Java generics help

I have class A and a class B that extends A.

I have an abstract class X and a concrete class Y that extends X.

In X I have an abstract method with the following signature:

public abstract Collection<? extends A> getStuff();

In Y I implement the abstract method with the following signature:

public Collection<B> getStuff();

When I try to do the following: classX.getStuff(开发者_如何学Go).add(B)

It gave the following complaint: add (capture ) in Collection cannot be applied to B

Any help would be appreciated.

You can use the following example as your X class.

For example,

public abstract <T extends A> Collection<T> getStuff();

You can also refer to this threads.