Question

Please give me a hint as to what is going on here:

List<? extends Number> a = new ArrayList<Number>();
List<? extends Number> b = new ArrayList<Number>();

a.addAll(b); // ouch! compiler yells at me, see the block below:
/*
  incompatible types
  found   : java.util.List<capture#714 of ? extends java.lang.Number>
  required: java.util.List<java.lang.Number>
 */

This simple code does not compile. I vaguely remember something related to type captures, like those should be mostly used in interface specs, not the actual code, but I never got dumbfounded like that.

This of course might be fixed brute-forcefully, like that:

List<? extends Number> a = new ArrayList<Number>();
List<? extends Number> b = new ArrayList<Number>();

@SuppressWarnings({"unchecked"})
List<Number> aPlain = (List<Number>) a;
@SuppressWarnings({"unchecked"}) 
List<Number> bPlain = (List<Number>) b;

aPlain.addAll(bPlain); 

So, do I really have to either give up captures in the declaration (the capture came to me from an interface, so I’ll have to change some API), or stick with type casts with suppression annotations (which generally suck and complicates code a bit)?

Answer 1

You have essentially two lists of possibly different types. Because ? extends Number means a class which extends Number. So for list a it can be classA and for list b it can be for example classB. They are not compatible, they can be totally different.