RxJava 2でNoSuchElementExceptionに遭う件

NoSuchElementException

RxJava 2では、CompletableをtoMaybe()、flatMapSingle()と繋ぐと、NoSuchElementExceptionに遭遇します。
CompletableからSingleへ繋げたいパターンです。

サンプル

private Completable hoge() {
    return Completable.complete();
}

private Single<String> fuga() {
    return Single
            .fromCallable(new Callable<String>() {
                @Override
                public String call() throws Exception {
                    return "fuga";
                }
            });
}

hoge()
        .toMaybe()
        .flatMapSingle(new Function<Object, SingleSource<String>>() {
            @Override
            public SingleSource<String> apply(Object o) throws Exception {
                return fuga();
            }
        })
        .subscribe(new SingleObserver<String>() {
            @Override
            public void onSubscribe(Disposable d) {

            }

            @Override
            public void onSuccess(String s) {
                Log.d("onSuccess", String.valueOf(s));
            }

            @Override
            public void onError(Throwable e) {
                Log.d("onError", e.toString());
            }
        });

出力結果

onError: java.util.NoSuchElementException

回避方法

CompletableをMaybeに変換せずにSingleに繋ぎます。

hoge()
        .toSingleDefault(new Object())
        .flatMap(new Function<Object, SingleSource<String>>() {
            @Override
            public SingleSource<String> apply(Object o) throws Exception {
                return fuga();
            }
        })
        .subscribe(new SingleObserver<String>() {
            @Override
            public void onSubscribe(Disposable d) {

            }

            @Override
            public void onSuccess(String s) {
                Log.d("onSuccess", String.valueOf(s));
            }

            @Override
            public void onError(Throwable e) {
                Log.d("onError", e.toString());
            }
        });

出力結果

onSuccess: fuga