Subtypes

Just as everything is a subtype of Object, anything which implements an interface is a subtype of that interface.

For the following code this means that any field or variable which holds a Dog can be assigned to an instance of Mutt.

interface Dog {
    void bark();

    String fetch(String ball);
}

class Mutt implements Dog {
    @Override
    public void bark() {
        IO.println("Bark");
    }

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

void main() {
    Dog dog = new Mutt();
}

Through a Dog variable you will be able to call any methods defined on the interface. These will use the actual underlying implementation - in this case from Mutt.

interface Dog {
    void bark();

    String fetch(String ball);
}

class Mutt implements Dog {
    @Override
    public void bark() {
        IO.println("Bark");
    }

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

void main() {
    Dog dog = new Mutt();

    dog.bark();

    IO.println(dog.fetch("Ball"));
}