code: public synchronized CounterGroup getGroup(String groupName) {
      CounterGroup grp = groups.get(groupName);
      if (grp == null) {
        grp = new CounterGroup(groupName);
        groups.put(groupName, grp);
      }
      return grp;
    }
docs: Returns the named counter group, or an empty group if there is none
  with the specified name.