Android rxJava - Null object wrapped in Observable

Question

I have a use case that is as simple as this: Look for a Book in the Remote source, if it doesn't exist, create it. So this is my approach that used to work with rxJava1:

public void handleBook(int id) {
    getBookById(id) 
        .flatMap(new Func1<Book, Observable<Book>> {
            @Override
            public Observable<Book> call(Book book) {
                if(book != null) // it exists
                ...
                else // it's null - it doesn't exist ...
            }
        }
}


public Observable<Book> getBookById(int id) {
    Book book = remoteSource.book(id)
    return Observable.just(book)
}

In this case if the object book is null, the Observable.just call throws an exception. It's explicitly checking against null values. Back for rxJava 1 I could pass a null as value to Observable.just (or other accesoars), then in the map or flatMap operators I'd check if the value I get is null (meaning result does not exist) or not (meaning I got some result).

With this limitation it seems that I cannot do this check anymore. I tried returning Observable.empty() in case the book object is null but then the whole thread would complete and finish when the returned value is Observable.empty().

How can I check in a rx-chain of execution if something I need is there and branch the chain of execution afterwards?

Either use Single and check for errors or wrap the book object inside an Optional — Pelocho

Pelocho Pelocho · Accepted Answer · 2017-07-10T12:29:48

Instead of using Observable<Book> as a return type use Single<Book>. Single type emits either an object or an error

NOTE: Right now I don't have an IDE so the code will probably have some compiler fails. I assume you'll fix it easily

public Single<Book> getBookById(int id) {
    return Single.create((e) => {
        Book book = remoteSource.book(id)
        if (book != null)
            e.emit(book);
        else
            e.fail();
    }
}

public void handleBook(int id) {
    getBookById(id) 
        .onErrorResume(e => createBook(...))
        ...
}

Like a said earlier I'm not sure about the exact code but you can read it as if it was pseudo-code

Android rxJava - Null object wrapped in Observable

1 Answers