How to convert rxJava2's Observable to Completable?

The fluent way is to use Observable.ignoreElements().

Observable.just(1, 2, 3)
.ignoreElements()

Convert it back via toObservable if needed.

You can do something like below.

Observable<Integer> observable = Observable.just(1, 2, 3);
Completable completable = Completable.fromObservable(observable);

Like on an Observable, you will have to subscribe to the completable to start the asynchronous process that Observable wraps.

More details can be found here in the Java doc for the method.

As I understand all this solutions will work only if Observable call onComplete, which is not enough if you want your result Completable to trigger after first onNext or onError, so for this case I'd recommend this:

Observable<Integer> observable = Observable.just(1, 2, 3);
Completable completable = observable.firstOrError().ignoreElement()