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<String, Integer> m = new HashMap<>();
    HashMap<String, Integer> m2 = new HashMap<>();

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

ı tried putall

m.putAll(m2);

output is as follows {banana=6, apple=9, pear=3}

but its result is not true for this problem. ı want to output as

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

how can ı get this result in java?

Answer 1

If you are using Java 8, you can use the new merge method of Map.

m2.forEach((k, v) -> m.merge(k, v, (v1, v2) -> v1 + v2));

Answer 2

This is a very nice use case for Java 8 streams. You can concatentate the streams of entries and then collect them in a new map:

Map<String, Integer> combinedMap = Stream.concat(m1.entrySet().stream(), m2.entrySet().stream())
    .collect(Collectors.groupingBy(Map.Entry::getKey,
             Collectors.summingInt(Map.Entry::getValue)));

There are lots of nice things about this solution, including being able to make it parallel, expanding to as many maps as you want and being able to trivial filter the maps if required. It also does not require the orginal maps to be mutable.

Answer 3

This method should do it (in Java 5+)

public static <K> Map<K, Integer> mergeAndAdd(Map<K, Integer>... maps) {
    Map<K, Integer> result = new HashMap<>();
    for (Map<K, Integer> map : maps) {
        for (Map.Entry<K, Integer> entry : map.entrySet()) {
            K key = entry.getKey();
            Integer current = result.get(key);
            result.put(key, current == null ? entry.getValue() : entry.getValue() + current);
        }
    }
    return result;
}

Answer 4

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<String, Integer> m = new HashMap<>();
        HashMap<String, Integer> m2 = new HashMap<>();

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

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

    public Map<String, Integer> mergeSumOfMaps(Map<String, Integer>... maps) {
        final Map<String, Integer> resultMap = new HashMap<>();
        for (final Map<String, Integer> 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)