Question

So I need a 2-dimensional ConcurrentHashMap.

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.

Both the ‘x’ and ‘y’ indices are integers with a known range (0 through 40,000,000).

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:

ConcurrentHashMap<Integer, ConcurrentHashMap<Integer, ValueObj>> foo;

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 equals() and hashcode()? and will I wind up allocating too many new IntPairs? Could I keep a set of IntPairs 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?

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Update:

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.

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 new IntPair(x,y) each time I look up a key. Yes?

Answer 1

It depends on how sparse your (x,y) points are, in the 40,000,000 x 40,000,000 matrix. My guess is that the matrix is going to be quite sparse anyway, so creating a lot of ConcurrentHashMaps is going to be expensive.

Your (immutable) IntPair suggestion seems more attractive in comparison. As you’ve suggested, you can even cache some of these pairs to improve performance (see Integer.valueOf(int) to see how this can be implemented using a static nested class and a static factory method). Since the hashcode will always be required, you can pre-compute it in the constructor and save it as a final field. To compute equals, you could use the identity equality for objects in the cache, otherwise you’ll need to compare x and y individually.

EDIT: Here’s the source code (OpenJDK) for Integer.valueOf(int).