What is the difference between a HashMap and HashTable?

1. java.util.HashMap allows null keys and values whereas a java.util.Hashtable does not.
2. HashMap is unsynchronized and therefore much faster. However, its not suitable when there are multiple threads inserting/getting from the map. HashTable is synchronized and its slower because of overhead involved in maintaining locks.

Hashtable is old and is considered legacy. java.lang.concurrent.ConcurrentHashMap should be used instead. ConcurrentHashMap only locks certain section of HashMap during insertion. The remaining section of the map can be accessed lock free and therefore is as fast as a regular HashMap.