### Question:
SingleDefaultIfEmpty extends CompletionSingle<T> { @Override public void subscribe(Flow.Subscriber<? super T> subscriber) { source.subscribe(new DefaultIfEmptySubscriber<>(subscriber, defaultItem)); } SingleDefaultIfEmpty(Single<T> source, T defaultItem); @Override void subscribe(Flow.Subscriber<? super T> subscriber);  }### Answer:
@Test public void normal() { TestSubscriber<Integer> ts = new TestSubscriber<>(Long.MAX_VALUE); Single.just(1) .defaultIfEmpty(2) .subscribe(ts); ts.assertResult(1); }
@Test public void fallback() { TestSubscriber<Integer> ts = new TestSubscriber<>(Long.MAX_VALUE); Single.<Integer>empty() .defaultIfEmpty(2) .subscribe(ts); ts.assertResult(2); }
@Test public void fallbackBackpressured() { TestSubscriber<Integer> ts = new TestSubscriber<>(); Single.<Integer>empty() .defaultIfEmpty(2) .subscribe(ts); ts .assertEmpty() .request(1) .assertResult(2); }
@Test public void error() { TestSubscriber<Integer> ts = new TestSubscriber<>(Long.MAX_VALUE); Single.<Integer>error(new IOException()) .defaultIfEmpty(2) .subscribe(ts); ts.assertFailure(IOException.class); }
@Test public void cancel() { SubmissionPublisher<Integer> sp = new SubmissionPublisher<>(Runnable::run, 128); TestSubscriber<Integer> ts = new TestSubscriber<>(Long.MAX_VALUE); Single.create(sp) .defaultIfEmpty(2) .subscribe(ts); assertTrue(sp.hasSubscribers()); ts.assertEmpty(); ts.cancel(); assertFalse(sp.hasSubscribers()); }