Difference between HashMap and Hashtable ?

HashMap	Hashtable
HashMap lets you have null values as well as one null key.	HashTable  does not allows null values as key and value.
The iterator in the HashMap is fail-safe (If you change the map while iterating, you’ll know).	The enumerator for the Hashtable is not fail-safe.
HashMap is unsynchronized.	Hashtable is synchronized.

Note: Only one NULL is allowed as a key in HashMap. HashMap does not allow multiple keys to be NULL. Nevertheless, it can have multiple NULL values.