Object Oriented Programming in java
Declare an interface called “MyFirstInterface”. Declare integer type variable called “x”. Declare an abstract method called “display()”.
- Try to declare the variable with/without public, static and final keywords. Is there any difference between these two approaches? Why?
- Declare the abstract method with/without abstract keyword. Is there any difference between these two approaches? Why?
- 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?
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Best Answer
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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.
Syntax:
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 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)); } }
Output:
21 0
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