How to write Junit test for methods with local variables

I have written a method which asks user to press enter to continue and timeouts after some time. I am facing difficulty in writing Junit tests for this method use Mockito. Below is the method.

private static final ExecutorService l = Executors.newFixedThreadPool(1);

private static String getUserInputWithTimeout(int timeout) {
    Callable<String> k = () -> new Scanner(System.in).nextLine();
    LocalDateTime start = LocalDateTime.now();
    Future<String> g = l.submit(k);
    while (ChronoUnit.SECONDS.between(start, LocalDateTime.now()) < timeout) {
        if (g.isDone()) {
            try {
                String choice = g.get();
                return choice;
            } catch (InterruptedException | ExecutionException | IllegalArgumentException e) {
                logger.error("ERROR", e);
                g = l.submit(k);
            }
        }
    }
    logger.info("Timeout...");
    g.cancel(true);
    return null;
}

I tried mocking Callable and Future but as this method is creating them locally creating them in Test has no impact.

I tried few things but didnt work as expected I might be doing it wrong.

@Test
    public void testgetUserInputWithUserInput() throws Exception {
        
         Scanner scanner = new Scanner(System.in);
         Callable<String> callable = PowerMockito.mock(Callable.class);
        

         ExecutorService executorServiceMock = PowerMockito.mock(ExecutorService.class);
     
         Future<String> futureMock = PowerMockito.mock(Future.class);
         when(executorServiceMock.submit(any(Callable.class))).thenReturn(futureMock);


        assertEquals("", getUserInputWithTimeout(3));
    }

Solution 1:

I would say you need to slightly change your method, take the callable object out of method and pass it as a parameter, this should solve your problem with mocking.

private static final ExecutorService l = Executors.newFixedThreadPool(1);

private static String getUserInputWithTimeout(int timeout, Callable<String> k) {
    LocalDateTime start = LocalDateTime.now();
    Future<String> g = l.submit(k);
    while (ChronoUnit.SECONDS.between(start, LocalDateTime.now()) < timeout) {
        if (g.isDone()) {
            try {
                String choice = g.get();
                return choice;
            } catch (InterruptedException | ExecutionException | IllegalArgumentException e) {
                logger.error("ERROR", e);
                g = l.submit(k);
            }
        }
    }
    logger.info("Timeout...");
    g.cancel(true);
    return null;
}

Your tests should look like below :

@Test
public void testgetUserInputWithUserInput() throws Exception {
    
    String ENTER = " ";
    System.setIn(new ByteArrayInputStream(ENTER.getBytes()));

    Callable<String> callable = () -> new Scanner(System.in).nextLine();

    assertEquals(ENTER, getUserInputWithTimeout(5, callable));
}