Design A Parking Lot - Object-Oriented Design

Requirements

FR:

The system should assign a parking spot to an incoming vehicle
The system should add the vehicle to the WL if the driver agrees, else let them leave.
Calculate fare for the outgoing vehicle

NFR:

Easily support the addition of floors
Easily support the addition of new vehicle sizes
Classes should be loosely coupled

Define Core Objects

enum size {Compact, Large}
class Vehicle {
  string regd
  Size size
  Spot spot
  DateTime allotedAt
}
class SpotStatus {
  Size size
  int filled
  int capacity
  Object lock
}
class Lot {
  private Map<Size, SpotStatus> spots
  private Deque<Vehicle> waitingList;
  private FareCalculator fareCalculator;
  private int WLLimit;
  void setFareCalculator(FareCalculator fareCalculator) {
    this.fareCalculator= fareCalculator;
  }
  private boolean assignSpot(Vehicle vehicle) {
    SpotStatus spotStatus = getSpotStatus(vehicle.size)
    synchronized(spotStatus.lock) {
      if (spotStatus.capacity - spotStatus.filled > 0) {
        vehicle.allottedAt = java.time.LocalDateTime.now()
        spotStatus.filled++
        return true;
      }
      else return false;
    }
  }
  public void assignSpotForArrivingVehicle(Vehicle vehicle) {
    if (!assignSpot(vehicle)) {
      if (waitingList.size() > WLLimit) waitingList.offer(vehicle);
      else throw new WaitingListFullException();
    }
  }
  // Below function runs in intervals like cron
  public boolean assignSpotForWLVehice() {
    Vehicle vehicle = q.peek();
    if (assignSpot(vehicle)) waitingList.poll();
  }
  private SpotStatus getSpotStatus (Size size) {
    spots.get(size);
  }
  public int calculateFare(Vehicle vehicle) {
    return this.fareCalculator.calculateFare(java.time.LocalDateTime.now() - vehicle.allotedAt);
  }
}
interface FareCalculator {
  public int calculateFare()
}
class CompactFareCalculator implements FareCalculator {
  public int calculateFare(int minutes) {
      return 100 * minutes;
  }
}
class LargeFareCalculator implements FareCalculator {
public int calculateFare(int minutes) {
return 200 * minutes;
}
}

Analyze Relationships

Determine how these objects will interact with each other to fulfill the use cases...

Establish Hierarchy

Design inheritance trees where applicable to promote code reuse and polymorphism. This step involves identifying common attributes and behaviors that can be abstracted into parent classes...

Design Patterns

Consider using design patterns (e.g., Factory, Singleton, Observer, Strategy) that fit the problem...

Define Class Members (write code)

Attributes: For each class, define the attributes (data) it will hold...

Methods: Define the methods (functions) that operate on the attributes. Ensure they align with the object's responsibilities and adhere to the principle of encapsulation.

public class Car { ... } // Example

Adhere to SOLID Guidelines

Check and explain whether your design adheres to solid principles (Ask interviewer what SOLID principle is if you can not recall it.)...

Consider Scalability and Flexibility

Explain how your design can handle changes in scale and whether it would be easily to extend with new functionalities...

Create/Explain your diagram(s)

Try creating a class, flow, state and/or sequence diagram using the diagramming tool. Mermaid flow diagrams can be used to represent system use cases. You can ask the interviewer bot to create a starter diagram if unfamiliar with the tool. Briefly explain your diagrams if necessary...

Future improvements

Critically examine your design for any flaws or areas for future improvement...