Re: Generics and use of extends in HashMap

David Harrigan wrote:


 From Map<String, ? extends A> a, we know all the values of a extend A.
But there may be further constraints such that not all instances of A
can be values of a.

Suppose class C implements A. Then we could have had:

         Map<String, C> map = new HashMap<String, C>();
         Map<String, ? extends A> a = map;
         a.put("A Test", new B()); // ILLEGAL
         C c = map.get("A Test");

We have assigned a B to a C variable. Oops.

What you can write is:

         Map<String, ? super A> a = new HashMap<String, A>();

With a declared as such, it could either be a Map<String,A> or
Map<String,Object>. So we can definitely add an instance of B
(implements A). However, when we get an object from the map, we only
know that it is some kind of Object.

Tom Hawtin