how to easily sum two hashMap?

Question

I have two HashMap

How can I sum them easily?

Meaning that for String \"a\" the key will be sum of (value from Map1 + valu

Answer 1

in case you like Java 8:

Map sum(Map... maps) {
    return Stream.of(maps)    // Stream>
            .map(Map::entrySet)  // Stream>
            .flatMap(Collection::stream) // Stream>
            .collect(Collectors.toMap(Map.Entry::getKey,
                                      Map.Entry::getValue,
                                      Integer::sum));
}

can sum up arbitrary amounts of maps. It turns the array of maps into a Stream in the first few lines, then collects all the entries into a new Map while supplying a "merge function" in case of duplicate values.

alternatively something along the lines of

void addToA(HashMap a, HashMap b) {
    for (Entry entry : b.entrySet()) {
        Integer old = a.get(entry.getKey());
        Integer val = entry.getValue();
        a.put(entry.getKey(), old != null ? old + val : val);
    }
}