You can go through all the answers above to understand why the .add() is restricted to '', '', and partly to ''.
But here's the conclusion of it all if you want to remember it, and dont want to go exploring the answer every time:
List means this will accept any List of A and subclass of A.
But you cannot add anything to this list. Not even objects of type A.
List means this will accept any list of A and superclass of A.
You can add objects of type A and its subclasses.
