code: private static int mergeMaps(Map<String, Integer> map, Map<String, Integer> other)
      {
          int deltaSize = 0;
          for (Map.Entry<String, Integer> entry : other.entrySet()) {
              if (!map.containsKey(entry.getKey())) {
                  deltaSize += entry.getKey().getBytes().length + SIZE_OF_INT;
              }
              map.put(entry.getKey(), map.getOrDefault(entry.getKey(), 0) + other.getOrDefault(entry.getKey(), 0));
          }
          return deltaSize;
      }
docs: Returns the estimated memory increase in map