我正在springbootjunit中编写一些测试代码，并在一个使用存储库方法的测试案例中获得了成功，使用 "when, thenReturn" 就像下面一样。

@ExtendWith(SpringExtension.class) 
@WebMvcTest 
public class PictionarizerapiUserControllerTests {

  @MockBean 
  private UserRepository userRepository;

  @MockBean
  private UserController userController;

  @Autowired 
  private MockMvc mockMvc;

 @Test
  @DisplayName("When an update request is sent, the User data gets updated properly, and the updated User data gets returned in a form of JSON")
  public void testUpdateUser() throws Exception {
    // User before update
    User existingUser = new User();
    existingUser.setId(28);
    existingUser.setName("Alex");
    ......
    ......

    // return the User (before update) that is fetched by UserRepository#findById() with ID=28
    when(userRepository.findById(28)).thenReturn(Optional.of(existingUser));
    // UserRepository#save() returns the fetched entity as it is
    when(userRepository.save(any())).thenAnswer((invocation) -> invocation.getArguments()[0]);
    ......
    ......

我想我也可以为我自己编写的一个控制器方法编写一个测试用例，我试着做“when，thenreturn”，如下所示。

@Test
  @DisplayName("When correct login information is given and the matched user is fetched")
  public void testCheckIfValidUserFound() throws Exception {
      Integer userIdObj = Integer.valueOf(28);

      String requestEmail = "alex.armstrong@example.com";
      String requestPassword = "MajorStateAlchemist";

      when(userController.checkIfValidUser(requestEmail, requestPassword)).thenReturn(Optional.of(userIdObj));

      ......
      ......
  }

但是，我收到了一条错误消息 The method thenReturn(ResponseEntity<capture#1-of ?>) in the type OngoingStubbing<ResponseEntity<capture#1-of ?>> is not applicable for the arguments (Optional<Integer>) . 我做了一些研究，了解到你可以用 "when, thenReturn" 只有在测试存储库方法时才使用语法，这些方法是jpa中内置的方法，如 findById() 等等（除非我弄错了），在我的例子中，它不起作用，因为我要测试的是我自己创建的方法，而不是jpa的内置存储库方法。
我的问题来了。我怎样才能写出与 "when, thenReturn" 当我测试存储库方法以外的其他方法时？
更新
我自己的方法就是这样定义的。

@RequestMapping(value = "/login", method = RequestMethod.GET)
    public ResponseEntity<?> checkIfValidUser(
            @RequestParam("email") String email,
            @RequestParam("password") String password) {  
        int userId = 0;

        List<User> userList = repository.findAll();

        for(User user: userList) {
            String userEmail = user.getEmail();
            String userPassword = user.getPassword();
            String inputEmail = email;
            String inputPassword = password;
            if(userEmail.equals(inputEmail) && userPassword.equals(inputPassword)) {
                userId = user.getId();
            }
        }   

        if(userId > 0) {
            Integer userIdObj = Integer.valueOf(userId);
            return new ResponseEntity<>(userIdObj, HttpStatus.OK);
        } else {
            return new ResponseEntity<>(
                    new Error("The email address and the password don't match"),  
                    HttpStatus.NOT_FOUND
            );
        }
    }