How to run the forty-five minutes

class ParkingService {
  private final ParkingLot lot;
  private final SpotChooser chooser;
  private final TicketBook tickets;
  private final Pricing pricing;
  private final Clock clock;

  Ticket park(Vehicle vehicle) {
    ParkingSpot spot = chooser.choose(vehicle)
        .orElseThrow(LotFullException::new);
    spot.occupy(vehicle);
    return tickets.issue(vehicle, spot, clock.instant());
  }

  Receipt exit(TicketId ticketId) {
    Ticket ticket = tickets.require(ticketId);
    Money fee = pricing.fee(ticket, clock.instant());
    ticket.spot().vacate();
    tickets.close(ticket);
    return new Receipt(ticket.id(), fee);
  }
}

class LotFullException extends RuntimeException {
  LotFullException(VehicleType type) {
    super("no free spot for " + type);
  }
}

class UnknownTicketException extends RuntimeException {
  UnknownTicketException(TicketId id) {
    super("unknown ticket " + id.value());
  }
}

Optional<ParkingSpot> choose(Vehicle vehicle); // empty means full

Ticket require(TicketId id) {
  Ticket ticket = byId.get(id);
  if (ticket == null || ticket.closed()) {
    throw new UnknownTicketException(id);
  }
  return ticket;
}