Animal.java

public interface Animal {
    String salutation();
}

Cage.java

public final class Cage<T> {
    public final T value;
    public Cage(T value) {
        this.value = value;
    }
}

Dog.java

public final class Dog implements Animal {
    @Override
    public String salutation() {
        return "bark";
    }

    public static void main(String[] args) {
        Cage<? extends Animal> cage = new Cage(new Dog());
        System.out.println(cage.value.salutation());
    }
}