@Override
  public List<AggregatorFactory> getRequiredColumns()
  {
    return Collections.singletonList(new HllSketchBuildAggregatorFactory(fieldName, fieldName, lgK, tgtHllType.toString()));
  }
@ Override public List < AggregatorFactory > getRequiredColumns ( ) { return Collections . singletonList ( new HllSketchBuildAggregatorFactory ( fieldName , fieldName , lgK , tgtHllType . toString ( ) ) ) ; }
This is a convoluted way to return a list of input field names this aggregator needs.
Currently the returned factories are only used to obtain a field name by calling getName() method.