> Hashtable is synchronized, slow and thread-safe, but doesn't mind NULL. The issue with Hashtable or rather all legacy collection classes is that the API is r

> Hashtable is synchronized, slow and thread-safe, but doesn't mind NULL. The issue with Hashtable or rather all legacy collection classes is that the API is rarely useful without additional synchronization. So you might as well use a HashMap wrapped by Collections.synchronizedMap or use a ConcurrentHashMap with a placeholder object instead of null.