Careeroza

Multiple Interfaces

One Class, Many Roles

Example

A smartphone is a camera, a phone and a music player at the same time. Each interface is a separate role.

public class Main {
    public static void main(String[] args) {
        SmartPhone phone = new SmartPhone();
        phone.call("9876543210");
        phone.click();
        phone.play();

        Camera cam = phone;          // use only the Camera role
        cam.click();
    }
}

interface Phone  { void call(String number); }
interface Camera { void click(); }
interface MusicPlayer { void play(); }

class SmartPhone implements Phone, Camera, MusicPlayer {
    public void call(String number) { System.out.println("Calling " + number); }
    public void click() { System.out.println("Photo captured"); }
    public void play() { System.out.println("Playing music"); }
}

Output

Calling 9876543210
Photo captured
Playing music
Photo captured

Default Method Conflict

Example

If two interfaces give the same default method, the class must override it and can pick one with Interface.super.method().

public class Main {
    public static void main(String[] args) {
        new Duck().move();
    }
}

interface Swimmer {
    default void move() { System.out.println("Swimming"); }
}

interface Flyer {
    default void move() { System.out.println("Flying"); }
}

class Duck implements Swimmer, Flyer {
    @Override
    public void move() {
        Swimmer.super.move();
        Flyer.super.move();
        System.out.println("Duck can do both");
    }
}

Output

Swimming
Flying
Duck can do both

Interfaces Extending Interfaces

interface Readable  { String read(); }
interface Writable  { void write(String data); }

// an interface can extend several interfaces
interface ReadWrite extends Readable, Writable { }

class FileStore implements ReadWrite {
    private String data = "";
    public String read() { return data; }
    public void write(String d) { data = d; }
}