Java – generics – cannot be added to a list with unbounded wildcards

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 does not work I received a compiler error However, the following work should be done:

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

I know you can add objects to a collection containing unbounded wildcards instead of bounded wildcards

But why doesn't that work? Object is a supertype of integer. Why can't I add it?

Solution

This declares that it is a list of super - typed things, not that the list can contain any super - typed integer In other words, for the compiler, it can be list < integer >, list < number > Or list < Object >, but it doesn't know which, so you can't add anything to the list The only thing you can safely add is integer, because it is guaranteed to be a subtype of any type that list may have

let me put it another way,? Represents a type, not any type This is a non - obvious but important difference