Question

So I have this method:

protected void collectSelectedItems(ListSelectionModel lsm, 
         Collection<? super MyItemClass> result) {
    for (int i : GUI.getSelectionIndices(lsm))
    {
        result.add(getItemByDisplayIndex(i));
    }
}

I’d like to return the collection instead of having a void method:

protected <T super MyItemClass> Collection<T> 
  collectSelectedItems(ListSelectionModel lsm, Collection<T> result) {
    for (int i : GUI.getSelectionIndices(lsm))
    {
        result.add(getItemByDisplayIndex(i));
    }
    return result;
}

with the intent of doing something like this (where MyItemClass extends MyItemBaseClass):

List<MyItemBaseClass> list = 
   collectSelectedItems(lsm, new ArrayList<MyItemBaseClass>());

but I get a syntax error on the super:

Syntax error on token “super”, , expected

What gives? Can I fix this?

Answer 1

Here’s one link that explains why this is not allowed:

http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeParameters.html#FAQ107

It basically just says that use super in type parameters “does not buy you anything”, since if this is allowed, erasure will probably just erase it to Object, which does not make much sense.