ConcatMapIssue.java

public class Main {

    public static void main(String[] args) {

        final Emitter<String>[] emitter = new Emitter[]{null};

        Observable<Integer> integerObservable = Observable.just(0, 1, 2, 3, 4)
                .concatMap(integer -> {
                    if (emitter[0] != null) {
                        emitter[0].onCompleted();
                    }
                    return Observable.just(integer);
                });

        Observable<String> result = integerObservable.concatMap(integer ->
                Observable.fromEmitter(
                        (Action1<Emitter<String>>) stringEmitter -> {
                            emitter[0] = stringEmitter;
                            stringEmitter.onNext("Result: " + integer);
                        }, Emitter.BackpressureMode.BUFFER)
        );

        result.subscribe(s -> System.out.println(s));

    }
}