Implementation

All interfaces do¹ is hold method declarations.

If you want to have a class which "implements" the interface you can do so by writing implements followed by the interface name.

interface Dog {
    void bark();

    String fetch(String ball);
}

class Mutt implements Dog {

}

Then you all you need to do is declare methods which match up with the methods defined in the interface. Keep in mind that while you didn't write public in the interface, you need to write public when implementing a method from an interface.²

interface Dog {
    void bark();

    String fetch(String ball);
}

class Mutt implements Dog {
    public void bark() {
        System.out.println("Bark");
    }

    public String fetch(String ball) {
        return ball + " (with drool)";
    }
}