public static <T_ROW> FilterFieldMapper<T_ROW> of(
            final FilterFieldDefinition fieldDefinition,
            final Function<T_ROW, String> valueExtractor) {
        return new FilterFieldMapper<>(fieldDefinition, valueExtractor);
    }
Defines the mapping between a FilterFieldDefinition and a property
of T_ROW. The valueExtractor should return a value for the field
from an instance of T_ROW.
stroom-util/src/main/java/stroom/util/filter/FilterFieldMapper.java