How do I convert async task/ rx java code to rxjava2?

Issue

I tried converting my AsyncTask code below to Rxjava2, but apparently Rxjava2 doesn't deal with null values and hence my app crashes. Here's my code for AsyncTask:

new AsyncTask<Void, Void, Void>() {
            @Override
            protected Void doInBackground(Void... params) {
                Set<Map.Entry<String, Participant>> entries = pool.entrySet();
                for (Map.Entry<String, Participant> entry : entries) {
                    Participant participant = entry.getValue();
                    participant.release();
                }
                return null;
            }

            @Override
            protected void onPostExecute(Void aVoid) {
                cb.event(new Spin.Event<Void>());
            }
        }.execute();

And here's the converted code to Rxjava (NOT Rxjava2) :

 Observable.defer(new Func0<Observable<Void>>() {
        @Override
        public Observable<Void> call() {
            Set<Map.Entry<String, Participant>> entries = pool.entrySet();
            for (Map.Entry<String, Participant> entry : entries) {
                Participant participant = entry.getValue();
                participant.release();
            }
            return Observable.just(null);
        }
    }).doOnCompleted(new Action0() {
        @Override
        public void call() {
            cb.event(new Spin.Event<Void>());
        }
    })
    .subscribeOn(Schedulers.computation())
    .subscribe();

What would be the best approach to convert it to Rxjava without it crashing on returning null. Also, how does the .execute() play with respect to Rxjava2? not sure if that even works in Rxjava?

Here's the crash log:

FATAL EXCEPTION: RxComputationThreadPool-3

                                                                           io.reactivex.exceptions.OnErrorNotImplementedException: null ObservableSource supplied
                                                                               at io.reactivex.internal.functions.Functions$OnErrorMissingConsumer.accept(Functions.java:704)
                                                                               at io.reactivex.internal.functions.Functions$OnErrorMissingConsumer.accept(Functions.java:701)
                                                                               at io.reactivex.internal.observers.LambdaObserver.onError(LambdaObserver.java:74)
                                                                               at io.reactivex.internal.operators.observable.ObservableSubscribeOn$SubscribeOnObserver.onError(ObservableSubscribeOn.java:63)
                                                                               at io.reactivex.internal.disposables.EmptyDisposable.error(EmptyDisposable.java:63)
                                                                               at io.reactivex.internal.operators.observable.ObservableDefer.subscribeActual(ObservableDefer.java:35)
                                                                               at io.reactivex.Observable.subscribe(Observable.java:10842)
                                                                               at io.reactivex.internal.operators.observable.ObservableSubscribeOn$SubscribeTask.run(ObservableSubscribeOn.java:96)
                                                                               at io.reactivex.internal.schedulers.ScheduledDirectTask.call(ScheduledDirectTask.java:38)
                                                                               at io.reactivex.internal.schedulers.ScheduledDirectTask.call(ScheduledDirectTask.java:26)
                                                                               at java.util.concurrent.FutureTask.run(FutureTask.java:237)
                                                                               at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:154)
                                                                               at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:269)
                                                                               at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
                                                                               at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
                                                                               at java.lang.Thread.run(Thread.java:818)
                                                                            Caused by: java.lang.NullPointerException: null ObservableSource supplied
                                                                               at io.reactivex.internal.functions.ObjectHelper.requireNonNull(ObjectHelper.java:39)
                                                                               at io.reactivex.internal.operators.observable.ObservableDefer.subscribeActual(ObservableDefer.java:32)
                                                                               at io.reactivex.Observable.subscribe(Observable.java:10842) 
                                                                               at io.reactivex.internal.operators.observable.ObservableSubscribeOn$SubscribeTask.run(ObservableSubscribeOn.java:96) 
                                                                               at io.reactivex.internal.schedulers.ScheduledDirectTask.call(ScheduledDirectTask.java:38) 
                                                                               at io.reactivex.internal.schedulers.ScheduledDirectTask.call(ScheduledDirectTask.java:26) 
                                                                               at java.util.concurrent.FutureTask.run(FutureTask.java:237) 
                                                                               at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:154) 
                                                                               at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:269) 
                                                                               at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) 
                                                                               at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) 
                                                                               at java.lang.Thread.run(Thread.java:818) 

Solution

Observable.defer(new Callable<ObservableSource<?>>() {
            @Override
            public ObservableSource<?> call() throws Exception {
                Set<Map.Entry<String, Participant>> entries = pool.entrySet();
                for (Map.Entry<String, Participant> entry : entries) {
                   Participant participant = entry.getValue();
                   participant.release();
                }
                return Completable.complete().toObservable();
            }
        }).doOnComplete(new Action() {
            @Override
            public void run() throws Exception {
                Log.d("Complete", "Complete");
            }
        })
            .subscribeOn(Schedulers.computation())
            .observeOn(AndroidSchedulers.mainThread()).subscribe();

This code will work too. Calling subscribe() method will start the job.

Answered By - Glenn