Java generics to enforce return type of abstract method

Question

I have the following situation :

abstract class X { abstract X someMethod (...) {...} }.

Now I want to constrain any implementation of X to

Answer 1

This works as well;

abstract class X {
    public abstract T yourMethod();
}
class X1 extends X {
    public X1 yourMethod() {
        return this;
    }
}
class X2 extends X {
    public X2 yourMethod() {
        return this;
    }
}