RxExample.java

public void favoriteClicked(final long id) {
        handleFavorites(id)
                .subscribeOn(Schedulers.computation())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Subscriber<Boolean>() {
                    @Override
                    public void onCompleted() {
                    }

                    @Override
                    public void onError(Throwable e) {
                    }

                    @Override
                    public void onNext(Boolean favoriteWasAdded) {
                        if (favoriteWasAdded) {
                            // respond to the UI appropriately
                        } else {
                            // respond to the UI appropriately
                        }
                    }
                });
    }

    private rx.Observable<Boolean> handleFavorites(final long id) {
        return rx.Observable.create(new rx.Observable.OnSubscribe<Boolean>() {
            @Override
            public void call(Subscriber<? super Boolean> subscriber) {
                // check if you will need to delete or add the favorite into DB
                Favorite favorite = findFavoriteById(id);
                
                if (favorite == null) {
                    addFavorite(id);
                    subscriber.onNext(true);
                } else {
                    deleteFavorite(id);
                    subscriber.onNext(false);
                }
                
                // complete the subscriber
                subscriber.onCompleted();
            }
        });
    }