HashMap vs Hashtable

Two different

  • Hashtable is synchronized, HashMap is not
  • Hashtable does not allow null keys or values, HashMap does

Note: Should not use Hashtable for multiple threads application even it synchronized because Hashtable use synchronized for all methods (it’s not good for performance (check Hashtable code for more detail)). For multiple threads application, you can use ConcurrentHashMap.
Reference: https://stackoverflow.com/questions/40471/differences-between-hashmap-and-hashtable