Please be careful with blanket statements like this. HashMap, Hashtable and ConcurrentHashMap behave differently in certain (subtle?) ways. ConcurrentHashMap doesn't like NULL but is thread-safe. Hashtable is synchronized, slow and thread-safe, but doesn't mind NULL. HashMap is not thread safe and doesn't mind NULL.

Edit: pardon the dupes, on unreliable mobile link :-(

> 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.