Parking Lot — Detailed

classDiagram
  direction TB

  class ParkingLot {
    -levels: List~Level~
    -ticketRepo: TicketRepo
    -strategy: AllocationStrategy
    -paymentSvc: PaymentService
    +park(v: Vehicle) Ticket
    +unpark(t: Ticket) Receipt
    +findSlot(v: Vehicle) Slot
  }

  class Level {
    -slots: List~Slot~
    -id: int
    +freeSlots(type) List~Slot~
  }

  class Slot {
    -id: String
    -type: SlotType
    -occupied: boolean
    +occupy(v: Vehicle)
    +release()
  }

  class Vehicle {
    <<abstract>>
    +regNo: String
    +type(): VehicleType
  }
  class Car
  class Bike
  class Truck
  class EV {
    +chargingPort: boolean
  }

  class Ticket {
    +id: String
    +entryTs: Instant
    +slot: Slot
    +vehicle: Vehicle
  }

  class Receipt {
    +ticketId: String
    +exitTs: Instant
    +duration
    +amount
    +paymentRef
  }

  class AllocationStrategy {
    <<interface>>
    +pick(slots, vehicle) Slot
  }
  class NearestFirst
  class CompactSlot
  class ByVehicleType

  class PricingPolicy {
    <<interface>>
    +cost(duration, vehicle) Money
  }
  class FlatHourly
  class PeakOffPeak
  class DayPass

  class PaymentService {
    +charge(amount, method) PaymentRef
  }

  class GateController {
    -entryGates
    -exitGates
    +open(g)
    +close(g)
  }

  class DisplayBoard {
    +update(level, free)
  }

  class TicketRepo {
    <<interface>>
    +save(t)
    +findById(id) Ticket
  }

  ParkingLot "1" *-- "many" Level
  Level "1" *-- "many" Slot
  Vehicle <|-- Car
  Vehicle <|-- Bike
  Vehicle <|-- Truck
  Vehicle <|-- EV
  ParkingLot --> AllocationStrategy
  AllocationStrategy <|.. NearestFirst
  AllocationStrategy <|.. CompactSlot
  AllocationStrategy <|.. ByVehicleType
  ParkingLot --> PricingPolicy
  PricingPolicy <|.. FlatHourly
  PricingPolicy <|.. PeakOffPeak
  PricingPolicy <|.. DayPass
  ParkingLot --> PaymentService
  ParkingLot --> TicketRepo
  ParkingLot --> GateController
  ParkingLot --> DisplayBoard
  Ticket --> Slot
  Ticket --> Vehicle

Design notes

• Strategy pattern for allocation and pricing (swap without code change).
• Repository pattern hides DB; tests use in-memory.
• Observer pattern for DisplayBoard (subscribes to slot changes).
• State on Slot prevents double-occupy; CAS in repo.

Glossary & fundamentals

Concepts referenced in this design. Each row links to its canonical page; the tag column shows whether it is a high-level (HLD) or low-level (LLD) concept.

Tag	Concept	What it is	Page
LLD	Testing strategy	pyramid, doubles, TDD, contracts	testing-strategy
LLD	Behavioural patterns	Strategy, Observer, State, Command, Chain	behavioral-patterns
LLD	Concurrency primitives	mutex, semaphore, RW lock, atomic, CAS	concurrency-primitives