Generic class literal?

Question

I thought it would be useful if the class literal could return a generic class. For
e.g. something like this
Class<HashMap<Long,Long>> c = HashMap.class;
Obviously the above will not work by typecasting or templating the RHS. A
work around could be something like
HashMap<Long,Long> h = new HashMap<Long,Long> ();
Class<HashMap<Long,Long>> c = (HashMap<Long,Long>)h.getClass();
Are there any other better work arounds?

Answer 1

Hmmm, I was thinking of passing a generic Class to
the method so that I could use reflection to
instantiate the class (instead of new HashMap<K,
V>()). That way, the method would work not just for
HashMap but for anything implementing Map. For e.g. I
also use EnumMap simillarly.Okay, now I see where you're going with this. I don't see any reason the following wouldn't work if the client uses it correctly. You would need to document clearly the class must either be generic or implement the parameterized types specified and it must have a no-argument constructor.
    <K, V> Map<K, V> putIfMissing(final String key,
                                  final Map<String, Map<K, V>> h,
                                  final Class<? extends Map> clazz)
            throws IllegalAccessException, InstantiationException {
        Map<K, V> value = h.get(key);
        if (value == null) {
            // Unchecked assignment.  Will be fine if the class passed is a
            // generic Map or if it implements Map<K, V> with the same types.
            value = clazz.newInstance();
            h.put(key, value);
        return value;
    }You'll have to be very careful to document everything that could go wrong with this. Even then I'm not sure there's not a gotcha in there somewhere. A perhaps more object-oriented and safer solution, though more verbose, would be for the client to pass the mechanism for creating the map.
    interface MapFactory<K, V> {
        Map<K, V> createMap();
    <K, V> Map<K, V> putIfMissing(final String key,
                                  final Map<String, Map<K, V>> h,
                                  final MapFactory<K, V> factory) {
        Map<K, V> value = h.get(key);
        if (value == null) {
            value = factory.createMap();
            h.put(key, value);
        return value;
    }