HashMap vs ConcurrentHashMap

Difference between HashMap and ConcurrentHashMap:

• ConcurrentHashMap is thread-safe that is the code can be accessed by single thread at a time. while HashMap is not thread-safe
• HashMap can be synchronized by using synchronizedMap(HashMap) method. By using this method we get a HashMap object which is equivalent to the HashTable object. So every modification is performed on  Map is locked on Map object. ConcurrentHashMap is by default synchronized
• ConcurrentHashMap does not allow NULL values . So the key can not be null in ConcurrentHashMap. While In HashMap there can only be one null key.
• In multiple threaded environment HashMap is usually faster than ConcurrentHashMap. As only single thread can access the certain portion of the Map and thus reducing the performance. While in HashMap any number of threads can access the code at the same time.