Combine two Observables with the second using results from the first one in parameters

Question

I have two methods that return an Observable< Response< ResponseBody > >

firstAPI.getFirstInfo("1", "2");

secondApi.getSecondInfo(resultFirstObservable, "3");

I'm trying to get an information from the first one and use it as a parameter in the second one.

I started by trying to use a flatMap:

Observable<Integer> mergedObservers = firstAPI.getFirstInfo("1","2")
    .flatMap((Response<ResponseBody> resultFirstObservable) -> {
        try {
            return secondApi.getSecondInfo(transformToTheFormatNeeded(resultFirstObservable.body().string()), "3");
        } catch (IOException e) {
            e.printStackTrace();
            return secondApi.getSecondInfo("defaultValue", "3");
        }
    }, ((Response<ResponseBody> resultFirstObservable), (Response<ResponseBody> resultSecondObservable)) -> {
        try {
            return transformToWhatINeed(resultSecondObservable.body().string());
        } catch (IOException e) {
            e.printStackTrace();
        }
    });

I would try then to subscribeOn this new Observable and onNext I would send the returned value to my Activity.

Subscription sub = mergedObservers.subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread())
            .repeatWhen(completed -> completed.delay(30, TimeUnit.SECONDS))
            .subscribe(new Observer<Integer>() {
                @Override
                public void onCompleted() {

                }

                @Override
                public void onError(Throwable e) {

                }

                @Override
                public void onNext(Integer integer) {
                    view.updateInfo(integer);
                }
            });

My problem is that the flatMap throws 2 errors at compilation:

error: lambda body is neither value nor void compatible
error: no suitable method found for flatMap((__)->{ tr[...]; } },(.. ,..)->{ [...]; } })

What am I doing wrong ?

Suggested reading: github.com/ReactiveX/RxJava#dependent-sub-flows — akarnokd
when you have compile errors with lambdas, your types are wrong, please "expand lambdas to anonymous class" to get the full pictures, update the code so that everyone can help — ericn

akarnokd akarnokd · Accepted Answer · 2018-08-17T08:22:46

I don't think the API call actually throws an IOException while also returning an Observable. In addition, you have to return something from the second lambda but the try-catch there doesn't do that, causing the error. Try this:

Observable<Integer> mergedObservers = firstAPI.getFirstInfo("1","2")
.flatMap(resultFirstObservable -> {
    return secondApi.getSecondInfo(resultFirstObservable, "3")
        .onErrorResumeNext(e -> {
             e.printStackTrace();
             return secondApi.getSecondInfo("defaultValue", "3");
        });
}, (resultFirstObservable, resultSecondObservable) -> {
    try {
        return transformToWhatINeed(resultSecondObservable.body().string());
    } catch (IOException ex) {
        ex.printStackTrace();
        return "";
    }
});

Combine two Observables with the second using results from the first one in parameters

1 Answers