Having seen different ways of creating various types of Observable objects, subscribing to their events, and using the Subscription objects, we turn to composing Observable objects into larger programs. From what we have seen so far, the advantages of using Observable objects over a callback-based API are hardly worth the trouble.

The true power of Rx becomes apparent when we start composing Observable objects using various combinators. We can think of an Observable object in a similar way as we think of Scala sequence collections. In a Scala sequence, represented with the Seq[T] trait, elements of type T are ordered in the memory according to their indices. In an Observable[T] trait, events of type T are ordered in time.

Let's use the Observable.interval factory method in order to create an Observable object, which asynchronously emits a number every 0.5 seconds, and then output the first five odd numbers. To do this, we first call filter on the Observable object...