My problem is that of associating the type (Date.class, line 23) with
the type of my callback (line 15 and 24). There doesn't seem to be a way
of expressing this. I would be ok with the erasure (implicit casting)
issue, if only I got type safety and the developer got shielded from
having to do the casting.

However, generifying the callback into:

    interface Callback<T>
    {
        String format(T object);
    }

...and trying to use that inside installCallback:

    public <T> void installCallback(Callback<T> callback)
    {

callbacks.put(callback.getClass().getTypeParameters()[0].getClass(),
callback);
    }

...all I can get out is "T". Since this is by reflection, it is probably
an erasure issue but it could also be because I'm not smart enough.

/Casper

Thanks a lot Daniel for the lesson in wildcards. :)

/Casper

I managed to make the API just slightly easier to use, by extracting the
type from Callback<T> rather than requiring an explicit Class to be
supplied along with it. It's not pretty but I'm not sure of how to
extract type T, since I guess its not readily available on the account
of generics by erasure:

interface Callback<T> {
     String format(T object);
 }

class Somewhere {
     Map<Class<?>, Callback<?>> callbacks
          = new HashMap<Class<?>, Callback<?>>();

     public <T> void installCallback(Callback<T> callback) {

         callbacks.put(extractGenericType( callback ), callback);
     }

    private <T> Class extractGenericType(Callback<T> callback){
        Class clazz = Object.class;
        String interfaces =
callback.getClass().getGenericInterfaces()[0].toString();
        int genTypeStart = interfaces.indexOf("<")+1;
        int genTypeEnd = interfaces.lastIndexOf(">");
        String genType = interfaces.substring( genTypeStart, genTypeEnd);

        try{
            clazz = Class.forName(genType);
        }
        catch (ClassNotFoundException ex){}

        return clazz;
    }

     public <T> void doCallback(T obj) {
         final Callback<? super T> callback = getCallback(obj);
         if(callback != null)
             System.out.println( callback.format( obj ) );
     }

     @SuppressWarnings({"unchecked"})
     private <T> Callback<? super T> getCallback(T obj) {
         return (Callback<? super T>) callbacks.get(obj.getClass());
     }
 }

class Main {
    public static void main(String[] args) {
        new Somewhere().installCallback( new Callback<Date>(){
            public String format(Date date) {
                return SimpleDateFormat.getInstance().format(date);
            }
        });
    }
}

/Casper