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    <p>From the requirements you've given, I would personally just use a medium-quality random number generator that you know won't produce duplicates within the number of unique numbers that you require. Unless you have an extra requirement you haven't mentioned, it seems overkill to keep the set of all the previously generated numbers.</p> <p>For example, using a 32-bit XORShift generator will produce all 2^31 negative 4-byte integers in "random" order before repeating the pattern. If you need more numbers than that, you probably don't want to be putting them in a hash set anyway. So something like this (warning: off-top-of-head untested code...):</p> <pre><code>int seed = (int) System.nanoTime(); final int origSeed = seed; public int nextUniqueNegativeNumber() { int n = seed; do { n ^= (n &lt;&lt; 13); n ^= (n &gt;&gt;&gt; 17); n ^= (n &lt;&lt; 5); seed = n; if (n == origSeed) { throw new InternalError("Run out of numbers!"); } } while (n &gt; 0); return n; } </code></pre> <p>I'll leave it up to the reader to convert "seed" to use an AtomicInteger if concurrency is necessary...</p> <p>Edit: actually, to optimise the concurrent case you maybe only want to write back to "seed" after getting the next <em>negative</em> number.</p> <p>OK, by popular demand, the atomic version would then be something like this:</p> <pre><code> AtomicInteger seed = new AtomicInteger((int) System.nanoTime()); public int nextUniqueNegativeNumber() { int oldVal, n; do { do { oldVal = seed.get(); n = oldVal ^ (oldVal &lt;&lt; 13); // Added correction n ^= (n &gt;&gt;&gt; 17); n ^= (n &lt;&lt; 5); } while (seed.getAndSet(n) != oldVal); } while (n &gt; 0); return n; } </code></pre>
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    1. COI really like your approach here, even though it doesn't address my question of "can the algorithm be rewriten using nonblocking constructs?"
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    2. COSorry, didn't realise that making it atomic was the difficult bit. It's really just a trivial application of, say, AtomicInteger. But I've added the code.
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    3. COYour 'shuffle' generates lots of duplicates, even if not immediate ones due to the loop guard
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