JAVA HashSet

1.  not synchronized。 不是Thread Safe的。

2. 底层通过HashMap来实现。

3. This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element. 

4. This class offers constant time performance for the basic operations (add,remove,containsandsize)

HashSet is much faster than TreeSet (constant-time versus log-time for most operations like add, remove and contains) but offers no ordering guarantees like TreeSet.

HashSet:

• class offers constant time performance for the basic operations (add, remove, contains and size).
• it does not guarantee that the order of elements will remain constant over time
• iteration performance depends on the initial capacity and the load factor of the HashSet.
  • It's quite safe to accept default load factor but you may want to specify an initial capacity that's about twice the size to which you expect the set to grow.

TreeSet:

• guarantees log(n) time cost for the basic operations (add, remove and contains)
• guarantees that elements of set will be sorted (ascending, natural, or the one specified by you via it's constructor)
• doesn't offer any tuning parameters for iteration performance
• offers a few handy methods to deal with the ordered set likefirst(),last(),headSet(), andtailSet()etc

Important points:

• Both guarantee duplicate-free collection of elements
• It is generally faster to add elements to the HashSet and then convert the collection to a TreeSet for a duplicate-free sorted traversal.
• None of these implementation are synchronized. That is if multiple threads access a set concurrently, and at least one of the threads modifies the set, it must be synchronized externally.
• LinkedHashSet is in some sense intermediate betweenHashSetandTreeSet. Implemented as a hash table with a linked list running through it, however it provides insertion-ordered iteration which is not same as sorted traversal guaranteed by TreeSet.

So choice of usage depends entirely on your needs but I feel that even if you need an ordered collection then you should still prefer HashSet to create the Set and then convert it into TreeSet.

• e.g.Set<String> s = new TreeSet<String>(hashSet);