Interface

Interface:

• It consist of abstract, static and default methods
• It is created using interface keyword
• It consist of only public static final data members
• Class could extends another class but implements interfaces
• Those class who implements an interface have to provide implementations of all the abstract methods 
• It can also have main method
• It can not have Constructor

Advantage of Interface:

• Supports Multiple inheritance
• Method implementation restriction
• Code re-usability through static and default method

Syntax:

AcessSpecifiers interface InterfaceName {

     //Constant data members

     //abstract, static and defaults methods

}

Example: Create an interface Jet and a class Aeroplane that implements Jet interface?

Jet.java

package com.java.iter;

public interface Jet {
    int id = 10;
    void start();
    void fly();
    void stop();

    static void takeof(){
        System.out.println("Jet is taking off");
    }
    default void landing(){
        System.out.println("Jet is landing");
    }
    public static void main(String[] args) {
        System.out.println("Data member Id of interface Jet is " + id);
        Jet.takeof();
    }
}

Output:

Data member Id of interface Jet is 10
Jet is taking off

Aeroplane.java

package com.java.iter;

public class Aeroplane implements Jet {
    public void start() {
        System.out.println("Aeroplane is sarting");
    }
    public void fly() {
        System.out.println("Aeroplane is flying");
    }
    public void stop() {
        System.out.println("Aeroplane is stoping");
    }
    public static void main(String[] args) {
        //Jet j = new Jet();            //compiler error because interface cannot be instantiated
        Aeroplane a = new Aeroplane();
        a.start();
        Jet.takeof();
        a.fly();
        a.landing();
        a.stop();
        System.out.println("Id of interface Jet is " + a.id);
        System.out.println("Id of interface Jet is "+Jet.id);
        //a.id = 30;        //Compiler error because interface data members are final
    }
}

Output:

Aeroplane is sarting
Jet is taking off
Aeroplane is flying
Jet is landing
Aeroplane is stoping
Id of interface Jet is 10
Id of interface Jet is 10