What differences exist between HashMap and Hashtable ?

1. A HashMap allows the existence of null keys and values, 
2. Hashtable doesn’t allow neither null keys, nor null values.
3. 
   
1. A Hashtable is synchronized.
2. while a HashMap is not.
3. 
   
1.  HashMap is preferred in single-threaded environments.
2.  Hashtable is suitable for multi-threaded environments.
3. 
   
1. A HashMap provides its set of keys and a Java application can iterate over them.
2. Hashtable provides an Enumeration of its keys.