Application: Parking Lot Encapsulation

Domain: Parking Lot System
A parking lot must enforce capacity limits, prevent double-booking of spots, and manage vehicle assignments. Without encapsulation, these invariants can be violated.
ParkingLot
- floors: List<Floor>
- capacity: int
- occupiedCount: int
+ parkVehicle(vehicle): Ticket
+ unparkVehicle(ticket): Vehicle
+ isFull(): boolean
◆
ParkingSpot
- spotId: String
- isOccupied: boolean
- assignedVehicle: Vehicle
+ assignVehicle(vehicle): boolean
+ releaseVehicle(): Vehicle
+ isAvailable(): boolean
Encapsulation Decisions:
First, Protected Invariant: ParkingSpot.isOccupied must always match whether assignedVehicle is null. Making both private and providing assignVehicle() ensures they stay synchronized.
Second, Capacity Control: ParkingLot.occupiedCount is private. Only parkVehicle() and unparkVehicle() can modify it, preventing manual manipulation that could allow overbooking.
Third, Spot Assignment Logic: assignVehicle() checks availability before assignment. External code cannot bypass this check by directly setting assignedVehicle.
Critical Method: assignVehicle()
public assignVehicle(vehicle: Vehicle): boolean {
  // Check invariant before modification
  if (this.isOccupied) {
    return false  // Spot already taken
  }
  // Atomically update both fields
  this.assignedVehicle = vehicle
  this.isOccupied = true
  return true
}
Without Encapsulation
Public isOccupied field can be set to false while assignedVehicle still holds a vehicle, creating inconsistent state and double-booking.
→
With Encapsulation
Private fields with assignVehicle() method ensures both fields are always synchronized. Impossible to create invalid state.
Interview Tip: For domain objects, identify what can go wrong (double-booking, exceeding capacity) and explain how private fields + validation methods prevent these scenarios.

💡 Key Takeaways

✓Private fields prevent inconsistent state between related attributes

✓Public methods enforce domain rules like capacity limits

✓Encapsulation enables atomic updates to multiple related fields

✓Spot availability is computed logic, not directly settable state

✓Changes to spot assignment algorithm don't affect client code

📌 Examples

1Parking spot preventing double-booking through private assignment

2Library BookCopy preventing checkout if already borrowed

3Elevator preventing floor requests beyond building range

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