ConcurrentHashMap
What is ConcurrentHashMap?
As per current understanding, it is a thread-safe map, replacement of Hashtable.
It is much better than Hashtable because it does not synchronize the complete methods as done for Hashtable. How does it handle concurrency is by using some complex locking at some intermediate places by taking extreme care of JMM. The code is so complex that the code is peer reviewed by many JMM experts.
References:
http://stas-blogspot.blogspot.co.uk/2010/08/concurrenthashmap-revealed.html