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?

What do you want to compile, the first option? – Thomas Mar 05 '12 at 14:19 — Thomas, Mar 05 '12 at 14:19

score 11 · Accepted Answer · answered Mar 05 '12 at 14:21

11

You could use this:

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

answered Mar 05 '12 at 14:21

Thomas

score 1 · Answer 2 · answered Mar 05 '12 at 14:25

1

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

answered Mar 05 '12 at 14:25

Matthijs Bierman

2 Answers2