Understanding the use of generics in java

Question

Im trying to understand how class generics work and this bit just doesnt make sense to me.

So for instance if I have the following classes:

class A&l         


        

Answer 1

The error message is: The method go(capture#1-of ? extends A) in the type A is not applicable for the arguments (A)

The reason for it: You are not specific about reference a1. It is defined on a capture of "? extends A" that can never be duplicated. Again, when you create a new A() it passes another capture of the "? extends A", the A itself, that cannot be converted to the former.