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 D extends A.

I have the following ArrayLists each with a few elements in them:

ArrayList<B> b; ArrayList<? extends A> mix = b; 

I intended for the variable mix to contain elements of type B, C or D. I tried to add an element of type C into mix like this:

mix.add(anElementOfTypeC); 

But the IDE doesn't allow me to do so and it says:

anElementOfTypeC cannot be converted to CAP#1 by method of invocation conversion where CAP#1 is a fresh type-variable: CAP#1 extends A from capture of ? extends A

Did I use the <? extends A> correctly? How can I resolve this?

Answer 1

ArrayList<? extends A> 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<A>.

Answer 2

It is not possible to add elements in collection that uses ? extends.

ArrayList<? extends A> means that this is an ArrayList of type (exactly one type) that extends A. So you can be sure, that when you call get method, you'll get something that is A. But you can't add something because you don't know what exactly the ArrayList contains.