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

This method should do it (in Java 5+)

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