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    <p><strong>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.</strong></p> <h2><a href="http://docs.oracle.com/javase/8/docs/api/java/util/HashSet.html" rel="noreferrer">HashSet</a></h2> <ul> <li>the class offers constant time performance for the basic operations (add, remove, contains and size). </li> <li>it does not guarantee that the order of elements will remain constant over time</li> <li>iteration performance depends on the <em>initial capacity</em> and the <em>load factor</em> of the HashSet. <ul> <li>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.</li> </ul></li> </ul> <h2><a href="http://docs.oracle.com/javase/8/docs/api/java/util/TreeSet.html" rel="noreferrer">TreeSet</a></h2> <ul> <li>guarantees log(n) time cost for the basic operations (add, remove and contains)</li> <li>guarantees that elements of set will be sorted (ascending, natural, or the one specified by you via its constructor) (implements <a href="http://docs.oracle.com/javase/8/docs/api/java/util/SortedSet.html" rel="noreferrer"><code>SortedSet</code></a>)</li> <li>doesn't offer any tuning parameters for iteration performance</li> <li>offers a few handy methods to deal with the ordered set like <a href="http://docs.oracle.com/javase/8/docs/api/java/util/TreeSet.html#first--" rel="noreferrer"><code>first()</code></a>, <code>last()</code>, <a href="http://docs.oracle.com/javase/8/docs/api/java/util/TreeSet.html#headSet-E-" rel="noreferrer"><code>headSet()</code></a>, and <a href="http://docs.oracle.com/javase/8/docs/api/java/util/TreeSet.html#tailSet-E-" rel="noreferrer"><code>tailSet()</code></a> etc</li> </ul> <h2>Important points:</h2> <ul> <li>Both guarantee duplicate-free collection of elements</li> <li>It is generally faster to add elements to the HashSet and then convert the collection to a TreeSet for a duplicate-free sorted traversal.</li> <li>None of these implementations 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.</li> <li><strong>LinkedHashSet</strong> is in some sense intermediate between <code>HashSet</code> and <code>TreeSet</code>. Implemented as a hash table with a linked list running through it, however,<strong>it provides insertion-ordered iteration which is not same as sorted traversal guaranteed by TreeSet</strong>.</li> </ul> <p>So a 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.</p> <ul> <li>e.g. <code>SortedSet&lt;String&gt; s = new TreeSet&lt;String&gt;(hashSet);</code></li> </ul>
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    1. COMaybe it'll make more sense if your example assign the new TreeSet to a SortedSet type.
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    2. COWhat about memory consumption?
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    3. CO@matdumsa TreeSet has pointers in both children and parent directions, it will come at a cost, for sure.
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      1. USAhmet Alp Balkan - Google
 

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