Is there a way to have an interface with a generic type extend another interface with another generic type in Java?

Question

I would like to create a interface like this

public interface MyInterface<T extends OtherInterface<K>>{
    K doSomething(T o);
}

but the compiler won't recognize it. The other way around is that:

public interface MyInterface<T extends OtherInterface<K>,K>{
    K doSomething(T o);
}

My question is, though the second code works, is there a way like the first code so I don't have to put two types to announce the interface?

Answer 1

If you have an generic interface with two type arguments, then you need to declare them both in the class signature.

Alternatively if declare K as a wildcard ? and just return T , you will still be able to cast the output T to the correct interface. eg:

interface Foo<T> { }

interface Bar<T extends Foo<?>>{
    T doSomething(T o);
}

class IntegerFoo implements Foo<Integer> {}
...

public static void main(String[] args) {
    IntegerFoo integerFoo = new IntegerFoo();

    Bar<IntegerFoo> bar = t -> t;

    Foo<Integer> result = bar.doSomething(integerFoo);
}