Question

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?

Answer 1

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)