Can this callback mechanism be generified?

Casper Bang <>
Fri, 02 May 2008 04:51:44 +0200
Hello my fellow Java drinkers. I have a generics issue. Would it be
possible to devise an API, which would allow me to register a callback
*somewhere* associated with a class. Say I wish to allow a custom
formatter to be installed:

1 interface Callback
2 {
3 String format(Object object);
4 }
6 class Somewhere
7 {
8 Map<Class, Callback> callbacks = new HashMap<Class, Callback>();
10 public void installCallback(Class clazz, Callback callback)
11 {
12 callbacks.put(clazz, callback);
13 }
15 public void doCallback(Object obj)
16 {
17 Callback callback = callbacks.get( obj.getClass() );
18 if(callback != null)
19 System.out.println( callback.format( obj ) );
20 }
21 }
23 somewhereInstance.installCallback( Date.class, new Callback(){
24 public String format(Object obj)
25 {
26 Date date = (Date)obj;
27 return SimpleDateFormat.getInstance().format(date);
28 }
29 });

The code works, but it requires casting as it very much revolves around
a top-level Object. Is there any way to generify this, particularly the
callback itself, since the type actually is known (line 23).

Thanks in advance,

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