Internet programming text book

For More Visit : www.Learnengineering.in
Srividya College of Engineering and Technology, Virudhunagar Course material –Lecture Notes


UNIT I

Introduction to java

Java is an Object-Oriented Language. As a language that has the Object Oriented feature, Java
supports the following fundamental concepts:

Polymorphism
Inheritance
Encapsulation




n
Abstraction




g.i
Classes
Objects
Instance




n
Method
Message Parsing




eri
In this chapter, we will look into the concepts Classes and Objects.

e
Object - Objects have states and behaviors. Example: A dog has states - color, name,
gin
breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
Class - A class can be defined as a template/blue print that describes the behaviors/states
that object of its type support.
en

Objects in Java:

Let us now look deep into what are objects. If we consider the real-world we can find many
arn



objects around us, Cars, Dogs, Humans, etc. All these objects have a state and behavior.

If we consider a dog, then its state is - name, breed, color, and the behavior is - barking,
wagging, running
Le




If you compare the software object with a real world object, they have very similar
characteristics.
w.




Software objects also have a state and behavior. A software object's state is stored in fields and
behavior is shown via methods.
ww




So in software development, methods operate on the internal state of an object and the object-to-
object communication is done via methods.

Classes in Java:

A class is a blue print from which individual objects are created.

A sample of a class is given below:

CS6501 Internet Programming – Unit - I Page 1

For More Visit : www.Learnengineering.in

,For More Visit : www.Learnengineering.in
Srividya College of Engineering and Technology, Virudhunagar Course material –Lecture Notes


public class Dog{
String breed;
int age;
String color;

void barking(){
}

void hungry(){
}




n
g.i
void sleeping(){
}
}




n
A class can contain any of the following variable types.




eri
Local variables: Variables defined inside methods, constructors or blocks are called
local variables. The variable will be declared and initialized within the method and the
e
variable will be destroyed when the method has completed.
gin
Instance variables: Instance variables are variables within a class but outside any
method. These variables are instantiated when the class is loaded. Instance variables can
be accessed from inside any method, constructor or blocks of that particular class.
Class variables: Class variables are variables declared with in a class, outside any
en

method, with the static keyword.

A class can have any number of methods to access the value of various kinds of methods. In the
arn



above example, barking(), hungry() and sleeping() are methods.

Below mentioned are some of the important topics that need to be discussed when looking into
classes of the Java Language.
Le




Constructors:
w.




When discussing about classes, one of the most important sub topic would be constructors. Every
class has a constructor. If we do not explicitly write a constructor for a class the Java compiler
builds a default constructor for that class.
ww




Each time a new object is created, at least one constructor will be invoked. The main rule of
constructors is that they should have the same name as the class. A class can have more than one
constructor.

Example of a constructor is given below:

public class Puppy{
public Puppy(){

CS6501 Internet Programming – Unit - I Page 2

For More Visit : www.Learnengineering.in

,For More Visit : www.Learnengineering.in
Srividya College of Engineering and Technology, Virudhunagar Course material –Lecture Notes


}

public Puppy(String name){
// This constructor has one parameter, name.
}
}

Java also supports Singleton Classes where you would be able to create only one instance of a
class.




n
Creating an Object:




g.i
As mentioned previously, a class provides the blueprints for objects. So basically an object is
created from a class. In Java, the new key word is used to create new objects.




n
There are three steps when creating an object from a class:




eri
Declaration: A variable declaration with a variable name with an object type.
Instantiation: The 'new' key word is used to create the object.
e
Initialization: The 'new' keyword is followed by a call to a constructor. This call
gin
initializes the new object.

Example of creating an object is given below:
en

public class Puppy{

public Puppy(String name){
arn



// This constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
}
public static void main(String []args){
Le




// Following statement would create an object myPuppy
Puppy myPuppy = new Puppy( "tommy" );
}
w.




}

If we compile and run the above program, then it would produce the following result:
ww




Passed Name is :tommy

Accessing Instance Variables and Methods:

Instance variables and methods are accessed via created objects. To access an instance variable
the fully qualified path should be as follows:

/* First create an object */

CS6501 Internet Programming – Unit - I Page 3

For More Visit : www.Learnengineering.in

, For More Visit : www.Learnengineering.in
Srividya College of Engineering and Technology, Virudhunagar Course material –Lecture Notes


ObjectReference = new Constructor();

/* Now call a variable as follows */
ObjectReference.variableName;

/* Now you can call a class method as follows */
ObjectReference.MethodName();

Example:




n
This example explains how to access instance variables and methods of a class:




g.i
public class Puppy{




n
int puppyAge;




eri
public Puppy(String name){
// This constructor has one parameter, name.
System.out.println("Passed Name is :" + name );
} e
gin
public void setAge( int age ){
puppyAge = age;
}
en

public int getAge( ){
System.out.println("Puppy's age is :" + puppyAge );
return puppyAge;
arn



}
public static void main(String []args){
/* Object creation */
Puppy myPuppy = new Puppy( "tommy" );
Le




/* Call class method to set puppy's age */
myPuppy.setAge( 2 );
w.




/* Call another class method to get puppy's age */
myPuppy.getAge( );
ww




/* You can access instance variable as follows as well */
System.out.println("Variable Value :" + myPuppy.puppyAge );
}
}

If we compile and run the above program, then it would produce the following result:

Passed Name is :tommy

CS6501 Internet Programming – Unit - I Page 4

For More Visit : www.Learnengineering.in