"Attempting to use an incompatible return type" with Interface Inheritance

Question

I'm running into a problem with incompatible return types using inheritance.

public interface A { }
public interface B extends A { }

public interface C {
    Map<String, A> getMapping();
}

public interface D extends C {
    Map<String, B> getMapping();
}

Is there a way to make this work?

Right now the compiler tells me I'm 'Attempting to use an incompatible return type' on interface D.

Answer 1

I suggest you use

interface C {
    Map<String, ? extends A> getMapping();
}

This says "A map that maps String to A or a subtype of A". This is compatible with Map<String, B>.

Answer 2

Make the following changes:

interface C<E extends A> {
    Map<String, E> getMapping();
}

interface D extends C<B> {
    Map<String, B> getMapping();
}