Core Java 面试题与答案

Question: What is the difference between Synchronized Collection classes and Concurrent Collection Classes ? When to use what ?

Answer: 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 -- 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 ConcurrentModificationExceptions.

The 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.

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 indroduced 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.