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

Assume that you have many HashMaps: HashMap map1, map2, map3;

Then you can use Java 8 streams:

List> mapList = Arrays.asList(map1, map2, map3);

Map combinedMap = mapList.stream()
        .flatMap(map -> map.entrySet().stream())
        .collect(Collectors.groupingBy(Entry::getKey,
                Collectors.summingInt(Entry::getValue)));