泛型<？超级>通配符

Question

I was wondering why the following bit of code does not work:

Collection <? super String> col = new ArrayList<String>();
col.add(new Object());// does not compile
col.add("yo!");// compiles indeed;

If the type is <? super String> <? super String> it can contain anything which is super of String (String included) not?

Answer 1

Collection<? super String> Collection<? super String> , counter to your intuition, does not mean "a collection which contains objects of type String or its supertype". It means " col will be a collection holding some definite type, which itself is String or its supertype" (such as Object , Serializable , or CharSequence ).

The best way to think about Collection<? super String> Collection<? super String> is that it is not a type, like you are used to in Java, but a pattern against which specific types are matched.

Now, the only thing you can safely add to any collection which matches the above pattern is a String or its subclass (if there were any). Quite the opposite of what you would have expected, isn't it? That's Generics for you.

Answer 2

With a Collection <? super String> Collection <? super String> , we don't know what kind of objects it contains exactly, but we know that it must be a collection of either String or some superclass of String , ie it is safe to put a String into it but not necessarily an Object . Conversely for methods that take things out of the collection (eg when iterating) we can't be sure we'll get strings back.

On the other hand with Collection<? extends Foo> Collection<? extends Foo> we know it's a collection of something that is either Foo or some subclass of Foo , so we can safely take something from the collection and know it will be assignable to Foo , but we can't put anything in because we have no way of knowing what types would be safe.