docs: public FieldFilter build() throws IllegalStateException {
          if (filters.isEmpty()) {
              throw new IllegalStateException("No field filters configured!");
          }
          if (filters.size() == 1) {
             return filters.get(0); 
          }
          FieldFilter[] fieldFilters = new FieldFilter[filters.size()];
          filters.toArray(fieldFilters);
          return new FieldFilterList(fieldFilters);
      }