code: public static Mapping<Integer> intv(Constraint... constraints) {
          return new FieldMapping(
                  InputMode.SINGLE,
                  mkSimpleConverter(s ->
                      isEmptyStr(s) ? 0 : Integer.parseInt(s)
                  ), new MappingMeta(MAPPING_INT, Integer.class)
              ).constraint(checking(Integer::parseInt, "error.number", true))
                  .constraint(constraints);
          }
docs: (convert to Integer) mapping
  @param constraints constraints
  @return new created mapping