Difference between Hashtable & HashMap?

Hash table is a Collection .To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method and the equals method.  Hashtable  is Synchronized and permit not null values only.
Hashtable maps keys to values. Any non-null object can be used as a key or as a value.
The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.