In RxJava an object that implements theObserver interfacesubscribes to an object of theObservable class. Then that subscriber reacts to whatever item or sequence of items the Observable objectemits. This pattern facilitates concurrent operations because it does not need to block while waiting for the Observable to emit objects, but instead it creates a sentry in the form of a subscriber that stands ready to react appropriately at whatever future time the Observable does so.

For information about the Observable class, seethe Observable documentation page at ReactiveX.io.