### Question:
UserMapper { public User userFromId(String id) { if (id == null) { return null; } User user = new User(); user.setId(id); return user; }  List<UserDTO> usersToUserDTOs(List<User> users); UserDTO userToUserDTO(User user); List<User> userDTOsToUsers(List<UserDTO> userDTOs); User userDTOToUser(UserDTO userDTO); User userFromId(String id);  }### Answer:
@Test public void testUserFromId() { assertThat(userMapper.userFromId(DEFAULT_ID).getId()).isEqualTo(DEFAULT_ID); assertThat(userMapper.userFromId(null)).isNull(); }