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POJava HashMap performance optimization / alternative
 

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    <p>I want to create a large HashMap but the <code>put()</code> performance is not good enough. Any ideas?</p> <p>Other data structure suggestions are welcome but I need the lookup feature of a Java Map:</p> <p><code>map.get(key)</code></p> <p>In my case I want to create a map with 26 million entries. Using the standard Java HashMap the put rate becomes unbearably slow after 2-3 million insertions. </p> <p>Also, does anyone know if using different hash code distributions for the keys could help? </p> <p>My hashcode method:</p> <pre><code>byte[] a = new byte[2]; byte[] b = new byte[3]; ... public int hashCode() { int hash = 503; hash = hash * 5381 + (a[0] + a[1]); hash = hash * 5381 + (b[0] + b[1] + b[2]); return hash; } </code></pre> <p>I am using the associative property of addition to ensure that equal objects have the same hashcode. The arrays are bytes with values in the range 0 - 51. Values are only used once in either array. The objects are equal if the a arrays contain the same values (in either order) and the same goes for the b array. So a = {0,1} b = {45,12,33} and a = {1,0} b = {33,45,12} are equal.</p> <p>EDIT, some notes:</p> <ul> <li><p>A few people have criticized using a hash map or other data structure to store 26 million entries. I cannot see why this would seem strange. It looks like a classic data structures and algorithms problem to me. I have 26 million items and I want to be able to quickly insert them into and look them up from a data structure: give me the data structure and algorithms.</p></li> <li><p>Setting the initial capacity of the default Java HashMap to 26 million <em>decreases</em> the performance.</p></li> <li><p>Some people have suggested using databases, in some other situations that is definitely the smart option. But I am really asking a data structures and algorithms question, a full database would be overkill and much slower than a good datastructure solution (after all the database is just software but would have communication and possibly disk overhead).</p></li> </ul>
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    1. COIf HashMap becomes slow, in all likelihood your hash function is not good enough.
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    2. COdoctor, it hurts when I do *this*
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    3. COWould you mind posting your hashCode() function?
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