code: public synchronized void incrAllCounters(Counters other) {
      for(Map.Entry<String, CounterGroup> rightEntry: other.groups.entrySet()) {
        CounterGroup left = groups.get(rightEntry.getKey());
        CounterGroup right = rightEntry.getValue();
        if (left == null) {
          left = new CounterGroup(right.getName(), right.getDisplayName());
          groups.put(rightEntry.getKey(), left);
        }
        left.incrAllCounters(right);
      }
    }
docs: Increments multiple counters by their amounts in another Counters
  instance.
  @param other the other Counters instance