I think by "native" they mean "implemented-in-c++-by-the-JVM", which potentially vary; not java.util.HashTable, which should be pretty standard across JVM implementations.
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.
