Difference between Hashtable and ConcurrentHashMap and HashMap

Difference between Hashtable and ConcurrentHashMap and HashMap

HashMap	HashTable	ConcurrentHashMap
Not Synchronized	Synchronized	ConcurrentHashMap and CopyOnWriteArrayList implementations provide much higher concurrency while preserving thread safety, with some minor compromises in their promises to caller
Hashmap allow one null key and Multiple null value	Not allow null keys or null values	ConcurrentHashMap do not allow null keys or null values
COLLECTION CLASS	LEGACY CLASSS	COLLECTION CLASS
Works in only Single thread models access	Once the size of hashtable becomes considerable large performance degrade because for iteration it has to be locked for longer duration.	Since ConcurrentHashMap introduced concept of segmentation , how large it becomes only certain part of it get locked to provide thread safety so many other readers can still access map without waiting for iteration to complete.
Not  Loked	Locked on Whole  Map	In Summary ConcurrentHashMap only locked certain portion of Map
You can make HashMap synchronized by wrapping it on Collections.synchornizedMap(HashMap) which will return a collection which is almost equivalent to Hashtable, where every modification operation on Map is locked on Map object		while in case of ConcurrentHashMap, thread-safety is achieved by dividing whole Map into different partition based upon Concurrency level and only locking particular portion instead of locking whole Map. by default  size is 16
HashMap only slightly better. in Single thread Environment	bad performance	ConcurrentHashMap is more scalable and performs better than Synchronized HashMap in multi-threaded environment
Collections.synchronizedMap and Collections.synchronizedList, provide a basic conditionally thread-safe implementation of Map and List. However, several factors make them unsuitable for use in highly concurrent applications  for example their single collection-wide lock is an impediment to scalability and it often becomes necessary to lock a collection for a considerable time during iteration to prevent ConcurrentModificationException.
Does not maintain Insertion order	it maintain Insertion order

Reference :-

http://howtodoinjava.com/2013/06/14/popular-hashmap-and-concurrenthashmap-interview-questions/?subscribe=already#blog_subscription-6