UPDATE:

Car.java
- Fixed Problem with toString() method. Brand was missing.

CashDesk.java
- Added toString() method

ParkingLot.java
- Added setters for cashDesk, receiptHistory, parkingSpots and registeredUsers attributes

ParkingSpot.java
- Added Car toString() method to this toString() method.

XMLDataStorage.java
- loadParkingLotInfo() method now returns a ParkingLot.
- loadCashDeskInfo() method now returns a CashDesk.
- loadReceiptHistory() method now returns a List<Receipt>.
- loadParkingSpots() method now returns a List<ParkingSpot>.
- Added loadCar() method. Returns a Car.