RxJava - Using flatmap when return could be null

Question

Let's use this class for the example:

 public static class CurrentUser{
    public static Observable<User> get(){
         //code basically returns the currently logged in User object
         //but sometimes there may not be a logged in user 
    }

    public static Observable<PutResult> logOut(){
        return get()

                //I only want to execute the following if user != null

                .flatMap(new Func1<User, Observable<PutResult>>() {
                    @Override
                    public Observable<PutResult> call(User user) {
                        //delete the session token and save
                        user.removeSessionToken();
                        return DatabaseModule.getStorIOSQLite()
                                .put()
                                .object(user)
                                .prepare()
                                .asRxObservable();
                    }
                });
    }

}

Should I just return null inside the flatmap? Are there any repercussions to this since it's expecting an Observable?

Answer 1

You should not return null but Observable.empty() instead.

.flatMap(new Func1<User, Observable<PutResult>>() {
    @Override
    public Observable<PutResult> call(User user) {
        if (user == null) return Observable.empty();

        //delete the session token and save
        user.removeSessionToken();
        return DatabaseModule.getStorIOSQLite()
                .put()
                .object(user)
                .prepare()
                .asRxObservable();
    }
});

Answer 2

It isn't clear what null you're talking about.

If you're asking whether the Observable<PutResult> can emit a PutResult that's null, then yes. This would result in a null being emitted by the outer observable.

If you're asking whether the Observable<PutResult> returned by the Func1 can be null, then no. Return Observable.empty() instead, (or Observable.just(null) or similar if you need to keep track of the number of emitted items).