Java Generics (Wildcards)

Question

I have a couple of questions about generic wildcards in Java:

1. What is the difference between List and List

Answer 1

In general,

If a structure contains elements with a type of the form ? extends E, we can get elements out of the structure, but we cannot put elements into the structure

List ints = new ArrayList();
ints.add(1);
ints.add(2);
List nums = ints;
nums.add(3.14); // compile-time error
assert ints.toString().equals("[1, 2, 3.14]"); 

To put elements into the structure we need another kind of wildcard called Wildcards with super,

 List