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

You can only insert instances of T into a List<T>.

The type List<? extends Map<X,Y>> stands for a list of an unknown type T which extends Map<X,Y>. For instance, it could stand for a List<LinkedHashMap<X,Y>>. Obviously, you may not insert an ordinary HashMap<X,Y> into such a list.

You probably want:

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

Or if you want to be really flexible you could do:

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

Which would allow you to do crazy things like:

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

You might also wish to read Angelika Langer’s Generics FAQ, particularly the section about wildcards.