Is iterating through a TreeSet slower than iterating through a HashSet in Java?

问题

I'm running some benchmarks. One of my tests depends on order, so I'm using a TreeSet for that. My second test doesn't, so I'm using a HashSet for it.

I know that insertion is slower for the TreeSet. But what about iterating through all elements?

回答1:

From a similar post (Hashset vs Treeset):

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 like first(), last(), headSet(), and tailSet() 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 between HashSet and TreeSet. 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);

回答2:

TreeSets internally uses TreeMaps which are Red Black Trees (special type of BST) .

BST Inorder Traversal is O(n)

HashSets internally uses HashMaps which use an array for holding Entry objects.

Here also traversal should be O(n) .

Unless you write a benchmark it is going to be difficult to prove which is faster.

回答3:

If you want stable ordering with (nearly) the performance of a HashSet, then use a LinkedHashSet. You will still get constant-time operations, whereas I would assume a TreeSet will get you logarithmic time.

来源：https://stackoverflow.com/questions/19798347/is-iterating-through-a-treeset-slower-than-iterating-through-a-hashset-in-java