When do you use map vs flatMap in RxJava?

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When do you use map vs flatMap in RxJava?

Say, for example, we want to map Files containing JSON into Strings that contain the JSON–

Using map, we have to deal with the Exception somehow. But how?:

Observable.from(jsonFile).map(new Func1<File, String>() {
    @Override public String call(File file) {
        try {
            return new Gson().toJson(new FileReader(file), Object.class);
        } catch (FileNotFoundException e) {
            // So Exception. What to do ?
        }
        return null; // Not good :(
    }
});

Using flatMap, it’s much more verbose, but we can forward the problem down the chain of Observables and handle the error if we choose somewhere else and even retry:

Observable.from(jsonFile).flatMap(new Func1<File, Observable<String>>() {
    @Override public Observable<String> call(final File file) {
        return Observable.create(new Observable.OnSubscribe<String>() {
            @Override public void call(Subscriber<? super String> subscriber) {
                try {
                    String json = new Gson().toJson(new FileReader(file), Object.class);

                    subscriber.onNext(json);
                    subscriber.onCompleted();
                } catch (FileNotFoundException e) {
                    subscriber.onError(e);
                }
            }
        });
    }
});

I like the simplicity of the map, but the error handling of flatmap (not the verbosity). I haven’t seen any best practices on this floating around and I’m curious how this is being used in practice.

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