Core Java - What is the difference between HashMap and ConcurrentHashMap ?

This is also one of the most popular java collections interview question . Make sure this question is in your to do list before appearing for the interview .
Main differences between HashMap and ConcurrentHashMap are :
a. HashMap is not synchronized while ConcurrentHashMap is synchronized.
b. HashMap can have one null key and any number of null values while ConcurrentHashMap does not allow null keys and null values .