Rxjava: The mapper function returned a null value.

[AntonLunegovPerm]

Thanks for this library!

I upgraded RxJava to 2.x in my Android app and now I get an error:
NullPointerException: the mapper function returned a null value. Exception is thrown by ObjectHelper.
It appear inside this part of code (FlowableMap.java):
return t != null ? ObjectHelper.<U>requireNonNull(mapper.apply(t), "The mapper function returned a null value.") : null;
Seems that mapper.apply(t) return null...

I call onNext using enum object

public class SystemBus {

    private PublishProcessor<SystemEvent> mSystemEventPublishSubject;

    SystemBus() {
        mSystemEventPublishSubject = PublishProcessor.create();
    }

    public void onNext(SystemEvent systemEvent) {
        mSystemEventPublishSubject.onNext(systemEvent);  // ---> this line lead to an error
    }

    public PublishProcessor<SystemEvent> getObservable() {
        return mSystemEventPublishSubject;
    }
}

Also SystemEvent class :

public enum SystemEvent {
    FINISH_ACTIVITY,
    PERMISSIONS_REQUEST,
    PERMISSIONS_RESPONSE,
    UPDATE_CARDS,
    PDL,
    MENU_REFRESH
}

Here the stack trace:

Fatal Exception: io.reactivex.exceptions.OnErrorNotImplementedException: The mapper function returned a null value.
       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.subscribers.LambdaSubscriber.onError(LambdaSubscriber.java:79)
       at io.reactivex.internal.subscribers.BasicFuseableSubscriber.onError(BasicFuseableSubscriber.java:101)
       at io.reactivex.internal.subscribers.BasicFuseableConditionalSubscriber.onError(BasicFuseableConditionalSubscriber.java:100)
       at io.reactivex.internal.subscribers.BasicFuseableConditionalSubscriber.fail(BasicFuseableConditionalSubscriber.java:110)
       at io.reactivex.internal.operators.flowable.FlowableMap$MapConditionalSubscriber.tryOnNext(FlowableMap.java:126)
       at io.reactivex.internal.operators.flowable.FlowableFilter$FilterConditionalSubscriber.tryOnNext(FlowableFilter.java:141)
       at io.reactivex.internal.operators.flowable.FlowableFilter$FilterConditionalSubscriber.onNext(FlowableFilter.java:119)
       at io.reactivex.processors.PublishProcessor$PublishSubscription.onNext(PublishProcessor.java:315)
       at io.reactivex.processors.PublishProcessor.onNext(PublishProcessor.java:197)
       at ai.#####.SystemBus.onNext(SystemBus.java:20)
       at ai.#####.App$2.onReceive(App.java:151)
       at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:922)
       at android.os.Handler.handleCallback(Handler.java:815)
       at android.os.Handler.dispatchMessage(Handler.java:104)
       at android.os.Looper.loop(Looper.java:207)
       at android.app.ActivityThread.main(ActivityThread.java:5728)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)
Caused by java.lang.NullPointerException: The mapper function returned a null value.
       at io.reactivex.internal.functions.ObjectHelper.requireNonNull(ObjectHelper.java:39)
       at io.reactivex.internal.operators.flowable.FlowableMap$MapConditionalSubscriber.tryOnNext(FlowableMap.java:124)
       at io.reactivex.internal.operators.flowable.FlowableFilter$FilterConditionalSubscriber.tryOnNext(FlowableFilter.java:141)
       at io.reactivex.internal.operators.flowable.FlowableFilter$FilterConditionalSubscriber.onNext(FlowableFilter.java:119)
       at io.reactivex.processors.PublishProcessor$PublishSubscription.onNext(PublishProcessor.java:315)
       at io.reactivex.processors.PublishProcessor.onNext(PublishProcessor.java:197)
       at ai.#####.SystemBus.onNext(SystemBus.java:20)
       at ai.#####.App$2.onReceive(App.java:151)
       at android.app.LoadedApk$ReceiverDispatcher$Args.run(LoadedApk.java:922)
       at android.os.Handler.handleCallback(Handler.java:815)
       at android.os.Handler.dispatchMessage(Handler.java:104)
       at android.os.Looper.loop(Looper.java:207)
       at android.app.ActivityThread.main(ActivityThread.java:5728)
       at java.lang.reflect.Method.invoke(Method.java)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)

[akarnokd]

Nulls are not allowed in 2.x. Check your mapper function why it returns null.

[AntonLunegovPerm]

Thank you! I'm find the error.

[ezamelczyk]

Maybe add better logging to this? I have multiple streams running in background and can't pinpoint where is this mapper function returning null value

[akarnokd]

You could use the function tagging support in the extensions project, or simply define your lambdas in a way that throw on null return:

source.map(v -> {
   // some computation
   return Objects.requireNonNull(result);
});