code: private FacetList calculateFacetList(AbstractDataTable table, String[] facetFields) {
          if(facetFields == null || facetFields.length == 0){
               throw new NullPointerException("Facet field list empty");
          }
          FacetList facetList = new FacetList();
          for (String facetField : facetFields) {
              Set<String> distinctColumnValues = dataTableExtensions.getDistinctColumnValues(table, schema.getColumnIndex(facetField));
              facetList.addFacetField(facetField, distinctColumnValues);
          }
          return facetList;
      }
docs: facet field -> list of facet field values
  @param table gwt datatable as internal table data source
  @param facetFields list of field to calculate facet values for