Re: Using generics and annotations to resolve implementation interface

From:

Tom Hawtin <usenet@tackline.plus.com>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 08 Jun 2007 13:27:53 +0100

Message-ID:

<466949ec$0$8710$ed2619ec@ptn-nntp-reader02.plus.net>

Meidos wrote:

interface Handler<A extends Annotation> {
handle(A annotation);
}

Map<Class<? extends Annotation>, Handler<?>> handlerMap;
handlerMap.put(SomeAnnotation.class, new SomeAnnotationHandler());

The problem is that when I get the Handler from the map, it will be a
Handler of type Handler<? extends Annotation>, and I cannot call
handle() on it because the type is unspecified. How can I solve this?

You are going to need an unchecked cast.

class AnnotationHandlerMap {
     private final Map<
         Class<? extends Annotation>, Handler<? extends Annotation>
     > map = new java.util.HashMap<
         Class<? extends Annotation>, Handler<? extends Annotation>
     >();

     public <A extends Annotation> void put(
         Class<A> type, Handler<A> handler
     ) {
         map.put(type, handler);
     }

     @SuppressWarnings("unchecked")
     public <A extends Annotation> Handler<A> get(Class<A> type) {
         return (Handler<A>)map.get(type);
     }
     ...
}

(Dislaimer: Not tested or even compiled.)