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    <blockquote> <p>It strikes me as odd that an empty Multiset would have hash code 0</p> </blockquote> <p>Why? All empty collections probably have hash code 0. Even if not, it would have to be a fixed value (since all empty collections are equal), so what is wrong with 0?</p> <blockquote> <p>what would be an efficient hash code calculation?</p> </blockquote> <p>Yours is more efficient (which just means faster to calculate), not too that bad in terms of effectiveness (which means yielding results that work well), either. If I understand it correctly, it adds up the hash codes of all elements (with duplicate elements being added twice). This is exactly what a regular Set does, so if you have no duplicates, you get the same hashCode as with a Set, which could be an advantage (if you fix the empty set to have hashCode 0, not 1).</p> <p>Google's version is a little more complicated, I suppose in order to avoid some otherwise frequent collisions. Of course it probably causes some other collisions that are considered less frequent to happen instead.</p> <p>In particular, using XOR spreads the hashCodes all over the available range, even if the individual input hashCodes do not (which they for example do not for Integers from a limited range, which is a frequent use-case).</p> <p>Consider the hashCode for the Set [ 1, 2, 3]. It is 6. Likely to collide with similar Sets, for example [ 6], [ 4, 2] , [5, 1]. Throwing some XOR in there helps. If it is necessary and worth the extra cost is a tradeoff you have to make.</p>
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    1. POEfficient hash code for multiset in Java
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    1. COThe 0 striked me as odd because I thought it's good practice to reserve it for hashing nulls, that's all. Of course there's nothing wrong with it.
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    2. COYou understood my hash function perfectly. And if I understand you correctly, it would (or could) lead to more collisions compared to the XOR method. I'm afraid I don't really understand why. (Doesn't HashMap have optimizations for this?) I also don't understand why adding would be more efficient in general. In my particular case it probably is, but if getting count is cheap I'd say the XOR method is more efficient.
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    3. CO...and thanks for the answer of course :)
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