We can test what we get by simply subscribing to the source Observable instance and collecting all of the incoming notifications. In order to demonstrate that, we'll develop a factory method for creating a new Observable instance and will test its behavior.

The method will receive a Comparator instance and multiple items, and will return Observable instance, emitting these items as a sorted sequence. The items will be sorted according to the Comparator instance passed.

We can develop the method using TDD. Let's first define the test as follows:

public class SortedObservableTest {
  private Observable<String> tested;
  private List<String> expected;
  @Before
  public void before() {
    tested = CreateObservable.<String>sorted(
      (a, b) -> a.compareTo(b),
      "Star", "Bar", "Car", "War", "Far", "Jar");
    expected = Arrays.asList(
      "Bar", "Car", "Far", "Jar", "Star", "War"
    );
  }
  TestData data = new TestData();
  tested...