I have an HTTP executor class:
Future<? extends Response> future = service.apply(request).toJavaFuture();
Now I want to remove the ? extends part of this, because I don't want to make it so generic for the caller. Basically, I want to return Future<Response>.
From my point of view:
x extends y
Would mean x is y, and y is not x.
That would mean x can be casted to y.
In my eyes, this can be done safely, because x always extends y.
Why is the following unsafe?
Future<? extends Response> future = service.apply(request).toJavaFuture();
Future<Response> futureResponse = (Future<Response>) future;
A Future<? extends Response> variable can be assigned a Future<Response> or a Future<ResponseSubClass1> or a Future<ResponseSubClass2>.
A Future<ResponseSubClass1> instance cannot be safely assigned to a Future<Response> variable, since that would allow assigning a ResponseSubClass2 to a Future<ResponseSubClass1>.
Therefore, your attempted casting is unsafe.