flatMap in RxJava basic introduction for Beginners

Observable.flatMap() takes the emissions of one Observable and returns the emissions of another Observable to take its place.

Here’s how it works:

query("Hello, world!")
    .flatMap(new Func1<List<String>, Observable<String>>() {
        @Override
        public Observable<String> call(List<String> urls) {
            return Observable.from(urls);
        }
    })
    .subscribe(url -> System.out.println(url));

I’m showing the full function just so you can see exactly what happened, but simplified with lambdas it looks awesome:

query("Hello, world!")
    .flatMap(urls -> Observable.from(urls))
    .subscribe(url -> System.out.println(url));

Why is it returning another Observable? The key concept here is that the new Observable returned is what the Subscriber sees. It doesn’t receive a List<String> – it gets a series of individual Strings as returned by Observable.from().