Limited wildcards in Java

Question

This not normal

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

This is normal

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

How to make it the first to compile?

Answer 1

You can 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;