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I want to mock the following code snippet using Mockito.

Future<Optional<List<User>>> getUser =
       executorService.submit(() -> userRepository.findById(user.getUserId()));
I have tried with the following code, but it didn't work
    @Mock
    private ExecutorService executorService;
    @Mock
    private userRepository userRepository;
    when(executorService.submit(() -> userRepository.findById(USER_ID)))
           .thenReturn(ConcurrentUtils.constantFuture(userList));
Can anyone give me a solution for this?
Thanks for your answers. I have found a solution for this scenario.
We can mock executor service call using the following code snippet.
when(executorService.submit(any(Callable.class)))
      .thenReturn(ConcurrentUtils.constantFuture(userList()));
And if you have more than one ExecutorService calls in your method call, you can mock every response by adding them as a comma-separated list to the Mockito call as follows.
when(executorService.submit(any(Callable.class)))
      .thenReturn(ConcurrentUtils.constantFuture(userList()),
           ConcurrentUtils.constantFuture(Optional.of(departmentList())));
You don't want to fiddle with the ExecutorService itself but rather mock findById to get the result. As long as the mock returns the result immediately (unless you let it Thread.sleep for a while) the call itself in the ExecutorService is quick and the result is hence wrapped inside the Future.
Mockito.when(userRepository.findById(Mockito.any()).thenReturn(userList);
Then you don't need to mock the ExecutorService at all, you want to use a real service, or else it doesn't do what it's supposed to do.
                Don't mock the ExecuteService (remove @Mock), otherwise it doesn't do what it should do. (see my edit)
– Nikolas Charalambidis
                Oct 21, 2020 at 15:11
                Thanks. When I remove the @Mock from ExecuteService I'm getting NullPointerException when invoking the executorService.submit part
– slkhan
                Oct 21, 2020 at 15:20
        
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