How to create a deep unmodifiable collection?

Question

I often make a collection field unmodifiable before returning it from a getter method:

private List<X> _xs;
....
List<X> getXs(){
  return Collections.unmodifiableList(_xs);
}

But I can't think of a convenient way of doing that if the X above is itself a List:

private List<List<Y>> _yLists;
.....
List<List<Y>> getYLists() {
  return Collections.unmodifiableList(_yLists);
}

The problem in the above is of course that though the client cannot modify the List of lists, it can add/delete Y objects from the embedded lists.

Any thoughts?

Answer 1

The best I could come up with uses ForwardingList from Google Collections. Comments are welcome.

private static <T> List<List<T>> unmodifiableList2(final List<List<T>> input) {
    return Collections.unmodifiableList(new ForwardingList<List<T>>() {
        @Override protected List<List<T>> delegate() {
            return Collections.unmodifiableList(input);
        }
        @Override public List<T> get(int index) {
            return Collections.unmodifiableList(delegate().get(index));
        }
    });
}

Answer 2

unfortunately, there is no easy way to get deep const-ness in java. you would have to hack around it by always making sure that the list inside the list is also unmodifiable.

i'd be interested too to know any elegant solution.