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PO2-D (concurrent) HashMap: 2-property key type? hashmap of hashmaps? [update]
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  1. PO2-D (concurrent) HashMap: 2-property key type? hashmap of hashmaps? [update]
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    copied!<p>So I need a 2-dimensional <code>ConcurrentHashMap</code>.</p> <p>It has to be as blazing fast as possible, as I'm going to be adding to and updating its values extremely frequently. It's in a multithreaded application, hence the choice to use ConcurrentHashMap instead of just HashMap.</p> <p>Both the "x" and "y" indices are integers with a known range (0 through 40,000,000).</p> <p>What I need to know is: What's the most efficient way to implement this so it'll be as speedy as possible? The most obvious route is to do a literal 2-D hashmap:</p> <p><code>ConcurrentHashMap&lt;Integer, ConcurrentHashMap&lt;Integer, ValueObj&gt;&gt; foo;</code></p> <p>Or I could make a private class "IntPair" with two properties x and y, and use that as a key... though if I do that, what's the most efficient way to do <code>equals()</code> and <code>hashcode()</code>? and will I wind up allocating too many new <code>IntPair</code>s? Could I keep a set of <code>IntPair</code>s for each x/y I've assigned, and then use a purely reflexive equals() such that I'm just checking for the exact same object instance?</p> <hr> <h2>Update:</h2> <p>Now that I've taken a closer look at Integer.valueOf(int), the specific caching model it uses wouldn't make sense here, since I'm dealing with a very sparse matrix with unpredictable entries. I really need to be caching all those IntPairs which are used, not a prespecified subset.</p> <p>Intuitively, it seems to me that looking up an IntPair in a big map to see if I've already created it would, in fact, be more-or-less the same as just looking it up in the big "2-D" ConcurrentHashMap anyway, wouldn't it? So it seems the solution here is really to just use <code>new IntPair(x,y)</code> each time I look up a key. Yes?</p>
 

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