Efficient way to subtract values in two HashMaps by key

开发者_运维知识库I am wondering how to efficiently subtract the values of two maps when their keys match. Currently I have 2 HashMap<String,Integer> and do it like this:

for (String key: map1.keySet()){
   if (map2.keySet().contains(key)){
       //subtract
   }
}

Is there a better way to do it?

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Theoretically speaking, this is about as fast as it can be done unless you can somehow do a faster than O(n) way of finding the matching keys between the two HashMaps.

• Iterate over keys in first map's keySet() - O(n)
  • See if key is in other map - O(1)
  • Do your operation - O(1)

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Realise this is an old thread but do check out guava from google

https://code.google.com/p/guava-libraries/wiki/CollectionUtilitiesExplained#Maps

You can use Map.difference and then get the entries in common, only in left, right etc.

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I think there isn't a better method unless you use a different approach, and/or different data structures. You can for example create a class named ValuePair that can contain (up to) two values, which represent the values you are currently storing in two different maps, but you instead store all the pairs in a single map, and when it comes to "subtract" you can iterate in a single set of keys. Please note that a pair can be incomplete, so that no subtraction is done.

But that's probabily overkill.

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have you considered using Apache Commons Collections?

CollectionUtils.subtract( collection1, collection2 );