@Test
void mapString() {

    final ToString<String> toString = string -> string;
    final ToStringMapper<String, ToString<String>, UnaryOperator<String>> toStringMapper =
            (ToStringMapper<String, ToString<String>, UnaryOperator<String>>)
                    MapperUtil.mapString(toString, string -> string);
    assertEquals(MapperType.STRING_TO_STRING, toStringMapper.mapperType());
}