Design a parking lot

Mastering the art of designing scalable and efficient systems is paramount in object-oriented design (OOD) interviews. One common scenario presented to software engineers during these interviews is designing a parking lot system. This seemingly straightforward problem encompasses various design principles and considerations, making it an excellent litmus test for a candidate’s OOD skills and a staple among software engineer system design interview questions.

This blog delves into the intricacies of designing a parking lot system using object-oriented principles, patterns, and best practices. We’ll leverage a comprehensive implementation that covers various parking lot system components, including vehicle management, parking spot allocation, ticketing, payment processing, etc. Additionally, we’ll explore an OOD interview guide, providing insights and guidelines for effectively navigating low-level design interviews.

Here's a detailed guide on System Design.

Problem definition#

A parking lot system is an essential infrastructure in various public venues like shopping malls, stadiums, and offices, providing designated vehicle spaces. These lots contain a fixed number of parking spots tailored for different vehicle types, each charged based on the parking duration. Upon entry, vehicles receive a parking ticket to track their stay. When exiting, drivers can settle their fees at automated exit panels, utilizing payment methods such as cards or cash. This system ensures efficient vehicle management and revenue collection within parking facilities.

Requirements collection#

Let’s define the requirements for the parking lot problem:

R1: Four vehicles should be allowed to park in the parking lot: Car, Truck, Van, and Motorcycle.
R2: The parking lot should have three spots: CompactSpot, LargeSpot, and MotorcycleSpot.
R3: The parking lot should have a display board that shows its status, such as the number of spots, free spots, and occupied spots.
R4: If the parking lot is completely occupied, the system should display a message on the entrance and the parking lot display board.
R5: The system should generate a receipt at the exit.
R6: Customers should be able to collect a parking ticket from the entrance and pay at the exit.
R7: Payment should be calculated hourly and based on vehicle type.
R8: Payment can be made using either a credit/debit card or cash.

Class diagram#

In the class diagram for the parking lot system, we’ll identify and design classes, abstract classes, and interfaces based on the requirements.

Components of a parking lot system#

As we take a bottom-up approach to designing the parking lot system, we’ll begin by identifying and designing the classes for smaller components like vehicles and parking spots. Subsequently, we’ll integrate these smaller components into the class representing the parking lot system.

The Vehicle class#

The Vehicle class serves as the base class for a generic vehicle, with attributes for licensePlate and type, along with corresponding getters and setters. Subclasses Motorcycle, Car, Van, and Truck extend the Vehicle class, each initializing the licensePlate attribute and the vehicle type.

Relationship between the classes#

The relationship between the classes in the parking lot system implementation can be summarized as follows:

Association#

The class diagram exhibits the following association relationships:

One-way association#

The VehicleFactory has a one-way association with Vehicle.
The ParkingSpotFactory has a one-way association with ParkingSpot.
The ParkingSpot has a one-way association with Vehicle.
The ParkingSpotManager has a one-way association with Vehicle and ParkingSpot.
The DisplayBoard has a one-way association with ParkingLot.
The EntrancePanel has a one-way association with ParkingSpotManager and ParkingLot.
The ExitPanel has a one-way association with ParkingSpotManager, Receipt, CentralPaymentSystem, and ParkingLot.
The CentralPaymentSystem has a one-way association with PaymentStrategy.
The FairCalculator has a one-way association with ParkingTicket.

Inheritance#

The following classes show an inheritance relationship:

The Car, Truck, Van, and Motorcycle classes inherit from the Vehicle class.
The CompactSpot, LargeSpot, and MotorcycleSpot classes inherit from the ParkingSpot class.
The CashPayment and CardPayment classes implement the PaymentStrategy interface.

Note: We have already discussed the inheritance relationship between classes in the class diagram section above.

Class diagram of the parking lot system#

Here is the complete class diagram for our parking lot system:

Design patterns	Description
Singleton pattern	The `CentralPaymentSystem` class employs the Singleton pattern to ensure that only one instance of the payment system exists throughout the application. This guarantees centralized control over payment transactions, facilitating better management and coordination.
Factory method pattern	The Factory Method pattern is implemented in the `VehicleFactory` and `ParkingSpotFactory` classes to create instances of various vehicle and parking spot subclasses, respectively. By encapsulating the object creation process, these factories allow the client code to create objects without knowing the specific subclass implementations, promoting code flexibility and scalability.
Strategy pattern	The Strategy pattern is employed in the `PaymentStrategy` interface and its concrete implementations (`CardPayment`, `CashPayment`). This pattern encapsulates different payment strategies into separate classes, allowing clients to dynamically choose a strategy at runtime. Decoupling the payment logic from the client code enables easier integration of new payment methods and promotes code reusability and maintainability.
Observer pattern	The Observer pattern is utilized in the `DisplayBoard` class, where it observes changes in the parking lot status and updates the display accordingly. By decoupling the display functionality from the parking lot logic, this pattern ensures that the display is automatically updated whenever there are changes in the parking lot status, such as vehicles entering or exiting. This enhances the system’s modularity and extensibility.
Command pattern	The Command pattern is implemented in the handler classes (`EnterVehicleHandler`, `ExitVehicleHandler`, `DisplayCurrentVehiclesHandler`, `MenuOptionHandler`), where each handler encapsulates a specific request as an object. This allows clients to parameterize objects with requests, facilitating queueing, logging, and support for undoable operations. By separating the request from its execution, this pattern enhances the flexibility and maintainability of the system architecture.

Design Principle	Description
Single responsibility principle (SRP)	The `Vehicle` class in the parking lot system adheres to the SRP by encapsulating a vehicle’s license plate and type information. It focuses solely on representing a generic vehicle without being concerned with parking or payment functionalities. Similarly, the `ParkingTicket` class handles the responsibility of storing ticket information such as ticket number, vehicle, entry time, and exit time without being involved in payment processing or parking spot management.
Open/Closed principle (OCP)	The `ParkingSpotFactory` class exemplifies the OCP by allowing the system to be extended with new types of parking spots without modifying existing code. Using the factory pattern, new subclasses of `ParkingSpot` can be created to represent different types of parking spots (e.g., handicapped spots) without altering the `ParkingLotInitializer` or other parts of the system.
Liskov substitution principle (LSP)	The `Vehicle` hierarchy demonstrates adherence to the LSP by enabling subclasses like `Car`, `Van`, `Truck`, and `Motorcycle` to be seamlessly substituted for their parent class, `Vehicle`. This principle ensures that objects of the `Vehicle` superclass can be replaced with objects of its subclasses without affecting the correctness of the program. For example, any method that accepts a `Vehicle` object can also work correctly with instances of its subclasses, such as a `Car` or `Truck`.
Interface segregation principle (ISP)	The `PaymentStrategy` interface embodies the ISP by defining a single method of `pay(double amount)`, which encapsulates the behavior of different payment strategies. Concrete implementations like `CardPayment` and `CashPayment` must only implement this single method, ensuring that clients are not forced to depend on methods they do not use. This principle promotes the creation of cohesive interfaces that are tailored to specific client requirements.
Dependency inversion principle (DIP)	The `ParkingLot` class adheres to the DIP by depending on abstractions rather than concrete implementations. For instance, it relies on the `ParkingSpotManager` interface for parking spot management operations, allowing different implementations of spot management strategies to be used interchangeably. This promotes loose coupling between classes and facilitates easier testing, maintenance, and extension of the system.

Code for the parking lot system#

We’ve gone over the different aspects of the parking lot system and observed the attributes attached to the problem using the class diagram. Let’s explore the more practical side of things, where we will work on implementing the parking lot system using multiple languages. This is usually the last step in an object-oriented design interview process.

We have selected the Java programming languages to implement the parking lot system.

The Vehicle hierarchy#

The Vehicle hierarchy includes Car, Motorcycle, Van, and Truck, all inherited from a base Vehicle class.

Java

class Vehicle {
    private String licensePlate;
    private String type;
    public Vehicle(String licensePlate, String type) {
        this.licensePlate = licensePlate;
        this.type = type;
    }
    // Getters and setters
    public String getLicensePlate() {
        return licensePlate;
    }
    public void setLicensePlate(String licensePlate) {
        this.licensePlate = licensePlate;
    }
    public String getType() {
        return type;
    }
    public void setType(String type) {
        this.type = type;
    }
}
class Motorcycle extends Vehicle {
    public Motorcycle(String licensePlate) {
        super(licensePlate, "Motorcycle");
    }
}
class Car extends Vehicle {
    public Car(String licensePlate) {
        super(licensePlate, "Car");
    }
}
class Van extends Vehicle {
    public Van(String licensePlate) {
        super(licensePlate, "Van");
    }
}
class Truck extends Vehicle {
    public Truck(String licensePlate) {
        super(licensePlate, "Truck");
    }
}
// VehicleFactory to create instances of Vehicle subclasses
class VehicleFactory {
    public Vehicle createVehicle(String type, String licensePlate) {
        // Convert type to uppercase
        type = type.toUpperCase();
        switch (type) {
            case "VAN":
                return new Van(licensePlate);
            case "TRUCK":
                return new Truck(licensePlate);
            case "CAR":
                return new Car(licensePlate);
            case "MOTORCYCLE":
                return new Motorcycle(licensePlate);
            default:
                throw new IllegalArgumentException("Invalid vehicle type: " + type);
        }
    }
}

Java

class ParkingSpot {
    private Vehicle vehicle;
    private String type; 
    private boolean occupied;
    public ParkingSpot() {
        this.occupied = false;
    }
    // Getters and setters
    public Vehicle getVehicle() {
        return vehicle;
    }
    public void setVehicle(Vehicle vehicle) {
        this.vehicle = vehicle;
    }
    public String getType() {
        return type;
    }
    public void setType(String type) {
        this.type = type;
    }
    public void setOccupied(boolean occupied) {
        this.occupied = occupied;
    }
    public boolean getOccupied() {
        return occupied;
    }
}
class MotorcycleSpot extends ParkingSpot {
    public MotorcycleSpot() {
        super();
    }
}
class CompactSpot extends ParkingSpot {
    public CompactSpot() {
        super();
    }
}
class LargeSpot extends ParkingSpot {
    public LargeSpot() {
        super();
    }
}
// ParkingSpotFactory to create instances of ParkingSpot subclasses
class ParkingSpotFactory {
    public ParkingSpot createParkingSpot(String type) {
        switch (type) {
            case "Compact":
                return new CompactSpot();
            case "Large":
                return new LargeSpot();
            case "Motorcycle":
                return new MotorcycleSpot();
            default:
                throw new IllegalArgumentException("Invalid parking spot type: " + type);
        }
    }
}
// ParkingSpotManager class to manage parking spot operations
class ParkingSpotManager {
    public boolean isOccupied(ParkingSpot spot) {
        return spot.getVehicle() != null;
    }
    // Member function to park a vehicle in a parking spot
    public void parkVehicle(ParkingSpot spot, Vehicle vehicle, String type) {
        spot.setVehicle(vehicle);
        spot.setType(type);
    }
    // Member function to remove a vehicle from a parking spot
    public void removeVehicle(ParkingSpot spot) {
        spot.setVehicle(null);
        spot.setType(""); // Reset type
    }
    // Member function to get a parking spot for a specific vehicle
    public ParkingSpot getSpotForVehicle(ParkingLot parkingLot, Vehicle vehicle) {
        for (ParkingFloor floor : parkingLot.getFloors()) {
            for (ParkingSpot spot : floor.getSpots()) {
                if (isOccupied(spot) && spot.getVehicle().equals(vehicle)) {
                    return spot;
                }
            }
        }
        return null; // Return null if no spot found for the vehicle
    }
    // Member function to free up a parking spot
    public void freeParkingSpot(ParkingSpot spot) {
        spot.setVehicle(null);
        spot.setOccupied(false);
    }
}

Java

// PaymentStrategy interface for different payment methods
interface PaymentStrategy {
    void pay(double amount);
}
class CashPayment implements PaymentStrategy {
    public void pay(double amount) {
        // Implementation for cash payment
    }
}
class CardPayment implements PaymentStrategy {
    public void pay(double amount) {
        // Implementation for card payment
    }
}
// CentralPaymentSystem to handle payments
class CentralPaymentSystem {
    private static CentralPaymentSystem instance;
    private CentralPaymentSystem() {
        // Private constructor to prevent instantiation
    }
    public static CentralPaymentSystem getInstance() {
        if (instance == null) {
            instance = new CentralPaymentSystem();
        }
        return instance;
    }
    // Methods for payment processing
    public void processPayment(PaymentStrategy paymentStrategy, double amount) {
        paymentStrategy.pay(amount);
    }
}

Java

// ParkingFloor class representing a floor in the parking lot
class ParkingFloor {
    private List<ParkingSpot> spots;
    // Constructors
    public ParkingFloor() {
        this.spots = new ArrayList<>();
    }
    // Getters and setters
    public List<ParkingSpot> getSpots() {
        return spots;
    }
    public void setSpots(List<ParkingSpot> spots) {
        this.spots = spots;
    }
    // Member function
    public void addParkingSpot(ParkingSpot spot) {
        spots.add(spot);
    }
    public void removeParkingSpot(ParkingSpot spot) {
        spots.remove(spot);
    }
    public int getNumberOfSpots() {
        return spots.size();
    }
}

Java

// FareCalculator class for calculating fare
class FareCalculator {
    private double carRatePerHour;
    private double vanRatePerHour;
    private double truckRatePerHour;
    private double motorcycleRatePerHour;
    // Default constructor with default values
    public FareCalculator() {
        this.carRatePerHour = 10.0; // Default rate for cars
        this.vanRatePerHour = 15.0; // Default rate for vans
        this.truckRatePerHour = 20.0; // Default rate for trucks
        this.motorcycleRatePerHour = 5.0; // Default rate for motorcycles
    }
    // Constructor with custom rates
    public FareCalculator(double carRatePerHour, double vanRatePerHour, double truckRatePerHour, double motorcycleRatePerHour) {
        this.carRatePerHour = carRatePerHour;
        this.vanRatePerHour = vanRatePerHour;
        this.truckRatePerHour = truckRatePerHour;
        this.motorcycleRatePerHour = motorcycleRatePerHour;
    }
    // Getters and setters
    public double getCarRatePerHour() {
        return carRatePerHour;
    }
    public void setCarRatePerHour(double carRatePerHour) {
        this.carRatePerHour = carRatePerHour;
    }
    public double getVanRatePerHour() {
        return vanRatePerHour;
    }
    public void setVanRatePerHour(double vanRatePerHour) {
        this.vanRatePerHour = vanRatePerHour;
    }
    public double getTruckRatePerHour() {
        return truckRatePerHour;
    }
    public void setTruckRatePerHour(double truckRatePerHour) {
        this.truckRatePerHour = truckRatePerHour;
    }
    public double getMotorcycleRatePerHour() {
        return motorcycleRatePerHour;
    }
    public void setMotorcycleRatePerHour(double motorcycleRatePerHour) {
        this.motorcycleRatePerHour = motorcycleRatePerHour;
    }
    // Member function to calculate fare
    public double calculateFare(ParkingTicket ticket) {
        LocalDateTime entryTime = ticket.getEntryTime();
        LocalDateTime exitTime = ticket.getExitTime();
        long hoursParked = ChronoUnit.HOURS.between(entryTime, exitTime);
        // Adding 1 hour to the parked duration to ensure at least one hour is charged for payment
        if (hoursParked < 1)
            hoursParked=1;
        
        double ratePerHour;
        switch (ticket.getVehicle().getType()) {
            case "Car":
                ratePerHour = carRatePerHour;
                break;
            case "Van":
                ratePerHour = vanRatePerHour;
                break;
            case "Truck":
                ratePerHour = truckRatePerHour;
                break;
            case "Motorcycle":
                ratePerHour = motorcycleRatePerHour;
                break;
            default:
                throw new IllegalArgumentException("Invalid vehicle type");
        }
        return hoursParked * ratePerHour;
    }
}

Java

class ParkingTicket {
    private String ticketNumber;
    private Vehicle vehicle;
    private LocalDateTime entryTime;
    private LocalDateTime exitTime;
    // Constructors
    public ParkingTicket(String ticketNumber, Vehicle vehicle, LocalDateTime entryTime) {
        this.ticketNumber = ticketNumber;
        this.vehicle = vehicle;
        this.entryTime = entryTime;
    }
    // Getters and setters
    public String getTicketNumber() {
        return ticketNumber;
    }
    public void setTicketNumber(String ticketNumber) {
        this.ticketNumber = ticketNumber;
    }
    public Vehicle getVehicle() {
        return vehicle;
    }
    public void setVehicle(Vehicle vehicle) {
        this.vehicle = vehicle;
    }
    public LocalDateTime getEntryTime() {
        return entryTime;
    }
    public void setEntryTime(LocalDateTime entryTime) {
        this.entryTime = entryTime;
    }
    public LocalDateTime getExitTime() {
        return exitTime;
    }
    public void setExitTime(LocalDateTime exitTime) {
        this.exitTime = exitTime;
    }
}

Java

// EntrancePanel class for vehicle entrance
class EntrancePanel {
    private ParkingLot parkingLot;
    private ParkingSpotManager spotManager; 
    // Constructors
    public EntrancePanel(ParkingLot parkingLot, ParkingSpotManager spotManager) {
        this.parkingLot = parkingLot;
        this.spotManager = spotManager; 
    }
    // Member function
    public ParkingTicket issueParkingTicket(Vehicle vehicle) {
        // Check if spotManager is not null before accessing its methods
        
        // Check if there are available spots in the parking lot
        // Generate ticket details and return the issued parking ticket
        // Return the issued parking ticket
    }
    // Helper method to check if the spot is compatible with the vehicle
    private boolean isCompatibleSpot(Vehicle vehicle, ParkingSpot spot) {
        if (spot instanceof MotorcycleSpot) {
            return vehicle instanceof Motorcycle;
        } else if (spot instanceof CompactSpot) {
            return vehicle instanceof Car;
        } else if (spot instanceof LargeSpot) {
            return true; // Large spots allow any type of vehicle
        }
        return false;
    }
}
// ExitPanel class for vehicle exit
class ExitPanel {
    private ParkingLot parkingLot;
    private ParkingSpotManager spotManager;
    // Constructors
    public ExitPanel(ParkingLot parkingLot, ParkingSpotManager spotManager) {
        this.parkingLot = parkingLot;
        this.spotManager = spotManager; 
    }
    // Getters and setters
    public ParkingLot getParkingLot() {
        return parkingLot;
    }
    public void setParkingLot(ParkingLot parkingLot) {
        this.parkingLot = parkingLot;
    }
    // Member function to process vehicle exit
    public void processExit(String licensePlate) {
        // Check if spotManager is not null before accessing its methods
        // Find the ticket associated with the provided license plate
        // Set exit time
        // Calculate the total hours parked
        // Calculate the fare using fare calculator
        
        // Generate the receipt
        // Free up the parking spot
        // Remove the ticket from the parking lot
        // Prompt user for payment method
        Scanner scanner = new Scanner(System.in);
        System.out.println("\nChoose an option for payment:");
        System.out.println("1. Cash");
        System.out.println("2. Card");
        System.out.println("3. Exit program");
        int choice = scanner.nextInt();
        scanner.nextLine(); // Consume newline
        switch (choice) {
            case 1:
                PaymentStrategy cashPayment = new CashPayment();
                CentralPaymentSystem.getInstance().processPayment(cashPayment, fare);
                break;
            case 2:
                PaymentStrategy cardPayment = new CardPayment();
                CentralPaymentSystem.getInstance().processPayment(cardPayment, fare);
                break;
            case 3:
                System.out.println("Exiting program...");
                scanner.close();
                return;
            default:
                System.out.println("Invalid choice of payment. Please try again.");
        }
    }
    // Helper function to find the parking ticket associated with the provided license plate
    private ParkingTicket findTicketByLicensePlate(String licensePlate) {
        for (ParkingTicket ticket : parkingLot.getTicketMap().values()) {
            if (ticket.getVehicle().getLicensePlate().equals(licensePlate)) {
                return ticket;
            }
        }
        return null; // Return null if no ticket found for the provided license plate
    }
}

Java

public class Receipt {
    public static void displayReceipt(ParkingTicket ticket, double fare, long totalHoursParked) {
        System.out.println("\nReceipt:");
        System.out.println("Ticket Number: " + ticket.getTicketNumber());
        System.out.println("Vehicle Type: " + ticket.getVehicle().getType());
        System.out.println("Entry Time: " + ticket.getEntryTime().format(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss")));
        System.out.println("Exit Time: " + ticket.getExitTime().format(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss")));
        System.out.println("Total Hours Parked: " + totalHoursParked);
        System.out.println("Fare: $" + fare);
    }
}

Java

class DisplayBoard {
    private ParkingLot parkingLot;
    // Constructors
    public DisplayBoard(ParkingLot parkingLot) {
        this.parkingLot = parkingLot;
    }
    // Getters and setters
    public ParkingLot getParkingLot() {
        return parkingLot;
    }
    public void setParkingLot(ParkingLot parkingLot) {
        this.parkingLot = parkingLot;
    }
    // Member function
    public void displayParkingLotStatus() {
        System.out.println("\nParking Lot Status:");
        System.out.println("Total Spots: " + parkingLot.getTotalSpots());
        System.out.println("Free Spots: " + parkingLot.getTotalFreeSpots());
        System.out.println("Occupied Spots: " + parkingLot.getOccupiedSpots());
        System.out.println("Current Vehicles:");
        List<Vehicle> vehicles = parkingLot.getCurrentVehicles();
        for (Vehicle vehicle : vehicles) {
            System.out.println("License Plate: " + vehicle.getLicensePlate() + ", Type: " + vehicle.getType());
        }
    }
}

Java

class ParkingLot {
    private List<ParkingFloor> floors;
    private Map<String, ParkingTicket> ticketMap;
    private ParkingSpotManager spotManager; 
    // Constructors
    public ParkingLot() {
        this.floors = new ArrayList<>();
        this.ticketMap = new HashMap<>();
        this.spotManager = new ParkingSpotManager(); 
    }
    // Getters and setters
    public List<ParkingFloor> getFloors() {
        return floors;
    }
    public void setFloors(List<ParkingFloor> floors) {
        this.floors = floors;
    }
    public Map<String, ParkingTicket> getTicketMap() {
        return ticketMap;
    }
    public void setTicketMap(Map<String, ParkingTicket> ticketMap) {
        this.ticketMap = ticketMap;
    }
    public ParkingTicket getTicket(String ticketNumber) {
        return ticketMap.get(ticketNumber);
    }
    // Method to get all tickets
    public Map<String, ParkingTicket> getTickets() {
        return ticketMap;
    }
    // Updated method to get total free spots
    public int getTotalFreeSpots() {
        // definition
    }
    // Updated method to get total free spots by type
    public int getTotalFreeSpotsByVehicleType(String vehicleType) {
        // definition
    }
    public List<Vehicle> getCurrentVehicles() {
        // definition
    }
    // Additional methods for managing parking tickets
    public void addTicket(String ticketNumber, ParkingTicket ticket) {
        ticketMap.put(ticketNumber, ticket);
    }
    public void removeTicket(String ticketNumber) {
        ticketMap.remove(ticketNumber);
    }
    // Updated method to get the parking spot for a given vehicle
    public ParkingSpot getSpotForVehicle(Vehicle vehicle) {
        // definition
    }
    public int getTotalSpots() {
        int totalSpots = 0;
        for (ParkingFloor floor : floors) {
            totalSpots += floor.getSpots().size();
        }
        return totalSpots;
    }
    // Updated method to get the total occupied spots
    public int getOccupiedSpots() {
        int occupiedSpots = 0;
        for (ParkingFloor floor : floors) {
            for (ParkingSpot spot : floor.getSpots()) {
                if (spotManager.isOccupied(spot)) {
                    occupiedSpots++;
                }
            }
        }
        return occupiedSpots;
    }
    public boolean isLicensePlatePresent(String licensePlate) {
        for (ParkingTicket ticket : ticketMap.values()) {
            if (ticket.getVehicle().getLicensePlate().equals(licensePlate)) {
                return true; // License plate already present
            }
        }
        return false; // License plate not found
    }
}

Java

// ParkingLotInitializer class for initializing parking lot
class ParkingLotInitializer {
    private ParkingLot parkingLot;
    private ParkingSpotFactory spotFactory;
    // Constructors
    public ParkingLotInitializer(ParkingLot parkingLot) {
        this.parkingLot = parkingLot;
        this.spotFactory = new ParkingSpotFactory();
    }
    // Getters and setters
    public ParkingLot getParkingLot() {
        return parkingLot;
    }
    public void setParkingLot(ParkingLot parkingLot) {
        this.parkingLot = parkingLot;
    }
    // Member function
    public void initializeParkingLot(int numberOfFloors, int spotsPerFloor) {
        List<ParkingFloor> floors = new ArrayList<>();
        List<String> spotTypes = List.of("Compact", "Large", "Motorcycle"); 
        for (int i = 0; i < numberOfFloors; i++) {
            ParkingFloor floor = new ParkingFloor();
            for (int j = 0; j < spotsPerFloor; j++) {
                String spotType = spotTypes.get(j % spotTypes.size()); // Alternate spot types
                ParkingSpot spot = spotFactory.createParkingSpot(spotType);
                floor.addParkingSpot(spot);
            }
            floors.add(floor);
        }
        parkingLot.setFloors(floors);
    }
}

Java

class EnterVehicleHandler {
    private Scanner scanner;
    private ParkingLot parkingLot;
    private ParkingSpotManager spotManager;
    public EnterVehicleHandler(Scanner scanner, ParkingLot parkingLot, ParkingSpotManager spotManager) {
        this.scanner = scanner;
        this.parkingLot = parkingLot;
        this.spotManager = spotManager;
    }
    public void execute() {
        // Implementation to enter a vehicle
        Scanner scanner = new Scanner(System.in);
        String vehicleType;
        String licensePlate;
        // Prompt for vehicle type until a non-empty and valid input is provided
        do {
            System.out.println("Enter vehicle type (Car, Van, Truck, Motorcycle):");
            vehicleType = scanner.nextLine().trim().toLowerCase(); // Trim leading and trailing whitespace and convert to lowercase
            if (vehicleType.isEmpty()) {
                System.out.println("Vehicle type cannot be empty. Please enter a valid vehicle type.");
            } else if (!isValidVehicleType(vehicleType)) {
                System.out.println("Invalid vehicle type. Please enter a valid vehicle type (Car, Van, Truck, Motorcycle).");
            }
        } while (vehicleType.isEmpty() || !isValidVehicleType(vehicleType));
        // Prompt for license plate until a non-empty input is provided
        do {
            System.out.println("Enter license plate:");
            licensePlate = scanner.nextLine().trim(); // Trim leading and trailing whitespace
            if (licensePlate.isEmpty()) {
                System.out.println("License plate cannot be empty. Please enter a valid license plate.");
            }
        } while (licensePlate.isEmpty());
        // Check if the license plate is already present in the parking lot
        if (parkingLot.isLicensePlatePresent(licensePlate)) {
            System.out.println("Vehicle with the provided license plate is already parked. Please enter a different license plate.");
            return; // Exit the function
        }
        VehicleFactory vehicleFactory = new VehicleFactory();
        Vehicle vehicle = vehicleFactory.createVehicle(vehicleType, licensePlate);
        EntrancePanel entrancePanel = new EntrancePanel(parkingLot, spotManager);
        entrancePanel.issueParkingTicket(vehicle);
    }
    private boolean isValidVehicleType(String vehicleType) {
        return vehicleType.equals("car") || vehicleType.equals("van") || vehicleType.equals("truck") || vehicleType.equals("motorcycle");
    }
}
class ExitVehicleHandler {
    private Scanner scanner;
    private ParkingLot parkingLot;
    private ParkingSpotManager spotManager;
    public ExitVehicleHandler(Scanner scanner, ParkingLot parkingLot, ParkingSpotManager spotManager) {
        this.scanner = scanner;
        this.parkingLot = parkingLot;
        this.spotManager = spotManager;
    }
    public void execute() {
        // Implementation to exit a vehicle
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter license plate number:");
        String licensePlate = scanner.nextLine();
        ExitPanel exitPanel = new ExitPanel(parkingLot, spotManager);
        exitPanel.processExit(licensePlate);
    }
}
class DisplayCurrentVehiclesHandler {
    private ParkingLot parkingLot;
    public DisplayCurrentVehiclesHandler(ParkingLot parkingLot) {
        this.parkingLot = parkingLot;
    }
    public void execute() {
        // Implementation to display current vehicles
        DisplayBoard displayBoard = new DisplayBoard(parkingLot);
        List<Vehicle> currentVehicles = displayBoard.getParkingLot().getCurrentVehicles();
        for (Vehicle vehicle : currentVehicles) {
            System.out.println("Vehicle Type: " + vehicle.getType() + ", License Plate: " + vehicle.getLicensePlate());
        }
    }
}

Java

public class DriverMain {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        ParkingLot parkingLot = new ParkingLot();
        ParkingLotInitializer parkingLotInitializer = new ParkingLotInitializer(parkingLot);
        parkingLotInitializer.initializeParkingLot(3, 30);
        ParkingSpotManager spotManager = new ParkingSpotManager();
        DisplayBoard displayBoard = new DisplayBoard(parkingLot);
        displayBoard.displayParkingLotStatus();
        MenuOptionHandler menuOptionHandler = new MenuOptionHandler(scanner, parkingLot, spotManager, displayBoard);
        menuOptionHandler.runMenu();
    }
}

The above-implemented parking lot system offers comprehensive functionalities for vehicle entry and exit, payment processing, and real-time status display. Employing modular design patterns like Strategy and Factory ensures flexibility to adapt to evolving requirements. This design establishes a robust foundation for a scalable and adaptable parking management solution, allowing customization and enhancement. You can modify and improve the design according to your needs and preferences.

OOD interview guide#

Object-oriented design (OOD) interviews test your ability to solve complex problems using OOD principles and design patterns. Here’s a simple guide to help you prepare:

Understand OOD concepts: Learn the key principles of OOD, like abstraction, encapsulation, inheritance, and polymorphism for scalable and maintainable systems.
Practice solving problems: Regularly solve coding problems that require object-oriented solutions. Practice designing object models for real-world scenarios, like a parking lot or a banking system.
Know design patterns: Familiarize yourself with design patterns like Singleton, Factory, Strategy, Observer, and State, and learn how and when to use them.
System design skills: Focus on designing systems that are scalable, perform well, and can be easily extended with new features. Be prepared to discuss why you made certain design choices based on constraints.
Effective communication: Clearly explain your problem-solving process and design decisions. Be open to suggestions and able to discuss alternative solutions.
Review past projects: Reflect on previous projects and identify patterns and challenges you encountered. Be ready to discuss these projects with interviewers, explaining how you applied OOD principles.
Stay updated: Stay informed about the latest OOD and software design trends.
Mock interviews: Participate in realistic mock interviews to simulate the interview experience. Use feedback from these mock interviews to improve your skills. You can visit the Educative AI-based mock interview to prepare for FAANG/MAANG object-oriented design, system design, API design, and coding interviews.
Continuous learning: Engage with the software development community by joining forums, attending meetups, and participating in discussions. Continuously practice your skills to stay sharp and improve over time.

By following this guide and dedicating time to practice, you can build confidence to do well in OOD interviews and handle tough design problems effectively. Also consider exploring the following courses as an excellent starting point and very helpful for your system design interviews:

Conclusion#

Mastering object-oriented design principles is essential for tackling complex system design challenges like designing a parking lot. We can create scalable and maintainable solutions by breaking down the problem, applying SOLID principles, and utilizing design patterns effectively. Remember to practice problem-solving, understand design patterns, prioritize code readability, hone system design skills, and foster effective communication to excel in interviews and real-world projects. Keep learning and practicing to become a proficient and versatile developer.

Table of Contents

Design a parking lot

Problem definition#

Requirements collection#

Class diagram#

Components of a parking lot system#

The Vehicle class#

The ParkingSpot class#

The PaymentStrategy interface#

The VehicleFactory class#

The ParkingSpotFactory class#

The CentralPaymentSystem class#

The ParkingFloor class#

The ParkingTicket class#

The FareCalculator class#

The EntrancePanel class#

The Receipt class#

The ExitPanel class#

The DisplayBoard class#

The ParkingLot class#

The ParkingLotInitializer class #

The ParkingSpotManager class#

Relationship between the classes#

Association#

One-way association#

Two-way association#

Aggregation#

Inheritance#

Class diagram of the parking lot system#

Design patterns#

SOLID design principles#

Code for the parking lot system#

The Vehicle hierarchy#

Parking spot management#

Payment system#

Parking floor#

Fare calculator#

Ticket management#

Entrance and exit panels#

Receipt generation#

Display board#

Parking lot#

Parking lot initialization#

User interaction#

Main execution #

OOD interview guide#

Conclusion#

System Design Interview Questions by Company#

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