Bounded Wildcards in Java

Question

This is not fine

     List<List<? extends Number>> a;
     List<List<Integer>> b;
     a = b;

This is fine

     List<? extends Number> c;
     List<Integer> d;
     c = d;

How can make it compile first one?

Answer 1

You could use this:

List<? extends List<? extends Number>> a;
List<List<Integer>> b;
a = b;

Answer 2

List<? extends List<? extends Number>> a = null;
List<List<Integer>> b = null;
a = b;