how to merge more than one hashmaps also sum the values of same key in java

Question

ı am trying to merge more than one hashmaps also sum the values of same key, ı want to explain my problem with toy example as follows

    HashMap

        

Answer 1

Here's my quick and dirty implementation:

import java.util.HashMap;
import java.util.Map;

public class MapMerger {

    public static void main(String[] args) {
        HashMap m = new HashMap<>();
        HashMap m2 = new HashMap<>();

        m.put("apple", 2);
        m.put("pear", 3);
        m2.put("apple", 9);
        m2.put("banana", 6);

        final Map result = (new MapMerger()).mergeSumOfMaps(m, m2);
        System.out.println(result);
    }

    public Map mergeSumOfMaps(Map... maps) {
        final Map resultMap = new HashMap<>();
        for (final Map map : maps) {
            for (final String key : map.keySet()) {
                final int value;
                if (resultMap.containsKey(key)) {
                    final int existingValue = resultMap.get(key);
                    value = map.get(key) + existingValue;
                }
                else {
                    value = map.get(key);
                }
                resultMap.put(key, value);
            }
        }
        return resultMap;
    }
}

Output:

{banana=6, apple=11, pear=3}

There are some things you should do (like null checking), and I'm not sure if it's the fastest. Also, this is specific to integers. I attempted to make one using generics of the Number class, but you'd need this method for each type (byte, int, short, longer, etc)