Question

I want to create a mapping from (a) class type to (b) long (the identifier of the object of the defined class type) to (c) the object itself.

I have the following:

 protected HashMap<Class<?>, HashMap<Long, ?>> obj = new HashMap<Class<?>, HashMap<Long, ?>>();

Is it possible to somehow denote that the first ? must be of the same type than the second ?? I would expect something like this, but this is ofcourse not possible:

protected <T> HashMap<Class<T>, HashMap<Long, T>> obj = new HashMap<Class<T>, HashMap<Long, T>>();

Answer 1

As an alternative, you could use a small amount of not-type-safe code encapsulated in a way that enforces your constraint:

class Cache {
    private Map<Class<?>, Map<Long, ?>> items = new HashMap<Class<?>, Map<Long, ?>>();

    private <T> Map<Long, T> getItems(Class<T> type) {
        @SuppressWarnings("unchecked")
        Map<Long, T> result = (Map<Long, T>) items.get(type);
        if (result == null) {
            result = new HashMap<Long, T>();
            items.put(type, result);
        }
        return (Map<Long, T>) result;
    }

    public <T> void addItem(Class<T> type, Long id, T item) {
        getItems(type).put(id, item);
    }

    public <T> T getItem(Class<T> type, Long id) {
        return type.cast(getItems(type).get(id));
    }
}

The type.cast() in getItem() isn’t necessary for the compiler to not complain, but it would help catch an object of the wrong type getting into the cache early.