Hi,

I'm trying to understand a warning message that Eclipse is giving me.

I have the following code:

1: public class Test {
2:    class Foo {}
3:    class Bar extends Foo {}
4:    public static Collection<? extends Foo> getStuff() {
5:        return new ArrayList<Bar>();
6:    }
7:    public static void main(String args[]) {
8:        Collection<Bar> stuff = (Collection<Bar>)getStuff();
9:    }
10: }

The compiler has no problem with that but Eclipse thinks the cast on
line 8 might be dangerous.  The IDE gives me the following error:

"Type safety: The cast from Collection<capture-of ? extends Test.Foo>
to Collection<Test.Bar> is actually checking against the erased type
Collection"

Can anyone explain exactly what this means and why the above code may
be dangerous?

This is just example code, of course.  The real code that I ran into
this warning on was on a factory method that creates a collectoin of
objects of a given type that I pass as a parameter to the factory
method. Thus, the client of the factory method knows what type of
objects the returned collection is going to contain but the factory
method itself can't know so it must be declared as returning
Collecton<? extends BaseType>.