Why is the net-mail subscriber of the signature of the future return type <? super v>

Question

Method signature addListenerforio.netty.util.concurrent.Promise<V>

Promise<V> addListener(GenericFutureListener<? extends Future<? super V>> listener)

Please explain why the argument is not type GenericFutureListener<? extends Future<? extends V>> because it seems that the result of the promise should be Veither its subclasses, but not its superclasses.

But why is the return type of the future <? super V>?

Answer 1

Here we need something that we can listen Vto at least. He might be able to listen more, but that would not be enough if he could only listen to a subclass V.

, Animal → Mammal → Dog, Listener Mammal, Listener Animal , Listener a Dog , my Mammal Cat.