如何在jdk1.6下构造一个ConcurrentHashSet

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一直很奇怪，为什么concurrent包下只有ConcurrentHashMap而没有ConcurrentHashSet。虽然JDK提供了其他两种并发的Set实现（CopyOnWriteArraySet与ConcurrentSkipListSet），但这两种Set的使用场景与我们平时使用HashSet的方式并不一样。今天google了一下，发现了一篇好文章，贴下来供参考：

（不想看英文的话，只要知道通过下面这个方法构造ConcurrentHashSet就可以了。

Collections.newSetFromMap(new ConcurrentHashMap<Object,Boolean>())

）

Concurrency Pattern: Concurrent Set implementations in Java 6

by DHRUBA BANDOPADHYAY on WED, 05 AUG 2009 00:17:03 +0000

An interesting question came up on the jsr166 concurrency interest mailing list recently which I felt was worthy of mention. Why is there no ConcurrentHashSet equivalent of the ConcurrentHashMap data structure and how does one achieve the same concurrency and performance characteristics of the latter while maintaining the uniqueness semantics of the former? Currently there exist a few different ways of making a Set concurrent.

CopyOnWriteArraySet
ConcurrentSkipListSet
Collections.synchronizedSet(Set<T> s)

However none of these exhibit the lock striped minimal blocking high concurrency characteristics of a ConcurrentHashMap. In actual fact all Set implementations with the exception of EnumSet are little more than wrappers around a second kind of backing implementation. Here are all Set implementations and their corresponding backing implementations in brackets.

HashSet (HashMap)
TreeSet (TreeMap)
LinkedHashSet (LinkedHashMap)
CopyOnWriteArraySet (CopyOnWriteArrayList)
ConcurrentSkipListSet (ConcurrentSkipListMap)
JobStateReasons (HashMap)

Following on from that, for those map implementations for which there aren’t already Set equivalents, the JDK from version 1.6 onwards provides a wayfor you to create a set with your own choice of backing map implementation. For example one can create a ConcurrentHashSet or a WeakHashSet simply by doing the following.

Collections.newSetFromMap(new ConcurrentHashMap<Object,Boolean>())Collections.newSetFromMap(new WeakHashMap<Object, Boolean>())

With this knowledge some may opt to use the underlying map implementations directly by themselves and this is a trade off between a solely theoretical performance optimisation and making your choice of collection a semantically correct one.