Difference between HashMap and Hashtable in Java

• HashMap allows null values as key and value whereas Hashtable doesn't allow nulls 

• HashMap is non synchronized whereas Hashtable is synchronized.  

• Java 5 introduces ConcurrentHashMap which is an alternative of Hashtable and provides better scalability than Hashtable in Java. 

• Iterator in the HashMap is  a fail-fast iterator  while the enumerator for the Hashtable is not. 

• HashMap throw ConcurrentModificationException if any other Thread modifies the map structurally  by adding or removing any element except Iterator's own remove() method.  

• But this is not a guaranteed behavior and will be done by JVM on best effort. 

• One more notable difference between Hashtable and HashMap is that because of thread-safety and synchronization Hashtable is much slower than HashMap if used in Single threaded environment.  

• HashMap does not guarantee that the order of the map will remain constant over time.