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.
