Difference between ConcurrentHashMap and Collections.synchronizedMap and Hashtable in Java

Collections classes are heart of java API though I feel using them judiciously is an art. Its my personal experience where I have improved performance by using ArrayList where legacy codes are unnecessarily used Vector etc.

JDK 1.5 introduces some good concurrent collections which is highly efficient for high volume, low latency system electronic trading systems In general those are backbone for Concurrent fast access of stored data. In this tutorial we will look on ConcurrentHashMap, Hashtable and HashMap and Collections.synchronizedMap and see difference between ConcurrentHashMap and Hashtable and synchronizedMap.

This Java Collection tutorial is in continuation of my article How HashMap works in Java  and difference between HashMap and Hashtable in Java  if you haven’t read already you may find some useful information based on my experience in Java Collections.

Why we need ConcurrentHashMap and CopyOnWriteArrayList

The synchronized collections classes, Hashtable and Vector, and the synchronized wrapper classes, 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.

ConcurrentHashMap and CopyOnWriteArrayList implementations provide much higher concurrency while preserving thread safety, with some minor compromises in their promises to callers. ConcurrentHashMap and CopyOnWriteArrayList are not necessarily useful everywhere you might use HashMap or ArrayList, but are designed to optimize specific common situations. Many concurrent applications will benefit from their use. 

Difference between ConcurrentHashMap and Hashtable

So what is the difference between Hashtable and ConcurrentHashMap , both can be used in multithreaded environment but 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.

In Summary ConcurrentHashMap only locked certain portion of Map while Hashtable lock full map while doing iteration. 

Difference between ConcurrentHashMap and Collections.synchronizedMap

ConcurrentHashMap is designed for concurrency and improve performance while HashMap which is non synchronized by nature can be synchronized by applying a wrapper using Collections.synchronizedMap. Here are some of common differences between ConcurrentHashMap and Collections.synchronizedMap

ConcurrentHashMap do not allow null keys or null values while HashMap allows null keys.

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