Completable

Completable is simply concerned with an action being executed, but it does not receive any emissions. Logically, it does not have  onNext() or onSuccess() to receive emissions, but it does have onError() and onComplete():

    interface CompletableObserver<T> {
    void onSubscribe(Disposable d);
    void onComplete();
    void onError(Throwable error);
    }

Completable is something you likely will not use often. You can construct one quickly by calling Completable.complete() or Completable.fromRunnable(). The former will immediately call onComplete() without doing anything, while fromRunnable() will execute the specified action before calling onComplete():

    import io.reactivex.Completable;

    public class Launcher {

      public static void main(String[] args) {

        Completable.fromRunnable(() -> runProcess())
        .subscribe(() -> System.out.println("Done!"));

      }
      public static void runProcess() {
        //run process here
       }
     }

 The output is as follows:

    Done!