<? super E> and <? extends E> for List

Question

Having the following simple class structure:

class A {
}

class B extends A {
}

class C extends B {
}

I\'m creating an ArrayList to keep o

Answer 1

"? extends A" means "some type derived from A (or A itself)". So for instance, a List is compatible with List - but you shouldn't be able to add a FileOutputStream to such a list - it's meant to be a List! All you know is that anything you fetch from the list will be an OutputStream of some kind.

"? super A" means "some type which is a superclass of A (or A itself)". So for instance, a List is compatible with List. You can definitely add a ByteArrayOutputStream to such a list - but if you fetch an item from the list, you can't really guarantee much about it.

See Angelika Langer's Generics FAQ for much more information.