code: public static final DefaultTupleMapper newStringDefaultTupleMapper( ) {
          return new DefaultTupleMapper(new TupleMapper<String>() {
              @Override
              public String map(Tuple input) {
                  return input.getStringByField(FIELD_SOURCE);
              }
          });
      }
docs: Returns a new {@link DefaultTupleMapper} that accept String as source field value.