How can I add to List<? extends Number> data structures?

Question

I have a List which is declared like this :

 List foo3 = new ArrayList();

I tried to add 3 to foo3.

Answer 1

"List '<' ? extends Number> is actually an upper bound wildcard !

The upper-bounded wildcard says that any class that extends Number or Number itself can be used as the formal parameter type: The problem stems from the fact that Java doesn’t know what type List really is. It has to be an EXACT and UNIQUE Type. I hope it helps :)