Download Unit 2 - Erode Sengunthar Engineering College

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B. Sc Computer Science
Subject : Internet and Java Programming
Unit
: II
Semester : 6
Unit – II - OBJECT ORIENTED FUNDAMENTALS AND JAVA REVOLUTION
Object oriented programming – Encapsulation – Inheritance – Polymorphism –
Java Genesis – Characteristics – Java Programming techniques – Reserved
words – Identifiers – Literals – Operators – Separators – Variables – Types –
Arrays – Operator Precedence
OBJECT ORIENTED PROGRAMMING.
Object Oriented Programming is a method of implementation in which programs
are organized as a collection of objects each of which represents an instance of
some class.
TWO PARADIGMS OF OBJECT ORIENTED PROGRAMMING
All programs consists of two elements code and data. A program can be
conceptually organized around its code or around the data. The process –
oriented approach or model characterizes a program as a series of linear steps
. This approach can be thought of as code acting on data. Procedural
languages like C employ this model to considerable success. Problems with
model appear as programs grow larger and complex. To manage the increasing
complexity the second approach called object – oriented programming was
conceived. Object oriented programming organizes a program around its data
and a set of well defined interfaces to that data. An object oriented program can
be characterized as data controlling access to code.
ABSTRACTION
Humans manage complexity through abstraction. For ex, people do not think of a
car as a set of individual parts. They think of it as a well defined object. This
abstraction allows people to drive a car without being overwhelmed by its
complexity. They can ignore the details of how the engine, braking systems work.
A powerful way to manage abstraction is through the use of hierarchical
classifications. The data from a traditional process- oriented program can be
transformed by abstraction into its components. A sequence of process steps
can become a collection of messages between these objects. Each of these
objects describes its own behavior. These objects are treated as concrete
entities that respond to messages telling them to do something. This is the
essence of object-oriented programming.
OOP PRINCIPLES
ENCAPSULATION
It is the process of localizing data definition, i.e. of binding together (wrapping)
code and data specifically related to a single entity to form a logical unit called a
class. This mechanism keeps the data safe from outside interference and
misuse. Encapsulation is a protective wrapper that prevents the code and data
from being arbitrarily accessed by other code defined outside the wrapper.
The basis of encapsulation is the ‘class’. Object is an instance of a class. Class is
a logical construct and object is a physical reality. The code and data within a
class are called as members of the class. The data are referred to as the
instance variables or member variables. The code that access or operate the
data are referred to as member methods or methods. Each method in a class
may be marked as private or public. The public interface of a class represents
everything that a external user can know. The private methods and data can only
be accessed by code that is a member of the class. So any other code that is not
a member of a class cannot access a private method or variable.
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INHERITANCE
Inheritance is the ability of a class to acquire the characteristics and behavior i.e.
nothing but the properties another class. The most common method of using
inheritance is to create class hierarchies that move from the most general
concept to the most specific representation. The class that inherits the properties
from other is called the derived class or subclass and the one from which it is
derived is called the base class or the super class. Each subclass has an “is a
relationship” to its super class.
Transportation
Public
Bus
Plane
Transport
Train
Private Transport
Automobile
Motorcycle
Yamaha
Honda
In the above example, Yamaha is a bike. A bike is a private transport. A private
transport ‘is a ‘ form of transportation.
POLYMORPHISM
Polymorphism is a feature that allows one interface to be used for a general
class of actions. Ie a concrete operation inherits its definition and properties from
a generic operation. It allows a method to have multiple implementations that are
selected based on the type of the object that is passed to the method invocation.
i.e. one interface multiple methods.
Consider a stack. You might have a program that requires three types of stacks.
One stack is used for integers, one for floating point values, and one for
characters. The algorithm that implements each stack is the same, even though
the data being stored differs. In a procedural language, you have to create three
different sets of stack routines, with each set using different name. With
polymorphism, you can specify a general set of stack routines that all share the
same names.
JAVA GENESIS
Java is related to C++. Much of the character of Java is inherited from c, and
c++. Java derives its syntax from c and many of its object-oriented features
from c++. Java was influenced by C++ and not an enhanced version of C++.
Java was conceived by James Gosling, Patrick Noughton, Chris Wrath, Ed Frank
and Mike Sheridan at Sun Microsystems in 1991. It took 18 months to develop
the first working version. The language was initially called Oak but was renamed
Java in 1995. The primary motivation was the need for a platform-independent
language that could be used to create software to be embedded in various
consumer electronic devices, such as microwave ovens and remote controls.
A Java compiler converts the Java source code into a binary program consisting
of byte codes. Byte codes are optimized set of instructions designed to be
executed by the Java run-time system, which is called the Java Virtual Machine
(JVM). JVM is an interpreted for byte code. This interpreter inspects and
deciphers the byte codes and executes the actions that the byte codes specify. A
Java interpreter can run stand-alone or it can be a part of a web browser such as
Netscape Navigator or IE where it can be invoked automatically to run applets in
a web page.
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Translating a Java program into byte code makes it easier to run a program in a
wide variety of environments. Only JVM needs to be implemented for each
platform. Although the details of JVM differ from platform to platform, all interpret
the same Java byte code. If a Java program were compiled to native code, then
different versions of the same program would have to exist for each type of CPU
connected to the Internet.
When a program is interpreted, it generally runs substantially slower than it
would run if compiled to executable code. However with Java, the difference is
not great. The use of byte code enables the Java run-time system to execute
program much faster than you might expect.
Sun has just completed its Just In Time (JIT) compiler for byte code, which is
included in Java 2. JIT is a part of JVM, it compiles the byte code into executable
code in real time. It is not possible to compile a entire Java program into
executable code all at once, as Java performs various run-time checks that can
be done only at run time. Instead JIT compiles the code as needed only during
execution.
JAVA CHARACTERISTICS
Simple – Java removes many c, c++ features that were either redundant,
sources of confusion, i.e. no use of pointers, no multiple inheritance, no need to
explicitly free memory. This has reduced the complexity and has increased the
reliability of the language.
Robust and secure – Java is robust in its many safeguards to ensure reliable
code.
1. There is no Java preprocessor, i.e. the program will execute just as it is
written without worry that a variable is #defined to mean something else.
2. Java has strict compile time and runtime checking for the proper use of
the type system.
3. Free from numerous memory bugs as it is garbage-collected (explicit deallocation, Garbage collection is a technique that eliminates the need for
memory management by periodically reclaiming unused memory)
Security features
The automatic downloading of program across the insecure Internet made
security a high priority for the Java Development Team.
1. The first security feature is that computer memory is not directly
accessible by any Java program.
2. Byte codes are verified for accuracy and tested for security violation
3. Java’s network package allows the user to set the level of access for each
system.
Architecture Neutral
If you run a program today there is no assurance that it will execute tomorrow, on
the same machine. Operating system upgrades, processor upgrades and
changes in core system resources can all collude to make a program cease to
function. The Java designers made several hard decisions in the Java Language
and runtime so you can truly write once run anywhere. Write a good program and
Java will make sure that it works on Macintosh, UNIX etc.
Multi- threaded
Java was designed to meet the real-world requirement of creating interactive,
networked programs. To accomplish this Java supports multithreaded
programming which allows you to write programs that do many things
simultaneously. Java runtime system supports multi process synchronization
which enables you to construct smoothly running interactive systems.
Distributed
Java is designed for the distributed environment of the Internet, because it
handles TCP/IP protocols. The original version of Java included features for
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intra-address-space messaging. This allows objects on two different computers
to execute procedures remotely. Java has revived these interfaces in a package
called ‘Remote Method Invocation (RMI).
Dynamic
Java programs carry with them substantial amounts of run-time type information
that is used to verify and resolve accesses to objects at run-time. This makes it
possible to dynamically link code in a safe and expedient manner. This is crucial
to the robustness of the applet environment, in which small fragments of byte
code may be dynamically updated on a running system.
JAVA PROGRAMMING TECHNIQUES
A simple Java program can be as follows
class Sample{
public static void main(String args[ ]){
System.out.println(“Welcome”);
}
}
The reserved word ‘class’ is used to declare a class.
public - main() must be declared public since it must be accessed by code
outside of its class when the program is started
static – the method can be called independent of an instance of a class, i.e. its
not necessary to instantiate the class to invoke the method.
void – specifies that the method main() doesn’t return any value
String args[ ] declares a parameter named args, which is an array of instances
of the class String. Objects of type String store character strings. args[ ] receives
any command line arguments .
RESERVED WORDS
Reserved words are special identifiers, and these keywords can only be used for
their intended purpose and cannot be used as an identifier for a variable, class or
method name.
The Java reserved words are as follows
abstract
case
const
else
for
instanceof
new
return
switch
void
boolean
cast
continue
extends
goto
int
package
short
this
volatile
break
catch
default
final
if
interface
private
static
throws
while
byte
char
do
finally
implements
long
protected
strictfp
transient
byvalue
class
double
float
import
native
public
synchronized
try
IDENTIFIERS
Identifiers are used for class names, method names and variable names. An
identifier may be any descriptive sequence of uppercase and lowercase letters,
numbers or the underscore and dollar sign characters. They must not begin with
a number, they should not be confused with a numeric literal. Java is case
sensitive so ‘MATRIX’ is a different identifier from ‘matrix’.
Some examples of valid identifiers are
value
b5 $sname
dt_of_join
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Invalid identifiers
8value max-level
or/and
SEPARATORS
The most commonly used separator is semicolon. The other separators are
Symbol
purpose
()
Used to specify the list of parameters in method definition
and invocation
{}
To represent the values of initialized arrays. To define a
block code for classes, methods and local scopes.
[]
Use to declare array types. Also used when dereferencing
array values.
;
semicolon is used to terminate statements
,
Comma separates consecutive identifiers in a variable
declaration. Also used to chain statements together inside a
for statement
.
Period is used to separate package names from
subpackages and classes. Also used to separate a variable
or method from a reference variable
TYPES
Java defines eight simple types of data : byte, short, int, long, char, float , double
and boolean.
These can be put in four groups.
Integers - this group includes byte, short, int and long which are for wholevalued signed numbers
Floating - point numbers : This includes float and double
Characters - this includes char
Boolean - this includes boolean, which is a special type for representing
true/false values
Integers
All the four integer types are signed, positive and negative values. Java doesn’t
support unsigned, positive-only integers.
The width and the range of the integer types can be specified
Type width
long
64
int
32
short 16
byte
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Range
-9,233,372,036,854,775,808 to 9,233,372,036,854,775,807
-2,147,483,648 to 2,147,483,647
-32,768 to 32,768
-128 to 127
byte- This is a signed 8-bit type that has a range from –128 to 127. The
variables of type byte are used when you work with a stream of data from a
network or file. The variables can be declared as - byte a ;
short - This is a signed 16-bit type that range from -32,768 to 32,768.
int - It is a signed 32 bit type that ranges from -2,147,483,648 to 2,147,483,647
long - long is a signed 64 bit type and is useful when an int type is not large
enough to hold the desired value.
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Floating –point types
The two kinds of floating point types are float and double which represent single
and double precision numbers
type
double
long
width
64 bits
32 bits
range
1.7e-308 to 1.7e+308
3.4e-038 to 3.4e+038.
float - specifies single precision values. Single precision is faster on some
processors and takes half as much space as double precision. float can be useful
when representing dollars and cents
double – It uses 64 bits to store a value. double precision is actually faster than
single precision. All mathematic, trigonometric functions such as sqrt(), sin(),
cos() return double values. double is the best choice if you need to maintain
accuracy.
Characters
The data type used to store characters is char. Java uses Unicode to represent
characters. Unicode defines fully international char set that represents characters
of all languages, such as Latin, Greek. Arabic etc. It requires 16 bits. The range
of char is 0 to 65536. There are no negative chars.
Ex
class Chardemo{
public static void main(String args[ ]){
char ch1, ch2;
ch1 = 88;
ch2 = “Y”;
System.out.println(“ch1 and ch2 : ” + ch1+” “+ch2);
}
}
The output is
ch1 and ch2 : X Y
where 88 is the ASCII value that corresponds to X.
Booleans
boolean is the type used for logical values. It can have any one of the two
possible values true or false. This is the type returned by all relational operators
such as a > b. boolean is the type required by conditional expressions that
govern the control statements such as if or for.
LITERALS
Literals are nothing but constants. A literal represents a value of certain type,
whereas the type itself describes how the value behaves and how it is stored.
The various literals are as follows
Integer literals
Any integral numeric value is an integer value. Ex are 5, 7 etc. These are decimal
values i.e. of base 10.
The other two bases used in integral literals are octal and hexadecimal. Octal
values are denoted in Java with a leading zero. Decimal numbers cannot have
leading zero. A hexadecimal constant can be specified with a leading 0x or 0X.
Floating Point literals
Floating Point numbers represent decimal values with a fractional component.
They can be expressed in two ways
Standard notation – use some number followed by a decimal point and then
some fractional digit.
Ex – 5768.33333333
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Scientific notation – a standard notation floating point number with a additional
notation that indicates a multiplication times ten to the specified exponent. The
exponent is given by a E or e followed by decimal number. Ex – 45.34e12, 348e01.
Floating point literals in Java default to double precision, you need to append an
F or f to force the constant to be small enough to store into a variable declared as
a float.
Boolean Literals
The Boolean literals can have two values i.e. true or false.
Character Literals
A literal character is represented inside a pair of Parentheses (‘ ‘). For characters
that are impossible to enter directly, there are several escape sequences which
allow to enter the character such as (‘\’), for the single quote character itself and
‘\n’ for new line character.
String literals
String literals are arbitrary text enclosed in double quotes. Ex – “name”,
“Command”. Java strings must begin and end in the same line. There is no line
continuation escape sequence as in other language.
ARRAYS
Array is a collection of values of the same data type referred by a common value.
One-dimensional arrays
The general form of one-dimensional array is
type var-name[ ];
type is the data type of the each element that comprises the array.
Ex –
int marks[ ];
marks is a array variable, no array actually exists. The value of marks is set to
null.
A operator “new” is used to allocated space for an array.
marks = new int[6];
marks refers to an array of 6 integers. The individual elements of the array can
be accessed starting from the index or subscript 0.
For Ex.
class Student{
public static void main(String args[ ]){
int marks[ ] = new int[6];
marks[0] = 50;
marks[1] = 55;
marks[2] = 43;
marks[3] = 61;
marks[4] = 77;
marks[5] = 58;
System.out.println(“The marks scored in subject1 is :”+marks[0]);
}
An array initializer is a list of comma separated expressions surrounded by curly
braces. The commas separated the values of the array elements. The array will
be created large enough to hold exactly the number of elements you specify in
the array initializer.
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Java strictly checks that you don’t store or reference values outside the range of
the array. Java runtime will check to be sure that all array indices are in the
correct range.
Multi- dimensional arrays
These are arrays within arrays. The multidimensional arrays are allocated in
blocks. If it is an X by Y by Z three-dimensional matrix, then the storage required
will be X times Y times Z times the size of the type stored in each cell. In Java
you can declare a variable to be three- dimensional but leave out the second and
the thIrd dimension then allocate the Y and Z directions separately.
Ex
double matrix [ ] [ ] = new double [4] [4];
matrix[0] = new double[4];
matrix[1] = new double[4];
matrix[2] = new double[4];
matrix[3] = {0,1,2,3};
Ex
class Exarray{
public static void main(String args[ ]){
int twodim = new int [4] [5];
int I, j, k = 0;
for(i = 0; i < 4 ; i++ ){
for(j = 0 ; j<5; j++){
twodim[i][j] = k ;
k++;
}
}
for(i = 0; i < 4 ; i++ ){
for(j = 0 ; j<5; j++){
System.out.println( twodim[i][j] ) ;
k++;
}
}
}
}
VARIABLES
The variable is the basic unit of storage in Java. A variable is defined by the
combination of an identifier, a type and a optional identifier. All variables have a
scope, which defines their visibility and a lifetime.
Declaring a variable
The variables must be declared before using them. The format for declaration is
type identifier [= value][, identifier [=value]…]
Examples : int a, b, c;
int a = 8, b = 7;
double x = 49.90;
char c = ‘j’;
Dynamic initialization
The variables can be initialized dynamically, using any expression valid at the
time the variable is declared.
Class Leng{
public static void main(String args[ ]){
double a = 3.0 , b = 4.0;
double c = Math.sqrt(a * a + b * b) ;
System.out.println(“ Hypotenuse “ +c);
}
}
a and b are declared as constants, c is initialized dynamically to the length of the
hypotenuse.
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The scope and lifetime of variables
A block defines the scope. A scope determines what objects are visible to other
parts of the program. It determines the lifetime of those objects. In Java the two
major scopes are those defined by a class and those defined by a method.
When you declare a variable within a scope, you are localizing the variable and
protecting it from unauthorized access and/or modification. Scopes can be
nested. Objects declared within inner scope will not be visible outside it.
class Scope{
public static void main(String args[ ]){
int x;
x = 12;
if ( x == 12){
int y = 6;
// y is visible only to this block
System.out.println(“the value of y :” +y);
x = y + 4;
}
y = 5;
// error as y is not visible here
System.out.println(“the value of x : +x);
}
}
y is defined within the if block. So its value is not visible outside that block. Within
the if block x can be used as code within a block has access to the variables
declared by an enclosing scope.
Variables are created when their scope is entered, and destroyed when their
scope is left. So the variables declared within a method will not hold their values
between calls to that method.
Type conversion and casting
It is common to assign a value of one type to a variable of another type. If the two
types are compatible, then Java will perform the conversion automatically. It is
possible to assign a int value to a long variable. Not all types are compatible, so
not all type conversions are implicitly allowed. i.e. there is no conversion defined
from double to byte. It is possible to get a conversion between incompatible types
using the ‘casting’
Automatic conversion
Automatic conversion occur under the following conditions
1. The two types are compatible
2. The destination type is larger than the source type
A widening conversion takes place when the conditions are met. Numeric types
are compatible with each other. The numeric types are incompatible with char or
boolean. char and boolean are not compatible with each other.
Type casting
When you assign an int value to a byte variable the conversion will not be
performed automatically as a byte is smaller than an int. The conversion is called
a narrowing conversion as you are explicitly making the value narrower to fit
into the target type.
A cast is a explicit type conversion. The form is
(target-type)value
The target-type specifies the desired type to convert the specified value to.
Ex –
int a;
byte b;
b = (byte) a;
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Type promotion rules
1. All bytes and short values are promoted to int
2. if one operand is long, the whole expression is promoted to long
3. If one operand is float, the entire expression is promoted to float.
4. If any of the operands is double, the result is double.
OPERATORS
Operators are characters that instruct the compiler that you wish to perform an
operation on some operands. Operators specify operation instructions, while
operands are variables, expressions or literal values. Some operators take a
single operand these are called unary operators. Some operators come before
the operand, these are called prefix operators. And those that come after the
operands are called postfix operators. Most operators are between operands and
are called as infix binary operators. An operator that takes 3 operands called as
ternary operator.
The basic types of operators are – arithmetic, bitwise, relational and logical.
Arithmetic Operators
The operands must be a numeric type, and cannot be used on boolean types.
We can use it on char type since char is a subset of int in Java.
Four function calculator operators
class Calculate{
public static void main(String args[ ]){
int a = 5;
int b = a + 2;
int c = a * b;
int d = c – b;
int e = c / d;
int f = - b;
System.out.println(“value of b =” +b);
System.out.println(“value of c =” +c);
System.out.println(“value of d =” +d);
System.out.println(“value of e =” +e);
System.out.println(“value of f =” +f);
}
}
Modulus operator
this operator returns the remainder of a division operation. It can be applied to
floating-point types as well as integer types.
Arithmetic assignment operator
The statement is of the form
var = var op expression
which can be written as
var op = expression
ie a = a + 5 ; can be written as a + = 5 ;
the other operators can be given as
a % = 5 ; which is nothing but a = a % 5;
a/=7;
a * = 10 ;
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BITWISE LOGICAL OPERATORS
Bitwise NOT
This is the unary NOT operator ^. which inverts all the bits of its operand .
EX
the number 18 can be represented in binary as 10010 which becomes 01101
after the NOT operator is applied.
Bitwise AND
The AND operator (&) produces 1 when both the operands are 1 , 0 zero is
produced in all other cases.
ex
1010
& 1110
1010
Bitwise OR ( | )
In this case , the result is 1 if any one of the operands is 1.
Ex
1010
| 1001
1011
Bitwise XOR ( ^ )
The result is 1 if exactly one operand is 1, else the result is zero.
Ex 1110
^ 1100
1011
Right shift ( >> )
This operator shifts all the bits in a value to the right to a specified number of
times. The syntax is
value >> num ;
Ex int a = 16 ;
a = a>>2 ;
The result is 4 . ie the binary value of 16 is 10000 which after right shift
becomes 00100 . The decimal value of the result is 4.
The operator shifts the value of 16 to the right two positions, which causes 2 low
– order bits to be lost resulting in number being set to 4.
Left shift ( << )
The syntax is value << num ;
Ex
int a = 64 ;
a = a << 2 ;
the result is 256.
ie the binary value of 64 is 0100 0000 is shifted left two positions, and the
result is 1 0000 0000 which is 256.
unsigned right shift ( >>> )
>> operator automatically fills two high order bits with its previous contents each
time a shift occurs. This preserves the sign of the value. If you are shifting
something that doesn’t represent a numeric value, you may not want sign
extension to take place. In this case you need to shift a zero to the high order bit
no matter what the initial value was. This is known as unsigned shift.
Relational operator
These operators determine the relationship that one operand has to the other.
They determine the equality and the ordering.
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operators
==
!=
>
>=
<
<=
equal to
not equal to
greater than
greater than and equal to
less than
less than and equal to
Boolean logical operators
They operate on boolean operands.
&
|
^
||
&&
!
==
?:
logical AND
logical OR
logical XOR
short circuit OR
short circuit AND
logical unary NOT
assignment
ternary
Ternary operator
Java includes a special ternary operator(three way ) that can replace certain
types of if then else statements. It has the form
exp1 ? exp2 : exp3
ie if expression1 is true then expression2 will be evaluated else expression3 will
be evaluated
OPERATOR PRECEDENCE
The precedence or priority of the operators is given as follows
Highest
()
++
*
+
>>
>
==
&
^
&&
||
?:
=
[]
-/
>>>
>=
!=
~
%
<<
<
!
<=
op=
Lowest
Square brackets provide the array indexing. The dot operator is used to
dereference objects.
Using Parentheses
Parentheses raise the precedence of the operations that are inside them. This is
often necessary to obtain the result you desire. For ex, consider the following
expression
a >> b + 3 ;
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This expression first adds 3 to b and then shifts right by that results. ie. the
expression can be written using redundant parentheses like
a >>(b + 3 ) ;
If you want to first shift right by b positions and then add 3 to that result, you will
need to parenthesize like,
(a >> b ) + 3 ;
QUESTION BANK
Give short notes
1. Define object oriented programming.
2. what are three basic principles or features of OOP?
3. Give the difference between OOP and procedure oriented programming.
4. Give examples for Object Oriented Language
5. Define Abstraction
6. Define class.
7. Define object
8. Give the expansion of JVM. what is the work of JVM?
9. What do you mean by bytecode?
10. Give the representation of ternary operator.
11. What was Java initially called?
12. What are the two types of programs that can be created in Java?
13. What do you mean by applet?
14. Give the expansion of JIT
15. How can you justify the robust feature of Java programs
16. What is RMI
17. What do you mean by multiline comment
18. Explain the use of the keywords in the declaration
public static void main(String args[ ] )
19. Give the purpose of the following separators
i)
{}
ii)
.
20. What is the name given to Java character set
Answer in detail
1. What is encapsulation?
2. Define Inheritance
3. Define Polymorphism. Give examples.
4. What are the differences between C++ and Java
5. Give a note on the data types supported by Java
6. What is Dynamic Initialization
7. Explain Type conversion
8. When do you use type casting
9. Explain multi-dimensional arrays with example
10. Explain bitwise operators in Java
11. Give a note on logical Boolean operators
12. Specify the precedence of operators
13. Write a note on the characteristics of Java
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