Interfaces

1 Learn

An interface is a contract: it lists methods a class promises to provide. A class uses implements to sign the contract.

A class can extend one class but implement many interfaces. Interfaces are great for unrelated classes that share an ability, like Printable.

Want more detail? Open the full lesson

2 See the Code

Java

interface Playable {
    void play();
}

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

class Drum implements Playable {
    public void play() { System.out.println("Boom boom"); }
}

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

Playable declares play(); both classes implement it.
Interface methods are public, so the implementing methods must be public too.
An array of Playable can hold any class that implements it.

Output

Strum strum
Boom boom

3 Practice

Create an interface Shape with double area(), and implement it in Square and Rectangle.

Try it in your own Java setup or write it in the Practice Notebook.

4 Quick Check

1. Which keyword does a class use with an interface?
2. How many interfaces can a class implement?

Step 5

Answer and check the Quick Check above to unlock this.

Day 21