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Reactive Programming with RxJava

Observable and Observer

Explore the relationship between Observable and Observer in RxJava. Understand how to subscribe to Observables, manage events like onNext, onComplete, and onError, and grasp the synchronous and push-based nature of RxJava streams.

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An Observable can emit any number of items. As mentioned, to receive or listen to these emitted items, an Observer needs to subscribe to the Observable. The .subscribe() method is defined by the ObservableSource interface, which is implemented by Observable.

Java
public interface ObservableSource<T> {
    void subscribe(Observer<? super T> observer);
}

Once the Observable and Observer have been paired via the .subscribe() method, the Observer receives the following events through its defined methods:

  • The .onSubscribe() method is called along with a Disposable object that may be used to unsubscribe from the Observable to stop receiving items.
  • The .onNext() method is called when an item is emitted by the Observable.
  • The .onComplete() method is called when the Observable has finished sending items.
  • The .onError() method is called when an error is encountered within the Observable. The specific error is passed to this method.
Java
public interface Observer<T> {
    void onSubscribe(Disposable d);
    void onNext(T value);
    void onError(Throwable e);
    void onComplete();
}

.Next()

The method .onNext() can be invoked zero, one, or multiple times by the Observable whereas the .onComplete() and ...