Null handling in rx-java2 flatMap -

as explained in docs rxjava 2.x no longer accepts null values. not surprising both of following 2 lines terminate onerror called:

observable.fromcallable(() -> null); observable.just(1).flatmap(i -> observable.error(new runtimeexception())); 

what unclear why

observable.just(1).flatmap(i -> observable.fromcallable(() -> null)) 

terminates success , no items emitted. seams reasonable expect behave in same fashion 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 happening in terms of code, still have 2 questions:

1) intended behavior or bug?

2) if intended, there reason this?

this bug observable.fromcallable , fixed pr 5517.

if, reason can't avoid null return in setup, can apply hide() workaround bug:

observable.just(1).flatmap(i -> observable.fromcallable(() -> null).hide()) 

or rxjava throw:

observable.just(1)     .flatmap(i -> observable.fromcallable(() ->           java.util.objects.requirenonnull(apireturningnull()))     )