Difference between HashMap and ConcurrentHashMap.

HashMap	ConcurrentHashMap
it is not thread safe.	it is Thread safe .
Relatively performance high because thread are not required to wait to operate on HashMap.	Relatively performances is low because some time Thread are required to wait to operate on ConcurrentHashMap.
While one thread iterating HashMap the other Threads are not allowed to modify Map object otherwise we will get Runtime exception saying ConcurrentModificationException.	While one Thread is  iterating ConcurrentHashMap the other Thread are allowed to modify Map object in safe manner and it won't throws ConcurrentModificationException.
null is allowed for both keys and values.	null is not allowed for both keys and values .otherwise we will get NullPointerException.
introduced in 1.2 version.	introduced in 1.5 version.
Iterator of HashMap is Fail-Fast and it throws ConcurrentModificationException.	Iterator of ConcurrentHashMap is Fail-Safe and it won't throws ConcurrentModificationException.