Incompatible return type when overriding methods of Generic classes in Java

Question

I was playing around with generics, and I found a problem I really can't figure out. Please take a look at this code:

class Operation<R> {
}

class P<O extends Operation<R>, R> {
}

class MyResource {
}

class MyO extends Operation<MyResource> {
}

class MyP extends P<MyO, MyResource> {
}

abstract class A<R> {
    abstract P<Operation<R>, R> doIt();
}

class B extends A<MyResource> {
    @Override
    MyP doIt() {
    /*
        the compiler is complaining about the above line:
        The return type is incompatible with A<MyResource>.doIt()
    */
        return null;
    }
}

So basically, MyP is a subclass of P<MyO, MyResource> which in turn is a subclass of P<Operation<MyResource>, MyResource>. In other words, we are returning a subclass of A.doIt return type in B.doIt. But nevertheless, the compiler is complaining.

Can you help me to understand this ? Thanks.

Answer 1

The problem is that MyP is a P<MyO,>, which is not the same as a P<Operation<MyResource>,> (even though it does inherit it).
You need to change the abstract method's return type to be covariant in that parameter:

abstract class A<R> {
    abstract P<? extends Operation<R>, R> doIt();
}

Now, MyP is convertible to the base return type, so everything works fine.