Object-Oriented Design: Parking Lot System

📌 Problem Statement

"Design a parking lot system that supports different vehicle types, multiple entrances and exits, ticketing, and payment."

This question is a favorite in object-oriented design interviews. It challenges you to model real-world entities, their interactions, and support extensible behaviors using OOP principles.

✅ Requirements

Functional Requirements

  • Support multiple types of vehicles: car, bike, truck, electric vehicle
  • Different parking spot types: compact, large, handicapped, EV-charging
  • Allocate nearest available spot when a vehicle enters
  • Issue a ticket on entry and calculate fees on exit
  • Allow multiple entrances and exits
  • Track available spots per level and type

Non-Functional Requirements

  • Scalable and maintainable design
  • Extensible: should support new spot types or pricing policies
  • Handle concurrency for multiple vehicles entering/exiting simultaneously

🎯 Key Object-Oriented Concepts

  • Inheritance: for vehicle and parking spot hierarchies
  • Composition: to associate tickets with vehicles, spots with floors
  • Encapsulation: internal state of floors, spots, or payment logic
  • Interfaces / Abstract Classes: for payment or pricing strategies

🧱 Core Classes and Responsibilities

Vehicle

  • licensePlate
  • vehicleType: car, bike, truck, EV
  • Can be subclassed: Car, Motorcycle, Truck, ElectricCar

ParkingSpot

  • spotId, spotType, isOccupied
  • Associated with a floor and optionally a chargingUnit
  • Subclasses: CompactSpot, LargeSpot, EVSpot, HandicappedSpot

ParkingFloor

  • Contains multiple parking spots
  • Tracks available and occupied counts
  • Handles allocation requests from entrances

Ticket

  • ticketId, entryTime, vehicle, assignedSpot
  • Tracks time-in and used for calculating payment

Payment

  • paymentId, ticket, amount, status, method
  • Subtypes or strategies: credit card, cash, online

EntranceGate / ExitGate

  • Devices or modules that validate entry/exit
  • Trigger ticket generation and fee calculation

ParkingLot

  • Aggregates all floors, gates, and configuration logic
  • Main system controller
  • Supports methods like assignSpot(vehicle), freeSpot(spot)

🔁 Spot Allocation Logic

The system must:

  • Match vehicle type to eligible spot types
  • Prefer nearest or most optimal spot (e.g., lowest floor)
  • Mark spot as occupied and issue a ticket
  • Reject if no spot is available for the type

Could be enhanced with:

  • Dynamic pricing based on availability
  • Preferred parking for monthly users

💸 Payment and Fee Calculation

  • On exit, retrieve ticket and calculate time parked
  • Apply pricing policy (e.g., hourly rate, daily cap)
  • Integrate with different payment methods
  • Mark ticket as paid and free the parking spot

Pricing strategies can vary by spot type, floor, or user type, modeled via strategy pattern.

🔐 Concurrency and Synchronization

  • Synchronize spot allocation to avoid double booking
  • Use thread-safe collections or locks on critical methods
  • Logging and monitoring for entry/exit anomalies

🔍 Extensions and Variations

  • Add online reservations and spot pre-booking
  • Allow license plate recognition for auto-entry
  • Track EV charging unit status
  • Add monthly subscriptions or loyalty points
  • Generate reports for occupancy or revenue

🧠 Interviewer Evaluation Criteria

AreaWhat They’re Looking For
Class ModelingClear object responsibilities and clean abstraction
Inheritance UsageProper use of polymorphism and hierarchy for vehicles/spots
ExtensibilityCan the design grow without rewriting core logic?
Real-world LogicDoes the spot allocation mirror realistic constraints?
Error HandlingHandling edge cases like full lot, lost tickets, payment fail

✅ Summary

Designing a parking lot system tests a wide range of OOD principles, including class hierarchy, encapsulation, interaction between components, and real-time decisions. A robust system supports:

  • Flexible spot allocation
  • Vehicle diversity
  • Ticketing and pricing logic
  • Easy future extensions