Re: Generics and use of extends in HashMap

From:

Thomas Hawtin <usenet@tackline.plus.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 02 Oct 2006 14:54:12 +0100

Message-ID:

<452119cf$0$16563$ed2619ec@ptn-nntp-reader01.plus.net>

David Harrigan wrote:

public interface A {

public class B implements A {

    public void doIt() {
        Map<String, ? extends A> a = new HashMap<String, A>();
        a.put("A Test", new B());
    }

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