public final T convertFromEntity(final U entity) {
    return fromEntity.apply(entity);
  }
@Test
  void testCustomConverter() {
    var converter = new Converter<UserDto, User>(
        userDto -> new User(
            userDto.firstName(),
            userDto.lastName(),
            userDto.active(),
            String.valueOf(new Random().nextInt())
        ),
        user -> new UserDto(
            user.firstName(),
            user.lastName(),
            user.active(),
            user.firstName().toLowerCase() + user.lastName().toLowerCase() + "@whatever.com")
    );
    var u1 = new User("John", "Doe", false, "12324");
    var userDto = converter.convertFromEntity(u1);
    assertEquals("johndoe@whatever.com", userDto.email());
  }