RxJava makes it possible to generate not only one event in the future, but an asynchronous sequence of events based, for example, on a time interval, as shown in the following code:

Observable.interval(1, TimeUnit.SECONDS)
   .subscribe(e -> System.out.println("Received: " + e));
Thread.sleep(5000);                                                  // (1)

In that case, the output is as following:

Received: 0
Received: 1
Received: 2
Received: 3
Received: 4

Also, if we remove Thread.sleep(...) (1), our application will exit without any output. This happens because events would be generated and therefore consumed in a separate daemon thread. So, to prevent the main thread from finishing the execution, we may sleep() or do some other useful tasks.

Of course, there is something that controls the Observer-Subscriber cooperation. This is called Subscription, and has the following interface declaration:

interface Subscription {
   void unsubscribe();
   boolean isUnsubscribed();
}

The unsubscribe() method allows the Subscriber to inform Observable that there is no need to send new events. In other words, the aforementioned code is a subscription cancellation. On the other hand, Observable uses isUnsubscribed() to check that the Subscriber is still waiting for events.

To understand the mentioned unsubscribe functionality, let's consider the case where a subscriber is the only party interested in the events, and consumes them until an external signal is propagated by CountDawnLatch (1). The incoming stream generates a new event every 100 milliseconds, and these events produce the endless sequence—0, 1, 2, 3... (3). The following code demonstrates how to get a Subscription (2) when defining a reactive stream. It also shows how to unsubscribe from a stream (4):

CountDownLatch externalSignal = ...;                                 // (1)

Subscription subscription = Observable                               // (2)
        .interval(100, MILLISECONDS)                                 // (3)
        .subscribe(System.out::println);

externalSignal.await();
subscription.unsubscribe();                                          // (4)

So here, the subscriber receives the events 0, 1, 2, 3, and then the externalSignal invocation occurs, which leads to the subscription cancellation.

At this point, we have already learned that reactive programming consists of an Observable stream, a Subscriber, and some sort of Subscription that communicates the intention of the Subscriber to receive events from the Observable producer. It is now time to transform the data flowing through the reactive streams.