Interfaces

A list of abilities

An interface is a list of methods a class promises to have. It says what can be done, not how. A class implements an interface and writes the methods:

interface Playable {
    void play();
}

class Guitar implements Playable {
    public void play() {
        System.out.println("Strum strum");
    }
}

class Piano implements Playable {
    public void play() {
        System.out.println("Plink plonk");
    }
}

public class Main {
    public static void main(String[] args) {
        Playable[] band = {new Guitar(), new Piano()};
        for (Playable p : band) {
            p.play();
        }
    }
}

Output

Strum strum
Plink plonk

Think of it like this: A power socket is an interface. A phone charger, a lamp and a TV are very different, but they all have a plug that fits. The socket doesn't care what's on the other end.

Interface methods are automatically public, so the class must write them as public too.

Many interfaces at once

A class can extend only one class, but it can implement many interfaces:

interface Swimmer {
    void swim();
}

interface Flyer {
    void fly();
}

class Duck implements Swimmer, Flyer {
    public void swim() { System.out.println("Duck paddles"); }
    public void fly() { System.out.println("Duck flaps"); }
}

public class Main {
    public static void main(String[] args) {
        Duck d = new Duck();
        d.swim();
        d.fly();
    }
}

Output

Duck paddles
Duck flaps

Default methods

Since Java 8 an interface can include a ready-made method marked default. Classes get it for free, but can override it:

interface Greeter {
    String name();

    default void greet() {
        System.out.println("Hello from " + name());
    }
}

class Robot implements Greeter {
    public String name() { return "Robo"; }
}

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

Output

Hello from Robo

Abstract class or interface?