Download Java - WordPress.com

CORE-JAVA IMPORTANT POINTS TO REMEMBER
Q: Why C++ is Object Based Programming Language and Why Java is Object Oriented
Programming Language?
Ans: C++ is also an OOP Language!
Java syntax is C++ minus pointers, templates, multiple inheritance, operator
overloading....
Maybe java enforces OO programming in a stricter manner than C++, whereas C++ is
more flexible and has to be used more sensibly
The main difference between object based and object oraiented language is the
language which supports concepts of inheritance and dynamic binding is known as
object oriented and the language which doesn’t support these 2 concepts is called
object based language.
Q: Is true that JAVA is not 100% object oriented language?
Ans:Java is not 100% object oriented because it does not support multiple inheritance.
Java is not 100% OOP's, because it uses primitive data types like Boolean, short, int,
long etc.
Q: What are the differences between script language and programming language?
Ans: Scripting languages run inside another program. Scripting languages are not
compiled. Scripting languages are easy to use and easy to write. Scripting languages
today are used to build complex software.
Define Java in one line?
Ans: Compiled once can run in anywhere. [Platform independent]
(write once run anywhere.)
Class Def: A blue print from which objects are created.
Object Def: An object is a software bundle of state (variables) and behaviors
(methods).
(Or)
Instance of class is object (using new operators)
Encapsulation : Encapsulation is a process that allows selective hiding of properties and
methods in a class.
Ex: private, public see /home/sivashankar/Documents/JAVA STUFF/Sun Java
2/session 1.ppt#Encapsulation
Abstraction: Data abstraction is a process of identifying properties and methods
related much to a particular entity as relevant to the application
Abstract
 Memory in Java is automatically garbage collected. You never have to worry
about memory corruption.
 Array initialization: int[] a = new int[100];
Translator: is a program that convert computer program into machine language. It is of
3-types
a. Interpreter: converts line by line at a time.
(or)
Convert bytecode into machine code. (in java)
b. Compiler: Read all lines and convert all lines into machine
language.
(or)
Convert source code into bytecode. (in java)
c. Assemble: assembly level language is converted into m/c level lang.
Stream : represents flow of data from one plac to another place. (~pipe)
three types of streams are there: In system class we have 3 fields they are
System.out ----display o/p on monitor etc.,
System.in -----by default is keyboard
System.err ----to o/p error msg
How can i attach keyboard to InputstreamReader?
A: InputStreamReader obj=new InputStreamReader(System.in);
BufferedReader br=new BufferdReader(obj);
char ch = br.read(); ----reads single char.
Static Method: is a method that is called and executed without using any object.
Syntax: Classname.methodName();
Ex:System.out.print(x);
Accepting a float/double/int value from Keyboard:
float n=Float.parseFloat(str);
Command line arguments:
Ex:c:\> java Arr 10 11 12
args[0]=10
| args[0]=Arr
------------(in C lang)
args[1]=11
args[2]=12
|
args[1]=10
| args[2]=11, args[3]=12
String: string is a group of characters.
String is class in java.lang.String. (user defined data type)
Ex: String str=”Helo”;
String s1=new string();
 JVM is written in C-language.
 .class or byte code is System independent. It contains byte code instructions
understandable by JVM.
 .exe is contains m/c code understandable by the microprocessor, it is system
dependent
 virus is self replicating. It will not spread in text file. Virus cannot spread in
.class because it is text file.
 java is case sensitive.
 Exception represents runtime error. If any runtime error occur it does not
terminate,but in C/C++ if any runtime error occur then the program will
abnormal termination occur.
 String getName() - to get the name of the class
 Each word of class names and Interface names start with a Capital letter.
Method names start with small letter, then each word start with capital letter.
Ex: parseInt();
What is the difference b/w the #include and import ?
 #include makes the compiler to copy the entire header file code into a C/C++
program hence, the program size increases abnormally thus it wastes memory
and processor time.
 import statement makes JVM to goto a particular package, execute the code
there and substitute the results into the java program. JVM will not copy any
Java program therefore import is more efficient than #include.
Can you write main() without any arguments?
Ans: No, JVM recognize only “main(String args[])”
What is hash code?
It is the unique ID number given to every object by the JVM. Hash code is also called
reference number.
What is difference b/w == operator and equal method?
== operator compares the reference of the string object.
equal method compares contents of the string objects.
What is the difference b/w object oriented PL and object based PL?
Object oriented PL follow all the features of OOP's Eg:C++, Java, Smalltalk,simula67.
Object based program language follow all the features of OOP's except Inheritance .
Ex: Java script, VB script.
What is the difference between instance variables and static variable?
Instance variable is a variable whose separate copy is available to each object. If the
value of instance variable is changed in one object it will not effect the other object.
Static variable is a variable whose single copy is shared by all the object, any
modification to the static variable will effect all th objects.
What is the difference between Assignment and Initialization?
Assignment can be done as many times as desired whereas initialization can be done
only once.
Wrapper Classes: Wrapped classes are classes that allow primitive types to be
accessed as objects..[ Casting primitive data type.]
or
wrapper classes are those which are used to change primitive datatype values into
object references. So that they can be serializable. some examples of wrapper
classes are:- Integer,Float,Byte,Character,Long..
1. widening : casting a lower data type into higher data type.
2. Narrowing : converting a higher data type into a lower data type .
Rule : primitive data types represent single value. whereas advanced data types
(reference) represents a group of values.
Rule: we cannot convert one class type into another type unless those classes have
relationship by the way of inheritance.
Explain the differences between interface and class in Java
Interface has no implementation where class provides implementation for interface
What is the use of bin and lib in JDK?
 Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib
contains API and all packages.
Binding: Connecting a method call( i.e. ) to a method body( i.e. Function) is called
binding.
What is final, finalize() and finally?
final : final keyword can be used for class, method and variables. A final class cannot
be subclassed and it prevents other programmers from subclassing a secure class to
invoke insecure methods. A final method can’t be overridden. A final variable can’t
change from its initialized value.
finalize() : finalize() method is used just before an object is destroyed and can be called
just prior to garbage collection.
finally : finally, a key word used in exception handling, creates a block of code that will
be executed after a try/catch block has completed and before the code following the
try/catch block. The finally block will execute whether or not an exception is thrown.
For example, if a method opens a file upon exit, then you will not want the code that
closes the file to be bypassed by the exception-handling mechanism. This finally
keyword is designed to address this contingency.
CONSTRUCTOR
Constructor is a special kind of method that determines how an object is initialized when
created.
a. A constructor has the same name as the class.
b. A constructor has no return value.
c. A constructor is always called with the new operator.
Controlling Access to Members of a Class
What are different types of access modifiers?
public: Any thing declared as public can be accessed from anywhere.
private: Any thing declared as private can’t be seen outside of its class.
protected: Any thing declared as protected can be accessed by classes in the same
package and subclasses in the other packages.
default modifier : Can be accessed only to classes in the same package.
Specifier
class
subclass
package
world
private
X
protected
X
X*
X
public
X
X
X
package
X
X
X
Method overriding:
Def: Creating two methods with the same name but,there is no change in type and
arguments. Example of Inheritance is Method overriding. It has super and subclass.
Is null, sizeof is a keyword?
The null, sizeof is not a keyword.
What are Transient and Volatile Modifiers?
Transient: The transient modifier applies to variables only and it is not stored as part
of its object’s Persistent state. Transient variables are not serialized.
Volatile: Volatile modifier applies to variables only and it tells the compiler that the
variable modified by volatile can be changed unexpectedly by other parts of the
program.
What is the Vector class?
The Vector class provides the capability to implement a grow able array of objects.
What are inner class and anonymous class?
Inner class : classes defined in other classes, including those defined in methods are
called inner classes. An inner class can have any accessibility including private.
Anonymous class : Anonymous class is a class defined inside a method without a name
and is instantiated and declared in the same place and cannot have explicit
constructors.
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..
Abstract Class---method without body
a. Inheritance.
b. Instance of class can not be created.
c. An abstract class has at least one abstract method.
Example:
A programmer uses abstract class when there are some common features implemented
in the same way in all of its objects. A programmer writes an interface when ever
features is having different implementation in different objects. Here implementation
leaved to third party vendors.
Concrete method: a method with body
final
1.prevents its content from being modified.(no inheritance).
2. Instance of class can be created.
3. Final is similar to constant in c/c++.
4. Final methods cannot be overridden.
Example:
public class Math
{
...
public static final double PI =3.14159265358979323846;
...
}
You can access this constant in your programs as Math.PI.
5. The use of final to a variable is similar to the use const in C.
AWT: The abstract windowing toolkit or group of classes for writing programs with
graphical user interfaces.
 Objects---call by reference
methods---call by value

If x=22.9 then
floor(x)=22
round(x)=23
ceil(x)=23
this() :
a. this() is used to invoke a constructor of the same class
Ex:
super() :
a. super() is method to call the constructor of the super class from the sub
class
b. super.methodnameInsuperclass(arg) is used to execute any method of the
superclass from the subclass
 Rule : Calling the superclass constructor must be the first statement in
subclass constructor.
Ex:
Q: The java.util package contains two classes that are designed to work with
hashtables. They are _______ and _________.
A: Hashtable , HashMap class
Q: Why is it necessary to hash?
A: Hashing assures that the format of keys is uniform and unique
Q: If a Runtime Exception is thrown in the finalize method
A: The exception is simply ignored and the object is garbage collected
Q: final handles exception is false.
Q: What is the output of the following program? class example { public static void
main(String args[]) { int j; do { j++; } while(j < 0); System.out.println(j); } }
A: The program does not compile as j is not initialized
Q: Which of the following is used by a thread to enter the waiting state?
A: wait() ,sleep(), suspend() methods.
Q: In order to use a servlet for handling the submission of a web form, subclass
HttpServlet() and put service code in
A: doGet() or doPost() method
Q: The list of locations for the compiler to search in order to resolve class names at
compile time is called
A: build path

Memory in Java is automatically garbage
collected. You never have to
worry about memory corruption.

Array initialization: int[] a = new int[100];

CONSTRUCTOR
a. A constructor is a special method whose purpose is to construct and
initialize objects.
b. A constructor has the same name as the class.
c. A constructor has no return value.
d. A constructor is always called with the new operator.

Polymorphism:
If same method is performing various tasks.

Example of polymorphism is Method overloading.

Ex1: Method overloading
Class sample {
Void add(int a,int b)
{
int c=a+b;
System.out.println (“add is =”+c);
}
Void add (int a, int b,int c) {
int d=a+b+c;
System.out.println (“add is =”+d);
}
}
Class main {
Public static void main(string args[]) {
Sample obj=new sample();
Obj.add(5,10);
Obj.add(5,10,15);
}
}

Ex2: you can assign a subclass object to a superclass
variable.
Employee e;
e = new Employee(. . .); // Employee object expected
e = new Manager(. . .); // OK, Manager can be used as
well



Package:
Def: Java allows you to group classes in a collection called a package.
Using packages:
import java.util.*;
Then you can use
Date today = new Date();
 Adding a class into a package
To place classes inside a package, you must put the name of the
package at the top of your source file, before the code that defines
the classes in the package. For example, the file Employee.java
starts out like this:
package com.horstmann.corejava;
public class Employee
{
...
}

What is a transient variable?
Ans: A transient variable is a variable that may not be
serialized. .
11. Is null a keyword?
Ans: The null value is not a keyword..

Is sizeof a keyword?
Ans: The sizeof operator is not a keyword..

What is the Vector class?
Ans: The Vector class provides the capability to
implement a growable array of objects.

What is the difference between yielding and
sleeping?
Ans: 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 are wrapped classes?
Ans : Wrapped classes are classes that allow primitive
types to be accessed as objects..

Abstract Class


Inheritance.
Instance of class can not be created.
Example:

final
prevents its content from being modified.(no inheritance).
Instance of class can be created.
 Final is similar to constant in c/c++.
 Final methods cannot be overridden.
Example:
public class Math
{


...
public static final double PI =3.14159265358979323846;
...
}
You can access this constant in your programs as Math.PI.