (parameters);
variable := PACKAGE NAME.FUNCTION NAME (arguments);
EXEC SQL EXECUTE
b.BEGIN
PACKAGE NAME.PROCEDURE NAME (parameters)
variable := PACKAGE NAME.FUNCTION NAME (arguments);
END;
END EXEC;
c. EXECUTE PACKAGE NAME.PROCEDURE if the procedures does not have any
out/in-out parameters. A function can not be called.
What is a stored procedure ?
A stored procedure is a sequence of statements that perform specific function.
What are the components of a PL/SQL block ?
A set of related declarations and procedural statements is called block.
What is difference between a PROCEDURE & FUNCTION ?
A FUNCTION is always returns a value using the return statement.
A PROCEDURE may return one or more values through parameters or may not return
at all.
What is difference between a Cursor declared in a procedure and Cursor
declared in a package specification ?
A cursor declared in a package specification is global and can be accessed by other
procedures or procedures in a package.
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A cursor declared in a procedure is local to the procedure that can not be accessed by
other procedures.
What are the cursor attributes used in PL/SQL ?
%ISOPEN - to check whether cursor is open or not
% ROWCOUNT - number of rows fetched/updated/deleted.
% FOUND - to check whether cursor has fetched any row. True if rows are fetched.
% NOT FOUND - to check whether cursor has fetched any row. True if no rows are
featched.
These attributes are proceeded with SQL for Implicit Cursors and with Cursor name
for Explicit Cursors.
What are % TYPE and % ROWTYPE ? What are the advantages of using these
over datatypes?
% TYPE provides the data type of a variable or a database column to that variable.
% ROWTYPE provides the record type that represents a entire row of a table or view
or columns selected in the cursor.
The advantages are :
I. Need not know about variable's data type
ii. If the database definition of a column in a table changes, the data type of a variable
changes accordingly.
What is difference between % ROWTYPE and TYPE RECORD ?
% ROWTYPE is to be used whenever query returns a entire row of a table or view.
TYPE rec RECORD is to be used whenever query returns columns of different table
or views and variables.
E.g. TYPE r_emp is RECORD (eno emp.empno% type,ename emp ename %type );
e_rec emp% ROWTYPE
cursor c1 is select empno,deptno from emp;
e_rec c1 %ROWTYPE.
What are the different types of PL/SQL program units that can be defined and
stored in ORACLE database ?
Procedures and Functions,Packages and Database Triggers.
What are the advantages of having a Package ?
Increased functionality (for example,global package variables can be declared and
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used by any proecdure in the package) and performance (for example all objects of the
package are parsed compiled, and loaded into memory once)
What are the uses of Database Trigger ?
Database triggers can be used to automatic data generation, audit data modifications,
enforce complex Integrity constraints, and customize complex security authorizations.
What is a Procedure ?
A Procedure consist of a set of SQL and PL/SQL statements that are grouped together
as a unit to solve a specific problem or perform a set of related tasks.
What is a Package ?
A Package is a collection of related procedures, functions, variables and other package
constructs together as a unit in the database.
What is difference between Procedures and Functions ?
A Function returns a value to the caller where as a Procedure does not.
What is Database Trigger ?
A Database Trigger is procedure (set of SQL and PL/SQL statements) that is
automatically executed as a result of an insert in, update to, or delete from a table.
Can the default values be assigned to actual parameters?
Yes
Can a primary key contain more than one columns?
Yes
What is an UTL_FILE.What are different procedures and functions associated
with it?
UTL_FILE is a package that adds the ability to read and write to operating system
files. Procedures associated with it are FCLOSE, FCLOSE_ALL and 5 procedures to
output data to a file PUT, PUT_LINE, NEW_LINE, PUTF, FFLUSH.PUT,
FFLUSH.PUT_LINE,FFLUSH.NEW_LINE. Functions associated with it are FOPEN,
ISOPEN.
What are ORACLE PRECOMPILERS?
Using ORACLE PRECOMPILERS, SQL statements and PL/SQL blocks can be
contained inside 3GL programs written in C,C++,COBOL,PASCAL, FORTRAN,PL/1
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AND ADA. The Precompilers are known as Pro*C,Pro*Cobol,... This form of
PL/SQL is known as embedded pl/sql,the language in which pl/sql is embedded is
known as the host language. The prcompiler translates the embedded SQL and pl/sql
statements into calls to the precompiler runtime library. The output must be compiled
and linked with this library to creator an executable.
Differentiate between TRUNCATE and DELETE?
TRUNCATE deletes much faster than DELETE
TRUNCATE
DELETE
It is a DDL statement
It is a DML statement
It is a one way trip, cannot ROLLBACK
One can Rollback
Doesn't have selective features (where clause)
Has
Doesn't fire database triggers
Does
It requires disabling of referential constraints.
What is difference between a formal and an actual parameter?
The variables declared in the procedure and which are passed, as arguments are called
actual, the parameters in the procedure declaration. Actual parameters contain the
values that are passed to a procedure and receive results. Formal parameters are the
placeholders for the values of actual parameters
What should be the return type for a cursor variable. Can we use a scalar data
type as return type?
The return type for a cursor must be a record type.It can be declared explicitly as a
user-defined or %ROWTYPE can be used. eg TYPE t_studentsref IS REF CURSOR
RETURN students%ROWTYPE
What are different Oracle database objects?
-TABLES
-VIEWS
-INDEXES
-SYNONYMS
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-SEQUENCES
-TABLESPACES etc
What is difference between SUBSTR and INSTR?
SUBSTR returns a specified portion of a string eg SUBSTR('BCDEF',4) output BCDE
INSTR provides character position in which a pattern is found in a string. eg
INSTR('ABC-DC-F','-',2) output 7 (2nd occurence of '-')
Display the number value in Words?
SQL> select sal, (to_char(to_date(sal,'j'), 'jsp'))
from emp;
the output like,
SAL (TO_CHAR(TO_DATE(SAL,'J'),'JSP'))
--------- ---------------------------------------800 eight hundred
1600 one thousand six hundred
1250 one thousand two hundred fifty
If you want to add some text like, Rs. Three Thousand only.
SQL> select sal "Salary ",
(' Rs. '|| (to_char(to_date(sal,'j'), 'Jsp'))|| ' only.'))
"Sal in Words" from emp
/
Salary Sal in Words
------- ----------------------------------------------800 Rs. Eight Hundred only.
1600 Rs. One Thousand Six Hundred only.
1250 Rs. One Thousand Two Hundred Fifty only.
What is difference between SQL and SQL*PLUS?
SQL*PLUS is a command line tool where as SQL and PL/SQL language interface and
reporting tool. Its a command line tool that allows user to type SQL commands to be
executed directly against an Oracle database. SQL is a language used to query the
relational database(DML,DCL,DDL). SQL*PLUS commands are used to format
query result, Set options, Edit SQL commands and PL/SQL.
What are various joins used while writing SUBQUERIES?
Self join-Its a join foreign key of a table references the same table. Outer Join--Its a
join condition used where One can query all the rows of one of the tables in the join
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condition even though they don't satisfy the join condition.
Equi-join--Its a join condition that retrieves rows from one or more tables in which
one or more columns in one table are equal to one or more columns in the second
table.
What a SELECT FOR UPDATE cursor represent.?
SELECT......FROM......FOR......UPDATE[OF column-reference][NOWAIT]
The processing done in a fetch loop modifies the rows that have been retrieved by the
cursor. A convenient way of modifying the rows is done by a method with two parts:
the FOR UPDATE clause in the cursor declaration, WHERE CURRENT OF
CLAUSE in an UPDATE or declaration statement.
What are various privileges that a user can grant to another user?
-SELECT
-CONNECT
-RESOURCES
Display the records between two range?
select rownum, empno, ename from emp where rowid in (select rowid from emp
where rownum <=&upto minus select rowid from emp where rownum<&Start);
minvalue.sql Select the Nth lowest value from a table?
select level, min('col_name') from my_table where level = '&n' connect by prior
('col_name') < 'col_name')
group by level;
Example:
Given a table called emp with the following columns:
-- id number
-- name varchar2(20)
-- sal number
--- For the second lowest salary:
-- select level, min(sal) from emp
-- where level=2
-- connect by prior sal < sal
-- group by level
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What is difference between Rename and Alias?
Rename is a permanent name given to a table or column whereas Alias is a temporary
name given to a table or column which do not exist once the SQL statement is
executed.
Difference between an implicit & an explicit cursor.?
only one row. However,queries that return more than one row you must declare an
explicit cursor or use a cursor FOR loop. Explicit cursor is a cursor in which the cursor
name is explicitly assigned to a SELECT statement via the CURSOR...IS statement.
An implicit cursor is used for all SQL statements Declare, Open, Fetch, Close. An
explicit cursors are used to process multirow SELECT statements An implicit cursor is
used to process INSERT, UPDATE, DELETE and single row SELECT. .INTO
statements.
What is a OUTER JOIN?
Outer Join--Its a join condition used where you can query all the rows of one of the
tables in the join condition even though they don‘t satisfy the join condition.
What is a cursor?
Oracle uses work area to execute SQL statements and store processing information
PL/SQL construct called a cursor lets you name a work area and access its stored
information A cursor is a mechanism used to fetch more than one row in a Pl/SQl
block.
What is the purpose of a cluster?
Oracle does not allow a user to specifically locate tables, since that is a part of the
function of the RDBMS. However, for the purpose of increasing performance, oracle
allows a developer to create a CLUSTER. A CLUSTER provides a means for storing
data from different tables together for faster retrieval than if the table placement were
left to the RDBMS.
What is OCI. What are its uses?
Oracle Call Interface is a method of accesing database from a 3GL program. Uses--No
precompiler is required,PL/SQL blocks are executed like other DML statements.
The OCI library provides
--functions to parse SQL statemets
--bind input variables
--bind output variables
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--execute statements
--fetch the results
How you open and close a cursor variable. Why it is required?
OPEN cursor variable FOR SELECT...Statement
CLOSE cursor variable In order to associate a cursor variable with a particular
SELECT statement OPEN syntax is used. In order to free the resources used for the
query CLOSE statement is used.
Display Odd/ Even number of records?
Odd number of records:
select * from emp where (rowid,1) in (select rowid, mod(rownum,2) from emp);
Output:1
3
5
Even number of records:
select * from emp where (rowid,0) in (select rowid, mod(rownum,2) from emp)
Output:2
4
6
What are various constraints used in SQL?
-NULL
-NOT NULL
-CHECK
-DEFAULT
Can cursor variables be stored in PL/SQL tables. If yes how. If not why?
No, a cursor variable points a row which cannot be stored in a two-dimensional
PL/SQL table.
Difference between NO DATA FOUND and %NOTFOUND?
NO DATA FOUND is an exception raised only for the SELECT....INTO statements
when the where clause of the querydoes not match any rows. When the where clause
of the explicit cursor does not match any rows the %NOTFOUND attribute is set to
TRUE instead.
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Can you use a commit statement within a database trigger?
No
What WHERE CURRENT OF clause does in a cursor?
LOOP
SELECT num_credits INTO v_numcredits FROM classes
WHERE dept=123 and course=101;
UPDATE students
FHKO;;;;;;;;;SET current_credits=current_credits+v_numcredits
WHERE CURRENT OF X;
There is a string 120000 12 0 .125 , how you will find the position of the decimal
place?
INSTR('120000 12 0 .125',1,'.')
output 13
What are different modes of parameters used in functions and procedures?
-IN -OUT -INOUT
How you were passing cursor variables in PL/SQL 2.2?
In PL/SQL 2.2 cursor variables cannot be declared in a package.This is because the
storage for a cursor variable has to be allocated using Pro*C or OCI with version 2.2,
the only means of passing a cursor variable to a PL/SQL block is via bind variable or a
procedure parameter.
When do you use WHERE clause and when do you use HAVING clause?
HAVING clause is used when you want to specify a condition for a group function
and it is written after GROUP BY clause. The WHERE clause is used when you want
to specify a condition for columns, single row functions except group functions and it
is written before GROUP BY clause if it is used.
Difference between procedure and function.?
Functions are named PL/SQL blocks that return a value and can be called with
arguments procedure a named block that can be called with parameter. A procedure all
is a PL/SQL statement by itself, while a Function call is called as part of an
expression.
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Which is more faster - IN or EXISTS?
EXISTS is more faster than IN because EXISTS returns a Boolean value whereas IN
returns a value.
What is syntax for dropping a procedure and a function .Are these operations
possible?
Drop Procedure procedure_name
Drop Function function_name
How will you delete duplicating rows from a base table?
delete from table_name where rowid not in (select max(rowid) from table group by
duplicate_values_field_name); or delete duplicate_values_field_name dv from
table_name ta where rowid <(select min(rowid) from table_name tb where
ta.dv=tb.dv);
Difference between database triggers and form triggers?
-Data base trigger(DBT) fires when a DML operation is performed on a data base
table. Form trigger(FT) Fires when user presses a key or navigates between fields on
the screen
-Can be row level or statement level No distinction between row level and statement
level.
-Can manipulate data stored in Oracle tables via SQL Can manipulate data in Oracle
tables as well as variables in forms.
-Can be fired from any session executing the triggering DML statements. Can be fired
only from the form that define the trigger.
-Can cause other database triggers to fire. Can cause other database triggers to fire, but
not other form triggers.
What is a cursor for loop?
Cursor For Loop is a loop where oracle implicitly declares a loop variable, the loop
index that of the same record type as the cursor's record.
How you will avoid duplicating records in a query?
By using DISTINCT
What is a view ?
A view is stored procedure based on one or more tables, it‘s a virtual table.
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What is difference between UNIQUE and PRIMARY KEY constraints?
A table can have only one PRIMARY KEY whereas there can be any number of
UNIQUE keys. The columns that compose PK are automatically define NOT NULL,
whereas a column that compose a UNIQUE is not automatically defined to be
mandatory must also specify the column is NOT NULL.
What is use of a cursor variable? How it is defined?
A cursor variable is associated with different statements at run time, which can hold
different values at run time. Static cursors can only be associated with one run time
query. A cursor variable is reference type (like a pointer in C).
Declaring a cursor variable:
TYPE type_name IS REF CURSOR RETURN return_type type_name is the name of
the reference type,return_type is a record type indicating the types of the select list that
will eventually be returned by the cursor variable.
How do you find the numbert of rows in a Table ?
A bad answer is count them (SELECT COUNT(*) FROM table_name)
A good answer is :'By generating SQL to ANALYZE TABLE table_name COUNT STATISTICS by
querying Oracle System Catalogues (e.g. USER_TABLES or ALL_TABLES).
The best answer is to refer to the utility which Oracle released which makes it
unnecessary to do ANALYZE TABLE for each Table individually.
What is the maximum buffer size that can be specified using the
DBMS_OUTPUT.ENABLE function?
1,000,00
What are cursor attributes?
-%ROWCOUNT
-%NOTFOUND
-%FOUND
-%ISOPEN
There is a % sign in one field of a column. What will be the query to find it?
'' Should be used before '%'.
What is ON DELETE CASCADE ?
When ON DELETE CASCADE is specified ORACLE maintains referential integrity
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by automatically removing dependent foreign key values if a referenced primary or
unique key value is removed.
What is the fastest way of accessing a row in a table ?
Using ROWID.CONSTRAINTS
What is difference between TRUNCATE & DELETE ?
TRUNCATE commits after deleting entire table i.e., can not be rolled back. Database
triggers do not fire on TRUNCATEDELETE allows the filtered deletion. Deleted
records can be rolled back or committed. Database triggers fire on DELETE.
What is a transaction ?
Transaction is logical unit between two commits and commit and rollback.
What are the advantages of VIEW ?
To protect some of the columns of a table from other users.To hide complexity of a
query.To hide complexity of calculations.
How will you a activate/deactivate integrity constraints ?
The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE
constraint/DISABLE constraint.
Where the integrity constraints are stored in Data Dictionary ?
The integrity constraints are stored in USER_CONSTRAINTS.
What is the Subquery ?
Sub query is a query whose return values are used in filtering conditions of the main
query.
How to access the current value and next value from a sequence ? Is it possible to
access the current value in a session before accessing next value ?
Sequence name CURRVAL, Sequence name NEXTVAL.It is not possible. Only if
you access next value in the session, current value can be accessed.
What are the usage of SAVEPOINTS ?value in a session before accessing next
value ?
SAVEPOINTS are used to subdivide a transaction into smaller parts. It enables rolling
back part of a transaction. Maximum of five save points are allowed.
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What is ROWID ?in a session before accessing next value ?
ROWID is a pseudo column attached to each row of a table. It is 18 character long,
blockno, rownumber are the components of ROWID.
Explain Connect by Prior ?in a session before accessing next value ?
Retrieves rows in hierarchical order.e.g. select empno, ename from emp where.
How many LONG columns are allowed in a table ? Is it possible to use LONG
columns in WHERE clause or ORDER BY ?
Only one LONG columns is allowed. It is not possible to use LONG column in
WHERE or ORDER BY clause.
What is Referential Integrity ?
Maintaining data integrity through a set of rules that restrict the values of one or more
columns of the tables based on the values of primary key or unique key of the
referenced table.
What is a join ? Explain the different types of joins ?
Join is a query which retrieves related columns or rows from multiple tables.Self Join Joining the table with itself.Equi Join - Joining two tables by equating two common
columns.Non-Equi Join - Joining two tables by equating two common columns.Outer
Join - Joining two tables in such a way that query can also retrieve rows that do not
have corresponding join value in the other table.
If an unique key constraint on DATE column is created, will it validate the rows
that are inserted with SYSDATE ?
It won't, Because SYSDATE format contains time attached with it.
How does one stop and start the OMS? (for DBA)
Use the following command sequence to stop and start the OMS (Oracle Management
Server):
oemctl start oms
oemctl status oms sysman/oem_temp
oemctl stop oms sysman/oem_temp
Windows NT/2000 users can just stop and start the required services. The default
OEM administrator is "sysman" with a password of "oem_temp".
NOTE: Use command oemctrl instead of oemctl for Oracle 8i and below.
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What is an Integrity Constraint ?
Integrity constraint is a rule that restricts values to a column in a table.
How does one create a repository? (for DBA)
For OEM v2 and above, start the Oracle Enterprise Manager Configuration Assistant
(emca on Unix) to create and configure the management server and repository.
Remember to setup a backup for the repository database after creating it.
If a View on a single base table is manipulated will the changes be reflected on the
base table ?
If changes are made to the tables which are base tables of a view will the changes be
reference on the view.
The following describes means to create a OEM V1.x (very old!!!) repository on
WindowsNT:
. Create a tablespace that would hold the repository data. A size between 200- 250 MB
would be ideal. Let us call it Dummy_Space.
. Create an Oracle user who would own this repository. Assign DBA, SNMPAgent,
Exp_Full_database, Imp_Full_database roles to this user. Lets call this user
Dummy_user. Assign Dummy_Space as the default tablespace.
. Create an operating system user with the same name as the Oracle username. I.e.
Dummy_User. Add 'Log on as a batch job' under advanced rights in User manager.
. Fire up Enterprise manager and log in as Dummy_User and enter the password. This
would trigger the creation of the repository. From now on, Enterprise manager is ready
to accept jobs.
What is a database link ?
Database Link is a named path through which a remote database can be accessed.
How does one list one's databases in the OEM Console? (for DBA)
Follow these steps to discover databases and other services from the OEM Console:
1. Ensure the GLOBAL_DBNAME parameter is set for all databases in your
LISTENER.ORA file (optional). These names will be listed in the OEM Console.
Please note that names entered are case sensitive. A portion of a listener.ora file:
(SID_DESC =
(GLOBAL_DBNAME = DB_name_for_OEM)
(SID_NAME = ...
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2. Start the Oracle Intelligent Agent on the machine you want to discover. See section
"How does one start the Oracle Intelligent Agent?".
3. Start the OEM Console, navigate to menu "Navigator/ Discover Nodes". The OEM
Discovery Wizard will guide you through the process of discovering your databases
and other services.
What is CYCLE/NO CYCLE in a Sequence ?
CYCLE specifies that the sequence continues to generate values after reaching either
maximum or minimum value. After pan ascending sequence reaches its maximum
value, it generates its minimum value. After a descending sequence reaches its
minimum, it generates its maximum.NO CYCLE specifies that the sequence cannot
generate more values after reaching its maximum or minimum value.
What is correlated sub-query ?
Correlated sub query is a sub query which has reference to the main query.
What are the data types allowed in a table ?
CHAR,VARCHAR2,NUMBER,DATE,RAW,LONG and LONG RAW.
What is difference between CHAR and VARCHAR2 ? What is the maximum
SIZE allowed for each type ?
CHAR pads blank spaces to the maximum length. VARCHAR2 does not pad blank
spaces. For CHAR it is 255 and 2000 for VARCHAR2.
Can a view be updated/inserted/deleted? If Yes under what conditions ?
A View can be updated/deleted/inserted if it has only one base table if the view is
based on columns from one or more tables then insert, update and delete is not
possible.
What are the different types of Coordinations of the Master with the Detail
block?
POPULATE_GROUP(function)
POPULATE_GROUP_WITH_QUERY(function)
SET_GROUP_CHAR_CELL(procedure)
SET_GROUPCELL(procedure)
SET_GROUP_NUMBER_CELL(procedure)
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Use the ADD_GROUP_COLUMN function to add a column to a record group
that was created at design time?
I) TRUE II) FALSE
II) FALSE
Use the ADD_GROUP_ROW procedure to add a row to a static record group?
I) TRUE II) FALSE
I) FALSE
maxvalue.sql Select the Nth Highest value from a table?
select level, max('col_name') from my_table where level = '&n' connect by prior
('col_name') > 'col_name')
group by level;
Example:
Given a table called emp with the following columns:
-- id number
-- name varchar2(20)
-- sal number
--- For the second highest salary:
-- select level, max(sal) from emp
-- where level=2
-- connect by prior sal > sal
-- group by level
Find out nth highest salary from emp table?
SELECT DISTINCT (a.sal) FROM EMP A WHERE &N = (SELECT COUNT
(DISTINCT (b.sal)) FROM EMP B WHERE a.sal<=b.sal);
For E.g.:Enter value for n: 2
SAL
--------3700
Suppose a customer table is having different columns like customer no,
payments.What will be the query to select top three max payments?
SELECT customer_no, payments from customer C1
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WHERE 3<=(SELECT COUNT(*) from customer C2
WHERE C1.payment <= C2.payment)
How you will avoid your query from using indexes?
SELECT * FROM emp
Where emp_no+' '=12345;
i.e you have to concatenate the column name with space within codes in the where
condition.
SELECT /*+ FULL(a) */ ename, emp_no from emp
where emp_no=1234;
i.e using HINTS
What utility is used to create a physical backup?
Either rman or alter tablespace begin backup will do..
What are the Back ground processes in Oracle and what are they.
This is one of the most frequently asked question.There are basically 9 Processes but
in a general system we need to mention the first five background processes.They do
the house keeping activities for the Oracle and are common in any system.
The various background processes in oracle are
a) Data Base Writer(DBWR) :: Data Base Writer Writes Modified blocks from
Database buffer cache to Data Files.This is required since the data is not written
whenever a transaction is committed.
b)LogWriter(LGWR) :: LogWriter writes the redo log entries to disk. Redo Log data is
generated in redo log buffer of SGA. As transaction commits and log buffer fills,
LGWR writes log entries into a online redo log file.
c) System Monitor(SMON) :: The System Monitor performs instance recovery at
instance startup. This is useful for recovery from system failure
d)Process Monitor(PMON) :: The Process Monitor performs process recovery when
user Process fails. Pmon Clears and Frees resources that process was using.
e) CheckPoint(CKPT) :: At Specified times, all modified database buffers in SGA are
written to data files by DBWR at Checkpoints and Updating all data files and control
files of database to indicate the most recent checkpoint
f)Archieves(ARCH) :: The Archiver copies online redo log files to archival storal
when they are busy.
g) Recoveror(RECO) :: The Recoveror is used to resolve the distributed transaction in
network
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h) Dispatcher (Dnnn) :: The Dispatcher is useful in Multi Threaded Architecture
i) Lckn :: We can have upto 10 lock processes for inter instance locking in parallel sql.
How many types of Sql Statements are there in Oracle
There are basically 6 types of sql statments.They are
a) Data Definition Language(DDL) :: The DDL statements define and maintain
objects and drop objects.
b) Data Manipulation Language(DML) :: The DML statements manipulate database
data.
c) Transaction Control Statements :: Manage change by DML
d) Session Control :: Used to control the properties of current session enabling and
disabling roles and changing .e.g. :: Alter Statements, Set Role
e) System Control Statements :: Change Properties of Oracle Instance .e.g.:: Alter
System
f) Embedded Sql :: Incorporate DDL, DML and T.C.S in Programming Language.e.g::
Using the Sql Statements in languages such as 'C', Open, Fetch, execute and close
What is a Transaction in Oracle
A transaction is a Logical unit of work that compromises one or more SQL Statements
executed by a single User. According to ANSI, a transaction begins with first
executable statement and ends when it is explicitly committed or rolled back.
Key Words Used in Oracle
The Key words that are used in Oracle are ::
a) Committing :: A transaction is said to be committed when the transaction makes
permanent changes resulting from the SQL statements.
b) Rollback :: A transaction that retracts any of the changes resulting from SQL
statements in Transaction.
c) SavePoint :: For long transactions that contain many SQL statements, intermediate
markers or savepoints are declared. Savepoints can be used to divide a transaction into
smaller points.
d) Rolling Forward :: Process of applying redo log during recovery is called rolling
forward.
e) Cursor :: A cursor is a handle ( name or a pointer) for the memory associated with a
specific stamen. A cursor is basically an area allocated by Oracle for executing the Sql
Statement. Oracle uses an implicit cursor statement for Single row query and Uses
Explicit cursor for a multi row query.
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f) System Global Area(SGA) :: The SGA is a shared memory region allocated by the
Oracle that contains Data and control information for one Oracle Instance. It consists
of Database Buffer Cache and Redo log Buffer.
g) Program Global Area (PGA) :: The PGA is a memory buffer that contains data and
control information for server process.
g) Database Buffer Cache :: Database Buffer of SGA stores the most recently used
blocks of database data. The set of database buffers in an instance is called Database
Buffer Cache.
h) Redo log Buffer :: Redo log Buffer of SGA stores all the redo log entries.
i) Redo Log Files :: Redo log files are set of files that protect altered database data in
memory that has not been written to Data Files. They are basically used for backup
when a database crashes.
j) Process :: A Process is a 'thread of control' or mechanism in Operating System that
executes series of steps.
What are Procedure, functions and Packages ?
Procedures and functions consist of set of PL/SQL statements that are grouped
together as a unit to solve a specific problem or perform set of related tasks.
Procedures do not Return values while Functions return one One Value Packages ::
Packages Provide a method of encapsulating and storing related procedures, functions,
variables and other Package Contents
What are Database Triggers and Stored Procedures
Database Triggers :: Database Triggers are Procedures that are automatically executed
as a result of insert in, update to, or delete from table.
Database triggers have the values old and new to denote the old value in the table
before it is deleted and the new indicated the new value that will be used. DT are
useful for implementing complex business rules which cannot be enforced using the
integrity rules.We can have the trigger as Before trigger or After Trigger and at
Statement or Row level. e.g:: operations insert,update ,delete 3 before ,after 3*2 A
total of 6 combinatons
At statment level(once for the trigger) or row level( for every execution ) 6 * 2 A total
of 12. Thus a total of 12 combinations are there and the restriction of usage of 12
triggers has been lifted from Oracle 7.3 Onwards.
Stored Procedures :: Stored Procedures are Procedures that are stored in Compiled
form in the database.The advantage of using the stored procedures is that many users
can use the same procedure in compiled and ready to use format.
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How many Integrity Rules are there and what are they
There are Three Integrity Rules. They are as follows ::
a) Entity Integrity Rule :: The Entity Integrity Rule enforces that the Primary key
cannot be Null
b) Foreign Key Integrity Rule :: The FKIR denotes that the relationship between the
foreign key and the primary key has to be enforced.When there is data in Child Tables
the Master tables cannot be deleted.
c) Business Integrity Rules :: The Third Intigrity rule is about the complex business
processes which cannot be implemented by the above 2 rules.
What are the Various Master and Detail Relation ships.
The various Master and Detail Relationship are
a) NonIsolated :: The Master cannot be deleted when a child is exisiting
b) Isolated :: The Master can be deleted when the child is exisiting
c) Cascading :: The child gets deleted when the Master is deleted.
What are the Various Block Coordination Properties
The various Block Coordination Properties are
a) Immediate Default Setting. The Detail records are shown when the Master Record
are shown.
b) Deffered with Auto Query Oracle Forms defer fetching the detail records until the
operator navigates to the detail block.
c) Deffered with No Auto Query The operator must navigate to the detail block and
explicitly execute a query
What are the Different Optimization Techniques
The Various Optimisation techniques are
a) Execute Plan :: we can see the plan of the query and change it accordingly based on
the indexes
b) Optimizer_hint ::
set_item_property('DeptBlock',OPTIMIZER_HINT,'FIRST_ROWS');
Select /*+ First_Rows */ Deptno,Dname,Loc,Rowid from dept
where (Deptno > 25)
c) Optimize_Sql ::
By setting the Optimize_Sql = No, Oracle Forms assigns a single cursor for all SQL
statements.This slow downs the processing because for evertime the SQL must be
parsed whenver they are executed.
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f45run module = my_firstform userid = scott/tiger optimize_sql = No
d) Optimize_Tp ::
By setting the Optimize_Tp= No, Oracle Forms assigns seperate cursor only for each
query SELECT statement. All other SQL statements reuse the cursor.
f45run module = my_firstform userid = scott/tiger optimize_Tp = No
How does one change an Oracle user's password?(for DBA)
Issue the following SQL command:
ALTER USER <username> IDENTIFIED BY <new_password>;
From Oracle8 you can just type "password" from SQL*Plus, or if you need to change
another user's password, type "password user_name". Look at this example:
SQL> password
Changing password for SCOTT
Old password:
New password:
Retype new password:
How does one create and drop database users?
Look at these examples:
CREATE USER scott
IDENTIFIED BY tiger -- Assign password
DEFAULT TABLESACE tools -- Assign space for table and index segments
TEMPORARY TABLESPACE temp; -- Assign sort space
DROP USER scott CASCADE; -- Remove user
After creating a new user, assign the required privileges:
GRANT CONNECT, RESOURCE TO scott;
GRANT DBA TO scott; -- Make user a DB Administrator
Remember to give the user some space quota on its tablespaces:
ALTER USER scott QUOTA UNLIMITED ON tools;
Who created all these users in my database?/ Can I drop this user? (for DBA)
Oracle creates a number of default database users or schemas when a new database is
created. Below are a few of them:
SYS/CHANGE_ON_INSTALL or INTERNAL
Oracle Data Dictionary/ Catalog
Created by: ?/rdbms/admin/sql.bsq and various cat*.sql scripts
Can password be changed: Yes (Do so right after the database was created)
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Can user be dropped: NO
SYSTEM/MANAGER
The default DBA user name (please do not use SYS)
Created by: ?/rdbms/admin/sql.bsq
Can password be changed: Yes (Do so right after the database was created)
Can user be dropped: NO
OUTLN/OUTLN
Stored outlines for optimizer plan stability
Created by: ?/rdbms/admin/sql.bsq
Can password be changed: Yes (Do so right after the database was created)
Can user be dropped: NO
SCOTT/TIGER, ADAMS/WOOD, JONES/STEEL, CLARK/CLOTH and
BLAKE/PAPER.
Training/ demonstration users containing the popular EMP and DEPT tables
Created by: ?/rdbms/admin/utlsampl.sql
Can password be changed: Yes
Can user be dropped: YES - Drop users cascade from all production environments
HR/HR (Human Resources), OE/OE (Order Entry), SH/SH (Sales History).
Training/ demonstration users containing the popular EMPLOYEES and
DEPARTMENTS tables
Created by: ?/demo/schema/mksample.sql
Can password be changed: Yes
Can user be dropped: YES - Drop users cascade from all production environments
CTXSYS/CTXSYS
Oracle interMedia (ConText Cartridge) administrator user
Created by: ?/ctx/admin/dr0csys.sql
TRACESVR/TRACE
Oracle Trace server
Created by: ?/rdbms/admin/otrcsvr.sql
DBSNMP/DBSNMP
Oracle Intelligent agent
Created by: ?/rdbms/admin/catsnmp.sql, called from catalog.sql
Can password be changed: Yes - put the new password in snmp_rw.ora file
Can user be dropped: YES - Only if you do not use the Intelligent Agents
ORDPLUGINS/ORDPLUGINS
Object Relational Data (ORD) User used by Time Series, etc.
Created by: ?/ord/admin/ordinst.sql
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ORDSYS/ORDSYS
Object Relational Data (ORD) User used by Time Series, etc
Created by: ?/ord/admin/ordinst.sql
DSSYS/DSSYS
Oracle Dynamic Services and Syndication Server
Created by: ?/ds/sql/dssys_init.sql
MDSYS/MDSYS
Oracle Spatial administrator user
Created by: ?/ord/admin/ordinst.sql
AURORA$ORB$UNAUTHENTICATED/INVALID
Used for users who do not authenticate in Aurora/ORB
Created by: ?/javavm/install/init_orb.sql called from ?/javavm/install/initjvm.sql
PERFSTAT/PERFSTAT
Oracle Statistics Package (STATSPACK) that supersedes UTLBSTAT/UTLESTAT
Created by: ?/rdbms/admin/statscre.sql
Remember to change the passwords for the SYS and SYSTEM users immediately
after installation!
Except for the user SYS, there should be no problem altering these users to use a
different default and temporary tablespace.
How does one enforce strict password control? (for DBA)
By default Oracle's security is not extremely good. For example, Oracle will allow
users to choose single character passwords and passwords that match their names and
userids. Also, passwords don't ever expire. This means that one can hack an account
for years without ever locking the user.
From Oracle8 one can manage passwords through profiles. Some of the things that one
can restrict:
. FAILED_LOGIN_ATTEMPTS - failed login attempts before the account is locked
. PASSWORD_LIFE_TIME - limits the number of days the same password can be
used for authentication
. PASSWORD_REUSE_TIME - number of days before a password can be reused
. PASSWORD_REUSE_MAX - number of password changes required before the
current password can be reused
. PASSWORD_LOCK_TIME - number of days an account will be locked after
maximum failed login attempts
. PASSWORD_GRACE_TIME - number of days after the grace period begins during
which a warning is issued and login is allowed
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. PASSWORD_VERIFY_FUNCTION - password complexity verification script
Look at this simple example:
CREATE PROFILE my_profile LIMIT
PASSWORD_LIFE_TIME 30;
ALTER USER scott PROFILE my_profile;
How does one switch to another user in Oracle? (for DBA)
Users normally use the "connect" statement to connect from one database user to
another. However, DBAs can switch from one user to another without a password. Of
course it is not advisable to bridge Oracle's security, but look at this example: SQL>
select password from dba_users where username='SCOTT';
PASSWORD
F894844C34402B67
SQL> alter user scott identified by lion;
User altered.
SQL> connect scott/lion
Connected.
REM Do whatever you like...
SQL> connect system/manager
Connected.
SQL> alter user scott identified by values 'F894844C34402B67';
User altered.
SQL> connect scott/tiger
Connected.
Note: Also see the su.sql script in the Useful Scripts and Sample Programs Page
What are snap shots and views
Snapshots are mirror or replicas of tables. Views are built using the columns from one
or more tables. The Single Table View can be updated but the view with multi table
cannot be updated
What are the OOPS concepts in Oracle.
Oracle does implement the OOPS concepts. The best example is the Property Classes.
We can categorize the properties by setting the visual attributes and then attach the
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property classes for the objects. OOPS supports the concepts of objects and classes
and we can consider the property classes as classes and the items as objects
What is the difference between candidate key, unique key and primary key
Candidate keys are the columns in the table that could be the primary keys and the
primary key is the key that has been selected to identify the rows. Unique key is also
useful for identifying the distinct rows in the table.)
What is concurrency
Concurrency is allowing simultaneous access of same data by different users. Locks
useful for accesing the database are
a) Exclusive
The exclusive lock is useful for locking the row when an insert,update or delete is
being done.This lock should not be applied when we do only select from the row.
b) Share lock
We can do the table as Share_Lock as many share_locks can be put on the same
resource.
Previleges and Grants
Previleges are the right to execute a particulare type of SQL statements. e.g :: Right to
Connect, Right to create, Right to resource Grants are given to the objects so that the
object might be accessed accordingly.The grant has to be given by the owner of the
object
Table Space,Data Files,Parameter File, Control Files
Table Space :: The table space is useful for storing the data in the database.When a
database is created two table spaces are created.
a) System Table space :: This data file stores all the tables related to the system and
dba tables
b) User Table space :: This data file stores all the user related tables
We should have seperate table spaces for storing the tables and indexes so that the
access is fast.
Data Files :: Every Oracle Data Base has one or more physical data files.They store
the data for the database.Every datafile is associated with only one database.Once the
Data file is created the size cannot change.To increase the size of the database to store
more data we have to add data file.
Parameter Files :: Parameter file is needed to start an instance.A parameter file
contains the list of instance configuration parameters e.g.::
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db_block_buffers = 500
db_name = ORA7
db_domain = u.s.acme lang
Control Files :: Control files record the physical structure of the data files and redo log
files
They contain the Db name, name and location of dbs, data files ,redo log files and time
stamp.
Physical Storage of the Data
The finest level of granularity of the data base are the data blocks.
Data Block :: One Data Block correspond to specific number of physical database
space
Extent :: Extent is the number of specific number of contigious data blocks.
Segments :: Set of Extents allocated for Extents. There are three types of Segments
a) Data Segment :: Non Clustered Table has data segment data of every table is stored
in cluster data segment
b) Index Segment :: Each Index has index segment that stores data
c) Roll Back Segment :: Temporarily store 'undo' information
What are the Pct Free and Pct Used
Pct Free is used to denote the percentage of the free space that is to be left when
creating a table. Similarly Pct Used is used to denote the percentage of the used space
that is to be used when creating a table
eg.:: Pctfree 20, Pctused 40
What is Row Chaining
The data of a row in a table may not be able to fit the same data block.Data for row is
stored in a chain of data blocks .
What is a 2 Phase Commit
Two Phase commit is used in distributed data base systems. This is useful to maintain
the integrity of the database so that all the users see the same values. It contains DML
statements or Remote Procedural calls that reference a remote object. There are
basically 2 phases in a 2 phase commit.
a) Prepare Phase :: Global coordinator asks participants to prepare
b) Commit Phase :: Commit all participants to coordinator to Prepared, Read only or
abort Reply
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What is the difference between deleting and truncating of tables
Deleting a table will not remove the rows from the table but entry is there in the
database dictionary and it can be retrieved But truncating a table deletes it completely
and it cannot be retrieved.
What are mutating tables
When a table is in state of transition it is said to be mutating. eg :: If a row has been
deleted then the table is said to be mutating and no operations can be done on the table
except select.
What are Codd Rules
Codd Rules describe the ideal nature of a RDBMS. No RDBMS satisfies all the 12
codd rules and Oracle Satisfies 11 of the 12 rules and is the only Rdbms to satisfy the
maximum number of rules.
What is Normalisation
Normalisation is the process of organising the tables to remove the redundancy.There
are mainly 5 Normalisation rules.
a) 1 Normal Form :: A table is said to be in 1st Normal Form when the attributes are
atomic
b) 2 Normal Form :: A table is said to be in 2nd Normal Form when all the candidate
keys are dependant on the primary key
c) 3rd Normal Form :: A table is said to be third Normal form when it is not dependant
transitively
What is the Difference between a post query and a pre query
A post query will fire for every row that is fetched but the pre query will fire only
once.
Deleting the Duplicate rows in the table
We can delete the duplicate rows in the table by using the Rowid
Can U disable database trigger? How?
Yes. With respect to table
ALTER TABLE TABLE
[[ DISABLE all_trigger ]]
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What is pseudo columns ? Name them?
A pseudocolumn behaves like a table column, but is not actually stored in the table.
You can select from pseudocolumns, but you cannot insert, update, or delete their
values. This section describes these pseudocolumns:
* CURRVAL
* NEXTVAL
* LEVEL
* ROWID
* ROWNUM
How many columns can table have?
The number of columns in a table can range from 1 to 254.
Is space acquired in blocks or extents ?
In extents .
What is clustered index?
In an indexed cluster, rows are stored together based on their cluster key values . Can
not applied for HASH.
What are the datatypes supported By oracle (INTERNAL)?
Varchar2, Number,Char , MLSLABEL.
What are attributes of cursor?
%FOUND , %NOTFOUND , %ISOPEN,%ROWCOUNT
Can you use select in FROM clause of SQL select ?
Yes.
Which trigger are created when master -detail relay?
master delete property
* NON-ISOLATED (default)
a) on check delete master
b) on clear details
c) on populate details
* ISOLATED
a) on clear details
b) on populate details
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* CASCADE
a) per-delete
b) on clear details
c) on populate details
which system variables can be set by users?
SYSTEM.MESSAGE_LEVEL
SYSTEM.DATE_THRESHOLD
SYSTEM.EFFECTIVE_DATE
SYSTEM.SUPPRESS_WORKING
What are object group?
An object group is a container for a group of objects. You define an object group when
you want to package related objects so you can copy or reference them in another
module.
What are referenced objects?
Referencing allows you to create objects that inherit their functionality and appearance
from other objects. Referencing an object is similar to copying an object, except that
the resulting reference object maintains a link to its source object. A reference object
automatically inherits any changes that have been made to the source object when you
open or regenerate the module that contains the reference object.
Can you store objects in library?
Referencing allows you to create objects that inherit their functionality and appearance
from other objects. Referencing an object is similar to copying an object, except that
the resulting reference object maintains a link to its source object. A reference object
automatically inherits any changes that have been made to the source object when you
open or regenerate the module that contains the reference object.
Is forms 4.5 object oriented tool ? why?
yes , partially. 1) PROPERTY CLASS - inheritance property 2) OVERLOADING :
procedures and functions.
Can you issue DDL in forms?
yes, but you have to use FORMS_DDL.
Referencing allows you to create objects that inherit their functionality and appearance
from other objects. Referencing an object is similar to copying an object, except that
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the resulting reference object maintains a link to its source object. A reference object
automatically inherits any changes that have been made to the source object when you
open or regenerate the module that contains the reference object. Any string
expression up to 32K:
- a literal
- an expression or a variable representing the text of a block of dynamically created
PL/SQL code
- a DML statement or
- a DDL statement
Restrictions:
The statement you pass to FORMS_DDL may not contain bind variable references in
the string, but the values of bind variables can be concatenated into the string before
passing the result to FORMS_DDL.
What is SECURE property?
- Hides characters that the operator types into the text item. This setting is typically
used for password protection.
What are the types of triggers and how the sequence of firing in text item
Triggers can be classified as Key Triggers, Mouse Triggers ,Navigational Triggers.
Key Triggers :: Key Triggers are fired as a result of Key action.e.g :: Key-next-field,
Key-up,Key-Down
Mouse Triggers :: Mouse Triggers are fired as a result of the mouse navigation.e.g.
When-mouse-button-presed,when-mouse-doubleclicked,etc
Navigational Triggers :: These Triggers are fired as a result of Navigation. E.g. : PostText-item,Pre-text-item.
We also have event triggers like when ?new-form-instance and when-new-blockinstance.
We cannot call restricted procedures like go_to(?my_block.first_item?) in the
Navigational triggers
But can use them in the Key-next-item.
The Difference between Key-next and Post-Text is an very important question. The
key-next is fired as a result of the key action while the post text is fired as a result of
the mouse movement. Key next will not fire unless there is a key event. The sequence
of firing in a text item are as follows ::
a) pre - text
b) when new item
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c) key-next
d) when validate
e) post text
Can you store pictures in database? How?
Yes , in long Raw datatype.
What are property classes ? Can property classes have trigger?
Property class inheritance is a powerful feature that allows you to quickly define
objects that conform to your own interface and functionality standards. Property
classes also allow you to make global changes to applications quickly. By simply
changing the definition of a property class, you can change the definition of all objects
that inherit properties from that class.
Yes . All type of triggers .
If you have property class attached to an item and you have same trigger written
for the item . Which will fire first?
Item level trigger fires , If item level trigger fires, property level trigger won't fire.
Triggers at the lowest level are always given the first preference. The item level
trigger fires first and then the block and then the Form level trigger.
What are record groups ? * Can record groups created at run-time?
A record group is an internal Oracle Forms data structure that has a column/row
framework similar to a database table. However, unlike database tables, record groups
are separate objects that belong to the form module in which they are defined. A
record group can have an unlimited number of columns of type CHAR, LONG,
NUMBER, or DATE provided that the total number of columns does not exceed 64K.
Record group column names cannot exceed 30 characters.
Programmatically, record groups can be used whenever the functionality offered by a
two-dimensional array of multiple data types is desirable.
TYPES OF RECORD GROUP:
Query Record Group A query record group is a record group that has an associated
SELECT statement. The columns in a query record group derive their default names,
data types, and lengths from the database columns referenced in the SELECT
statement. The records in a query record group are the rows retrieved by the query
associated with that record group.
Non-query Record Group A non-query record group is a group that does not have an
associated query, but whose structure and values can be modified programmatically at
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runtime.
Static Record Group A static record group is not associated with a query; rather, you
define its structure and row values at design time, and they remain fixed at runtime.
What are ALERT?
An ALERT is a modal window that displays a message notifying operator of some
application condition.
Can a button have icon and label at the same time ?
-NO
What is mouse navigate property of button?
When Mouse Navigate is True (the default), Oracle Forms performs standard
navigation to move the focus to the item when the operator activates the item with the
mouse.
When Mouse Navigate is set to False, Oracle Forms does not perform navigation (and
the resulting validation) to move to the item when an operator activates the item with
the mouse.
What is FORMS_MDI_WINDOW?
forms run inside the MDI application window. This property is useful for calling a
form from another one.
What are timers ? when when-timer-expired does not fire?
The When-Timer-Expired trigger can not fire during trigger, navigation, or transaction
processing.
Can object group have a block?
Yes , object group can have block as well as program units.
How many types of canvases are there.
There are 2 types of canvases called as Content and Stack Canvas. Content canvas is
the default and the one that is used mostly for giving the base effect. Its like a plate on
which we add items and stacked canvas is used for giving 3 dimensional effect.
What are user-exits?
It invokes 3GL programs.
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Can you pass values to-and-fro from foreign function ? how ?
Yes . You obtain a return value from a foreign function by assigning the return value
to an Oracle Forms variable or item. Make sure that the Oracle Forms variable or item
is the same data type as the return value from the foreign function.
After assigning an Oracle Forms variable or item value to a PL/SQL variable, pass the
PL/SQL variable as a parameter value in the PL/SQL interface of the foreign function.
The PL/SQL variable that is passed as a parameter must be a valid PL/SQL data type;
it must also be the appropriate parameter type as defined in the PL/SQL interface.
What is IAPXTB structure ?
The entries of Pro * C and user exits and the form which simulate the proc or user_exit
are stored in IAPXTB table in d/b.
Can you call WIN-SDK thru user exits?
YES.
Does user exits supports DLL on MSWINDOWS ?
YES .
What is path setting for DLL?
Make sure you include the name of the DLL in the FORMS45_USEREXIT variable of
the ORACLE.INI file, or rename the DLL to F45XTB.DLL. If you rename the DLL to
F45XTB.DLL, replace the existing F45XTB.DLL in the ORAWINBIN directory with
the new F45XTB.DLL.
How is mapping of name of DLL and function done?
The dll can be created using the Visual C++ / Visual Basic Tools and then the dll is
put in the path that is defined the registry.
What is precompiler?
It is similar to C precompiler directives.
Can you connect to non - oracle datasource ?
Yes .
What are key-mode and locking mode properties? level ?
Key Mode : Specifies how oracle forms uniquely identifies rows in the database.This
is property includes for application that will run against NON-ORACLE datasources .
Key setting unique (default.)
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dateable
n-updateable.
Locking mode :
Specifies when Oracle Forms should attempt to obtain database locks on rows that
correspond to queried records in the form. a) immediate b) delayed
What are savepoint mode and cursor mode properties ? level?
Specifies whether Oracle Forms should issue savepoints during a session. This
property is included primarily for applications that will run against non-ORACLE data
sources. For applications that will run against ORACLE, use the default setting.
Cursor mode - define cursor state across transaction Open/close.
What is transactional trigger property?
Identifies a block as transactional control block. i.e. non - database block that oracle
forms should manage as transactional block.(NON-ORACLE datasource) default FALSE.
What is OLE automation ?
OLE automation allows an OLE server application to expose a set of commands and
functions that can be invoked from an OLE container application. OLE automation
provides a way for an OLE container application to use the features of an OLE server
application to manipulate an OLE object from the OLE container environment.
(FORMS_OLE)
What does invoke built-in do?
This procedure invokes a method.
Syntax:
PROCEDURE OLE2.INVOKE
(object obj_type,
method VARCHAR2,
list list_type := 0);
Parameters:
object Is an OLE2 Automation Object.
method Is a method (procedure) of the OLE2 object.
list Is the name of an argument list assigned to the OLE2.CREATE_ARGLIST
function.
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What are OPEN_FORM,CALL_FORM,NEW_FORM? diff?
CALL_FORM : It calls the other form. but parent remains active, when called form
completes the operation , it releases lock and control goes back to the calling form.
When you call a form, Oracle Forms issues a savepoint for the called form. If the
CLEAR_FORM function causes a rollback when the called form is current, Oracle
Forms rolls back uncommitted changes to this savepoint.
OPEN_FORM : When you call a form, Oracle Forms issues a savepoint for the called
form. If the CLEAR_FORM function causes a rollback when the called form is
current, Oracle Forms rolls back uncommitted changes to this savepoint.
NEW_FORM : Exits the current form and enters the indicated form. The calling form
is terminated as the parent form. If the calling form had been called by a higher form,
Oracle Forms keeps the higher call active and treats it as a call to the new form. Oracle
Forms releases memory (such as database cursors) that the terminated form was
using.
Oracle Forms runs the new form with the same Runform options as the parent form. If
the parent form was a called form, Oracle Forms runs the new form with the same
options as the parent form.
What is call form stack?
When successive forms are loaded via the CALL_FORM procedure, the resulting
module hierarchy is known as the call form stack.
Can u port applictions across the platforms? how?
Yes we can port applications across platforms.Consider the form developed in a
windows system.The form would be generated in unix system by using f45gen
my_form.fmb scott/tiger
What is a visual attribute?
Visual attributes are the font, color, and pattern properties that you set for form and
menu objects that appear in your application's interface.
Diff. between VAT and Property Class?
Named visual attributes define only font, color, and pattern attributes; property classes
can contain these and any other properties.
You can change the appearance of objects at runtime by changing the named visual
attribute programmatically; property class assignment cannot be changed
programmatically. When an object is inheriting from both a property class and a
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named visual attribute, the named visual attribute settings take precedence, and any
visual attribute properties in the class are ignored.
Which trigger related to mouse?
When-Mouse-Click
When-Mouse-DoubleClick
When-Mouse-Down
When-Mouse-Enter
When-Mouse-Leave
When-Mouse-Move
When-Mouse-Up
What is Current record attribute property?
Specifies the named visual attribute used when an item is part of the current record.
Current Record Attribute is frequently used at the block level to display the current
row in a multi-record If you define an item-level Current Record Attribute, you can
display a pre-determined item in a special color when it is part of the current record,
but you cannot dynamically highlight the current item, as the input focus changes.
Can u change VAT at run time?
Yes. You can programmatically change an object's named visual attribute setting to
change the font, color, and pattern of the object at runtime.
Can u set default font in forms?
Yes. Change windows registry(regedit). Set form45_font to the desired font.
_break
What is Log Switch ?
The point at which ORACLE ends writing to one online redo log file and begins
writing to another is called a log switch.
What is On-line Redo Log?
The On-line Redo Log is a set of tow or more on-line redo files that record all
committed changes made to the database. Whenever a transaction is committed, the
corresponding redo entries temporarily stores in redo log buffers of the SGA are
written to an on-line redo log file by the background process LGWR. The on-line redo
log files are used in cyclical fashion.
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Which parameter specified in the DEFAULT STORAGE clause of CREATE
TABLESPACE cannot be altered after creating the tablespace?
All the default storage parameters defined for the tablespace can be changed using the
ALTER TABLESPACE command. When objects are created their INITIAL and
MINEXTENS values cannot be changed.
What are the steps involved in Database Startup ?
Start an instance, Mount the Database and Open the Database.
Rolling forward to recover data that has not been recorded in data files, yet has been
recorded in the on-line redo log, including the contents of rollback segments. Rolling
back transactions that have been explicitly rolled back or have not been committed as
indicated by the rollback segments regenerated in step a. Releasing any resources
(locks) held by transactions in process at the time of the failure. Resolving any
pending distributed transactions undergoing a two-phase commit at the time of the
instance failure.
Can Full Backup be performed when the database is open ?
No.
What are the different modes of mounting a Database with the Parallel Server ?
Exclusive Mode If the first instance that mounts a database does so in exclusive mode,
only that Instance can mount the database.
Parallel Mode If the first instance that mounts a database is started in parallel mode,
other instances that are started in parallel mode can also mount the database.
What are the advantages of operating a database in ARCHIVELOG mode over
operating it in NO ARCHIVELOG mode ?
Complete database recovery from disk failure is possible only in ARCHIVELOG
mode. Online database backup is possible only in ARCHIVELOG mode.
What are the steps involved in Database Shutdown ?
Close the Database, Dismount the Database and Shutdown the Instance.
What is Archived Redo Log ?
Archived Redo Log consists of Redo Log files that have archived before being
reused.
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What is Restricted Mode of Instance Startup ?
An instance can be started in (or later altered to be in) restricted mode so that when the
database is open connections are limited only to those whose user accounts have been
granted the RESTRICTED SESSION system privilege.
Can u have OLE objects in forms?
Yes.
Can u have VBX and OCX controls in forms ?
Yes.
What r the types of windows (Window style)?
Specifies whether the window is a Document window or a Dialog window.
What is OLE Activation style property?
Specifies the event that will activate the OLE containing item.
Can u change the mouse pointer ? How?
Yes. Specifies the mouse cursor style. Use this property to dynamically change the
shape of the cursor.
How many types of columns are there and what are they
Formula columns :: For doing mathematical calculations and returning one value
Summary Columns :: For doing summary calculations such as summations etc. Place
holder Columns :: These columns are useful for storing the value in a variable
Can u have more than one layout in report
It is possible to have more than one layout in a report by using the additional layout
option in the layout editor.
Can u run the report with out a parameter form
Yes it is possible to run the report without parameter form by setting the PARAM
value to Null
What is the lock option in reports layout
By using the lock option we cannot move the fields in the layout editor outside the
frame. This is useful for maintaining the fields .
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What is Flex
Flex is the property of moving the related fields together by setting the flex property
on
What are the minimum number of groups required for a matrix report
The minimum of groups required for a matrix report are 4 e ----What is a Synonym ?
A synonym is an alias for a table, view, sequence or program unit.
What is a Sequence ?
A sequence generates a serial list of unique numbers for numerical columns of a
database's tables.
What is a Segment ?
A segment is a set of extents allocated for a certain logical structure.
What is schema?
A schema is collection of database objects of a User.
Describe Referential Integrity ?
A rule defined on a column (or set of columns) in one table that allows the insert or
update of a row only if the value for the column or set of columns (the dependent
value) matches a value in a column of a related table (the referenced value). It also
specifies the type of data manipulation allowed on referenced data and the action to be
performed on dependent data as a result of any action on referenced data.
What is Hash Cluster ?
A row is stored in a hash cluster based on the result of applying a hash function to the
row's cluster key value. All rows with the same hash key value are stores together on
disk.
What is a Private Synonyms ?
A Private Synonyms can be accessed only by the owner.
What is Database Link ?
A database link is a named object that describes a "path" from one database to another.
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What is index cluster?
A cluster with an index on the cluster key.
What is hash cluster?
A row is stored in a hash cluster based on the result of applying a hash function to the
row's cluster key value. All rows with the same hash key value are stores together on
disk.
When can hash cluster used?
Hash clusters are better choice when a table is often queried with equality queries. For
such queries the specified cluster key value is hashed. The resulting hash key value
points directly to the area on disk that stores the specified rows.
When can hash cluster used?
Hash clusters are better choice when a table is often queried with equality queries. For
such queries the specified cluster key value is hashed. The resulting hash key value
points directly to the area on disk that stores the specified rows.
What are the types of database links?
Private database link, public database link & network database link.
What is private database link?
Private database link is created on behalf of a specific user. A private database link can
be used only when the owner of the link specifies a global object name in a SQL
statement or in the definition of the owner's views or procedures.
What is public database link?
Public database link is created for the special user group PUBLIC. A public database
link can be used when any user in the associated database specifies a global object
name in a SQL statement or object definition.
What is network database link?
Network database link is created and managed by a network domain service. A
network database link can be used when any user of any database in the network
specifies a global object name in a SQL statement or object definition.
What is data block?
Oracle database's data is stored in data blocks. One data block corresponds to a
specific number of bytes of physical database space on disk.
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How to define data block size?
A data block size is specified for each Oracle database when the database is created. A
database users and allocated free database space in Oracle data blocks. Block size is
specified in init.ora file and cannot be changed latter.
What is row chaining?
In circumstances, all of the data for a row in a table may not be able to fit in the same
data block. When this occurs, the data for the row is stored in a chain of data block
(one or more) reserved for that segment.
What is an extent?
An extent is a specific number of contiguous data blocks, obtained in a single
allocation and used to store a specific type of information.
What are the different types of segments?
Data segment, index segment, rollback segment and temporary segment.
What is a data segment?
Each non-clustered table has a data segment. All of the table's data is stored in the
extents of its data segment. Each cluster has a data segment. The data of every table in
the cluster is stored in the cluster's data segment.
What is an index segment?
Each index has an index segment that stores all of its data.
What is rollback segment?
A database contains one or more rollback segments to temporarily store "undo"
information.
What are the uses of rollback segment?
To generate read-consistent database information during database recovery and to
rollback uncommitted transactions by the users.
What is a temporary segment?
Temporary segments are created by Oracle when a SQL statement needs a temporary
work area to complete execution. When the statement finishes execution, the
temporary segment extents are released to the system for future use.
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What is a datafile?
Every Oracle database has one or more physical data files. A database's data files
contain all the database data. The data of logical database structures such as tables and
indexes is physically stored in the data files allocated for a database.
What are the characteristics of data files?
A data file can be associated with only one database. Once created a data file can't
change size. One or more data files form a logical unit of database storage called a
tablespace.
What is a redo log?
The set of redo log files for a database is collectively known as the database redo log.
What is the function of redo log?
The primary function of the redo log is to record all changes made to data.
What is the use of redo log information?
The information in a redo log file is used only to recover the database from a system
or media failure prevents database data from being written to a database's data files.
What does a control file contains?
- Database name
- Names and locations of a database's files and redolog files.
- Time stamp of database creation.
What is the use of control file?
When an instance of an Oracle database is started, its control file is used to identify the
database and redo log files that must be opened for database operation to proceed. It is
also used in database recovery.
Is it possible to split the print reviewer into more than one region?
Yes
Is it possible to center an object horizontally in a repeating frame that has a
variable horizontal size?
Yes
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For a field in a repeating frame, can the source come from the column which does
not exist in the data group which forms the base for the frame?
Yes
Can a field be used in a report without it appearing in any data group?
Yes
The join defined by the default data link is an outer join yes or no?
Yes
Can a formula column referred to columns in higher group?
Yes
Can a formula column be obtained through a select statement?
Yes
Is it possible to insert comments into sql statements return in the data model
editor?
Yes
Is it possible to disable the parameter from while running the report?
Yes
When a form is invoked with call_form, Does oracle forms issues a save point?
Yes
Explain the difference between a hot backup and a cold backup and the benefits
associated with each.
A hot backup is basically taking a backup of the database while it is still up and
running and it must be in archive log mode. A cold backup is taking a backup of the
database while it is shut down and does not require being in archive log mode. The
benefit of taking a hot backup is that the database is still available for use while the
backup is occurring and you can recover the database to any point in time. The benefit
of taking a cold backup is that it is typically easier to administer the backup and
recovery process. In addition, since you are taking cold backups the database does not
require being in archive log mode and thus there will be a slight performance gain as
the database is not cutting archive logs to disk.
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You have just had to restore from backup and do not have any control files. How
would you go about bringing up this database?
I would create a text based backup control file, stipulating where on disk all the data
files where and then issue the recover command with the using backup control file
clause.
How do you switch from an init.ora file to a spfile?
Issue the create spfile from pfile command.
Explain the difference between a data block, an extent and a segment.
A data block is the smallest unit of logical storage for a database object. As objects
grow they take chunks of additional storage that are composed of contiguous data
blocks. These groupings of contiguous data blocks are called extents. All the extents
that an object takes when grouped together are considered the segment of the database
object.
Give two examples of how you might determine the structure of the table DEPT.
Use the describe command or use the dbms_metadata.get_ddl package.
Where would you look for errors from the database engine?
In the alert log.
Compare and contrast TRUNCATE and DELETE for a table.
Both the truncate and delete command have the desired outcome of getting rid of all
the rows in a table. The difference between the two is that the truncate command is a
DDL operation and just moves the high water mark and produces a now rollback. The
delete command, on the other hand, is a DML operation, which will produce a
rollback and thus take longer to complete.
Give the reasoning behind using an index.
Faster access to data blocks in a table.
Give the two types of tables involved in producing a star schema and the type of
data they hold.
Fact tables and dimension tables. A fact table contains measurements while dimension
tables will contain data that will help describe the fact tables.
What type of index should you use on a fact table?
A Bitmap index.
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Give two examples of referential integrity constraints.
A primary key and a foreign key.
A table is classified as a parent table and you want to drop and re-create it. How
would you do this without affecting the children tables?
Disable the foreign key constraint to the parent, drop the table, re-create the table,
enable the foreign key constraint.
Explain the difference between ARCHIVELOG mode and NOARCHIVELOG
mode and the benefits and disadvantages to each.
ARCHIVELOG mode is a mode that you can put the database in for creating a backup
of all transactions that have occurred in the database so that you can recover to any
point in time. NOARCHIVELOG mode is basically the absence of ARCHIVELOG
mode and has the disadvantage of not being able to recover to any point in time.
NOARCHIVELOG mode does have the advantage of not having to write transactions
to an archive log and thus increases the performance of the database slightly.
What command would you use to create a backup control file?
Alter database backup control file to trace.
Give the stages of instance startup to a usable state where normal users may
access it.
STARTUP NOMOUNT - Instance startup
STARTUP MOUNT - The database is mounted
STARTUP OPEN - The database is opened
What column differentiates the V$ views to the GV$ views and how?
The INST_ID column which indicates the instance in a RAC environment the
information came from.
How would you go about generating an EXPLAIN plan?
Create a plan table with utlxplan.sql.
Use the explain plan set statement_id = 'tst1' into plan_table for a SQL statement
Look at the explain plan with utlxplp.sql or utlxpls.sql
How would you go about increasing the buffer cache hit ratio?
Use the buffer cache advisory over a given workload and then query the
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v$db_cache_advice table. If a change was necessary then I would use the alter system
set db_cache_size command.
Explain an ORA-01555
You get this error when you get a snapshot too old within rollback. It can usually be
solved by increasing the undo retention or increasing the size of rollbacks. You should
also look at the logic involved in the application getting the error message.
Explain the difference between $ORACLE_HOME and $ORACLE_BASE.
ORACLE_BASE is the root directory for oracle. ORACLE_HOME located beneath
ORACLE_BASE is where the oracle products reside.
How would you determine the time zone under which a database was operating?
select DBTIMEZONE from dual;
Explain the use of setting GLOBAL_NAMES equal to TRUE.
Setting GLOBAL_NAMES dictates how you might connect to a database. This
variable is either TRUE or FALSE and if it is set to TRUE it enforces database links to
have the same name as the remote database to which they are linking.
What command would you use to encrypt a PL/SQL application?
WRAP
Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.
A function and procedure are the same in that they are intended to be a collection of
PL/SQL code that carries a single task. While a procedure does not have to return any
values to the calling application, a function will return a single value. A package on
the other hand is a collection of functions and procedures that are grouped together
based on their commonality to a business function or application.
Explain the use of table functions.
Table functions are designed to return a set of rows through PL/SQL logic but are
intended to be used as a normal table or view in a SQL statement. They are also used
to pipeline information in an ETL process.
Name three advisory statistics you can collect.
Buffer Cache Advice, Segment Level Statistics, & Timed Statistics
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Where in the Oracle directory tree structure are audit traces placed?
In unix $ORACLE_HOME/rdbms/audit, in Windows the event viewer
Explain materialized views and how they are used.
Materialized views are objects that are reduced sets of information that have been
summarized, grouped, or aggregated from base tables. They are typically used in data
warehouse or decision support systems.
When a user process fails, what background process cleans up after it?
PMON
What background process refreshes materialized views?
The Job Queue Processes.
How would you determine what sessions are connected and what resources they
are waiting for?
Use of V$SESSION and V$SESSION_WAIT
Describe what redo logs are.
Redo logs are logical and physical structures that are designed to hold all the changes
made to a database and are intended to aid in the recovery of a database.
How would you force a log switch?
ALTER SYSTEM SWITCH LOGFILE;
Give two methods you could use to determine what DDL changes have been
made.
You could use Logminer or Streams
What does coalescing a tablespace do?
Coalescing is only valid for dictionary-managed tablespaces and de-fragments space
by combining neighboring free extents into large single extents.
What is the difference between a TEMPORARY tablespace and a PERMANENT
tablespace?
A temporary tablespace is used for temporary objects such as sort structures while
permanent tablespaces are used to store those objects meant to be used as the true
objects of the database.
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Name a tablespace automatically created when you create a database.
The SYSTEM tablespace.
When creating a user, what permissions must you grant to allow them to connect
to the database?
Grant the CONNECT to the user.
How do you add a data file to a tablespace
ALTER TABLESPACE <tablespace_name> ADD DATAFILE <datafile_name>
SIZE
How do you resize a data file?
ALTER DATABASE DATAFILE <datafile_name> RESIZE <new_size>;
What view would you use to look at the size of a data file?
DBA_DATA_FILES
What view would you use to determine free space in a tablespace?
DBA_FREE_SPACE
How would you determine who has added a row to a table?
Turn on fine grain auditing for the table.
How can you rebuild an index?
ALTER INDEX <index_name> REBUILD;
Explain what partitioning is and what its benefit is.
Partitioning is a method of taking large tables and indexes and splitting them into
smaller, more manageable pieces.
You have just compiled a PL/SQL package but got errors, how would you view
the errors?
SHOW ERRORS
How can you gather statistics on a table?
The ANALYZE command.
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How can you enable a trace for a session?
Use the DBMS_SESSION.SET_SQL_TRACE or
Use ALTER SESSION SET SQL_TRACE = TRUE;
What is the difference between the SQL*Loader and IMPORT utilities?
These two Oracle utilities are used for loading data into the database. The difference is
that the import utility relies on the data being produced by another Oracle utility
EXPORT while the SQL*Loader utility allows data to be loaded that has been
produced by other utilities from different data sources just so long as it conforms to
ASCII formatted or delimited files.
Name two files used for network connection to a database.
TNSNAMES.ORA and SQLNET.ORA
What is the function of Optimizer ?
The goal of the optimizer is to choose the most efficient way to execute a SQL
statement.
What is Execution Plan ?
The combinations of the steps the optimizer chooses to execute a statement is called an
execution plan.
Can one resize tablespaces and data files? (for DBA)
One can manually increase or decrease the size of a datafile from Oracle 7.2 using the
command.
ALTER DATABASE DATAFILE 'filename2' RESIZE 100M;
Because you can change the sizes of datafiles, you can add more space to your
database without adding more datafiles. This is beneficial if you are concerned about
reaching the maximum number of datafiles allowed in your database.
Manually reducing the sizes of datafiles allows you to reclaim unused space in the
database. This is useful for correcting errors in estimations of space requirements.
Also, datafiles can be allowed to automatically extend if more space is required. Look
at the following command:
CREATE TABLESPACE pcs_data_ts
DATAFILE 'c:\ora_apps\pcs\pcsdata1.dbf' SIZE 3M
AUTOEXTEND ON NEXT 1M MAXSIZE UNLIMITED
DEFAULT STORAGE (INITIAL 10240
NEXT 10240
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MINEXTENTS 1
MAXEXTENTS UNLIMITED
PCTINCREASE 0)
ONLINE
PERMANENT;
What is SAVE POINT ?
For long transactions that contain many SQL statements, intermediate markers or
savepoints can be declared which can be used to divide a transaction into smaller
parts. This allows the option of later rolling back all work performed from the current
point in the transaction to a declared savepoint within the transaction.
What are the values that can be specified for OPTIMIZER MODE Parameter ?
COST and RULE.
Can one rename a tablespace? (for DBA)
No, this is listed as Enhancement Request 148742. Workaround:
Export all of the objects from the tablespace
Drop the tablespace including contents
Recreate the tablespace
Import the objects
What is RULE-based approach to optimization ?
Choosing an executing planbased on the access paths available and the ranks of these
access paths.
What are the values that can be specified for OPTIMIZER_GOAL parameter of
the ALTER SESSION Command ?
CHOOSE,ALL_ROWS,FIRST_ROWS and RULE.
How does one create a standby database? (for DBA)
While your production database is running, take an (image copy) backup and restore it
on duplicate hardware. Note that an export will not work!!!
On your standby database, issue the following commands:
ALTER DATABASE CREATE STANDBY CONTROLFILE AS 'filename';
ALTER DATABASE MOUNT STANDBY DATABASE;
RECOVER STANDBY DATABASE;
On systems prior to Oracle 8i, write a job to copy archived redo log files from the
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primary database to the standby system, and apply the redo log files to the standby
database (pipe it). Remember the database is recovering and will prompt you for the
next log file to apply.
Oracle 8i onwards provide an "Automated Standby Database" feature, which will send
archived, log files to the remote site via NET8, and apply then to the standby
database.
When one needs to activate the standby database, stop the recovery process and
activate it:
ALTER DATABASE ACTIVATE STANDBY DATABASE;
How does one give developers access to trace files (required as input to tkprof)?
(for DBA)
The "alter session set sql_trace=true" command generates trace files in
USER_DUMP_DEST that can be used by developers as input to tkprof. On Unix the
default file mask for these files are "rwx r-- ---".
There is an undocumented INIT.ORA parameter that will allow everyone to read (rwx
r-r--) these trace files:
_trace_files_public = true
Include this in your INIT.ORA file and bounce your database for it to take effect.
What are the responsibilities of a Database Administrator ?
Installing and upgrading the Oracle Server and application tools. Allocating system
storage and planning future storage requirements for the database system. Managing
primary database structures (tablespaces) Managing primary objects
(table,views,indexes) Enrolling users and maintaining system security. Ensuring
compliance with Oralce license agreement Controlling and monitoring user access to
the database. Monitoring and optimizing the performance of the database. Planning for
backup and recovery of database information. Maintain archived data on tape Backing
up and restoring the database. Contacting Oracle Corporation for technical support.
What is a trace file and how is it created ?
Each server and background process can write an associated trace file. When an
internal error is detected by a process or user process, it dumps information about the
error to its trace. This can be used for tuning the database.
What are the roles and user accounts created automatically with the database?
DBA - role Contains all database system privileges.
SYS user account - The DBA role will be assigned to this account. All of the base
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tables and views for the database's dictionary are store in this schema and are
manipulated only by ORACLE. SYSTEM user account - It has all the system
privileges for the database and additional tables and views that display administrative
information and internal tables and views used by oracle tools are created using this
username.
What are the minimum parameters should exist in the parameter file (init.ora) ?
DB NAME - Must set to a text string of no more than 8 characters and it will be stored
inside the datafiles, redo log files and control files and control file while database
creation.
DB_DOMAIN - It is string that specifies the network domain where the database is
created. The global database name is identified by setting these parameters
(DB_NAME & DB_DOMAIN) CONTORL FILES - List of control filenames of the
database. If name is not mentioned then default name will be used.
DB_BLOCK_BUFFERS - To determine the no of buffers in the buffer cache in SGA.
PROCESSES - To determine number of operating system processes that can be
connected to ORACLE concurrently. The value should be 5 (background process) and
additional 1 for each user.
ROLLBACK_SEGMENTS - List of rollback segments an ORACLE instance acquires
at database startup. Also optionally
LICENSE_MAX_SESSIONS,LICENSE_SESSION_WARNING and
LICENSE_MAX_USERS.
Why and when should I backup my database? (for DBA)
Backup and recovery is one of the most important aspects of a DBAs job. If you lose
your company's data, you could very well lose your job. Hardware and software can
always be replaced, but your data may be irreplaceable!
Normally one would schedule a hierarchy of daily, weekly and monthly backups,
however consult with your users before deciding on a backup schedule. Backup
frequency normally depends on the following factors:
. Rate of data change/ transaction rate
. Database availability/ Can you shutdown for cold backups?
. Criticality of the data/ Value of the data to the company
. Read-only tablespace needs backing up just once right after you make it read-only
. If you are running in archivelog mode you can backup parts of a database over an
extended cycle of days
. If archive logging is enabled one needs to backup archived log files timeously to
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prevent database freezes
. Etc.
Carefully plan backup retention periods. Ensure enough backup media (tapes) are
available and that old backups are expired in-time to make media available for new
backups. Off-site vaulting is also highly recommended.
Frequently test your ability to recover and document all possible scenarios.
Remember, it's the little things that will get you. Most failed recoveries are a result of
organizational errors and miscommunications.
What strategies are available for backing-up an Oracle database? (for DBA)
The following methods are valid for backing-up an Oracle database:
Export/Import - Exports are "logical" database backups in that they extract logical
definitions and data from the database to a file.
Cold or Off-line Backups - Shut the database down and backup up ALL data, log, and
control files.
Hot or On-line Backups - If the databases are available and in ARCHIVELOG mode,
set the tablespaces into backup mode and backup their files. Also remember to backup
the control files and archived redo log files.
RMAN Backups - While the database is off-line or on-line, use the "rman" utility to
backup the database.
It is advisable to use more than one of these methods to backup your database. For
example, if you choose to do on-line database backups, also cover yourself by doing
database exports. Also test ALL backup and recovery scenarios carefully. It is better to
be save than sorry.
Regardless of your strategy, also remember to backup all required software libraries,
parameter files, password files, etc. If your database is in ARCGIVELOG mode, you
also need to backup archived log files.
What is the difference between online and offline backups? (for DBA)
A hot backup is a backup performed while the database is online and available for
read/write. Except for Oracle exports, one can only do on-line backups when running
in ARCHIVELOG mode.
A cold backup is a backup performed while the database is off-line and unavailable to
its users.
What is the difference between restoring and recovering? (for DBA)
Restoring involves copying backup files from secondary storage (backup media) to
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disk. This can be done to replace damaged files or to copy/move a database to a new
location.
Recovery is the process of applying redo logs to the database to roll it forward. One
can roll-forward until a specific point-in-time (before the disaster occurred), or rollforward until the last transaction recorded in the log files. Sql> connect SYS as
SYSDBA
Sql> RECOVER DATABASE UNTIL TIME '2001-03-06:16:00:00' USING
BACKUP CONTROLFILE;
How does one backup a database using the export utility? (for DBA)
Oracle exports are "logical" database backups (not physical) as they extract data and
logical definitions from the database into a file. Other backup strategies normally
back-up the physical data files.
One of the advantages of exports is that one can selectively re-import tables, however
one cannot roll-forward from an restored export file. To completely restore a database
from an export file one practically needs to recreate the entire database.
Always do full system level exports (FULL=YES). Full exports include more
information about the database in the export file than user level exports.
What are the built_ins used the display the LOV?
Show_lov
List_values
How do you call other Oracle Products from Oracle Forms?
Run_product is a built-in, Used to invoke one of the supported oracle tools products
and specifies the name of the document or module to be run. If the called product is
unavailable at the time of the call, Oracle Forms returns a message to the operator.
What is the main diff. bet. Reports 2.0 & Reports 2.5?
Report 2.5 is object oriented.
What are the Built-ins to display the user-named editor?
A user named editor can be displayed programmatically with the built in procedure
SHOW-EDITOR, EDIT_TETITEM independent of any particular text item.
How many number of columns a record group can have?
A record group can have an unlimited number of columns of type CHAR, LONG,
NUMBER, or DATE provided that the total number of column does not exceed 64K.
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What is a Query Record Group?
A query record group is a record group that has an associated SELECT statement. The
columns in a query record group derive their default names, data types, had lengths
from the database columns referenced in the SELECT statement. The records in query
record group are the rows retrieved by the query associated with that record group.
What does the term panel refer to with regard to pages?
A panel is the no. of physical pages needed to print one logical page.
What is a master detail relationship?
A master detail relationship is an association between two base table blocks- a master
block and a detail block. The relationship between the blocks reflects a primary key to
foreign key relationship between the tables on which the blocks are based.
What is a library?
A library is a collection of subprograms including user named procedures, functions
and packages.
What is an anchoring object & what is its use? What are the various sub events a
mouse double click event involves?
An anchoring object is a print condition object which used to explicitly or implicitly
anchor other objects to itself.
Use the add_group_column function to add a column to record group that was
created at a design time?
False
What are the various sub events a mouse double click event involves? What are
the various sub events a mouse double click event involves?
Double clicking the mouse consists of the mouse down, mouse up, mouse click, mouse
down & mouse up events.
What is the use of break group? What are the various sub events a mouse double
click event involves?
A break group is used to display one record for one group ones. While multiple related
records in other group can be displayed.
What tuning indicators can one use? (for DBA)
The following high-level tuning indicators can be used to establish if a database is
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performing optimally or not:
. Buffer Cache Hit Ratio
Formula: Hit Ratio = (Logical Reads - Physical Reads) / Logical Reads
Action: Increase DB_CACHE_SIZE (DB_BLOCK_BUFFERS prior to 9i) to increase
hit ratio
. Library Cache Hit Ratio
Action: Increase the SHARED_POOL_SIZE to increase hit ratio
What tools/utilities does Oracle provide to assist with performance tuning? (for
DBA)
Oracle provide the following tools/ utilities to assist with performance monitoring and
tuning:
. TKProf
. UTLBSTAT.SQL and UTLESTAT.SQL - Begin and end stats monitoring
. Statspack
. Oracle Enterprise Manager - Tuning Pack
What is STATSPACK and how does one use it? (for DBA)
Statspack is a set of performance monitoring and reporting utilities provided by Oracle
from Oracle8i and above. Statspack provides improved BSTAT/ESTAT functionality,
though the old BSTAT/ESTAT scripts are still available. For more information about
STATSPACK, read the documentation in file
$ORACLE_HOME/rdbms/admin/spdoc.txt.
Install Statspack:
cd $ORACLE_HOME/rdbms/admin
sqlplus "/ as sysdba" @spdrop.sql -- Install Statspack sqlplus "/ as sysdba" @spcreate.sql-- Enter tablespace names when prompted
Use Statspack:
sqlplus perfstat/perfstat
exec statspack.snap; -- Take a performance snapshots
exec statspack.snap;
o Get a list of snapshots
select SNAP_ID, SNAP_TIME from STATS$SNAPSHOT;
@spreport.sql -- Enter two snapshot id's for difference report
Other Statspack Scripts:
. sppurge.sql - Purge a range of Snapshot Id's between the specified begin and end
Snap Id's
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. spauto.sql - Schedule a dbms_job to automate the collection of STATPACK
statistics
. spcreate.sql - Installs the STATSPACK user, tables and package on a database (Run
as SYS).
. spdrop.sql - Deinstall STATSPACK from database (Run as SYS)
. sppurge.sql - Delete a range of Snapshot Id's from the database
. spreport.sql - Report on differences between values recorded in two snapshots
. sptrunc.sql - Truncates all data in Statspack tables
What are the common RMAN errors (with solutions)? (for DBA)
Some of the common RMAN errors are:
RMAN-20242: Specification does not match any archivelog in the recovery catalog.
Add to RMAN script: sql 'alter system archive log current';
RMAN-06089: archived log xyz not found or out of sync with catalog
Execute from RMAN: change archivelog all validate;
How can you execute the user defined triggers in forms 3.0 ?
Execute Trigger (trigger-name)
What ERASE package procedure does ?
Erase removes an indicated global variable.
What is the difference between NAME_IN and COPY ?
Copy is package procedure and writes values into a field.
Name in is a package function and returns the contents of the variable to which you
apply.
What package procedure is used for calling another form ?
Call (E.g. Call(formname)
When the form is running in DEBUG mode, If you want to examine the values of
global variables and other form variables, What package procedure command
you would use in your trigger text ?
Break.
SYSTEM VARIABLES
The value recorded in system.last_record variable is of type
a. Number
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b. Boolean
c. Character. ?
b. Boolean.
What is mean by Program Global Area (PGA) ?
It is area in memory that is used by a Single Oracle User Process.
What is hit ratio ?
It is a measure of well the data cache buffer is handling requests for data. Hit Ratio =
(Logical Reads - Physical Reads - Hits Misses)/ Logical Reads.
How do u implement the If statement in the Select Statement
We can implement the if statement in the select statement by using the Decode
statement. e.g. select DECODE (EMP_CAT,'1','First','2','Second'Null); Here the Null
is the else statement where null is done .
How many types of Exceptions are there
There are 2 types of exceptions. They are
a) System Exceptions
e.g. When no_data_found, When too_many_rows
b) User Defined Exceptions
e.g. My_exception exception
When My_exception then
What are the inline and the precompiler directives
The inline and precompiler directives detect the values directly
How do you use the same lov for 2 columns
We can use the same lov for 2 columns by passing the return values in global values
and using the global values in the code
How many minimum groups are required for a matrix report
The minimum number of groups in matrix report are 4
What is the difference between static and dynamic lov
The static lov contains the predetermined values while the dynamic lov contains
values that come at run time
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How does one manage Oracle database users? (for DBA)
Oracle user accounts can be locked, unlocked, forced to choose new passwords, etc.
For example, all accounts except SYS and SYSTEM will be locked after creating an
Oracle9iDB database using the DB Configuration Assistant (dbca). DBA's must
unlock these accounts to make them available to users.
Look at these examples:
ALTER USER scott ACCOUNT LOCK -- lock a user account
ALTER USER scott ACCOUNT UNLOCK; -- unlocks a locked users account
ALTER USER scott PASSWORD EXPIRE; -- Force user to choose a new password
What is the difference between DBFile Sequential and Scattered Reads?(for
DBA)
Both "db file sequential read" and "db file scattered read" events signify time waited
for I/O read requests to complete. Time is reported in 100's of a second for Oracle 8i
releases and below, and 1000's of a second for Oracle 9i and above. Most people
confuse these events with each other as they think of how data is read from disk.
Instead they should think of how data is read into the SGA buffer cache.
db file sequential read:
A sequential read operation reads data into contiguous memory (usually a single-block
read with p3=1, but can be multiple blocks). Single block I/Os are usually the result of
using indexes. This event is also used for rebuilding the control file and reading data
file headers (P2=1). In general, this event is indicative of disk contention on index
reads.
db file scattered read:
Similar to db file sequential reads, except that the session is reading multiple data
blocks and scatters them into different discontinuous buffers in the SGA. This statistic
is NORMALLY indicating disk contention on full table scans. Rarely, data from full
table scans could be fitted into a contiguous buffer area, these waits would then show
up as sequential reads instead of scattered reads.
The following query shows average wait time for sequential versus scattered reads:
prompt "AVERAGE WAIT TIME FOR READ REQUESTS"
select a.average_wait "SEQ READ", b.average_wait "SCAT READ"
from sys.v_$system_event a, sys.v_$system_event b
where a.event = 'db file sequential read'
and b.event = 'db file scattered read';
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What is the use of PARFILE option in EXP command ?
Name of the parameter file to be passed for export.
What is the use of TABLES option in EXP command ?
List of tables should be exported.ze)
What is the OPTIMAL parameter?
It is used to set the optimal length of a rollback segment.
How does one use ORADEBUG from Server Manager/ SQL*Plus? (for DBA)
Execute the "ORADEBUG HELP" command from svrmgrl or sqlplus to obtain a list
of valid ORADEBUG commands. Look at these examples:
SQLPLUS> REM Trace SQL statements with bind variables
SQLPLUS> oradebug setospid 10121
Oracle pid: 91, Unix process pid: 10121, image: oracleorcl
SQLPLUS> oradebug EVENT 10046 trace name context forever, level 12
Statement processed.
SQLPLUS> ! vi /app/oracle/admin/orcl/bdump/ora_10121.trc
SQLPLUS> REM Trace Process Statistics
SQLPLUS> oradebug setorapid 2
Unix process pid: 1436, image: ora_pmon_orcl
SQLPLUS> oradebug procstat
Statement processed.
SQLPLUS>> oradebug TRACEFILE_NAME
/app/oracle/admin/orcl/bdump/pmon_1436.trc
SQLPLUS> REM List semaphores and shared memory segments in use
SQLPLUS> oradebug ipc
SQLPLUS> REM Dump Error Stack
SQLPLUS> oradebug setospid <pid>
SQLPLUS> oradebug event immediate trace name errorstack level 3
SQLPLUS> REM Dump Parallel Server DLM locks
SQLPLUS> oradebug lkdebug -a convlock
SQLPLUS> oradebug lkdebug -a convres
SQLPLUS> oradebug lkdebug -r <resource handle> (i.e 0x8066d338 from convres
dump)
Are there any undocumented commands in Oracle? (for DBA)
Sure there are, but it is hard to find them. Look at these examples:
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From Server Manager (Oracle7.3 and above): ORADEBUG HELP
It looks like one can change memory locations with the ORADEBUG POKE
command. Anyone brave enough to test this one for us? Previously this functionality
was available with ORADBX (ls -l $ORACLE_HOME/rdbms/lib/oradbx.o; make -f
oracle.mk oradbx) SQL*Plus: ALTER SESSION SET CURRENT_SCHEMA = SYS
If the maximum record retrieved property of the query is set to 10 then a
summary value will be calculated?
Only for 10 records.
What are the different objects that you cannot copy or reference in object
groups?
Objects of different modules
Another object groups
Individual block dependent items
Program units.
What is an OLE?
Object Linking & Embedding provides you with the capability to integrate objects
from many Ms-Windows applications into a single compound document creating
integrated applications enables you to use the features form .
Can a repeating frame be created without a data group as a base?
No
Is it possible to set a filter condition in a cross product group in matrix reports?
No
What is Overloading of procedures ?
The Same procedure name is repeated with parameters of different datatypes and
parameters in different positions, varying number of parameters is called overloading
of procedures. e.g. DBMS_OUTPUT put_line
What are the return values of functions SQLCODE and SQLERRM ? What is
Pragma EXECPTION_INIT ? Explain the usage ?
SQLCODE returns the latest code of the error that has occurred.
SQLERRM returns the relevant error message of the SQLCODE.
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What are the datatypes a available in PL/SQL ?
Some scalar data types such as NUMBER, VARCHAR2, DATE, CHAR, LONG,
BOOLEAN. Some composite data types such as RECORD & TABLE.
What are the two parts of a procedure ?
Procedure Specification and Procedure Body.
What is the basic structure of PL/SQL ?
PL/SQL uses block structure as its basic structure. Anonymous blocks or nested
blocks can be used in PL/SQL
What is PL/SQL table ?
Objects of type TABLE are called "PL/SQL tables", which are modeled as (but not the
same as) database tables, PL/SQL tables use a primary PL/SQL tables can have one
column and a primary key. Cursors
WHAT IS RMAN ? (for DBA)
Recovery Manager is a tool that: manages the process of creating backups and also
manages the process of restoring and recovering from them.
WHY USE RMAN ? (for DBA)
No extra costs …Its available free
?RMAN introduced in Oracle 8 it has become simpler with newer versions and easier
than user managed backups
?Proper security
?You are 100% sure your database has been backed up.
?Its contains detail of the backups taken etc in its central repository
Facility for testing validity of backups also commands like crosscheck to check the
status of backup.
Faster backups and restores compared to backups without RMAN
RMAN is the only backup tool which supports incremental backups.
Oracle 10g has got further optimized incremental backup which has resulted in
improvement of performance during backup and recovery time
Parallel operations are supported
Better querying facility for knowing different details of backup
No extra redo generated when backup is taken..compared to online
backup without RMAN which results in saving of space in hard disk
RMAN an intelligent tool
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Maintains repository of backup metadata
Remembers backup set location
Knows what need to backed up
Knows what is required for recovery
Knows what backups are redundant
UNDERSTANDING THE RMAN ARCHITECTURE
An oracle RMAN comprises of
RMAN EXECUTABLE This could be present and fired even through client side
TARGET DATABASE This is the database which needs to be backed up .
RECOVERY CATALOG Recovery catalog is optional otherwise backup details are
stored in target database controlfile .
It is a repository of information queried and updated by Recovery Manager
It is a schema or user stored in Oracle database. One schema can support many
databases
It contains information about physical schema of target database datafile and archive
log ,backup sets and pieces Recovery catalog is a must in following scenarios
. In order to store scripts
. For tablespace point in time recovery
Media Management Software
Media Management software is a must if you are using RMAN for storing backup in
tape drive directly.
Backups in RMAN
Oracle backups in RMAN are of the following type
RMAN complete backup OR RMAN incremental backup
These backups are of RMAN proprietary nature
IMAGE COPY
The advantage of uing Image copy is its not in RMAN proprietary format..
Backup Format
RMAN backup is not in oracle format but in RMAN format. Oracle backup comprises
of backup sets and it consists of backup pieces. Backup sets are logical entity In oracle
9i it gets stored in a default location There are two type of backup sets Datafile backup
sets, Archivelog backup sets One more important point of data file backup sets is it do
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not include empty blocks. A backup set would contain many backup pieces.
A single backup piece consists of physical files which are in RMAN proprietary
format.
Example of taking backup using RMAN
Taking RMAN Backup
In non archive mode in dos prompt type
RMAN
You get the RMAN prompt
RMAN > Connect Target
Connect to target database : Magic
using target database controlfile instead of recovery catalog
Lets take a simple backup of database in non archive mode
shutdown immediate ; - - Shutdowns the database
startup mount
backup database ;- its start backing the database
alter database open;
We can fire the same command in archive log mode
And whole of datafiles will be backed
Backup database plus archivelog;
Restoring database
Restoring database has been made very simple in 9i .
It is just
Restore database..
RMAN has become intelligent to identify which datafiles has to be restored
and the location of backuped up file.
Oracle Enhancement for RMAN in 10 G
Flash Recovery Area
Right now the price of hard disk is falling. Many dba are taking oracle database
backup inside the hard disk itself since it results in lesser mean time between
recoverability.
The new parameter introduced is
DB_RECOVERY_FILE_DEST = /oracle/flash_recovery_area
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By configuring the RMAN RETENTION POLICY the flash recovery area will
automatically delete obsolete backups and archive logs that are no longer required
based on that configuration Oracle has introduced new features in incremental backup
Change Tracking File
Oracle 10g has the facility to deliver faster incrementals with the implementation of
changed tracking file feature.This will results in faster backups lesser space
consumption and also reduces the time needed for daily backups
Incrementally Updated Backups
Oracle database 10g Incrementally Updates Backup features merges the image copy of
a datafile with RMAN incremental backup. The resulting image copy is now updated
with block changes captured by incremental backups.The merging of the image copy
and incremental backup is initiated with RMAN recover command. This results in
faster recovery.
Binary compression technique reduces backup space usage by 50-75%.
With the new DURATION option for the RMAN BACKUP command, DBAs can
weigh backup performance against system service level requirements. By specifying a
duration, RMAN will automatically calculate the appropriate backup rate; in addition,
DBAs can optionally specify whether backups should minimize time or system load.
New Features in Oem to identify RMAN related backup like backup pieces, backup
sets and image copy
Oracle 9i New features Persistent RMAN Configuration
A new configure command has been introduced in Oracle 9i , that lets you configure
various features including automatic channels, parallelism ,backup options, etc.
These automatic allocations and options can be overridden by commands in a RMAN
command file.
Controlfile Auto backups
Through this new feature RMAN will automatically perform a controlfile auto backup.
after every backup or copy command.
Block Media Recovery
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If we can restore a few blocks rather than an entire file we only need few blocks.
We even dont need to bring the data file offline.
Syntax for it as follows
Block Recover datafile 8 block 22;
Configure Backup Optimization
Prior to 9i whenever we backed up database using RMAN our backup also used take
backup of read only table spaces which had already been backed up and also the same
with archive log too.
Now with 9i backup optimization parameter we can prevent repeat backup of read
only tablespace and archive log. The command for this is as follows Configure backup
optimization on
Archive Log failover
If RMAN cannot read a block in an archived log from a destination. RMAN
automatically attempts to read from an alternate location this is called as archive log
failover
There are additional commands like
backup database not backed up since time '31-jan-2002 14:00:00'
Do not backup previously backed up files
(say a previous backup failed and you want to restart from where it left off).
Similar syntax is supported for restores
backup device sbt backup set all Copy a disk backup to tape
(backing up a backup
Additionally it supports
. Backup of server parameter file
. Parallel operation supported
. Extensive reporting available
. Scripting
. Duplex backup sets
. Corrupt block detection
. Backup archive logs
Pitfalls of using RMAN
Previous to version Oracle 9i backups were not that easy which means you had to
allocate a channel compulsorily to take backup You had to give a run etc . The syntax
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was a bit complex …RMAN has now become very simple and easy to use..
If you changed the location of backup set it is compulsory for you to register it using
RMAN or while you are trying to restore backup It resulted in hanging situations
There is no method to know whether during recovery database restore is going to fail
because of missing archive log file.
Compulsory Media Management only if using tape backup
Incremental backups though used to consume less space used to be slower since it
used to read the entire database to find the changed blocks and also They have difficult
time streaming the tape device. .
Considerable improvement has been made in 10g to optimize the algorithm to handle
changed block.
Observation
Introduced in Oracle 8 it has become more powerful and simpler with newer version
of Oracle 9 and 10 g.
So if you really don't want to miss something critical please start using RMAN.
Explain UNION,MINUS,UNION ALL, INTERSECT ?
INTERSECT returns all distinct rows selected by both queries.MINUS - returns all
distinct rows selected by the first query but not by the second.UNION - returns all
distinct rows selected by either queryUNION ALL - returns all rows selected by either
query, including all duplicates.
Should the OEM Console be displayed at all times (when there are scheduled
jobs)? (for DBA)
When a job is submitted the agent will confirm the status of the job. When the status
shows up as scheduled, you can close down the OEM console. The processing of the
job is managed by the OIA (Oracle Intelligent Agent). The OIA maintains a .jou file in
the agent's subdirectory. When the console is launched communication with the Agent
is established and the contents of the .jou file (binary) are reported to the console job
subsystem. Note that OEM will not be able to send e-mail and paging notifications
when the Console is not started.
Difference between SUBSTR and INSTR ?
INSTR (String1,String2(n,(m)),INSTR returns the position of the mth occurrence of
the string 2 instring1. The search begins from nth position of string1.SUBSTR
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(String1 n,m)SUBSTR returns a character string of size m in string1, starting from nth
position of string1.
What kind of jobs can one schedule with OEM? (for DBA)
OEM comes with pre-defined jobs like Export, Import, run OS commands, run sql
scripts, SQL*Plus commands etc. It also gives you the flexibility of scheduling custom
jobs written with the TCL language.
What are the pre requisites ?
I. to modify data type of a column ? ii. to add a column with NOT NULL constraint ?
To Modify the datatype of a column the column must be empty. to add a column with
NOT NULL constrain, the table must be empty.
How does one backout events and jobs during maintenance slots? (for DBA)
Managemnet and data collection activity can be suspended by imposing a blackout.
Look at these examples:
agentctl start blackout # Blackout the entrire agent
agentctl stop blackout # Resume normal monitoring and management
agentctl start blackout ORCL # Blackout database ORCL
agentctl stop blackout ORCL # Resume normal monitoring and management
agentctl start blackout -s jobs -d 00:20 # Blackout jobs for 20 minutes
What are the types of SQL Statement ?
Data Definition Language :
CREATE,ALTER,DROP,TRUNCATE,REVOKE,NO AUDIT & COMMIT.
Data Manipulation Language:
INSERT,UPDATE,DELETE,LOCK
TABLE,EXPLAIN PLAN & SELECT.Transactional Control:
COMMIT & ROLLBACKSession Control: ALTERSESSION & SET
ROLESystem Control :
ALTER SYSTEM.
What is the Oracle Intelligent Agent? (for DBA)
The Oracle Intelligent Agent (OIA) is an autonomous process that needs to run on a
remote node in the network to make the node OEM manageable. The Oracle
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Intelligent Agent is responsible for:
. Discovering targets that can be managed (Database Servers, Net8 Listeners, etc.);
. Monitoring of events registered in Enterprise Manager; and
. Executing tasks associated with jobs submitted to Enterprise Manager.
How does one start the Oracle Intelligent Agent? (for DBA)
One needs to start an OIA (Oracle Intelligent Agent) process on all machines that will
to be managed via OEM.
For OEM 9i and above:
agentctl start agent
agentctl stop agent
For OEM 2.1 and below:
lsnrctl dbsnmp_start
lsnrctl dbsnmp_status
On Windows NT, start the "OracleAgent" Service.
If the agent doesn't want to start, ensure your environment variables are set correctly
and delete the following files before trying again:
1) In $ORACLE_HOME/network/admin: snmp_ro.ora and snmp_rw.ora.
2) Also delete ALL files in $ORACLE_HOME/network/agent/.
Can one write scripts to send alert messages to the console?
Start the OEM console and create a new event. Select option "Enable Unsolicited
Event". Select test "Unsolicited Event". When entering the parameters, enter values
similar to these:
Event Name: /oracle/script/myalert
Object: *
Severity: *
Message: *
One can now write the script and invoke the oemevent command to send alerts to the
console. Look at this example: oemevent /oracle/script/myalert DESTINATION alert
"My custom error message" where DESTINATION is the same value as entered in the
"Monitored Destinations" field when you've registered the event in the OEM Console.
Where can one get more information about TCL? (for DBA)
One can write custom event checking routines for OEM using the TCL (Tool
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Command Language) language. Check the following sites for more information about
TCL:
. The Tcl Developer Xchange - download and learn about TCL
. OraTCL at Sourceforge - Download the OraTCL package
. Tom Poindexter's Tcl Page - Oratcl was originally written by Tom Poindexter
Are there any troubleshooting tips for OEM? (for DBA)
. Create the OEM repository with a user (which will manage the OEM) and store it in
a tablespace that does not share any data with other database users. It is a bad practice
to create the repository with SYS and System.
. If you are unable to launch the console or there is a communication problem with the
intelligent agent (daemon). Ensure OCX files are registered. Type the following in the
DOS prompt (the current directory should be $ORACLE_HOME\BIN:
C:\Orawin95\Bin> RegSvr32 mmdx32.OCX
C:\Orawin95\Bin> RegSvr32 vojt.OCX
. If you have a problem starting the Oracle Agent
Solution A: Backup the *.Q files and Delete all the *.Q Files
($Oracle_home/network/agent folder)
Backup and delete SNMP_RO.ora, SNMP_RW.ora, dbsnmp.ver and services.ora files
($Oracle_Home/network/admin folder) Start the Oracle Agent service.
Solution B: Your version of Intelligent Agent could be buggy. Check with Oracle for
any available patches. For example, the Intelligent Agent that comes with Oracle 8.0.4
is buggy.
Sometimes you get a Failed status for the job that was executed successfully.
Check the log to see the results of the execution rather than relying on this status.
What is import/export and why does one need it? (for DBA)
The Oracle export (EXP) and import (IMP) utilities are used to perform logical
database backup and recovery. They are also used to move Oracle data from one
machine, database or schema to another.
The imp/exp utilities use an Oracle proprietary binary file format and can thus only be
used between Oracle databases. One cannot export data and expect to import it into a
non-Oracle database. For more information on how to load and unload data from files,
read the SQL*Loader FAQ.
The export/import utilities are also commonly used to perform the following tasks:
. Backup and recovery (small databases only)
. Reorganization of data/ Eliminate database fragmentation
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. Detect database corruption. Ensure that all the data can be read.
. Transporting tablespaces between databases
. Etc.
What is a display item?
Display items are similar to text items but store only fetched or assigned values.
Operators cannot navigate to a display item or edit the value it contains.
How does one use the import/export utilities? (for DBA)
Look for the "imp" and "exp" executables in your $ORACLE_HOME/bin directory.
One can run them interactively, using command line parameters, or using parameter
files. Look at the imp/exp parameters before starting. These parameters can be listed
by executing the following commands: "exp help=yes" or "imp help=yes".
The following examples demonstrate how the imp/exp utilities can be used:
exp scott/tiger file=emp.dmp log=emp.log tables=emp rows=yes indexes=no
exp scott/tiger file=emp.dmp tables=(emp,dept)
imp scott/tiger file=emp.dmp full=yes
imp scott/tiger file=emp.dmp fromuser=scott touser=scott tables=dept
exp userid=scott/tiger@orcl parfile=export.txt
... where export.txt contains:
BUFFER=100000
FILE=account.dmp
FULL=n
OWNER=scott
GRANTS=y
COMPRESS=y
NOTE: If you do not like command line utilities, you can import and export data with
the "Schema Manager" GUI that ships with Oracle Enterprise Manager (OEM).
What are the types of visual attribute settings?
Custom Visual attributes Default visual attributes Named Visual attributes. Window
Can one export a subset of a table? (for DBA)
From Oracle8i one can use the QUERY= export parameter to selectively unload a
subset of the data from a table. Look at this example:
exp scott/tiger tables=emp query=\"where deptno=10\"
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What are the two ways to incorporate images into a oracle forms application?
Boilerplate Images
Image_items
Can one monitor how fast a table is imported? (for DBA)
If you need to monitor how fast rows are imported from a running import job, try one
of the following methods:
Method 1:
select substr(sql_text,instr(sql_text,'INTO "'),30) table_name,
rows_processed,
round((sysdate-to_date(first_load_time,'yyyy-mm-dd hh24:mi:ss'))*24*60,1) minutes,
trunc(rows_processed/((sysdate-to_date(first_load_time,'yyyy-mm-dd
hh24:mi:ss'))*24*60)) rows_per_min
from sys.v_$sqlarea
where sql_text like 'INSERT %INTO "%'
and command_type = 2
and open_versions > 0;
For this to work one needs to be on Oracle 7.3 or higher (7.2 might also be OK). If the
import has more than one table, this statement will only show information about the
current table being imported.
Contributed by Osvaldo Ancarola, Bs. As. Argentina.
Method 2:
Use the FEEDBACK=n import parameter. This command will tell IMP to display a
dot for every N rows imported.
Can one import tables to a different tablespace? (for DBA)
Oracle offers no parameter to specify a different tablespace to import data into.
Objects will be re-created in the tablespace they were originally exported from. One
can alter this behaviour by following one of these procedures: Pre-create the table(s) in
the correct tablespace:
. Import the dump file using the INDEXFILE= option
. Edit the indexfile. Remove remarks and specify the correct tablespaces.
. Run this indexfile against your database, this will create the required tables in the
appropriate tablespaces
. Import the table(s) with the IGNORE=Y option.
Change the default tablespace for the user:
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. Revoke the "UNLIMITED TABLESPACE" privilege from the user
. Revoke the user's quota from the tablespace from where the object was exported.
This forces the import utility to create tables in the user's default tablespace.
. Make the tablespace to which you want to import the default tablespace for the user
. Import the table
What do you mean by a block in forms4.0?
Block is a single mechanism for grouping related items into a functional unit for
storing, displaying and manipulating records.
How is possible to restrict the user to a list of values while entering values for
parameters?
By setting the Restrict To List property to true in the parameter property sheet.
What is SQL*Loader and what is it used for? (for DBA)
SQL*Loader is a bulk loader utility used for moving data from external files into the
Oracle database. Its syntax is similar to that of the DB2 Load utility, but comes with
more options. SQL*Loader supports various load formats, selective loading, and
multi-table loads.
How does one use the SQL*Loader utility? (for DBA)
One can load data into an Oracle database by using the sqlldr (sqlload on some
platforms) utility. Invoke the utility without arguments to get a list of available
parameters. Look at the following example:
sqlldr scott/tiger control=loader.ctl
This sample control file (loader.ctl) will load an external data file containing delimited
data:
load data
infile 'c:\data\mydata.csv'
into table emp
fields terminated by "," optionally enclosed by '"'
( empno, empname, sal, deptno )
The mydata.csv file may look like this:
10001,"Scott Tiger", 1000, 40
10002,"Frank Naude", 500, 20
Another Sample control file with in-line data formatted as fix length records. The trick
is to specify "*" as the name of the data file, and use BEGINDATA to start the data
section in the control file.
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load data
infile *
replace
into table departments
( dept position (02:05) char(4),
deptname position (08:27) char(20)
)
begindata
COSC COMPUTER SCIENCE
ENGL ENGLISH LITERATURE
MATH MATHEMATICS
POLY POLITICAL SCIENCE
How can a cross product be created?
By selecting the cross products tool and drawing a new group surrounding the base
group of the cross products.
Is there a SQL*Unloader to download data to a flat file? (for DBA)
Oracle does not supply any data unload utilities. However, you can use SQL*Plus to
select and format your data and then spool it to a file:
set echo off newpage 0 space 0 pagesize 0 feed off head off trimspool on
spool oradata.txt
select col1 || ',' || col2 || ',' || col3
from tab1
where col2 = 'XYZ';
spool off
Alternatively use the UTL_FILE PL/SQL package:
rem Remember to update initSID.ora, utl_file_dir='c:\oradata' parameter
declare
fp utl_file.file_type;
begin
fp := utl_file.fopen('c:\oradata','tab1.txt','w');
utl_file.putf(fp, '%s, %s\n', 'TextField', 55);
utl_file.fclose(fp);
end;
/
You might also want to investigate third party tools like SQLWays from Ispirer
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Systems, TOAD from Quest, or ManageIT Fast Unloader from CA to help you unload
data from Oracle.
Can one load variable and fix length data records? (for DBA)
Yes, look at the following control file examples. In the first we will load delimited
data (variable length):
LOAD DATA
INFILE *
INTO TABLE load_delimited_data
FIELDS TERMINATED BY "," OPTIONALLY ENCLOSED BY '"'
TRAILING NULLCOLS
( data1,
data2
)
BEGINDATA
11111,AAAAAAAAAA
22222,"A,B,C,D,"
If you need to load positional data (fixed length), look at the following control file
example:
LOAD DATA
INFILE *
INTO TABLE load_positional_data
( data1 POSITION(1:5),
data2 POSITION(6:15)
)
BEGINDATA
11111AAAAAAAAAA
22222BBBBBBBBBB
Can one skip header records load while loading?
Use the "SKIP n" keyword, where n = number of logical rows to skip. Look at this
example:
LOAD DATA
INFILE *
INTO TABLE load_positional_data
SKIP 5
( data1 POSITION(1:5),
data2 POSITION(6:15)
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)
BEGINDATA
11111AAAAAAAAAA
22222BBBBBBBBBB
Can one modify data as it loads into the database? (for DBA)
Data can be modified as it loads into the Oracle Database. Note that this only applies
for the conventional load path and not for direct path loads.
LOAD DATA
INFILE *
INTO TABLE modified_data
( rec_no "my_db_sequence.nextval",
region CONSTANT '31',
time_loaded "to_char(SYSDATE, 'HH24:MI')",
data1 POSITION(1:5) ":data1/100",
data2 POSITION(6:15) "upper(:data2)",
data3 POSITION(16:22)"to_date(:data3, 'YYMMDD')"
)
BEGINDATA
11111AAAAAAAAAA991201
22222BBBBBBBBBB990112
LOAD DATA
INFILE 'mail_orders.txt'
BADFILE 'bad_orders.txt'
APPEND
INTO TABLE mailing_list
FIELDS TERMINATED BY ","
( addr,
city,
state,
zipcode,
mailing_addr "decode(:mailing_addr, null, :addr, :mailing_addr)",
mailing_city "decode(:mailing_city, null, :city, :mailing_city)",
mailing_state
)
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Can one load data into multiple tables at once? (for DBA)
Look at the following control file:
LOAD DATA
INFILE *
REPLACE
INTO TABLE emp
WHEN empno != ' '
( empno POSITION(1:4) INTEGER EXTERNAL,
ename POSITION(6:15) CHAR,
deptno POSITION(17:18) CHAR,
mgr POSITION(20:23) INTEGER EXTERNAL
)
INTO TABLE proj
WHEN projno != ' '
( projno POSITION(25:27) INTEGER EXTERNAL,
empno POSITION(1:4) INTEGER EXTERNAL
)
What is the difference between boiler plat images and image items?
Boiler plate Images are static images (Either vector or bit map) that you import from
the file system or database to use a graphical elements in your form, such as company
logos and maps. Image items are special types of interface controls that store and
display either vector or bitmap images. Like other items that store values, image items
can be either base table items(items that relate directly to database columns) or control
items. The definition of an image item is stored as part of the form module FMB and
FMX files, but no image file is actually associated with an image item until the item is
populate at run time.
What are the triggers available in the reports?
Before report, Before form, After form , Between page, After report.
Why is a Where clause faster than a group filter or a format trigger?
Because, in a where clause the condition is applied during data retrievalthan after
retrieving the data.
Can one selectively load only the records that one need? (for DBA)
Look at this example, (01) is the first character, (30:37) are characters 30 to 37:
LOAD DATA
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INFILE 'mydata.dat' BADFILE 'mydata.bad' DISCARDFILE 'mydata.dis'
APPEND
INTO TABLE my_selective_table
WHEN (01) <> 'H' and (01) <> 'T' and (30:37) = '19991217'
(
region CONSTANT '31',
service_key POSITION(01:11) INTEGER EXTERNAL,
call_b_no POSITION(12:29) CHAR
)
Can one skip certain columns while loading data? (for DBA)
One cannot use POSTION(x:y) with delimited data. Luckily, from Oracle 8i one can
specify FILLER columns. FILLER columns are used to skip columns/fields in the load
file, ignoring fields that one does not want. Look at this example: -- One cannot use
POSTION(x:y) as it is stream data, there are no positional fields-the next field begins
after some delimiter, not in column X. -->
LOAD DATA
TRUNCATE INTO TABLE T1
FIELDS TERMINATED BY ','
( field1,
field2 FILLER,
field3
)
How does one load multi-line records? (for DBA)
One can create one logical record from multiple physical records using one of the
following two clauses:
. CONCATENATE: - use when SQL*Loader should combine the same number of
physical records together to form one logical record.
. CONTINUEIF - use if a condition indicates that multiple records should be treated as
one. Eg. by having a '#' character in column 1.
How can get SQL*Loader to COMMIT only at the end of the load file? (for
DBA)
One cannot, but by setting the ROWS= parameter to a large value, committing can be
reduced. Make sure you have big rollback segments ready when you use a high value
for ROWS=.
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Can one improve the performance of SQL*Loader? (for DBA)
A very simple but easily overlooked hint is not to have any indexes and/or constraints
(primary key) on your load tables during the load process. This will significantly slow
down load times even with ROWS= set to a high value.
Add the following option in the command line: DIRECT=TRUE. This will effectively
bypass most of the RDBMS processing. However, there are cases when you can't use
direct load. Refer to chapter 8 on Oracle server Utilities manual.
Turn off database logging by specifying the UNRECOVERABLE option. This option
can only be used with direct data loads. Run multiple load jobs concurrently.
How does one use SQL*Loader to load images, sound clips and documents? (for
DBA)
SQL*Loader can load data from a "primary data file", SDF (Secondary Data file - for
loading nested tables and VARRAYs) or LOGFILE. The LOBFILE method provides
and easy way to load documents, images and audio clips into BLOB and CLOB
columns. Look at this example:
Given the following table:
CREATE TABLE image_table (
image_id NUMBER(5),
file_name VARCHAR2(30),
image_data BLOB);
Control File:
LOAD DATA
INFILE *
INTO TABLE image_table
REPLACE
FIELDS TERMINATED BY ','
(
image_id INTEGER(5),
file_name CHAR(30),
image_data LOBFILE (file_name) TERMINATED BY EOF
)
BEGINDATA
001,image1.gif
002,image2.jpg
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What is the difference between the conventional and direct path loader? (for
DBA)
The conventional path loader essentially loads the data by using standard INSERT
statements. The direct path loader (DIRECT=TRUE) bypasses much of the logic
involved with that, and loads directly into the Oracle data files. More information
about the restrictions of direct path loading can be obtained from the Utilities Users
Guide.
GENERAL INTERVIEW QUESTIONS
What are the various types of Exceptions ?
User defined and Predefined Exceptions.
Can we define exceptions twice in same block ?
No.
What is the difference between a procedure and a function ?
Functions return a single variable by value whereas procedures do not return any
variable by value. Rather they return multiple variables by passing variables by
reference through their OUT parameter.
Can you have two functions with the same name in a PL/SQL block ?
Yes.
Can you have two stored functions with the same name ?
Yes.
Can you call a stored function in the constraint of a table ?
No.
What are the various types of parameter modes in a procedure ?
IN, OUT AND INOUT.
What is Over Loading and what are its restrictions ?
OverLoading means an object performing different functions depending upon the no.
of parameters or the data type of the parameters passed to it.
Can functions be overloaded ?
Yes.
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Can 2 functions have same name & input parameters but differ only by return
datatype ?
No.
What are the constructs of a procedure, function or a package ?
The constructs of a procedure, function or a package are :
variables and constants
cursors
exceptions
Why Create or Replace and not Drop and recreate procedures ?
So that Grants are not dropped.
Can you pass parameters in packages ? How ?
Yes. You can pass parameters to procedures or functions in a package.
What are the parts of a database trigger ?
The parts of a trigger are:
A triggering event or statement
A trigger restriction
A trigger action
What are the various types of database triggers ?
There are 12 types of triggers, they are combination of :
Insert, Delete and Update Triggers.
Before and After Triggers.
Row and Statement Triggers.
(3*2*2=12)
What is the advantage of a stored procedure over a database trigger ?
We have control over the firing of a stored procedure but we have no control over the
firing of a trigger.
What is the maximum no. of statements that can be specified in a trigger
statement ?
One.
Can views be specified in a trigger statement ?
No
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What are the values of :new and :old in Insert/Delete/Update Triggers ?
INSERT : new = new value, old = NULL
DELETE : new = NULL, old = old value
UPDATE : new = new value, old = old value
What are cascading triggers? What is the maximum no of cascading triggers at a
time?
When a statement in a trigger body causes another trigger to be fired, the triggers are
said to be cascading. Max = 32.
What are mutating triggers ?
A trigger giving a SELECT on the table on which the trigger is written.
What are constraining triggers ?
A trigger giving an Insert/Update on a table having referential integrity constraint on
the triggering table.
Describe Oracle database's physical and logical structure ?
Physical : Data files, Redo Log files, Control file.
Logical : Tables, Views, Tablespaces, etc.
Can you increase the size of a tablespace ? How ?
Yes, by adding datafiles to it.
What is the use of Control files ?
Contains pointers to locations of various data files, redo log files, etc.
What is the use of Data Dictionary ?
Used by Oracle to store information about various physical and logical Oracle
structures e.g. Tables, Tablespaces, datafiles, etc
What are the advantages of clusters ?
Access time reduced for joins.
What are the disadvantages of clusters ?
The time for Insert increases.
Can Long/Long RAW be clustered ?
No.
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Can null keys be entered in cluster index, normal index ?
Yes.
Can Check constraint be used for self referential integrity ? How ?
Yes. In the CHECK condition for a column of a table, we can reference some other
column of the same table and thus enforce self referential integrity.
What are the min. extents allocated to a rollback extent ?
Two
What are the states of a rollback segment ? What is the difference between partly
available and needs recovery ?
The various states of a rollback segment are :
ONLINE, OFFLINE, PARTLY AVAILABLE, NEEDS RECOVERY and INVALID.
What is the difference between unique key and primary key ?
Unique key can be null; Primary key cannot be null.
An insert statement followed by a create table statement followed by rollback ?
Will the rows be inserted ?
No.
an you define multiple savepoints ?
Yes.
Can you Rollback to any savepoint ?
Yes.
What is the maximum no. of columns a table can have ?
254.
What is the significance of the & and && operators in PL SQL ?
The & operator means that the PL SQL block requires user input for a variable. The
&& operator means that the value of this variable should be the same as inputted by
the user previously for this same variable. If a transaction is very large, and the
rollback segment is not able to hold the rollback information, then will the transaction
span across different rollback segments or will it terminate ? It will terminate (Please
check ).
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Can you pass a parameter to a cursor ?
Explicit cursors can take parameters, as the example below shows. A cursor parameter
can appear in a query wherever a constant can appear. CURSOR c1 (median IN
NUMBER) IS SELECT job, ename FROM emp WHERE sal > median;
What are the various types of RollBack Segments ?
Public Available to all instances
Private Available to specific instance
Can you use %RowCount as a parameter to a cursor ?
Yes
Is the query below allowed :
Select sal, ename Into x From emp Where ename = 'KING'
(Where x is a record of Number(4) and Char(15))
Yes
Is the assignment given below allowed :
ABC = PQR (Where ABC and PQR are records)
Yes
Is this for loop allowed :
For x in &Start..&End Loop
Yes
How many rows will the following SQL return :
Select * from emp Where rownum < 10;
9 rows
How many rows will the following SQL return :
Select * from emp Where rownum = 10;
No rows
Which symbol preceeds the path to the table in the remote database ?
@
Are views automatically updated when base tables are updated ?
Yes
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Can a trigger written for a view ?
No
If all the values from a cursor have been fetched and another fetch is issued, the
output will be : error, last record or first record ?
Last Record
A table has the following data : [[5, Null, 10]]. What will the average function
return ?
7.5
Is Sysdate a system variable or a system function?
System Function
Consider a sequence whose currval is 1 and gets incremented by 1 by using the
nextval reference we get the next number 2. Suppose at this point we issue an
rollback and again issue a nextval. What will the output be ?
3
Definition of relational DataBase by Dr. Codd (IBM)?
A Relational Database is a database where all data visible to the user is organized
strictly as tables of data values and where all database operations work on these tables.
What is Multi Threaded Server (MTA) ?
In a Single Threaded Architecture (or a dedicated server configuration) the database
manager creates a separate process for each database user. But in MTA the database
manager can assign multiple users (multiple user processes) to a single dispatcher
(server process), a controlling process that queues request for work thus reducing the
databases memory requirement and resources.
Which are initial RDBMS, Hierarchical & N/w database ?
RDBMS - R system
Hierarchical - IMS
N/W - DBTG
What is Functional Dependency
Given a relation R, attribute Y of R is functionally dependent on attribute X of R if
and only if each X-value has associated with it precisely one -Y value in R
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What is Auditing ?
The database has the ability to audit all actions that take place within it.
a) Login attempts, b) Object Accesss, c) Database Action Result of Greatest(1,NULL)
or Least(1,NULL) NULL
While designing in client/server what are the 2 imp. things to be considered ?
Network Overhead (traffic), Speed and Load of client server
When to create indexes ?
To be created when table is queried for less than 2% or 4% to 25% of the table rows.
How can you avoid indexes ?
TO make index access path unavailable - Use FULL hint to optimizer for full table
scan - Use INDEX or AND-EQUAL hint to optimizer to use one index or set to
indexes instead of another. - Use an expression in the Where Clause of the SQL.
What is the result of the following SQL :
Select 1 from dual
UNION
Select 'A' from dual;
Error
Can database trigger written on synonym of a table and if it can be then what
would be the effect if original table is accessed.
Yes, database trigger would fire.
Can you alter synonym of view or view ?
No
Can you create index on view ?
No
What is the difference between a view and a synonym ?
Synonym is just a second name of table used for multiple link of database. View can
be created with many tables, and with virtual columns and with conditions. But
synonym can be on view.
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What is the difference between alias and synonym ?
Alias is temporary and used with one query. Synonym is permanent and not used as
alias.
What is the effect of synonym and table name used in same Select statement ?
Valid
What's the length of SQL integer ?
32 bit length
What is the difference between foreign key and reference key ?
Foreign key is the key i.e. attribute which refers to another table primary key.
Reference key is the primary key of table referred by another table.
Can dual table be deleted, dropped or altered or updated or inserted ?
Yes
If content of dual is updated to some value computation takes place or not ?
Yes
If any other table same as dual is created would it act similar to dual?
Yes
For which relational operators in where clause, index is not used ?
<> , like '% ...' is NOT functions, field +constant, field || ''
Assume that there are multiple databases running on one machine. How can you
switch from one to another ?
Changing the ORACLE_SID
What are the advantages of Oracle ?
Portability : Oracle is ported to more platforms than any of its competitors, running on
more than 100 hardware platforms and 20 networking protocols.
Market Presence : Oracle is by far the largest RDBMS vendor and spends more on R
& D than most of its competitors earn in total revenue. This market clout means that
you are unlikely to be left in the lurch by Oracle and there are always lots of third
party interfaces available.
Backup and Recovery : Oracle provides industrial strength support for on-line backup
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in-time recovery.
Performance : Speed of a 'tuned' Oracle Database and application is quite good, even
with large databases. Oracle can manage > 100GB databases.
Multiple database support : Oracle has a superior ability to manage multiple databases
within the same transaction using a two-phase commit protocol.
What is a forward declaration ? What is its use ?
PL/SQL requires that you declare an identifier before using it. Therefore, you must
declare a subprogram before calling it. This declaration at the start of a subprogram is
called forward declaration. A forward declaration consists of a subprogram
specification terminated by a semicolon.
What are actual and formal parameters ?
Actual Parameters : Subprograms pass information using parameters. The variables or
expressions referenced in the parameter list of a subprogram call are actual
parameters. For example, the following procedure call lists two actual parameters
named emp_num and amount:
Eg. raise_salary(emp_num, amount);
Formal Parameters : The variables declared in a subprogram specification and
referenced in the subprogram body are formal parameters. For example, the following
procedure declares two formal parameters named emp_id and increase: Eg.
PROCEDURE raise_salary (emp_id INTEGER, increase REAL) IS current_salary
REAL;
What are the types of Notation ?
Position, Named, Mixed and Restrictions.
What all important parameters of the init.ora are supposed to be increased if you
want to increase the SGA size ?
In our case, db_block_buffers was changed from 60 to 1000 (std values are 60, 550 &
3500) shared_pool_size was changed from 3.5MB to 9MB (std values are 3.5, 5 &
9MB) open_cursors was changed from 200 to 300 (std values are 200 & 300)
db_block_size was changed from 2048 (2K) to 4096 (4K) {at the time of database
creation}.
The initial SGA was around 4MB when the server RAM was 32MB and The new
SGA was around 13MB when the server RAM was increased to 128MB.
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If I have an execute privilege on a procedure in another users schema, can I
execute his procedure even though I do not have privileges on the tables within
the procedure ?
Yes
What are various types of joins ?
Equijoins, Non-equijoins, self join, outer join
What is a package cursor ?
A package cursor is a cursor which you declare in the package specification without an
SQL statement. The SQL statement for the cursor is attached dynamically at runtime
from calling procedures.
If you insert a row in a table, then create another table and then say Rollback. In
this case will the row be inserted ?
Yes. Because Create table is a DDL which commits automatically as soon as it is
executed. The DDL commits the transaction even if the create statement fails
internally (eg table already exists error) and not syntactically.
What are the various types of queries ??
Normal Queries
Sub Queries
Co-related queries
Nested queries
Compound queries
What is a transaction ?
A transaction is a set of SQL statements between any two COMMIT and ROLLBACK
statements.
What is implicit cursor and how is it used by Oracle ?
An implicit cursor is a cursor which is internally created by Oracle. It is created by
Oracle for each individual SQL.
Which of the following is not a schema object : Indexes, tables, public synonyms,
triggers and packages ?
Public synonyms
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What is PL/SQL?
PL/SQL is Oracle's Procedural Language extension to SQL. The language includes
object oriented programming techniques such as encapsulation, function overloading,
information hiding (all but inheritance), and so, brings state-of-the-art programming to
the Oracle database server and a variety of Oracle tools.
Is there a PL/SQL Engine in SQL*Plus?
No. Unlike Oracle Forms, SQL*Plus does not have a PL/SQL engine. Thus, all your
PL/SQL are send directly to the database engine for execution. This makes it much
more efficient as SQL statements are not stripped off and send to the database
individually.
Is there a limit on the size of a PL/SQL block?
Currently, the maximum parsed/compiled size of a PL/SQL block is 64K and the
maximum code size is 100K. You can run the following select statement to query the
size of an existing package or procedure.
SQL> select * from dba_object_size where name = 'procedure_name'
Can one read/write files from PL/SQL?
Included in Oracle 7.3 is a UTL_FILE package that can read and write files. The
directory you intend writing to has to be in your INIT.ORA file (see
UTL_FILE_DIR=... parameter). Before Oracle 7.3 the only means of writing a file
was to use DBMS_OUTPUT with the SQL*Plus SPOOL command.
DECLARE
fileHandler UTL_FILE.FILE_TYPE;
BEGIN
fileHandler := UTL_FILE.FOPEN('/home/oracle/tmp', 'myoutput','W');
UTL_FILE.PUTF(fileHandler, 'Value of func1 is %sn', func1(1));
UTL_FILE.FCLOSE(fileHandler);
END;
How can I protect my PL/SQL source code?
PL/SQL V2.2, available with Oracle7.2, implements a binary wrapper for PL/SQL
programs to protect the source code. This is done via a standalone utility that
transforms the PL/SQL source code into portable binary object code (somewhat larger
than the original). This way you can distribute software without having to worry about
exposing your proprietary algorithms and methods. SQL*Plus and SQL*DBA will
still understand and know how to execute such scripts. Just be careful, there is no
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"decode" command available.
The syntax is:
wrap iname=myscript.sql oname=xxxx.yyy
Can one use dynamic SQL within PL/SQL? OR Can you use a DDL in a
procedure ? How ?
From PL/SQL V2.1 one can use the DBMS_SQL package to execute dynamic SQL
statements.
Eg: CREATE OR REPLACE PROCEDURE DYNSQL
AS
cur integer;
rc integer;
BEGIN
cur := DBMS_SQL.OPEN_CURSOR;
DBMS_SQL.PARSE(cur,'CREATE TABLE X (Y DATE)', DBMS_SQL.NATIVE);
rc := DBMS_SQL.EXECUTE(cur);
DBMS_SQL.CLOSE_CURSOR(cur);
END;
======================================================
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C
Aptitude
Questions &
Answers
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C Aptitude Questions
Note :All the programs are tested under Turbo C/C++ compilers.
It is assumed that,
Programs run under DOS environment,
The underlying machine is an x86 system,
Program is compiled using Turbo C/C++ compiler.
The program output may depend on the information based on this assumptions
(for example sizeof(int) == 2 may be assumed).
Predict the output or error(s) for the following:
Questions 1. void main()
{
int const * p=5;
printf("%d",++(*p));
}
Answer:Compiler error: Cannot modify a constant value.
Explanation:
p is a pointer to a "constant integer". But we tried to change the value of the
"constant integer".
Questions 2.
main()
{
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char s[ ]="man";
int i;
for(i=0;s[ i ];i++)
printf("\n%c%c%c%c",s[ i ],*(s+i),*(i+s),i[s]);
}
Answer: mmmm
aaaa
nnnn
Explanation:
s[i], *(i+s), *(s+i), i[s] are all different ways of expressing the same idea.
Generally array name is the base address for that array. Here s is the base
address. i is the index number/displacement from the base address. So, indirecting
it with * is same as s[i]. i[s] may be surprising. But in the case of C it is same as
s[i].
Questions 3.
main()
{
float me = 1.1;
double you = 1.1;
if(me==you)
printf("I love U");
else
printf("I hate U");
}
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Answer:
I hate U
Explanation:
For floating point numbers (float, double, long double) the values cannot be
predicted exactly. Depending on the number of bytes, the precession with of the
value represented varies. Float takes 4 bytes and long double takes 10 bytes. So
float stores 0.9 with less precision than long double.
Rule of Thumb:
Never compare or at-least be cautious when using floating point numbers with
relational operators (== , >, <, <=, >=,!= )
Questions 4.
main()
{
static int var = 5;
printf("%d ",var--);
if(var)
main();
}
Answer:
54321
Explanation:
When static storage class is given, it is initialized once. The change in the value
of a static variable is retained even betweenthe function calls. Main is also treated
like any other ordinary function, which can be called recursively.
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Questions 5.
main()
{
int c[ ]={2.8,3.4,4,6.7,5};
int j,*p=c,*q=c;
for(j=0;j<5;j++) {
printf(" %d ",*c);
++q; }
for(j=0;j<5;j++){
printf(" %d ",*p);
++p; }
}
Answer:
2222223465
Explanation:
Initially pointer c is assigned to both p and q. In the first loop, since only q is
incremented and not c , the value 2 will be printed 5 times. In second loop p itself
is incremented. So the values 2 3 4 6 5 will be printed.
Questions 6.
main()
{
extern int i;
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i=20;
printf("%d",i);
}
Answer:
Linker Error : Undefined symbol '_i'
Explanation: extern storage class in the following
declaration,
extern int i;
specifies to the compiler that the memory for i is allocated in some other program
and that address will be given to the current program at the time of linking. But
linker finds that no other variable of name i is available in any other program with
memory space allocated for it. Hence a linker error has occurred .
Questions 7.
main()
{
int i=-1,j=-1,k=0,l=2,m;
m=i++&&j++&&k++||l++;
printf("%d %d %d %d %d",i,j,k,l,m);
}
Answer:
00131
Explanation :Logical operations always give a result of 1 or 0 . And also the
logical AND (&&) operator has higher priority over the logical OR (||) operator.
So the expression ‗i++ && j++ && k++‘ is executed first. The result of this
expression is 0 (-1 && -1 && 0 = 0). Now the expression is 0 || 2 which
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evaluates to 1 (because OR operator always gives 1 except for ‗0 || 0‘
combination- for which it gives 0). So the value of m is 1. The values
of other variables are also incremented by 1.
Questions 8.
main()
{
char *p;
printf("%d %d ",sizeof(*p),sizeof(p));
}
Answer:
12
Explanation:The sizeof() operator gives the number of bytes taken by its
operand. P is a character pointer, which needs one byte for storing its value (a
character). Hence sizeof(*p) gives a value of 1. Since it needs two bytes to store
the address of the character pointer sizeof(p) gives 2.
Questions 9.
main()
{
int i=3;
switch(i)
{
default:printf("zero");
case 1: printf("one");
break;
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case 2:printf("two");
break;
case 3: printf("three");
break;
}
}
Answer :
three
Explanation :
The default case can be placed anywhere inside the loop. It is executed only when
all other cases doesn't match.
Questions 10.
main()
{
printf("%x",-1<<4);
}
Answer: fff0
Explanation :-1 is internally represented as all 1's. When left shifted four times
the least significant 4 bits are filled with 0's.The %x format specified specifies
that the integer value be printed as a hexadecimal value.
Questions 11. main()
{
char string[]="Hello World";
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display(string);
}
void display(char *string)
{
printf("%s",string);
}
Answer: Compiler Error : Type mismatch in redeclaration of function display
Explanation :
In third line, when the function display is encountered, the compiler doesn't know
anything about the function display. It assumes the arguments and return types to
be integers, (which is the default type). When it sees the actual function display,
the arguments and type contradicts with what it has assumed previously. Hence a
compile time error occurs.
Questions 12. main()
{
int c=- -2;
printf("c=%d",c);
}
Answer: c=2;
Explanation:Here unary minus (or negation) operator is used twice. Same
maths rules applies, ie. minus * minus= plus.
Note: However you cannot give like --2. Because -- operator can only be applied
to variables as a decrement operator (eg., i--). 2 is a constant and not a variable.
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Questions 13.
#define int char
main()
{
int i=65;
printf("sizeof(i)=%d",sizeof(i));
}
Answer:
sizeof(i)=1
Explanation:Since the #define replaces the string int by the macro char
Questions 14.
main()
{
int i=10;
i=!i>14;
Printf ("i=%d",i);
}
Answer:
i=0
Explanation:
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In the expression !i>14 , NOT (!) operator has more precedence than ‗ >‘
symbol. ! is a unary logical operator. !i (!10) is 0 (not of true is false). 0>14 is
false (zero).
Questions 15.
#include<stdio.h>
main()
{
char s[]={'a','b','c','\n','c','\0'};
char *p,*str,*str1;
p=&s[3];
str=p;
str1=s;
printf("%d",++*p + ++*str1-32);
}
Answer:77
Explanation:
p is pointing to character '\n'. str1 is pointing to character 'a' ++*p. "p is pointing
to '\n' and that is incremented by one." theASCII value of '\n' is 10, which is then
incremented to 11. The value of ++*p is 11. ++*str1, str1 is pointing to 'a' that is
incremented by 1 and it becomes 'b'. ASCII value of 'b' is 98.
Now performing (11 + 98 – 32), we get 77("M");
So we get the output 77 :: "M" (Ascii is 77).
Questions 12. main()
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{
int c=- -2;
printf("c=%d",c);
}
Answer: c=2;
Explanation:Here unary minus (or negation) operator is used twice. Same
maths rules applies, ie. minus * minus= plus.
Note: However you cannot give like --2. Because -- operator can only be applied
to variables as a decrement operator (eg., i--). 2 is a constant and not a variable.
Questions 13.
#define int char
main()
{
int i=65;
printf("sizeof(i)=%d",sizeof(i));
}
Answer:
sizeof(i)=1
Explanation:Since the #define replaces the string int by the macro char
Questions 14.
main()
{
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int i=10;
i=!i>14;
Printf ("i=%d",i);
}
Answer:
i=0
Explanation:
In the expression !i>14 , NOT (!) operator has more precedence than ‗ >‘
symbol. ! is a unary logical operator. !i (!10) is 0 (not of true is false). 0>14 is
false (zero).
Questions 15.
#include<stdio.h>
main()
{
char s[]={'a','b','c','\n','c','\0'};
char *p,*str,*str1;
p=&s[3];
str=p;
str1=s;
printf("%d",++*p + ++*str1-32);
}
Answer:77
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Explanation:
p is pointing to character '\n'. str1 is pointing to character 'a' ++*p. "p is pointing
to '\n' and that is incremented by one." theASCII value of '\n' is 10, which is then
incremented to 11. The value of ++*p is 11. ++*str1, str1 is pointing to 'a' that is
incremented by 1 and it becomes 'b'. ASCII value of 'b' is 98.
Now performing (11 + 98 – 32), we get 77("M");
So we get the output 77 :: "M" (Ascii is 77).
Questions 20. main()
{
int i=5;
printf("%d%d%d%d%d%d",i++,i--,++i,--i,i);
}
Answer:45545
Explanation:
The arguments in a function call are pushed into the stack from left to right. The
evaluation is by popping out from the stack. and the evaluation is from right to
left, hence the result.
Questions 21.
#define square(x) x*x
main()
{
int i;
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i = 64/square(4);
printf("%d",i);
}
Answer: 64
Explanation:
the macro call square(4) will substituted by 4*4 so the expression becomes i =
64/4*4 . Since / and * has equal priority the expression will be evaluated as
(64/4)*4 i.e. 16*4 = 64
Questions 22.
main()
{
char *p="hai friends",*p1;
p1=p;
while(*p!='\0') ++*p++;
printf("%s %s",p,p1);
}
Answer:
ibj!gsjfoet
Explanation:
++*p++ will be parse in the given order
*p that is value at the location currently pointed by p will be taken
++*p the retrieved value will be incremented
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when ; is encountered the location will be incremented that is p++ will be
executed
Hence, in the while loop initial value pointed by p is ‗h‘, which is changed to ‗i‘
by executing ++*p and pointer moves to point, ‗a‘ which is similarly changed to
‗b‘ and so on. Similarly blank space is converted to ‗!‘. Thus, we obtain value in
p becomes ―ibj!gsjfoet‖ and since p reaches ‗\0‘ and p1 points to p thus p1doesnot
print anything.
Questions 23.
#include <stdio.h>
#define a 10
main()
{
#define a 50
printf("%d",a);
}
Answer: 50
Explanation:The preprocessor directives can be redefined anywhere in the
program. So the most recently assigned value will be taken.
Questions 24.
#define clrscr() 100
main()
{
clrscr();
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printf("%d\n",clrscr());
}
Answer:100
Explanation:Preprocessor executes as a seperate pass before the execution of the
compiler. So textual replacement of clrscr() to 100 occurs.The input program to
compiler looks like this :
main()
{
100;
printf("%d\n",100);
}
Note:100; is an executable statement but with no action. So it doesn't give any
problem.
Questions 25. main()
{
printf("%p",main);
}
Answer: Some address will be printed.
Explanation:Function names are just addresses (just like array names are
addresses).
main() is also a function. So the address of function main will be printed. %p
in printf specifies that the argument is an address. They are printed as
hexadecimal numbers.
Questions 27
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main()
{
clrscr();
}
clrscr();
Answer:
No output/error
Explanation:
The first clrscr() occurs inside a function. So it becomes a function call. In the
second clrscr(); is a function declaration (because it is not inside any function).
Questions 28
enum colors {BLACK,BLUE,GREEN}
main()
{
printf("%d..%d..%d",BLACK,BLUE,GREEN);
return(1);
}
Answer:
0..1..2
Explanation: enum assigns numbers starting from 0, if not explicitly defined.
Questions 29 void main()
{
char far *farther,*farthest;
printf("%d..%d",sizeof(farther),sizeof(farthest));
}
Answer:
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4..2
Explanation:the second pointer is of char type and not a far pointer
Questions 30
main()
{
int i=400,j=300;
printf("%d..%d");
}
Answer:
400..300
Explanation:
printf takes the values of the first two assignments of the program.
Any number of printf's may be given. All of them take only the first two values. If
more number of assignments given in the program, then printf will take garbage
values.
Questions 31
main()
{
char *p;
p="Hello";
printf("%c\n",*&*p);
}
Answer:
H
Explanation:
* is a dereference operator & is a reference operator. They can be applied
any number of times provided it is meaningful. Here p points to the first
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character in the string "Hello". *p dereferences it and so its value is H. Again &
references it to an address and * dereferences it to the value H.
Questions 32
main()
{
int i=1;
while (i<=5)
{
printf("%d",i);
if (i>2)
goto here;
i++;
}
}
fun()
{
here:
printf("PP");
}
Answer: Compiler error: Undefined label 'here' in function main
Explanation:
Labels have functions scope, in other words The scope of the labels is limited to
functions . The label 'here' is available in function fun() Hence it is not visible in
function main.
Questions 33
main()
{
static char names[5][20]={"pascal","ada","cobol","fortran","perl"};
int i;
char *t;
t=names[3];
names[3]=names[4];
names[4]=t;
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for (i=0;i<=4;i++)
printf("%s",names[i]);
}
Answer: Compiler error: Lvalue required in function main
Explanation:
Array names are pointer constants. So it cannot be modified.
Questions 34
void main()
{
int i=5;
printf("%d",i++ + ++i);
}
Answer:
Output Cannot be predicted exactly.
Explanation:
Side effects are involved in the evaluation of i
Questions 35
void main()
{
int i=5;
printf("%d",i+++++i);
}
Questions 36
#include<stdio.h>
main()
{
int i=1,j=2;
switch(i)
{
case 1: printf("GOOD");
break;
case j: printf("BAD");
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break;
}
}
Answer:
Compiler Error: Constant expression required in function main.
Explanation:
The case statement can have only constant expressions (this implies that we
cannot use variable names directly so an error).
Note:
Enumerated types can be used in case statements.
Questions 37
main()
{
int i;
printf("%d",scanf("%d",&i)); // value 10 is given as input here
}
Answer:
1
Explanation:
Scanf returns number of items successfully read and not 1/0. Here 10 is given as
input which should have been scanned successfully. So number of items read is 1.
Questions 38
#define f(g,g2) g##g2
main()
{
int var12=100;
printf("%d",f(var,12));
}
Answer:100
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Questions 39
main()
{
int i=0;
for(;i++;printf("%d",i)) ;
printf("%d",i);
}
Answer: 1
Explanation: before entering into the for loop the checking condition is
"evaluated". Here it evaluates to 0 (false) and comes out of the loop, and i is
incremented (note the semicolon after the for loop).
Questions 40
#include<stdio.h>
main()
{
char s[]={'a','b','c','\n','c','\0'};
char *p,*str,*str1;
p=&s[3];
str=p;
str1=s;
printf("%d",++*p + ++*str1-32);
}
Answer:
M
Explanation:
p is pointing to character '\n'.str1 is pointing to character 'a' ++*p meAnswer:"p
is pointing to '\n' and that is incremented by one." the ASCII value of '\n' is 10.
then it is incremented to 11. the value of ++*p is 11. ++*str1 meAnswer:"str1
is pointing to 'a' that is incremented by 1 and it becomes 'b'. ASCII value of 'b' is
98. both 11 and 98 is added and result is subtracted from 32.
i.e. (11+98-32)=77("M");
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Questions 41
#include<stdio.h>
main()
{
struct xx
{
int x=3;
char name[]="hello";
};
struct xx *s=malloc(sizeof(struct xx));
printf("%d",s->x);
printf("%s",s->name);
}
Answer:
Compiler Error
Explanation: Initialization should not be done for structure members inside the
structure declaration
Questions 42
#include<stdio.h>
main()
{
struct xx
{
int x;
struct yy
{
char s;
struct xx *p;
};
struct yy *q;
};
}
Answer:Compiler Error
Explanation: In the end of nested structure yy a member have to be declared.
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Questions 43
main()
{
extern int i;
i=20;
printf("%d",sizeof(i));
}
Answer:
Linker error: undefined symbol '_i'.
Explanation:
extern declaration specifies that the variable i is defined somewhere else. The
compiler passes the external variable to be resolved by the linker. So compiler
doesn't find an error. During linking the linker searches for the definition of i.
Since it is not found the linker flags an error.
Questions 44
main()
{
printf("%d", out);
}
int out=100;
Answer: Compiler error: undefined symbol out in function main.
Explanation:
The rule is that a variable is available for use from the point of declaration. Even
though a is a global variable, it is not available for main. Hence an error.
Questions 45
main()
{
extern out;
printf("%d", out);
}
int out=100;
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Answer:100
Explanation:
This is the correct way of writing the previous program.
Questions 46 main()
{
show();
}
void show()
{
printf("I'm the greatest");
}
Answer: Compier error: Type mismatch in redeclaration of show.
Explanation:
When the compiler sees the function show it doesn't know anything about it. So
the default return type (ie, int) is assumed. But when compiler sees the
actual definition of show mismatch occurs since it is declared as void. Hence the
error.
The solutions are as follows:
1. declare void show() in main() .
2. define show() before main().
3. declare extern void show() before the use of show().
Questions 47
main( )
{
int a[2][3][2] = {{{2,4},{7,8},{3,4}},{{2,2},{2,3},{3,4}}};
printf(―%u %u %u %d \n‖,a,*a,**a,***a);
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printf(―%u %u %u %d \n‖,a+1,*a+1,**a+1,***a+1);
}
Answer:
100, 100, 100, 2
114, 104, 102, 3
Explanation:
The given array is a 3-D one. It can also be viewed as a 1-D array.
2
4
7
8
3
4
2
2
2
3
3
4
100 102 104 106 108 110 112 114 116 118 120 122
thus, for the first printf statement a, *a, **a give address of first element . since
the indirection ***a gives the value. Hence, the first line of the output.
for the second printf a+1 increases in the third dimension thus points to value at
114, *a+1 increments in second dimension thus points to 104, **a +1 increments
the first dimension thus points to 102 and ***a+1 first gets the value at first
location and then increments it by 1. Hence, the output.
Questions 48
main( )
{
int a[ ] = {10,20,30,40,50},j,*p;
for(j=0; j<5; j++)
{
printf(―%d‖ ,*a);
a++;
}
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p = a;
for(j=0; j<5; j++)
{
printf(―%d ‖ ,*p);
p++;
}
}
Answer: Compiler error: lvalue required.
Explanation:
Error is in line with statement a++. The operand must be an lvalue and may be of
any of scalar type for the any operator, array name only when subscripted is an
lvalue. Simply array name is a non-modifiable lvalue.
Questions 49
main( ){
static int a[ ] = {0,1,2,3,4};
int *p[ ] = {a,a+1,a+2,a+3,a+4};
int **ptr = p;
ptr++;
printf(―\n %d %d %d‖, ptr-p, *ptr-a, **ptr);
*ptr++;
printf(―\n %d %d %d‖, ptr-p, *ptr-a, **ptr);
*++ptr;
printf(―\n %d %d %d‖, ptr-p, *ptr-a, **ptr);
++*ptr;
printf(―\n %d %d %d‖, ptr-p, *ptr-a, **ptr);
}
Answer:
111
222
333
344
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Explanation:
Let us consider the array and the two pointers with some address
a
0
1
100
2
3
102
4
104
106
108
p
100 102 104 106 108
1000 1002 1004 1006 1008
ptr
1000
2000
After execution of the instruction ptr++ value in ptr becomes 1002, if scaling
factor for integer is 2 bytes. Now ptr – p is value in ptr – starting location of array
p, (1002 – 1000) / (scaling factor) = 1, *ptr – a = value at address pointed by ptr
– starting value of array a, 1002 has a value 102 so the value is (102 –
100)/(scaling factor) = 1, **ptr is the value stored in the location pointed by the
pointer of ptr = value pointed by value pointed by 1002 = value pointed by 102 =
1. Hence the output of the firs printf is 1, 1, 1.
After execution of *ptr++ increments value of the value in ptr by scaling factor,
so it becomes1004. Hence, the outputs for the second printf are ptr – p = 2, *ptr –
a = 2, **ptr = 2.
After execution of *++ptr increments value of the value in ptr by scaling factor,
so it becomes1004. Hence, the outputs for the third printf are ptr – p = 3, *ptr – a
= 3, **ptr = 3.
After execution of ++*ptr value in ptr remains the same, the value pointed by the
value is incremented by the scaling factor. So the value in array p at location 1006
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changes from 106 10 108,. Hence, the outputs for the fourth printf are ptr – p =
1006 – 1000 = 3, *ptr – a = 108 – 100 = 4, **ptr = 4.
Questions 50
main( )
{
char *q;
int j;
for (j=0; j<3; j++) scanf(―%s‖ ,(q+j));
for (j=0; j<3; j++) printf(―%c‖ ,*(q+j));
for (j=0; j<3; j++) printf(―%s‖ ,(q+j));
}
Explanation:
Here we have only one pointer to type char and since we take input in the same
pointer thus we keep writing over in the same location, each time shifting the
pointer value by 1. Suppose the inputs are MOUSE, TRACK and VIRTUAL.
Then for the first input suppose the pointer starts at location 100 then the input
one is stored as
M
O
U
S
E
\0
When the second input is given the pointer is incremented as j value becomes 1,
so the input is filled in memory starting from 101.
M
T
R
A
C
K
\0
The third input starts filling from the location 102
M
T
V
I
R
T
U
A
L
\0
This is the final value stored .
The first printf prints the values at the position q, q+1 and q+2 = M T V
The second printf prints three strings starting from locations q, q+1, q+2
i.e MTVIRTUAL, TVIRTUAL and VIRTUAL.
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Questions 51 main( )
{
void *vp;
char ch = ‗g‘, *cp = ―goofy‖;
int j = 20;
vp = &ch;
printf(―%c‖, *(char *)vp);
vp = &j;
printf(―%d‖,*(int *)vp);
vp = cp;
printf(―%s‖,(char *)vp + 3);
}
Answer:
g20fy
Explanation:
Since a void pointer is used it can be type casted to any other type pointer. vp =
&ch stores address of char ch and the next statement prints the value stored in vp
after type casting it to the proper data type pointer. the output is ‗g‘. Similarly the
output from second printf is ‗20‘. The third printf statement type casts it to print
the string from the 4th value hence the output is ‗fy‘.
Questions 52
main ( )
{
static char *s[ ] = {―black‖, ―white‖, ―yellow‖, ―violet‖};
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char **ptr[ ] = {s+3, s+2, s+1, s}, ***p;
p = ptr;
**++p;
printf(―%s‖,*--*++p + 3);
}
Answer:ck
Explanation:
In this problem we have an array of char pointers pointing to start of 4 strings.
Then we have ptr which is a pointer to a pointer of type char and a variable p
which is a pointer to a pointer to a pointer of type char. p hold the initial value of
ptr, i.e. p = s+3. The next statement increment value in p by 1 , thus now value of
p = s+2. In the printf statement the expression is evaluated *++p causes gets
value s+1 then the pre decrement is executed and we get s+1 – 1 = s . the
indirection operator now gets the value from the array of s and adds 3 to the
starting address. The string is printed starting from this position. Thus, the output
is ‗ck‘.
Questions 53
main()
{
int i, n;
char *x = ―girl‖;
n = strlen(x);
*x = x[n];
for(i=0; i<n; ++i)
{
printf(―%s\n‖,x);
x++;
}
}
Answer:
(blank space)
irl
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rl
l
Explanation:
Here a string (a pointer to char) is initialized with a value ―girl‖. The strlen
function returns the length of the string, thus n has a value 4. The next statement
assigns value at the nth location (‗\0‘) to the first location. Now the string
becomes ―\0irl‖ . Now the printf statement prints the string after each iteration
it increments it starting position. Loop starts from 0 to 4. The first time x[0] =
‗\0‘ hence it prints nothing and pointer value is incremented. The second time
it prints from x[1] i.e ―irl‖ and the third time it prints ―rl‖ and the last time
it prints ―l‖ and the loop terminates.
Questions 54 int i,j;
for(i=0;i<=10;i++)
{
j+=5;
assert(i<5);
}
Answer: Runtime error: Abnormal program termination. assert failed (i<5), <file
name>,<line number>
Explanation:
asserts are used during debugging to make sure that certain conditions are
satisfied. If assertion fails, the program will terminate reporting the same. After
debugging use,
#undef NDEBUG
and this will disable all the assertions from the source code. Assertion
is a good debugging tool to make use of.
Questions 55
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main()
{
int i=-1;
+i;
printf("i = %d, +i = %d \n",i,+i);
}
Answer:
i = -1, +i = -1
Explanation:
Unary + is the only dummy operator in C. Where-ever it comes you can just
ignore it just because it has no effect in the expressions (hence the name dummy
operator).
Questions 56 : What are the files which are automatically opened when a C file
is executed?
Answer: stdin, stdout, stderr (standard input,standard output,standard error).
Questions 57: what will be the position of the file marker?
a: fseek(ptr,0,SEEK_SET);
b: fseek(ptr,0,SEEK_CUR);
Answer :
a: The SEEK_SET sets the file position marker to the starting of the file.
b: The SEEK_CUR sets the file position marker to the current position
of the file.
Questions 58:
main()
{
char name[10],s[12];
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scanf(" \"%[^\"]\"",s);
}
How scanf will execute?
Answer:First it checks for the leading white space and discards it.Then it
matches with a quotation mark and then it reads all character upto another
quotation mark.
Questions 59 What is the problem with the following code segment?
while ((fgets(receiving array,50,file_ptr)) != EOF)
;
Answer & Explanation: fgets returns a pointer. So the correct end of file check
is checking for != NULL.
Questions 60
main()
{
main();
}
Answer: Runtime error : Stack overflow.
Explanation: main function calls itself again and again. Each time the function is
called its return address is stored in the callstack. Since there is no condition to
terminate the function call, the call stack overflows at runtime. So it terminates
the program and results in an error.
Questions 61
main()
{
char *cptr,c;
void *vptr,v;
c=10; v=0;
cptr=&c; vptr=&v;
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printf("%c%v",c,v);
}
Answer:
Compiler error (at line number 4): size of v is Unknown.
Explanation: You can create a variable of type void * but not of type void, since
void is an empty type. In the second line you are creating variable vptr of type
void * and v of type void hence an error.
Questions 62
main()
{
char *str1="abcd";
char str2[]="abcd";
printf("%d %d %d",sizeof(str1),sizeof(str2),sizeof("abcd"));
}
Answer:
255
Explanation: In first sizeof, str1 is a character pointer so it gives you the size of
the pointer variable. In second sizeof the name str2 indicates the name of the
array whose size is 5 (including the '\0' termination character). The third sizeof is
similar to the second one.
Questions 63
main()
{
char not;
not=!2;
printf("%d",not);
}
Answer:
0
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Explanation:
! is a logical operator. In C the value 0 is considered to be the boolean value
FALSE, and any non-zero value is considered to be the boolean value TRUE.
Here 2 is a non-zero value so TRUE. !TRUE is FALSE (0) so it prints 0.
Questions 64
#define FALSE -1
#define TRUE 1
#define NULL 0
main() {
if(NULL)
puts("NULL");
else if(FALSE)
puts("TRUE");
else
puts("FALSE");
}
Answer:TRUE
Explanation:The input program to the compiler after processing by the
preprocessor is,
main(){
if(0)
puts("NULL");
else if(-1)
puts("TRUE");
else
puts("FALSE");
}
Preprocessor doesn't replace the values given inside the double quotes. The check
by if condition is boolean value false so it goes to else. In second if -1 is boolean
value true hence "TRUE" is printed.
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Questions 65
main()
{
int k=1;
printf("%d==1 is ""%s",k,k==1?"TRUE":"FALSE");
}
Answer:
1==1 is TRUE
Explanation:
When two strings are placed together (or separated by white-space) they are
concatenated (this is called as "stringization"operation). So the string is as if it is
given as "%d==1 is %s". The conditional operator( ?: ) evaluates to "TRUE".
Questions 66
main()
{
int y;
scanf("%d",&y); // input given is 2000
if( (y%4==0 && y%100 != 0) || y%100 == 0 )
printf("%d is a leap year");
else
printf("%d is not a leap year");
}
Answer:
2000 is a leap year
Explanation:
An ordinary program to check if leap year or not.
Questions 67
#define max 5
#define int arr1[max]
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main()
{
typedef char arr2[max];
arr1 list={0,1,2,3,4};
arr2 name="name";
printf("%d %s",list[0],name);
}
Answer:
Compiler error (in the line arr1 list = {0,1,2,3,4})
Explanation:arr2 is declared of type array of size 5 of characters. So it can be
used to declare the variable name of the type arr2. But it is not the case of arr1.
Hence an error.
Rule of Thumb:
#defines are used for textual replacement whereas typedefs are used for declaring
new types.
Questions 68
int i=10;
main()
{
extern int i;
{
int i=20;
{
const volatile unsigned i=30;
printf("%d",i);
}
printf("%d",i);
}
printf("%d",i);
}
Answer:
30,20,10
Explanation:
'{' introduces new block and thus new scope. In the innermost block i is declared
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as,
const volatile unsigned
which is a valid declaration. i is assumed of type int. So printf prints 30. In the
next block, i has value 20 and so printf prints 20. In the outermost block, i is
declared as extern, so no storage space is allocated for it. After compilation is
over the linker resolves it to global variable i (since it is the only variable visible
there). So it prints i's value as 10.
Questions 69
main()
{
int *j;
{
int i=10;
j=&i;
}
printf("%d",*j);
}
Answer:
10
Explanation:
The variable i is a block level variable and the visibility is inside that block only.
But the lifetime of i is lifetime of the function so it lives upto the exit of main
function. Since the i is still allocated space, *j prints the value stored in i since j
points i.
Questions 70
main()
{
int i=-1;
-i;
printf("i = %d, -i = %d \n",i,-i);
}
Answer:
i = -1, -i = 1
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Explanation:
-i is executed and this execution doesn't affect the value of i. In printf first you
just print the value of i. After that the value of the expression -i = -(-1) is printed.
Questions 71
#include<stdio.h>
main()
{
const int i=4;
float j;
j = ++i;
printf("%d %f", i,++j);
}
Answer:Compiler error
Explanation:
i is a constant. you cannot change the value of constant
Questions 72
#include<stdio.h>
main()
{
int a[2][2][2] = { {10,2,3,4}, {5,6,7,8} };
int *p,*q;
p=&a[2][2][2];
*q=***a;
printf("%d..%d",*p,*q);
}
Answer:
garbagevalue..1
Explanation:
p=&a[2][2][2] you declare only two 2D arrays. but you are trying to access the
third 2D(which you are not declared) it will print garbage values.
*q=***a starting address of a is assigned integer pointer. now q
is pointing to starting address of a.if you print *q meAnswer:it will print
first element of 3D array.
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Questions 73
#include<stdio.h>
main()
{
register i=5;
char j[]= "hello";
printf("%s %d",j,i);
}
Answer:
hello 5
Explanation:
if you declare i as register compiler will treat it as ordinary integer and it will
take integer value. i value may be stored either in register or in memory.
Questions 74
main()
{
int i=5,j=6,z;
printf("%d",i+++j);
}
Answer:
11
Explanation: the expression i+++j is treated as (i++ + j)
Questions 76
struct aaa{
struct aaa *prev;
int i;
struct aaa *next;
};
main()
{
struct aaa abc,def,ghi,jkl;
int x=100;
abc.i=0;abc.prev=&jkl;
abc.next=&def;
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def.i=1;def.prev=&abc;def.next=&ghi;
ghi.i=2;ghi.prev=&def;
ghi.next=&jkl;
jkl.i=3;jkl.prev=&ghi;jkl.next=&abc;
x=abc.next->next->prev->next->i;
printf("%d",x);
}
Answer:
2
Explanation:
above all statements form a double circular linked list;abc.next->next->prev>next->i this one points to "ghi" node the value of at particular node is 2.
Questions 77
struct point
{
int x;
int y;
};
struct point origin,*pp;
main()
{
pp=&origin;
printf("origin is(%d%d)\n",(*pp).x,(*pp).y);
printf("origin is (%d%d)\n",pp->x,pp->y);
}
Answer:
origin is(0,0)
origin is(0,0)
Explanation: pp is a pointer to structure. we can access the elements of
the structure either with arrow mark or with indirection operator.
Note: Since structure point is globally declared x & y are initialized as zeroes
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Questions 78
main()
{
int i=_l_abc(10);
printf("%d\n",--i);
}
int _l_abc(int i)
{
return(i++);
}
Answer:
9
Explanation:
return(i++) it will first return i and then increments. i.e. 10 will be returned.
Questions 79
main()
{
char *p;
int *q;
long *r;
p=q=r=0;
p++;
q++;
r++;
printf("%p...%p...%p",p,q,r);
}
Answer:
0001...0002...0004
Explanation:++ operator when applied to pointers increments address according
to their corresponding data-types.
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Questions 80
main()
{
char c=' ',x,convert(z);
getc(c);
if((c>='a') && (c<='z'))
x=convert(c);
printf("%c",x);
}
convert(z)
{
return z-32;
}
Answer:
Compiler error
Explanation:
declaration of convert and format of getc() are wrong.
Questions 81
main(int argc, char **argv)
{
printf("enter the character");
getchar();
sum(argv[1],argv[2]);
}
sum(num1,num2)
int num1,num2;
{
return num1+num2;
}
Answer:
Compiler error.
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Explanation:
argv[1] & argv[2] are strings. They are passed to the function sum without
converting it to integer values.
Questions 82
# include <stdio.h>
int one_d[]={1,2,3};
main()
{
int *ptr;
ptr=one_d;
ptr+=3;
printf("%d",*ptr);
}
Answer:
garbage value
Explanation: ptr pointer is pointing to out of the array range of one_d.
Questions 83
# include<stdio.h>
aaa() {
printf("hi");
}
bbb(){
printf("hello");
}
ccc(){
printf("bye");
}
main()
{
int (*ptr[3])();
ptr[0]=aaa;
ptr[1]=bbb;
ptr[2]=ccc;
ptr[2]();
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}
Answer:
bye
Explanation:
ptr is array of pointers to functions of return type int.ptr[0] is assigned to address
of the function aaa. Similarly ptr[1] and ptr[2] for bbb and ccc respectively.
ptr[2]() is in effect of writing ccc(), since ptr[2] points to ccc.
Questions 85
#include<stdio.h>
main()
{
FILE *ptr;
char i;
ptr=fopen("zzz.c","r");
while((i=fgetch(ptr))!=EOF)
printf("%c",i);
}
Answer:
contents of zzz.c followed by an infinite loop
Explanation:
The condition is checked against EOF, it should be checked against NULL.
Questions 86
main()
{
int i =0;j=0;
if(i && j++)
printf("%d..%d",i++,j);
printf("%d..%d,i,j);
}
Answer:
0..0
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Explanation:
The value of i is 0. Since this information is enough to determine the truth value
of the boolean expression. So the statement following the if statement is not
executed. The values of i and j remain unchanged and get printed.
Questions 87
main()
{
int i;
i = abc();
printf("%d",i);
}
abc()
{
_AX = 1000;
}
Answer:
1000
Explanation:
Normally the return value from the function is through the information from the
accumulator. Here _AH is the pseudo global variable denoting the accumulator.
Hence, the value of the accumulator is set 1000 so the function returns value
1000.
Questions 88
int i;
main(){
int t;
for ( t=4;scanf("%d",&i)-t;printf("%d\n",i))
printf("%d--",t--);
}
// If the inputs are 0,1,2,3 find the o/p
Answer:
4--0
3--1
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2--2
Explanation:Let us assume some x= scanf("%d",&i)-t the values during
execution
will be,
t
i
x
4
0
-4
3
1
-2
2
2
0
Questions 89
main(){
int a= 0;int b = 20;char x =1;char y =10;
if(a,b,x,y)
printf("hello");
}
Answer:
hello
Explanation:
The comma operator has associatively from left to right. Only the rightmost value
is returned and the other values are evaluated and ignored. Thus the value of last
variable y is returned to check in if. Since it is a non zero value if becomes true
so, "hello" will be printed.
Questions 90
main(){
unsigned int i;
for(i=1;i>-2;i--)
printf("c aptitude");
}
Explanation:
i is an unsigned integer. It is compared with a signed value. Since the both types
doesn't match, signed is promoted to unsigned value. The unsigned equivalent of 2 is a huge value so condition becomes false and control comes out of the loop.
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Questions 91
In the following pgm add a stmt in the function fun such that the address of 'a'
gets stored in 'j'.
main(){
int * j;
void fun(int **);
fun(&j);
}
void fun(int **k) {
int a =0;
/* add a stmt here*/
}
Answer:
*k = &a
Explanation:
The argument of the function is a pointer to a pointer.
Questions 92:
What are the following notations of defining functions known as?
int abc(int a,float b)
{
/* some code */
}
ii. int abc(a,b)
int a; float b;
{
/* some code*/
}
Answer:
i. ANSI C notation
ii. Kernighan & Ritche notation
Questions 93
main()
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{
char *p;
p="%d\n";
p++;
p++;
printf(p-2,300);
}
Answer:
300
Explanation:
The pointer points to % since it is incremented twice and again decremented by 2,
it points to '%d\n' and 300 is printed.
Questions 94
main(){
char a[100];
a[0]='a';a[1]]='b';a[2]='c';a[4]='d';
abc(a);
}
abc(char a[]){
a++;
printf("%c",*a);
a++;
printf("%c",*a);
}
Explanation:
The base address is modified only in function and as a result a points to 'b' then
after incrementing to 'c' so bc will beprinted.
Questions 95
func(a,b)
int a,b;
{
return( a= (a==b) );
}
main()
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{
int process(),func();
printf("The value of process is %d !\n ",process(func,3,6));
}
process(pf,val1,val2)
int (*pf) ();
int val1,val2;
{
return((*pf) (val1,val2));
}
Answer:
The value if process is 0 !
Explanation:The function 'process' has 3 parameters - 1, a pointer to
another function 2 and 3, integers. When this functionis invoked from main, the
following substitutions for formal parameters take place: func for pf, 3 for val1
and 6 for val2. Thisfunction returns the result of the operation performed
by the function 'func'. The function func has two integer parameters. The
formal parameters are substituted as 3 for a and 6 for b. since 3 is not equal to 6,
a==b returns 0. therefore the functionreturns 0 which in turn is returned by the
function 'process'.
Questions 96
void main()
{
static int i=5;
if(--i){
main();
printf("%d ",i);
}
}
Answer:
0000
Explanation:
The variable "I" is declared as static, hence memory for I will be allocated for
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only once, as it encounters the statement. Thefunction main() will be called
recursively unless I becomes equal to 0, and since main() is recursively called, so
the value of static I ie., 0 will be printed every time the control is returned.
Questions 97
void main()
{
int k=ret(sizeof(float));
printf("\n here value is %d",++k);
}
int ret(int ret)
{
ret += 2.5;
return(ret);
}
Answer:
Here value is 7
Explanation:
The int ret(int ret), ie., the function name and the argument name can be the
same. Firstly, the function ret() is called in which the sizeof(float) ie., 4 is
passed, after the first expression the value in ret will be 6, as ret is integer hence
the value stored in ret will have implicit type conversion from float to int. The ret
is returned in main() it is printed after and preincrement.
Questions 98
void main()
{
char a[]="12345\0";
int i=strlen(a);
printf("here in 3 %d\n",++i);
}
Answer:
here in 3 6
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Explanation:The char array 'a' will hold the initialized string, whose length will
be counted from 0 till the null character. Hence the 'I' will hold the value equal to
5, after the pre-increment in the printf statement, the 6 will be printed.
Questions 99
void main()
{
unsigned giveit=-1;
int gotit;
printf("%u ",++giveit);
printf("%u \n",gotit=--giveit);
}
Answer:
0 65535
100) void main()
{
int i;
char a[]="\0";
if(printf("%s\n",a))
printf("Ok here \n");
else
printf("Forget it\n");
}
Answer:
Ok here
Explanation:
Printf will return how many characters does it print. Hence printing a null
character returns 1 which makes the if statement true, thus "Ok here" is printed.
Questions 101
void main()
{
void *v;
int integer=2;
int *i=&integer;
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v=i;
printf("%d",(int*)*v);
}
Answer:
Compiler Error. We cannot apply indirection on type void*.
Explanation:
Void pointer is a generic pointer type. No pointer arithmetic can be done on it.
Void pointers are normally used for,
1. Passing generic pointers to functions and returning such pointers.
2. As a intermediate pointer type.
3. Used when the exact pointer type will be known at a later point of time.
Questions 102
void main()
{
int i=i++,j=j++,k=k++;
printf(―%d%d%d‖,i,j,k);
}
Answer:
Garbage values.
Explanation:
An identifier is available to use in program code from the point of its declaration.
So expressions such as i = i++ are valid statements. The i, j and k are automatic
variables and so they contain some garbage value. Garbage in is garbage out
(GIGO).
Questions 103
void main()
{
static int i=i++, j=j++, k=k++;
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printf(―i = %d j = %d k = %d‖, i, j, k);
}
Answer:
i=1j=1k=1
Explanation:
Since static variables are initialized to zero by default.
Questions 104
void main()
{
while(1){
if(printf("%d",printf("%d")))
break;
else
continue;
}
}
Answer:
Garbage values
Explanation:
The inner printf executes first to print some garbage value. The printf returns no
of characters printed and this value also cannot be predicted. Still the
outer printf prints something and so returns a non-zero value. So it encounters
the break statement and comes out of the while statement.
Questions 104
main()
{
unsigned int i=10;
while(i-->=0)
printf("%u ",i);
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}
Answer:
10 9 8 7 6 5 4 3 2 1 0 65535 65534…..
Explanation:
Since i is an unsigned integer it can never become negative. So the expression i->=0 will always be true, leading to aninfinite loop.
Questions 105
#include<conio.h>
main()
{
int x,y=2,z,a;
if(x=y%2) z=2;
a=2;
printf("%d %d ",z,x);
}
Answer:
Garbage-value 0
Explanation:
The value of y%2 is 0. This value is assigned to x. The condition reduces to if (x)
or in other words if(0) and so z goes uninitialized.
Thumb Rule: Check all control paths to write bug free code.
Questions 106
main()
{
int a[10];
printf("%d",*a+1-*a+3);
}
Answer:
4
Explanation:
*a and -*a cancels out. The result is as simple as 1 + 3 = 4 !
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Questions 107
#define prod(a,b) a*b
main()
{
int x=3,y=4;
printf("%d",prod(x+2,y-1));
}
Answer:
10
Explanation:
The macro expands and evaluates to as:
x+2*y-1 => x+(2*y)-1 => 10
Questions 108
main()
{
unsigned int i=65000;
while(i++!=0);
printf("%d",i);
}
Answer:
1
Explanation:
Note the semicolon after the while statement. When the value of i becomes 0 it
comes out of while loop. Due to post-increment on i the value of i while printing
is 1.
Questions 109
main()
{
int i=0;
while(+(+i--)!=0)
i-=i++;
printf("%d",i);
}
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Answer:
-1
Explanation:
Unary + is the only dummy operator in C. So it has no effect on the expression
and now the while loop is, while(i--!=0) which is false and so breaks out of
while loop. The value –1 is printed due to the post-decrement operator.
Questions 110
main()
{
float f=5,g=10;
enum{i=10,j=20,k=50};
printf("%d\n",++k);
printf("%f\n",f<<2);
printf("%lf\n",f%g);
printf("%lf\n",fmod(f,g));
}
Answer:
Line no 5: Error: Lvalue required
Line no 6: Cannot apply leftshift to float
Line no 7: Cannot apply mod to float
Explanation:
Enumeration constants cannot be modified, so you cannot apply ++.Bit-wise
operators and % operators cannot be applied on float values. fmod() is to find the
modulus values for floats as % operator is for ints.
Questions 110
main()
{
int i=10;
void pascal f(int,int,int);
f(i++,i++,i++);
printf(" %d",i);
}
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void pascal f(integer :i,integer:j,integer :k)
{
write(i,j,k);
}
Answer:
Compiler error: unknown type integer
Compiler error: undeclared function write
Explanation:
Pascal keyword doesn‘t mean that pascal code can be used. It means that the
function follows Pascal argument passing mechanism in calling the functions.
Questions 111
void pascal f(int i,int j,int k)
{
printf(―%d %d %d‖,i, j, k);
}
void cdecl f(int i,int j,int k)
{
printf(―%d %d %d‖,i, j, k);
}
main()
{
int i=10;
f(i++,i++,i++);
printf(" %d\n",i);
i=10;
f(i++,i++,i++);
printf(" %d",i);
}
Answer:
10 11 12 13
12 11 10 13
Explanation:
Pascal argument passing mechanism forces the arguments to be called from left to
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right. cdecl is the normal C argument passing mechanism where the arguments
are passed from right to left.
Questions 112. What is the output of the program given below
main()
{
signed char i=0;
for(;i>=0;i++) ;
printf("%d\n",i);
}
Answer
-128
Explanation
Notice the semicolon at the end of the for loop. THe initial value of the i is set to
0. The inner loop executes to increment the value from 0 to 127 (the positive
range of char) and then it rotates to the negative value of -128. The condition in
the for loop fails and so comes out of the for loop. It prints the current value of i
that is -128.
Questions 113
main()
{
unsigned char i=0;
for(;i>=0;i++) ;
printf("%d\n",i);
}
Answer
infinite loop
Explanation
The difference between the previous question and this one is that the char is
declared to be unsigned. So the i++ can never yield negative value and i>=0 never
becomes false so that it can come out of the for loop.
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Questions 114
main()
{
char i=0;
for(;i>=0;i++) ;
printf("%d\n",i);
}
Answer:Behavior is implementation dependent.
Explanation:The detail if the char is signed/unsigned by default is
implementation dependent. If the implementation treats the char to be signed by
default the program will print –128 and terminate. On the other hand if it
considers char to be unsigned by default, it goes to infinite loop.
Rule:
You can write programs that have implementation dependent behavior. But dont
write programs that depend on such behavior.
Questions 115 Is the following statement a declaration/definition. Find what
does it mean?
int (*x)[10];
Answer Definition.
x is a pointer to array of(size 10) integers.
Apply clock-wise rule to find the meaning of this definition.
Questions 116. What is the output for the program given below
typedef enum errorType{warning, error, exception,}error;
main()
{
error g1;
g1=1;
printf("%d",g1);
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}
Answer Compiler error: Multiple declaration for error
Explanation
The name error is used in the two meanings. One means that it is a enumerator
constant with value 1. The another use is that it is a type name (due to typedef)
for enum errorType. Given a situation the compiler cannot distinguish the
meaning of error to know in what sense the error is used:
error g1;
g1=error;
// which error it refers in each case?
When the compiler can distinguish between usages then it will not issue error (in
pure technical terms, names can only be overloaded in different namespaces).
Note: the extra comma in the declaration,
enum errorType{warning, error, exception,}
is not an error. An extra comma is valid and is provided just for programmer‘s
convenience.
Questions 117
typedef struct error{int warning, error, exception;}error;
main()
{
error g1;
g1.error =1;
printf("%d",g1.error);
}
Answer
1
Explanation
The three usages of name errors can be distinguishable by the compiler at any
instance, so valid (they are in different namespaces).
Typedef struct error{int warning, error, exception;}error;
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This error can be used only by preceding the error by struct kayword as in:
struct error someError;
typedef struct error{int warning, error, exception;}error;
This can be used only after . (dot) or -> (arrow) operator preceded by the variable
name as in :
g1.error =1;
printf("%d",g1.error);
typedef struct error{int warning, error, exception;}error;
This can be used to define variables without using the preceding struct keyword
as in:
error g1;
Since the compiler can perfectly distinguish between these three usages, it is
perfectly legal and valid.
Note
This code is given here to just explain the concept behind. In real programming
don‘t use such overloading of names. It reduces the readability of the code.
Possible doesn‘t mean that we should use it!
Questions 118
#ifdef something
int some=0;
#endif
main()
{
int thing = 0;
printf("%d %d\n", some ,thing);
}
Answer:
Compiler error : undefined symbol some
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Explanation:
This is a very simple example for conditional compilation. The name something
is not already known to the compiler making the declaration
int some = 0;
effectively removed from the source code.
Questions 119
#if something == 0
int some=0;
#endif
main()
{
int thing = 0;
printf("%d %d\n", some ,thing);
}
Answer
00
Explanation
This code is to show that preprocessor expressions are not the same as the
ordinary expressions. If a name is not known the preprocessor treats it to be equal
to zero.
Questions 120.
What is the output for the following program
main()
{
int arr2D[3][3];
printf("%d\n", ((arr2D==* arr2D)&&(* arr2D == arr2D[0])) );
}
Answer
1
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Explanation
This is due to the close relation between the arrays and pointers. N dimensional
arrays are made up of (N-1) dimensional arrays. arr2D is made up of a
3 single arrays that contains 3 integers each .The name arr2D refers to the
beginning of all the 3 arrays. *arr2D refers to the start of the first 1D array (of 3
integers) that is the same address as arr2D. So the expression(arr2D == *arr2D) is
true (1). Similarly, *arr2D is nothing but *(arr2D + 0), adding a zero doesn‘t
change the value/meaning. Again arr2D[0] is the another way of telling *(arr2D +
0). So the expression (*(arr2D + 0) == arr2D[0]) is true (1). Since both parts
of the expression evaluates to true the result is true(1) and the same is printed.
Questions 121
void main()
{
if(~0 == (unsigned int)-1)
printf(―You can answer this if you know how values are represented in
memory‖);
}
Answer
You can answer this if you know how values are represented in memory
Explanation
~ (tilde operator or bit-wise negation operator) operates on 0 to produce all ones
to fill the space for an integer. –1 is represented in unsigned value as all 1‘s and
so both are equal.
Questions 122
int swap(int *a,int *b)
{
*a=*a+*b;*b=*a-*b;*a=*a-*b;
}
main()
{
int x=10,y=20;
swap(&x,&y);
printf("x= %d y = %d\n",x,y);
}
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Answer
x = 20 y = 10
Explanation
This is one way of swapping two values. Simple checking will
help understand this.
Questions 123
main()
{
char *p = ―ayqm‖;
printf(―%c‖,++*(p++));
}
Answer:
b
Questions 124
main()
{
int i=5;
printf("%d",++i++);
}
Answer:
Compiler error: Lvalue required in function main
Explanation:
++i yields an rvalue. For postfix ++ to operate an lvalue is required.
Questions 125
main()
{
char *p = ―ayqm‖;
char c;
c = ++*p++;
printf(―%c‖,c);
}
Answer:
b
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Explanation:
There is no difference between the expression ++*(p++) and ++*p++. Parenthesis
just works as a visual clue for the reader to see which expression is first
evaluated.
Questions 126
int aaa() {printf(―Hi‖);}
int bbb(){printf(―hello‖);}
iny ccc(){printf(―bye‖);}
main()
{
int ( * ptr[3]) ();
ptr[0] = aaa;
ptr[1] = bbb;
ptr[2] =ccc;
ptr[2]();
}
Answer:
bye
Explanation:
int (* ptr[3])() says that ptr is an array of pointers to functions that takes no
arguments and returns the type int. By the assignment ptr[0] = aaa; it means that
the first function pointer in the array is initialized with the address of the function
aaa. Similarly, the other two array elements also get initialized with the addresses
of the functions bbb and ccc. Since ptr[2]contains the address of the function
ccc, the call to the function ptr[2]() is same as calling ccc(). So it results
in printing "bye".
Questions 127
main()
{
int i=5;
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printf(―%d‖,i=++i ==6);
}
Answer:
1
Explanation:
The expression can be treated as i = (++i==6), because == is of higher precedence
than = operator. In the inner expression, ++i is equal to 6 yielding true(1). Hence
the result.
Questions 128
main()
{
char p[ ]="%d\n";
p[1] = 'c';
printf(p,65);
}
Answer:
A
Explanation:
Due to the assignment p[1] = ‗c‘ the string becomes, ―%c\n‖. Since this string
becomes the format string for printf and ASCII value of 65 is ‗A‘, the same gets
printed.
Questions 129
void ( * abc( int, void ( *def) () ) ) ();
Answer::
abc is a ptr to a function which takes 2 parameters .(a).
an integer variable.(b).
a ptrto a funtion which returns void. the return type of
the function is void.
Explanation:
Apply the clock-wise rule to find the result.
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Questions 130
main()
{
while (strcmp(―some‖,‖some\0‖))
printf(―Strings are not equal\n‖);
}
Answer:
No output
Explanation:
Ending the string constant with \0 explicitly makes no difference. So ―some‖ and
―some\0‖ are equivalent. So, strcmp returns 0 (false) hence breaking out of the
while loop.
Questions 131
main()
{
char str1[] = {‗s‘,‘o‘,‘m‘,‘e‘};
char str2[] = {‗s‘,‘o‘,‘m‘,‘e‘,‘\0‘};
while (strcmp(str1,str2))
printf(―Strings are not equal\n‖);
}
Answer:
―Strings are not equal‖
―Strings are not equal‖
….
Explanation:
If a string constant is initialized explicitly with characters, ‗\0‘ is not
appended automatically to the string. Since str1 doesn‘t have null termination, it
treats whatever the values that are in the following positions as part of the string
until it randomly reaches a ‗\0‘. So str1 and str2 are not the same, hence the
result.
Questions 132
main()
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{
int i = 3;
for (;i++=0;) printf(―%d‖,i);
}
Answer:
Compiler Error: Lvalue required.
Explanation:
As we know that increment operators return rvalues and hence it cannot appear
on the left hand side of an assignment operation.
Questions 133
void main()
{
int *mptr, *cptr;
mptr = (int*)malloc(sizeof(int));
printf(―%d‖,*mptr);
int *cptr = (int*)calloc(sizeof(int),1);
printf(―%d‖,*cptr);
}
Answer:
garbage-value 0
Explanation:
The memory space allocated by malloc is uninitialized, whereas calloc returns the
allocated memory space initialized to zeros.
Questions 134
void main()
{
static int i;
while(i<=10)
(i>2)?i++:i--;
printf(―%d‖, i);
}
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Answer:
32767
Explanation:
Since i is static it is initialized to 0. Inside the while loop the conditional operator
evaluates to false, executing i--. Thiscontinues till the integer value rotates to
positive value (32767). The while condition becomes false and hence, comes out
of the while loop, printing the i value.
Questions 135
main()
{
int i=10,j=20;
j = i, j?(i,j)?i:j:j;
printf("%d %d",i,j);
}
Answer:
10 10
Explanation:
The Ternary operator ( ? : ) is equivalent for if-then-else statement. So the
question can be written as:
if(i,j)
{
if(i,j)
j = i;
else
j = j;
}
else
j = j;
Questions 136
1. const char *a;
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2. char* const a;
3. char const *a;
-Differentiate the above declarations.
Answer:
1. 'const' applies to char * rather than 'a' ( pointer to a constant char )
*a='F'
: illegal
a="Hi"
: legal
2. 'const' applies to 'a' rather than to the value of a (constant pointer to char )
*a='F'
: legal
a="Hi"
: illegal
3. Same as 1.
Questions 137
main()
{
int i=5,j=10;
i=i&=j&&10;
printf("%d %d",i,j);
}
Answer:
1 10
Explanation:
The expression can be written as i=(i&=(j&&10)); The inner expression (j&&10)
evaluates to 1 because j==10. i is 5. i = 5&1 is 1. Hence the result.
Questions 138
main()
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{
int i=4,j=7;
j = j || i++ && printf("YOU CAN");
printf("%d %d", i, j);
}
Answer:
41
Explanation:
The boolean expression needs to be evaluated only till the truth value of the
expression is not known. j is not equal to zero itself means that the expression‘s
truth value is 1. Because it is followed by || and true || (anything) => true where
(anything) will not be evaluated. So the remaining expression is not evaluated and
so the value of i remains the same.
Similarly when && operator is involved in an expression, when any of the
operands become false, the whole expression‘s truth value becomes false and
hence the remaining expression will not be evaluated.
false && (anything) => false where (anything) will not be evaluated.
Questions 139
main()
{
register int a=2;
printf("Address of a = %d",&a);
printf("Value of a = %d",a);
}
Answer:
Compier Error: '&' on register variable
Rule to Remember:
& (address of ) operator cannot be applied on register variables.
Questions 140
main()
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{
float i=1.5;
switch(i)
{
case 1: printf("1");
case 2: printf("2");
default : printf("0");
}
}
Answer:
Compiler Error: switch expression not integral
Explanation:
Switch statements can be applied only to integral types.
Questions 141
main()
{
extern i;
printf("%d\n",i);
{
int i=20;
printf("%d\n",i);
}
}
Answer:
Linker Error : Unresolved external symbol i
Explanation:
The identifier i is available in the inner block and so using extern has no use in
resolving it.
Questions 142
main()
{
int a=2,*f1,*f2;
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f1=f2=&a;
*f2+=*f2+=a+=2.5;
printf("\n%d %d %d",a,*f1,*f2);
}
Answer:
16 16 16
Explanation:
f1 and f2 both refer to the same memory location a. So changes through f1 and f2
ultimately affects only the value of a.
Questions 143
main()
{
char *p="GOOD";
char a[ ]="GOOD";
printf("\n sizeof(p) = %d, sizeof(*p) = %d, strlen(p) = %d", sizeof(p), sizeof(*p),
strlen(p));
printf("\n sizeof(a) = %d, strlen(a) = %d", sizeof(a), strlen(a));
}
Answer:
sizeof(p) = 2, sizeof(*p) = 1, strlen(p) = 4
sizeof(a) = 5, strlen(a) = 4
Explanation:
sizeof(p) => sizeof(char*) => 2
sizeof(*p) => sizeof(char) => 1
Similarly,
sizeof(a) => size of the character array => 5
When sizeof operator is applied to an array it returns the sizeof the array and it is
not the same as the sizeof the pointervariable. Here the sizeof(a) where a is the
character array and the size of the array is 5 because the space necessary for the
terminating NULL character should also be taken into account.
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Questions 144
#define DIM( array, type) sizeof(array)/sizeof(type)
main()
{
int arr[10];
printf(―The dimension of the array is %d‖, DIM(arr, int));
}
Answer:
10
Explanation:
The size of integer array of 10 elements is 10 * sizeof(int). The macro expands to
sizeof(arr)/sizeof(int) => 10 * sizeof(int) / sizeof(int) => 10.
Questions 145
int DIM(int array[])
{
return sizeof(array)/sizeof(int );
}
main()
{
int arr[10];
printf(―The dimension of the array is %d‖, DIM(arr));
}
Answer:
1
Explanation:
Arrays cannot be passed to functions as arguments and only the pointers can be
passed. So the argument is equivalent to int * array (this is one of the very few
places where [] and * usage are equivalent). The return statement becomes,
sizeof(int *)/ sizeof(int) that happens to be equal in this case.
Questions 146
main()
{
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static int a[3][3]={1,2,3,4,5,6,7,8,9};
int i,j;
static *p[]={a,a+1,a+2};
for(i=0;i<3;i++)
{
for(j=0;j<3;j++)
printf("%d\t%d\t%d\t%d\n",*(*(p+i)+j),
*(*(j+p)+i),*(*(i+p)+j),*(*(p+j)+i));
}
}
Answer:
1
1
1
1
2
4
2
4
3
7
7
4
2
4
2
5
5
5
5
6
9
3
8
6
8
7
3
7
3
8
6
8
6
9
9
9
Explanation:
*(*(p+i)+j) is equivalent to p[i][j].
Questions 147
main()
{
void swap();
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int x=10,y=8;
swap(&x,&y);
printf("x=%d y=%d",x,y);
}
void swap(int *a, int *b)
{
*a ^= *b, *b ^= *a, *a ^= *b;
}
Answer:
x=10 y=8
Explanation:
Using ^ like this is a way to swap two variables without using a temporary
variable and that too in a single statement.
Inside main(), void swap(); means that swap is a function that may take
any number of arguments (not no arguments) and returns nothing. So this doesn‘t
issue a compiler error by the call swap(&x,&y); that has two arguments.
This convention is historically due to pre-ANSI style (referred to as Kernighan
and Ritchie style) style of function declaration. In that style, the swap function
will be defined as follows,
void swap()
int *a, int *b
{
*a ^= *b, *b ^= *a, *a ^= *b;
}
where the arguments follow the (). So naturally the declaration for swap will look
like, void swap() which means the swap can take any number of arguments.
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Questions 148
main()
{
int i = 257;
int *iPtr = &i;
printf("%d %d", *((char*)iPtr), *((char*)iPtr+1) );
}
Answer:
11
Explanation:
The integer value 257 is stored in the memory as, 00000001 00000001, so the
individual bytes are taken by casting it to char * and get printed.
Questions 149
main()
{
int i = 258;
int *iPtr = &i;
printf("%d %d", *((char*)iPtr), *((char*)iPtr+1) );
}
Answer:
21
Explanation:
The integer value 257 can be represented in binary as, 00000001 00000001.
Remember that the INTEL machines are ‗small-endian‘ machines. Small-endian
means that the lower order bytes are stored in the higher memory addresses and
the higher order bytes are stored in lower addresses. The integer value 258
is stored in memory as: 00000001 00000010.
Questions 150
main()
{
int i=300;
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char *ptr = &i;
*++ptr=2;
printf("%d",i);
}
Answer:
556
Explanation:
The integer value 300 in binary notation is: 00000001 00101100. It is stored in
memory (small-endian) as: 00101100 00000001. Result of the expression *++ptr
= 2 makes the memory representation as: 00101100 00000010. So the integer
corresponding to it is 00000010 00101100 => 556.
Questions 151
#include <stdio.h>
main()
{
char * str = "hello";
char * ptr = str;
char least = 127;
while (*ptr++)
least = (*ptr<least ) ?*ptr :least;
printf("%d",least);
}
Answer:
0
Explanation:
After ‗ptr‘ reaches the end of the string the value pointed by ‗str‘ is ‗\0‘. So the
value of ‗str‘ is less than that of ‗least‘. So the value of ‗least‘ finally is 0.
Questions 152
Declare an array of N pointers to functions returning pointers to functions
returning pointers to characters?
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Answer:
(char*(*)( )) (*ptr[N])( );
Questions 153
main()
{
struct student
{
char name[30];
struct date dob;
}stud;
struct date
{
int day,month,year;
};
scanf("%s%d%d%d", stud.rollno, &student.dob.day,
&student.dob.month,
&student.dob.year);
}
Answer: Compiler Error: Undefined structure date
Explanation:
Inside the struct definition of ‗student‘ the member of type struct date is given.
The compiler doesn‘t have the definition ofdate structure (forward reference is
not allowed in C in this case) so it issues an error.
Questions 154
main()
{
struct date;
struct student
{
char name[30];
struct date dob;
}stud;
struct date
{
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int day,month,year;
};
scanf("%s%d%d%d", stud.rollno, &student.dob.day, &student.dob.month,
&student.dob.year);
}
Answer:
Compiler Error: Undefined structure date
Explanation:
Only declaration of struct date is available inside the structure definition
of ‗student‘ but to have a variable of type struct date the definition
of the structure is required.
Questions 155
There were 10 records stored in ―somefile.dat‖ but the following program printed
11 names. What went wrong?
void main()
{
struct student
{
char name[30], rollno[6];
}stud;
FILE *fp = fopen(―somefile.dat‖,‖r‖);
while(!feof(fp))
{
fread(&stud, sizeof(stud), 1 , fp);
puts(stud.name);
}
}
Explanation:
fread reads 10 records and prints the names successfully. It will return EOF only
when fread tries to read another record and fails reading EOF (and returning
EOF). So it prints the last record again. After this only the condition feof(fp)
becomes false, hence comes out of the while loop.
Questions 156
Is there any difference between the two declarations,
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1. int foo(int *arr[]) and
2. int foo(int *arr[2])
Answer:
No
Explanation:
Functions can only pass pointers and not arrays. The numbers that are allowed
inside the [] is just for more readability. So there is no difference between the two
declarations.
Questions 157
What is the subtle error in the following code segment?
void fun(int n, int arr[])
{
int *p=0;
int i=0;
while(i++<n)
p = &arr[i];
*p = 0;
}
Answer & Explanation:
If the body of the loop never executes p is assigned no address. So p remains
NULL where *p =0 may result in problem (may rise to runtime error ―NULL
pointer assignment‖ and terminate the program).
Questions 158
What is wrong with the following code?
int *foo()
{
int *s = malloc(sizeof(int)100);
assert(s != NULL);
return s;
}
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Answer & Explanation:
assert macro should be used for debugging and finding out bugs. The check s !=
NULL is for error/exception handling and for that assert shouldn‘t be used. A
plain if and the corresponding remedy statement has to be given.
Questions 159
What is the hidden bug with the following statement?
assert(val++ != 0);
Answer & Explanation:
Assert macro is used for debugging and removed in release version. In assert, the
experssion involves side-effects. So thebehavior of the code becomes different in
case of debug version and the release version thus leading to a subtle bug.
Rule to Remember:
Don‘t use expressions that have side-effects in assert statements.
Questions 160
void main()
{
int *i = 0x400; // i points to the address 400
*i = 0;
// set the value of memory location pointed by i;
}
Answer:
Undefined behavior
Explanation:
The second statement results in undefined behavior because it points to some
location whose value may not be available for modification. This type of pointer
in which the non-availability of the implementation of the referenced location is
known as 'incomplete type'.
Questions 161
#define assert(cond) if(!(cond)) \
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(fprintf(stderr, "assertion failed: %s, file %s, line %d \n",#cond,\
__FILE__,__LINE__), abort())
void main()
{
int i = 10;
if(i==0)
assert(i < 100);
else
printf("This statement becomes else for if in assert macro");
}
Answer:
No output
Explanation:
The else part in which the printf is there becomes the else for if in the
assert macro. Hence nothing is printed. The solution is to use conditional operator
instead of if statement,
#define assert(cond) ((cond)?(0): (fprintf (stderr, "assertion failed: \ %s, file %s,
line %d \n",#cond, __FILE__,__LINE__), abort()))
Note:
However this problem of ―matching with nearest else‖ cannot be solved by the
usual method of placing the if statement inside a block like this,
#define assert(cond) { \
if(!(cond)) \
(fprintf(stderr, "assertion failed: %s, file %s, line %d \n",#cond,\
__FILE__,__LINE__), abort()) \
}
Questions 162 Is the following code legal?
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struct a
{
int x;
struct a b;
}
Answer:
No
Explanation:
Is it not legal for a structure to contain a member that is of the same type as in this
case. Because this will cause the structure declaration to be
recursive without end.
Questions 163
Is the following code legal?
struct a
{
int x;
struct a *b;
}
Answer:
Yes.
Explanation:
*b is a pointer to type struct a and so is legal. The compiler knows, the size of the
pointer to a structure even before the sizeof the structure is determined(as you
know the pointer to any type is of same size). This type of structures is known as
‗self-referencing‘ structure.
Questions 164 Is the following code legal?
typedef struct a
{
int x;
aType *b;
}aType
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Answer:
No
Explanation:
The typename aType is not known at the point of declaring the structure
(forward references are not made for typedefs).
Questions 165
Is the following code legal?
typedef struct a aType;
struct a
{
int x;
aType *b;
};
Answer:
Yes
Explanation:
The typename aType is known at the point of declaring the structure, because it is
already typedefined.
Questions 166
Is the following code legal?
void main()
{
typedef struct a aType;
aType someVariable;
struct a
{
int x;
aType *b;
};
}
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Answer:
No
Explanation:
When the declaration, typedef struct a aType; is encountered body of
struct a is not known. This is known as ‗incomplete types‘.
Questions 167
void main()
{
printf(―sizeof (void *) = %d \n―, sizeof( void *));
printf(―sizeof (int *) = %d \n‖, sizeof(int *));
printf(―sizeof (double *) = %d \n‖, sizeof(double *));
printf(―sizeof(struct unknown *) = %d \n‖, sizeof(struct unknown *));
}
Answer :
sizeof (void *) = 2
sizeof (int *) = 2
sizeof (double *) = 2
sizeof(struct unknown *) = 2
Explanation:
The pointer to any type is of same size.
Questions 168
char inputString[100] = {0};
To get string input from the keyboard which one of the following is better?
1) gets(inputString)
2) fgets(inputString, sizeof(inputString), fp)
Answer & Explanation:
The second one is better because gets(inputString) doesn't know the size of the
string passed and so, if a very big input (here, more than 100 chars) the charactes
will be written past the input string. When fgets is used with stdin performs the
same operation as gets but is safe.
Questions 169
Which version do you prefer of the following two,
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1) printf(―%s‖,str);
// or the more curt one
2) printf(str);
Answer & Explanation:
Prefer the first one. If the str contains any format characters like %d then it
will result in a subtle bug.
Questions 170
void main()
{
int i=10, j=2;
int *ip= &i, *jp = &j;
int k = *ip/*jp;
printf(―%d‖,k);
}
Answer:
Compiler Error: ―Unexpected end of file in comment started in line 5‖.
Explanation:
The programmer intended to divide two integers, but by the ―maximum munch‖
rule, the compiler treats the operator sequence / and * as /* which happens to be
the starting of comment. To force what is intended by the programmer,
int k = *ip/ *jp;
// give space explicity separating / and *
//or
int k = *ip/(*jp);
// put braces to force the intention
will solve the problem.
Questions 171
void main()
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{
char ch;
for(ch=0;ch<=127;ch++)
printf(―%c %d \n―, ch, ch);
}
Answer:
Implementaion dependent
Explanation:
The char type may be signed or unsigned by default. If it is signed then ch++ is
executed after ch reaches 127 and rotates back to -128. Thus ch is always smaller
than 127.
Questions 172 Is this code legal?
int *ptr;
ptr = (int *) 0x400; Answer:
Yes
Explanation:
The pointer ptr will point at the integer in the memory location 0x400.
Questions 173
main()
{
char a[4]="HELLO";
printf("%s",a);
}
Answer:
Compiler error: Too many initializers
Explanation:
The array a is of size 4 but the string constant requires 6 bytes to get stored.
Questions 174
main()
{
char a[4]="HELL";
printf("%s",a);
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}
Answer:
HELL%@!~@!@???@~~!
Explanation:
The character array has the memory just enough to hold the string ―HELL‖ and
doesnt have enough space to store the terminating null character. So it prints the
HELL correctly and continues to print garbage values till it
accidentally
comes across a NULL character.
Questions 175)
main()
{
int a=10,*j;
void *k;
j=k=&a;
j++;
k++;
printf("\n %u %u ",j,k);
}
Answer:
Compiler error: Cannot increment a void pointer
Explanation:
Void pointers are generic pointers and they can be used only when the type is not
known and as an intermediate address storage type. No pointer arithmetic can be
done on it and you cannot apply indirection operator (*) on void pointers.
Questions 176
main()
{
extern int i;
{ int i=20;
{
const volatile unsigned i=30; printf("%d",i);
}
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printf("%d",i);
}
printf("%d",i);
}
int i;
Questions177
Printf can be implemented by using __________ list.
Answer:
Variable length argument lists
Questions 178
char *someFun()
{
char *temp = ―string constant";
return temp;
}
int main()
{
puts(someFun());
}
Answer:
string constant
Explanation:
The program suffers no problem and gives the output correctly because
the character constants are stored in code/data area and not allocated in stack, so
this doesn‘t lead to dangling pointers.
Questions 179
char *someFun1()
{
char temp[ ] = ―string";
return temp;
}
char *someFun2()
{
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char temp[ ] = {‗s‘, ‗t‘,‘r‘,‘i‘,‘n‘,‘g‘};
return temp;
}
int main()
{
puts(someFun1());
puts(someFun2());
}
Answer:
Garbage values.
Explanation:
Both the functions suffer from the problem of dangling pointers. In someFun1()
temp is a character array and so the space forit is allocated in heap and is
initialized with character string ―string‖. This is created dynamically as the
function is called, so is also deleted dynamically on exiting the function so the
string data is not available in the calling function main() leading to print some
garbage values. The function someFun2() also suffers from the same problem but
the problem can be easily identified in this case.
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Bibliography
Material referenced from :
www.techpreparation.com
www.stackoverflow.com
www.indiabix.com
www.geekinterview.com
www.roseindia.com
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Regards :
Team – IT Engg Portal
(www.itportal.in)
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