How to test for no interaction using Mockito

I'm currently studying the book written by Tomasz Kaczanowski: "Practical Unit Testing with JUnit and Mockito". Right now, I'm on the section concerning mocks, stubs, test doubles and the Mockito framework.

As part of the practice, there's a to-be-unit-tested program which sends messages to the subscribed clients. Here's the part of the exercise I'm currently struggling with.

• What should happen if a client that is not subscribed tries to unsubscribe? Make up your mind about it, write a test which verifies this behaviour, and make RaceResultsService behave accordingly.

I would like nothing to happen on such occasion. No exception, no messages... And I want to test it. But how do I do that? How do I test that nothing happens (without getting too deep into implementation and, for example, checking the size of collection containing subscribed clients)?

Here's the test I've written which does not do so:

@Test
void unsubscribingWhenAlreadyUnsubscribedShouldReturnVoid() {
	raceResults.removeSubscriber(clientA);

	verifyNoInteractions(clientA);
}

And here's the concerned method of the RaceResultsService.java:

public class RaceResultsService {

	private Collection<Client> clients = new HashSet<Client>();

    // other fields and methods

    public void removeSubscriber(Client client) {
	    clients.remove(client);
    }
}

Obviously, the removeSubscriber method will actually do something, not nothing. It will execute the HashSet.remove but, as the client is not in the clients collection (field), it will not modify anything. But how do I test for this without actually touching the associated Collection<Client>?

I'd appreciate any suggestions, thanks!

It would be hard to test that nothing happens. But you shouldn't: a unit test should only test observable behaviour. Whatever happens inside the implementation is not the test's concern.

For example, if the implementation is changed to write a warning to a log file to inform the developers that a client might be buggy, this should probably not break the test. (You could argue that logging is also observable behaviour, but it's not usually part of the contract.)

So, what should you test for in this case?

Your test is already testing something, namely, that raceResults.removeSubscriber(clientA); does not throw an exception. In other words, it verifies that it's not an error to remove a client that's not subscribed. This might already be enough!

If you also have a method like boolean isSubscribed(Client client), you could additionally verify that the client is not subscribed after unsubscribing.

You could also send a message and verify that the unsubscribed client does not receive it.