Java Collections - HashMap vs Hastable vs Collections.synchronizedMap vs ConcurrentHashMap

This is a very famous interview topic for the experienced persons who have "really" worked in java technologies. By this question, interviewer can verify weather you have really worked with collections or not also your knowledge on synchronization problem with Hash Map (Collections).

This article can help you for the following questions.
1.) How HashMap works in java.
2.) Have you work with collection classes(HashMap) in the multi threaded environment?
     What are the problems you faced in HashMap? What is ConcurrentModificationException?
     What is fail-fast and fail-safe iterator?
     Why do we need Hashtable or Collections.synchronizedMap Or ConcurrentHashMap?
3.) Similarity/difference between Hashtable and Collections.synchronizedMap.
     If functionality of both (Hashtable and Collections.synchronizedMap) is same then why do we need both?
4.) Similarity/difference between ConcurrentHashMap and Hashtable.
     Similarity/difference between ConcurrentHashMap and Collections.synchronizedMap.
5.) When one can use Hashtable, Collections.synchronizedMap and ConcurrentHashMap.

	HashMap	Hashtable	Collections.synchronizedMap	ConcurrentHashMap
Legacy class	-	Yes	-	Java 1.5 and above
Allow null key/ value	One null key and many null values are allowed.	Not allowed	Allowed	Not allowed (Neither key nor value)
Thread Safe	No	Yes	Yes	Yes
Lock the entire map?		Yes	Yes	No (Locks some portion(segment) of the map)
Speed		Slow (Compare to Concurrent HashMap)	Slow (Compare to Concurrent HashMap)	Fast (Compare to Hashtable and Collections.synchronizedMap)
Performance		Low (Compare to Concurrent HashMap)	Low (Compare to Concurrent HashMap)	High (Compare to Hashtable and Collections.synchronizedMap)
Multiple threads can access? (Is synchronized?)		Yes	Yes	Yes
Multiple threads can access (read) without blocking other thread?		No (At a time, only one thread can read)	No (At a time, only one thread can read)	Yes (At a time, more than one thread can read)
Multiple threads can access (write) without blocking other thread?		No	No	May be (If writers(threads) write on different segments than only)
Scalable		Less scalable then Concurrent HashMap	Less scalable then Concurrent HashMap	More scalable then Hashtable and Collections.synchronizedMap