Object Oriented Programming in java

Declare an interface called “MyFirstInterface”. Declare integer type variable called “x”. Declare an abstract method called “display()”.

  1. Try to declare the variable with/without public, static and final keywords. Is there any difference between these two approaches? Why?
  2. Declare the abstract method with/without abstract keyword. Is there any difference between these two approaches? Why?
  3. Implement this into a class called “IntefaceImplemented” . Override all the abstract methods. Try to change the value of x inside this method and print the value of x. Is it possible for you to change x? why?


  • Naveen RanasingheNaveen Ranasinghe Level 2Moderator

    How to declare an interface in Java?

    An interface is declared by using the interface keyword. It provides total abstraction; means all the methods in an interface are declared with the empty body, and all the fields are public, static and final by default. A class that implements an interface must implement all the methods declared in the interface.


      interface <MyFirstInterface>{        
        // declare constant fields  
        // declare methods that abstract  
        // by default.  

    Abstract method in interface

    All the methods of an interface are public abstract by default. You cannot have concrete (regular methods with body) methods in an interface.

    interface Multiply{
       //abstract methods
       public abstract int multiplyTwo(int n1, int n2);
       /* We need not to mention public and abstract in interface
        * as all the methods in interface are 
        * public and abstract by default so the compiler will
        * treat this as 
        * public abstract multiplyThree(int n1, int n2, int n3);
       int multiplyThree(int n1, int n2, int n3);
       /* Regular (or concrete) methods are not allowed in an interface
        * so if I uncomment this method, you will get compilation error
        * public void disp(){
        *    System.out.println("I will give error if u uncomment me");
        * }
    class Demo implements Multiply{
       public int multiplyTwo(int num1, int num2){
          return num1*num2;
       public int multiplyThree(int num1, int num2, int num3){
          return num1*num2*num3;
       public static void main(String args[]){
          Multiply obj = new Demo();
          System.out.println(obj.multiplyTwo(3, 7));
          System.out.println(obj.multiplyThree(1, 9, 0));



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