Java Generic with ArrayList <? extends A> add element

Question

I have classes A, B, C and D where B extends A, C extends A and

Answer 1

ArrayList means an ArrayList of some unknown type that extends A.
That type might not be C, so you can't add a C to the ArrayList.

In fact, since you don't know what the ArrayList is supposed to contain, you can't add anything to the ArrayList.

If you want an ArrayList that can hold any class that inherits A, use a ArrayList.