Parking Lot-LLD

[VishnuBhaarath]

In parkingLot.java
public Optional parkVehicle(Vehicle vehicle) {
Optional availableSpot = parkingStrategy.findSpot(floors, vehicle);

    if (availableSpot.isPresent()) {
        ParkingSpot spot = availableSpot.get();
        spot.parkVehicle(vehicle);
        ParkingTicket ticket = new ParkingTicket(vehicle, spot);
        activeTickets.put(vehicle.getLicenseNumber(), ticket);
        System.out.printf("%s parked at %s. Ticket: %s\n", vehicle.getLicenseNumber(), spot.getSpotId(), ticket.getTicketId());
        return Optional.of(ticket);
    }

    System.out.println("No available spot for " + vehicle.getLicenseNumber());
    return Optional.empty();
}

Won't the above code block provides race condition?
Timeline with TWO threads Thread A → findSpot() → Spot S1 (free)
Thread B → findSpot() → Spot S1 (still free)
Thread A → spot.parkVehicle() → occupies S1
Thread B → spot.parkVehicle() → overwrites

[avnis838]

Yes i agree too,but i think it needs to be handled

[divs711]

Makes sense, I think the solution would be to make parkVehicle() method synchronized.

Because if you make findSpot() synchronized, then two threads can still compete for the same spot.

Another way of handling this could be to have a time-based hold variable in each parking spot, and if they are accessed, then they go to hold for a min or so. Meaning they won't be included in the findSpot() result. This is what movie booking apps do.