class Flight {

}

class Observer {
    private String eMail;
    private Flight flight;

    Observer(String eMail, Flight flight) {
        this.eMail = eMail;
        this.flight = flight;
    }

    public void subscribe() {
        System.out.println("Subscribe this flight on this eMail");
    }

}

class Observable {

    private ArrayList<Observer> observers = new ArrayList<>();

    public void addObserver(Observer o) {
        observers.add(o);
    }

    public void removeObserverByFlight(Flight f) {
        observers.remove(f);
    }

    public void sendMessageByFlight(Flight flight) {
        System.out.println("Your flight is available");
    }
}

class ClientController {

    Observable observable;

    public void orderFlight(Flight flight, String email) {

        try {
            //flight is reserved
        } catch (Exception noPlaces) {
            Observer observer = new Observer(email, flight);
            observable.addObserver(observer);
            observer.subscribe();
        }
    }

}

class FlightRepository implements Repository<Flight> {

    Observable observable = new Observable();


    @Override
    public void add(Flight flight) {
        //add flight
        observable.sendMessageByFlight(flight);
        observable.removeObserverByFlight(flight);
    }

    @Override
    public void remove(Flight flight) {

    }
}