The TestSubscriber instance is a special Subscriber instance, which we can pass to the subscribe() method of any Observable instance.

We can retrieve all the received items and notifications from it. We can also look at the last thread on which the notifications have been received and the subscription state.

Let's rewrite our test using it, in order to demonstrate its capabilities and what it stores:

@Test
public void testUsingTestSubscriber() {
  TestSubscriber<String> subscriber =
    new TestSubscriber<String>();
  tested.subscribe(subscriber);
  Assert.assertEquals(expected, subscriber.getOnNextEvents());
  Assert.assertSame(1, subscriber.getOnCompletedEvents().size());
  Assert.assertTrue(subscriber.getOnErrorEvents().isEmpty());
  Assert.assertTrue(subscriber.isUnsubscribed());
}

The test is, again, very simple. We create a TestSubscriber instance and subscribe to the tested Observable instance with it. And we have access to the whole state after the Observable instance is completed. Let's take a look at the following term list:

The TestSubscriber instance has some assertion methods too. So, there is one more way in which the test could be written:

@Test
public void testUsingTestSubscriberAssertions() {
  TestSubscriber<String> subscriber = new TestSubscriber<String>();
  tested.subscribe(subscriber);
  subscriber.assertReceivedOnNext(expected);
  subscriber.assertTerminalEvent();
  subscriber.assertNoErrors();
  subscriber.assertUnsubscribed();
}

These are almost the same assertions, but done with the TestSubscriber instance's own assert* methods.

With these techniques, we can test different behaviors and states of our RxJava logic. There is one last thing left to learn in this chapter—testing asynchronous Observable instances, such as the ones created by the Observable.interval() method.