Observable.error()

This too is something you likely will only do with testing, but you can create an Observable that immediately calls onError() with a specified exception:

    import io.reactivex.Observable;

    public class Launcher {
      public static void main(String[] args) {
        
        Observable.error(new Exception("Crash and burn!"))
          .subscribe(i -> System.out.println("RECEIVED: " + i),
            Throwable::printStackTrace,
              () ->  System.out.println("Done!"));
      }
     }

The output is as follows:

    java.lang.Exception: Crash and burn!
    at Launcher.lambda$main$0(Launcher.java:7)
    at io.reactivex.internal.operators.observable.
      ObservableError.subscribeActual(ObservableError.java:32)
    at io.reactivex.Observable.subscribe(Observable.java:10514)
    at io.reactivex.Observable.subscribe(Observable.java:10500)
    ...

You can also provide the exception through a lambda so that it is created from scratch and separate exception instances are provided to each Observer:

    import io.reactivex.Observable;

    public class Launcher {
      public static void main(String[] args) {

        Observable.error(() -> new Exception("Crash and burn!"))
          .subscribe(i -> System.out.println("RECEIVED: " + i),
           Throwable::printStackTrace,
           () ->  System.out.println("Done!"));
      }
    }