code: public DBObject toDBObject() {
          DBObject args = new BasicDBObject("ns", collectionName).append("cond", condition)
                                                                 .append("$reduce", reduce)
                                                                 .append("initial", initial);
  
          if (keys != null) {
              args.put("key", keys);
          }
  
          if (keyf != null) {
              args.put("$keyf", keyf);
          }
  
          if (finalize != null) {
              args.put("finalize", finalize);
          }
          return new BasicDBObject("group", args);
      }
docs: Turns this group command into the DBObject format of the command.
  
  @return a DBObject containing the group command as a MongoDB document