How to properly handle onError inside RxJava (Android)?

Question

I'm getting a list of installed apps on the device. It's a costly operation, so I'm using Rx for that:

    Observable<List> observable = Observable.create(subscriber -> {         List result = getUserApps();          subscriber.onNext(result);         subscriber.onError(new Throwable());         subscriber.onCompleted();     });      observable             .map(s -> {                 ArrayList<String> list = new ArrayList<>();                 ArrayList<Application> applist = new ArrayList<>();                 for (Application p : (ArrayList<Application>) s) {                     list.add(p.getAppName());                     applist.add(p);                 }                 return applist;             })             .subscribeOn(Schedulers.newThread())             .observeOn(AndroidSchedulers.mainThread())             .doOnError(throwable -> L.e(TAG, "Throwable " + throwable.getMessage()))             .subscribe(s -> createListView(s, view)); 

However, my problem is with handling errors. Normally, user launches this screen, waits for apps to load, selects what is best and goes to next page. However, when user quickly changes the UI - app crashes with NullPointer.

Okay, so I implemented this onError. However it still doesn't work, and with above usecase it throws me this:

    04-15 18:12:42.530  22388-22388/pl.digitalvirgo.safemob E/AndroidRuntime﹕ FATAL EXCEPTION: main         java.lang.IllegalStateException: Exception thrown on Scheduler.Worker thread. Add `onError` handling.                 at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:52)                 at android.os.Handler.handleCallback(Handler.java:730)                 at android.os.Handler.dispatchMessage(Handler.java:92)                 at android.os.Looper.loop(Looper.java:176)                 at android.app.ActivityThread.main(ActivityThread.java:5419)                 at java.lang.reflect.Method.invokeNative(Native Method)                 at java.lang.reflect.Method.invoke(Method.java:525)                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)                 at dalvik.system.NativeStart.main(Native Method)          Caused by: rx.exceptions.OnErrorNotImplementedException                 at rx.Observable$31.onError(Observable.java:7134)                 at rx.observers.SafeSubscriber._onError(SafeSubscriber.java:154)                 at rx.observers.SafeSubscriber.onError(SafeSubscriber.java:111)                 at rx.internal.operators.OperatorDoOnEach$1.onError(OperatorDoOnEach.java:70)                 at rx.internal.operators.NotificationLite.accept(NotificationLite.java:147)                 at rx.internal.operators.OperatorObserveOn$ObserveOnSubscriber.pollQueue(OperatorObserveOn.java:177)                 at rx.internal.operators.OperatorObserveOn$ObserveOnSubscriber.access$000(OperatorObserveOn.java:65)                 at rx.internal.operators.OperatorObserveOn$ObserveOnSubscriber$2.call(OperatorObserveOn.java:153)                 at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:47)                 at android.os.Handler.handleCallback(Handler.java:730)                 at android.os.Handler.dispatchMessage(Handler.java:92)                 at android.os.Looper.loop(Looper.java:176)                 at android.app.ActivityThread.main(ActivityThread.java:5419)                 at java.lang.reflect.Method.invokeNative(Native Method)                 at java.lang.reflect.Method.invoke(Method.java:525)                 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1046)                 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:862)                 at dalvik.system.NativeStart.main(Native Method)          Caused by: java.lang.Throwable                 at pl.digitalvirgo.safemob.fragments.wizard.ApplicationsFragment.lambda$getAppList$25(ApplicationsFragment.java:267)                 at pl.digitalvirgo.safemob.fragments.wizard.ApplicationsFragment.access$lambda$2(ApplicationsFragment.java)                 at pl.digitalvirgo.safemob.fragments.wizard.ApplicationsFragment$$Lambda$3.call(Unknown Source)                 at rx.Observable$1.call(Observable.java:145)                 at rx.Observable$1.call(Observable.java:137)                 at rx.Observable.unsafeSubscribe(Observable.java:7304)                 at rx.internal.operators.OperatorSubscribeOn$1$1.call(OperatorSubscribeOn.java:62)                 at rx.internal.schedulers.ScheduledAction.run(ScheduledAction.java:47)                 at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:390)                 at java.util.concurrent.FutureTask.run(FutureTask.java:234)                 at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:153)                 at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:267)                 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080)                 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573)                 at java.lang.Thread.run(Thread.java:841) 

How should I properly handle this problem?

Answer 1

.doOnError() is an operator, and is not as such a part of the Subscriber.

Therefore, having a .doOnError() does not count as an implemented onError().

About the question in one of the comments, of course it is possible to use lambdas.

In this case simply replace

.doOnError(throwable -> L.e(TAG, "Throwable " + throwable.getMessage())) .subscribe(s -> createListView(s, view)) 

with

.subscribe(s -> createListView(s, view),     throwable -> L.e(TAG, "Throwable " + throwable.getMessage()))