Difference between HashMap and ConcurrentHashMap

HashMap and ConcurrentHashMap are two different types of hashMap in java.Both implements Map interface but their internal working is different.


                                                     HashMap Vs ConcurrentHashMap

	HashMap	ConcurrentHashMap
Null Value/Key	It allow null value as well as key.	It does not allow null values and keys.
Thread Safe	No.In order to make hashmap thread safe we need to use Collections.synchronized(Map) method	Yes
Concurrency	It does not allow concurrent access to multiple threads	It allows a very high level of concurrency. Multiple threads can work on different portion of ConcurrentHashMap
Iterator	It implements fail fast.	It implements fail safe.

Study internal working of both types of hashMap .It’s very important concept asked in interviews.