java generics and the addAll method

Question

What is the correct type of argument to the addAll(..) method in Java collections? If I do something like this:

List<? extends Map<String, Object[]>> currentList = new ArrayList<Map<String, Object[]>>();
Collection<HashMap<String, Object[]>> addAll = new ArrayList<HashMap<String, Object[]>>();
// add some hashmaps to the list..
currentList.addAll(addAll); 

I understand I need to initialize both variables. However, I get a compilation error (from eclipse):

Multiple markers at this line
    - The method addAll(Collection<? extends capture#1-of ? extends Map<String,Object[]>>) in the type List<capture#1-of ? extends Map<String,Object[]>> is not applicable for the arguments (List<capture#2-of ? extends 
     Map<String,Object[]>>)
    - The method addAll(Collection<? extends capture#1-of ? extends Map<String,Object[]>>) in the type List<capture#1-of ? extends Map<String,Object[]>> is not applicable for the arguments 
     (Collection<HashMap<String,Object[]>>)

what am I doing wrong?

Answer 1

Replace extends by super.

The addAll() itself takes an ? extends T argument and that won't work when T is ? extends Map<K, V>. With the change it will become effectively ? extends ? super Map<K, V>.

But still, better is just to remove that ? extends (or ? super). It's superfluous in this particular case. Just use List<Map<String, Object[]>>. The addAll() will then effectively take ? extends Map<String, Object[]>>.

Answer 2

just remember:

for <? extends ...>, you cannot put things (except null) into it

for <? super ...>, you cannot get things (except Object) out of it