Null handling in rx-java2 flatMap

Question

As explained in the docs RxJava 2.x no longer accepts null values. So it is not surprising that both of the following two lines terminate with onError called:

Observable.fromCallable(() -> null);
Observable.just(1).flatMap(i -> Observable.error(new RuntimeException()));

what is unclear is why

Observable.just(1).flatMap(i -> Observable.fromCallable(() -> null))

terminates with success and no items emitted. It seams reasonable to expect for it behave in the same fashion as Observable.error

I can see in source code of rx-java 2.1.2

 public final <R> Observable<R> flatMap(...) {
    if (this instanceof ScalarCallable) {
        @SuppressWarnings("unchecked")
        T v = ((ScalarCallable<T>)this).call();
        if (v == null) {
            return empty();
        }
        ...
 }

which explains why it is happening in terms of code, but I still have two questions:

1) Is this is an intended behavior or a bug?

2) If intended, is there a reason for this?

Answer 1

This is a bug with the Observable.fromCallable and will be fixed with PR 5517.

If, for some reason you can't avoid a null return in this setup, you can apply hide() to workaround this bug:

Observable.just(1).flatMap(i -> Observable.fromCallable(() -> null).hide())

or help RxJava throw:

Observable.just(1)
    .flatMap(i -> Observable.fromCallable(() -> 
         java.util.Objects.requireNonNull(apiReturningNull()))
    )