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    <p>Have you tried working with the initial capacity and load factor parameters while creating your HashSet?</p> <p><a href="http://docs.oracle.com/javase/6/docs/api/java/util/HashSet.html#HashSet%28int,%20float%29" rel="nofollow">HashSet doc</a></p> <p>Initial capacity, as you might think, refers to how big will the empty hashset be when created, and loadfactor is a threshhold that determines when to grow the hash table. Normally you would like to keep the ratio between used buckets and total buckets, below two thirds, which is regarded as the best ratio to achieve good stable performance in a hash table.</p> <p><a href="http://en.wikipedia.org/wiki/Hash_table#Dynamic_resizing" rel="nofollow">Dynamic rezing of a hash table</a></p> <p>So basically, try to set an initial capacity that will fit your needs (to avoid re-creating and reassigning the values of a hash table when it grows), as well as fiddling with the load factor until you find a sweet spot.</p> <p>It might be that for your particular data distribution and setting/getting values, a lower loadfactor could help (hardly a higher one will, but your milage may vary).</p>
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    1. COYes, I did try, but unfortunately when I know the size of a `HashSet` and tell it `new HashSet(n / 2)` it gives worse performance than without that capacity... Very strange, I guessed that it creates big arrays and spends a lot of time when doing lookup (during contains possibly), well, at least Bruce Eckel in Thinking in Java said something like that. Thank you for the answer anyway, I kind of accept it :)
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    2. COWhy would you do n / 2? If you know you are going to have exactly n elements, then use n. In an optimal case, each bucket will have exactly one element, but in case there happens to be a collision (unlikely if your data set is not very large) perfomance will not suffer much as long as you don't recalculate and reassign the hashtable (i.e. insert more than n elements, in your case, n/2). As per the hashtable doc (http://docs.oracle.com/javase/6/docs/api/java/util/Hashtable.html)
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    3. CO" "The initial capacity controls a tradeoff between wasted space and the need for rehash operations, which are time-consuming. No rehash operations will ever occur if the initial capacity is greater than the maximum number of entries the Hashtable will contain divided by its load factor. However, setting the initial capacity too high can waste space." So, if you are only holding ints, then the wasted space problem shouldn't be an issue. Try setting it to at least n, or even better, try n+1:
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