Difference Between HashMap and HashTable

Points To Remember

• HashTable have the legacy code and were introduced before HashMaps.
• There is nothing in a HashTable that a HashMap cannot do except multithreading.
• ConcurrentHashMap is a replacement for HashTable, it a synchronized HashMap.

Differences : HashMap and HashTable 

HashMap	HashTable
They implement Map Interface.	It extends Dictionary(obsolete from jdk 1.7) class and implements Map interface
They are a part of Collections API	They are not a part of Collections API
They allow a single null key and any number of null values.	They do not allow null values to be added.
They are not synchronized or thread-safe	They are synchronized or thread-safe
They use Iterator to iterate over objects	They use Enumeration to iterate over objects
They are faster and uses less memory	They are comparatively slower.

Similarities : HashMap and HashTable 

• Both of them implements Map interface.
• Both HashMap and Hashtable works on the Principle of Hashing.
• Both HashMap and HashTable do not guarantee that the order of the map will remain constant over time.

I recommend not to use HashTable and use HashMap since HashMap is much faster than the HashTables and it allows you to do everything that a HashTable allows you to do. In case you are in need of using a Map in a multithreaded environment than you should go for ConcurrentHashMap. A ConcurrentHashMap provides all the functions of a HAshMap and it is also thread safe or simple multithreaded.