List List ? extends T cannot be assigned to List List T , why?

Question

Can someone help we with why this fails to compile?
     List<List<? extends DrawableTile>> tiles;
     public TilePanel() {
          // This fails to compile:
          tiles = loadTestTiles();
     private static List<List<DrawableTile>> loadTestTiles() {
     }I know that this (below) works, but why not for the nested generics case?
List<? extends DrawableTile> tileList = new ArrayList<DrawableTile>();

Answer 1

List<DrawableTile> is a different type than List<? extends DrawableTile>. List of one type parameter cannot be assigned to List of another type parameter.
Suppose you try to add a List<SubTypeOfDrawableTile> to tiles; it would work. But then suppose that the reference returned by loadTestTiles() was stored somewhere else as a List<List<DrawableTile>>. Then you would expect to be able to retrieve List<DrawableTile> out of it. Well now suppose you try to add a DrawableTile to that sublist. See the problem?
You probably meant to do this
List<? extends List<? extends DrawableTile>> tiles;Edited by: spoon_ on Dec 10, 2007 5:02 PM