java - Generics - Cannot add to a List with unbounded wildcard

Question

I instantiate the following list:

// I am just revising generics again and the following is just cursory code!
List<? super Integer> someList = new ArrayList<Object>();
someList.add(new Object());

The above will not work. I get a compiler error. However, the following works:

List<? super Integer> someList = new ArrayList<Object>();
someList.add(11);

I am aware that you can add objects to a collection which contains an unbounded wildcard, not a bounded wildcard.

However, why does the above not work? An Object is a supertype of an Integer, so why can't I add it?

Answer 1

That declares that it's a list of something that's a supertype of Integer, not that the list can contain anything that's a supertype of Integer. In other words, to the compiler, it could be a List<Integer>, a List<Number> or a List<Object>, but it doesn't know which, so you can't add just anything to the List. The only thing you can safely add is an Integer, since that's guaranteed to be a subtype of any type the List could potentially hold.

In other words, the ? represents one type, not any type. It's a non-obvious but important distinction.