code: protected String extractGroupFromId(String columnName) {
          if (columnName == null || !columnName.contains(idSeparator)) return columnName;
          String[] tokens = columnName.split(idSeparator);
          if (tokens.length == 2) {
              return tokens[0];
          } else {
              throw new IllegalArgumentException(String.format("Cannot extract row key from column name string: %s", columnName));
          }
      }
docs: @warn method description missing
  @warn parameter columnName description missing
  @warn exception IllegalArgumentException description missing
  
  @param columnName
  @return
  @throws IllegalArgumentException