java - <? 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 objects of the earlier created classes:

List<? extends A> list1 = new ArrayList<A>();
List<? extends B> list2 = new ArrayList<B>();
List<? extends C> list3 = new ArrayList<C>();

List<? super A> list4 = new ArrayList<A>();
List<? super B> list5 = new ArrayList<B>();
List<? super C> list6 = new ArrayList<C>();

To each of those lists I'm trying to add 1 object of each earlier created class: A,B,C. The only possible combination is:

• adding object of class A,B,C to list4

• adding object of class B and C to list5

• adding object of class C to list list6. The rest of the tries gives compiler errors, such us:

The method add(capture#1-of ? extends A) in the type List is not applicable for the arguments (A)

Why can't I add any object of class A,B,C to list1/2/3? Why e.g. list4 accepts objects of classes A,B,C if they are supposed to be a super class of class A, as the list4 is defined?

Answer 1

"? extends A" means "some type derived from A (or A itself)". So for instance, a List<ByteArrayOutputStream> is compatible with List<? extends OutputStream> - but you shouldn't be able to add a FileOutputStream to such a list - it's meant to be a List<ByteArrayOutputStream>! 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<OutputStream> is compatible with List<? super ByteArrayOutputStream>. 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.