The onNext(), onComplete(), and onError() methods actually define the Observer type, an abstract interface implemented throughout RxJava to communicate these events. This is the Observer definition in RxJava shown in the code snippet. Do not bother yourself about onSubscribe() for now, as we will cover it at the end of this chapter. Just bring your attention to the other three methods:

    package io.reactivex;

    import io.reactivex.disposables.Disposable;

    public interface Observer<T> {
      void onSubscribe(Disposable d);
      void onNext(T value);
      void onError(Throwable e);
      void onComplete();
   }

Observers and source Observables are somewhat relative. In one context, a source Observable is where your Observable chain starts and where emissions originate. In our previous examples, you could say that the Observable returned from...