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Reputation: 2826

Most efficient way to access 2D Maps (Maps of Maps) in Java?

I'm implementing the WeightedSlopeOne prediciton algorithm for recommender system and at some point in the code I need to have 2 2D maps, one Map<Integer, Map<Integer, Integer>> and one Map<Integer, Map<Integer, Double>>

As you can understand accessing these and assigning values is a cumbersome procedure:

//The following 20 lines are 1 line in Python. Sigh...
HashMap<Integer, Integer> freqsForItem1 = frequencies.get(curItemID);
//See if we have a value for curItemID
if (freqsForItem1 == null) {
    freqsForItem1 = new HashMap<Integer, Integer>();
    freqsForItem1.put(curItemID_2, 1);
    frequencies.put(curItemID, freqsForItem1);
}
else {//See if we have a value for curItemID+curItemID_2
    Integer freqForItem1Item2 = freqsForItem1.get(curItemID_2);
    if (freqForItem1Item2 == null) {
        //If we don't have a value for item1+item2 we just put 1
        freqsForItem1.put(curItemID_2, 1);
    }
    else {//We already have a value for curItemID+curItemID_2
        //So we just increment it
        freqsForItem1.put(curItemID_2, freqForItem1Item2 + 1);
    }
}

So what should I be using here instead of a Map<K1, Map<K2, V>>, or if there is no better data structure available what is a better way to access and change the values of such a Map?

Upvotes: 0

Views: 110

Answers (3)

desperateCoder
desperateCoder

Reputation: 700

so your key is an integer in both dimensions. why dont you use a 

HashMap<Integer, Double> []

instead? or at least an

ArrayList<HashMap<Integer, Double>>

Would be much more performant

Upvotes: -1

Matt Ball
Matt Ball

Reputation: 359776

Instead of using a map of maps, you can create a new, immutable class (with properly implemented equals() and hashCode() methods!) that stores the two integer keys, and use that as the key for a simpler map.

class MyKey {
    int first;
    int second;
    // etc...
}

Map<MyKey, Integer> freqs = new HashMap<MyKey, Integer();

This will greatly simplify accessing & assigning values, and even moreso if you decide you need to make your key more complex.

Upvotes: 2

Michal Borek
Michal Borek

Reputation: 4624

You can use Table from Google's Guava to do this without worrying of implementation.

Check Guava Page: https://code.google.com/p/guava-libraries/wiki/NewCollectionTypesExplained#Table

Upvotes: 2

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