Java generics and addall methods

Addall (..) in Java collection What is the correct parameter type for the method? 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 know I need to initialize these two 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[]>> is not applicable for the arguments 
     (Collection<HashMap<String,Object[]>>)

What on earth did I do wrong?

Solution

You can only insert instances of T into list < T >

Type list > represents a list of unknown types T, and its extended map < x, Y > For example, it can represent list < LinkedHashMap < x, Y > > Obviously, you may not be able to insert an ordinary HashMap < x, Y > into such a list

You may want to:

List<Map<String,Object[]>> currentList;

Or if you want to be very flexible, you can do this:

List<? super Map<String,Object[]>> currentList;

This will allow you to do crazy things, such as:

currentList = new ArrayList<Map<? super String,? extends Object[]>>();

You may also want to read Angelika Langer's genetics FAQ, especially the section about wildcards