RxJava emit error on empty

Question

I want to "throw" custom error if an Observable does not emit exactly one value before completing.

Let me try to show an example:

Observable<SomeClass> stream = ...

stream
.filter(...)
.singleOrError(new MyCustomException())

So I have a stream of SomeClass objects. I want to emit custom error if fitler() does not emit exactly one element.

Answer 1

Since .singleOrError() throws NoSuchElementException if the source emits no items, you can check instance of thrown exception and return your custom one.

    stream.filter(...)
            .singleOrError()
            .onErrorResumeNext(throwable -> {
                if (throwable instanceof NoSuchElementException) {
                    return Single.error(new MyCustomException());
                } else {
                    return Single.error(throwable);
                }
            });

Note that if the filter() emits more than one item, singleOrError() will throw IllegalArgumentException. This can either be handled in the onErrorResumeNext(), or by simply adding take(1) before singleOrError().