• Class is a structure.
  • Binding the data with its related and corresponding functions.
  • Class is the base for encapsulation.
  • Class is a user defined data type in java.
  • Class will act as the base for encapsulation and implement the concept of encapsulation through objects.
  • Any java applications looks like collection of classes but where as c- application looks like collection of functions.
public class Example{

         //variable declaration
          int id;

      // methods
        public int getId() {
          return id;

    public void setId(int id) { = id;



  • Object is nothing but instance (dynamic memory allocation) of a class.
  • The dynamic memory allocated at run time for the members [non-static variables] of the class is known as object.

What is a Reference variable?

  • Reference variable is a variable which would be representing the address of the object.
  • Reference will act as a pointer and handler to the object.
  • Since reference variable always points an object.
  • In practice we call the reference variable also as an object.
  • We can create an object by using new operator.
  • If we want to call any method inside the class we need object
               Syntax: A obj= new A();

Example program:

      public class A {
         int a;
     public  void Print(){
        System.out.println("value of a="+a);
    public static void main(String[] args) {
   A obj=new A();

Output: value of a=0
What Object Contains?
  • Object of any class contains only data.
  • Apart from the data object of a class would not contains anything else.
  • Object of a class would not contain any functionalities or logic.
  • Thus object of a class would be representing only data and not represent logic.

What is state of the Object?

  • The data present inside object of a class at that point of time is known as state of the object

What is behavior of the object? 

  • The functionalities associated with the object => Behavior of the object.
     The state of the object changes from time-to-time depending up on the functionaities that are executed on that object but whereas behavior of the object would not change.

Naming conventions for declaring a class:

  • Class name should be relevant.
  • Use UpperCamelCase for class names. //StudentDemoClass
  • For Methods names follow camelCase(); //getName();
  • Declare variables lowercase first letter. // int rollNumber
  • To declare final static variables means which will acts like constants

Instance Of Java

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