code: public void add(long timestamp, Map<String, Object> data) {
          for (String k : data.keySet()) {
              if (!columns.contains(k)) {
                  throw new UnknownFieldException(k);
              }
          }
  
          Map<String, Object> curr = this.rows.get(timestamp);
          if (curr == null) {
              this.rows.put(timestamp, new HashMap<String, Object>(data));
          } else {
              curr.putAll(data);
          }
      }
docs: Add a data row to the batch at a particular timestamp, merging with previous values if
  needed.
  
  This method will throw an `UnknownFieldException` if `data` contains a key that is not in the
  set of columns this batch was constructed with.
  
  @param timestamp Timestamp for all data points
  @param data      Map that has field names as keys and the data value as values.