How to use Discrepancy class of org.mockito.internal.reporting package

You can use the inline mocking method by default, by changing your Gradle dependency from the normal Mockito dependency:

compile "org.mockito:mockito-core:$mockito_version"

... to the following:

compile "org.mockito:mockito-inline:$mockito_version"

This way you won't have to rely on activating inline mocking with the "file in the resources folder" method, which I have found to be flaky sometimes.

You could capture the value, e.g.

final ArgumentCaptor<Double> captor = ArgumentCaptor.forClass(Double.class)
...
verify(myManager).method1(captor.capture());

Then assert:

assertEquals(expected, captor.getValue(), delta)

Or, perhaps, use an argument matcher which does the assertion:

verify(myManager).method1(doubleThat(new ArgumentMatcher<Double>() 
{
    @Override 
    public boolean matches(final Object actual)
    {
        return Math.abs(expected - (Double) actual) <= delta;
    }
}));

Update:

Instead of using either of the methods above, you could use AdditionalMatchers.eq(double, double) instead, e.g.:

verify(myManager).method1(AdditionalMatchers.eq(expected, delta));

Although use AdditonalMatchers matchers wisely, as per the documentation:

AdditionalMatchers provides rarely used matchers, kept only for somewhat compatibility with EasyMock. Use additional matchers very judiciously because they may impact readability of a test. It is recommended to use matchers from Matchers and keep stubbing and verification simple.

You can try the following idea (taken from similar discussion on github):

.doAnswer { throw ServiceException("Bad Request") }

The try-with-resources statement can define multiple AutoCloseable instances, both of which will be closed at the end:

try (MockedStatic<ObjectFactory> objectFactory = mockStatic(ObjectFactory.class);
     MockedStatic<Context> contextMock = mockStatic(Context.class)) {

If I understand correctly, the problem is not really to test the observer, but to test the result of an asynchronous method call. To do that, create an observer which blocks until its update() method has been called. Something like the following:

public class BlockingGameObserver extends GameInstanceObserver {
    private CountDownLatch latch = new CountDownLatch(1);

    @Override
    public void update() {
        latch.countDown();
    }

    public void waitUntilUpdateIsCalled() throws InterruptedException {
        latch.await();
    }
}

And in your test:

private BlockingGameObserver observer;

@Before
public void setUp() throws IOException, Exception {
    observer = new BlockingGameObserver();
    GameInstance.getInstance().addObserver(observer); 
}

@Test
public void getRoomUserData_usingNullKey_shouldThrowUserDataDoesntExist() throws InterruptedException, UserDataDoesntExistException {
    serverService.createRoom("exampleRoom");   
    observer.waitUntilUpdateIsCalled();
    assertEquals("exampleRoom",
                 GameInstance.getInstance().getRoom("exampleRoom").getRoomId());
}

I got this when combining PowerMock with Mockito. Fixed by using compatible version as shown here: https://github.com/powermock/powermock/wiki/Mockito#supported-versions

I had this issue and my problem was that I was calling my method with any() instead of anyInt(). So I had:

doAnswer(...).with(myMockObject).thisFuncTakesAnInt(any())

and I had to change it to:

doAnswer(...).with(myMockObject).thisFuncTakesAnInt(anyInt())

I have no idea why that produced a NullPointerException. Maybe this will help the next poor soul.

I've finally been able to solve my problem. The key to the answer is the line

java.lang.IllegalStateException: Could not self-attach to current VM using external process

For Java 9 and more, one must add -Djdk.attach.allowAttachSelf=true as VM argument to avoid this exception. When I did, unit tests ran perfectly.

Here are the links where I've found this :

https://github.com/raphw/byte-buddy/issues/612

https://github.com/mockk/mockk/issues/254

As Object.getClass() is final, you cannot mock that method with Mockito. I would strongly advice you to refactor your code to inject the class in another way. If that's not possible, you could try out powermock, where you could mock any final method. Object.getClass() is a bit special, so be sure to set MockGateway.MOCK_GET_CLASS_METHOD = true in powermock.