Class identification (ParkingLot, Floor, Spot, Vehicle) in LLD - System Design Exercise

Design: Parking Lot Management System

Design the core classes and their relationships to model the parking lot, floors, spots, and vehicles. Out of scope: payment processing, user authentication, mobile app UI.

Functional Requirements

FR1: Allow vehicles to enter and find a parking spot

FR2: Support multiple floors with multiple spots per floor

FR3: Track which spots are occupied and which are free

FR4: Support different vehicle types (car, motorcycle, truck)

FR5: Allow vehicles to leave and free up spots

FR6: Provide a way to check availability of spots

Non-Functional Requirements

NFR1: System should handle up to 1000 vehicles simultaneously

NFR2: Spot allocation and release should be done within 200ms

NFR3: System should be designed for easy extension (e.g., new vehicle types)

NFR4: Maintain consistency of spot occupancy status

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

ParkingLot class to represent the entire parking structure

Floor class to represent each level in the parking lot

Spot class to represent individual parking spots

Vehicle class hierarchy for different vehicle types

Spot allocation and release logic

Design Patterns

Factory pattern for creating different vehicle types

Composite pattern for ParkingLot containing Floors and Spots

Strategy pattern for different spot allocation algorithms

Observer pattern to notify when spots become free or occupied

Reference Architecture

ParkingLot
  ├─ Floor 1
  │    ├─ Spot 1 (Car)
  │    ├─ Spot 2 (Motorcycle)
  │    └─ Spot 3 (Truck)
  ├─ Floor 2
  │    ├─ Spot 1 (Car)
  │    └─ Spot 2 (Car)
  └─ Floor N
       └─ Spot M

Vehicle
  ├─ Car
  ├─ Motorcycle
  └─ Truck

Components

ParkingLot

Class

Represents the entire parking lot containing multiple floors

Floor

Class

Represents a single floor in the parking lot containing multiple spots

Spot

Class

Represents an individual parking spot with a type and occupancy status

Vehicle

Abstract Class or Interface

Base class for different vehicle types

Car, Motorcycle, Truck

Classes inheriting Vehicle

Specific vehicle types with possible unique attributes

Request Flow

1. Vehicle arrives and requests a spot from ParkingLot

2. ParkingLot delegates request to Floors to find an available Spot matching vehicle type

3. Floor checks its Spots and returns an available Spot if any

4. ParkingLot assigns the Spot to the Vehicle and marks it occupied

5. When Vehicle leaves, ParkingLot marks the Spot as free

6. Availability queries check Floors and Spots for free spots

Database Schema

Entities: - ParkingLot (id, name, location) - Floor (id, parking_lot_id, floor_number) - Spot (id, floor_id, spot_type, is_occupied) - Vehicle (id, vehicle_type, license_plate) Relationships: - ParkingLot 1:N Floor - Floor 1:N Spot - Spot 0..1 Vehicle (occupied by one vehicle or none)

Scaling Discussion

Bottlenecks

Spot allocation latency increases with large number of floors and spots

Concurrency issues when multiple vehicles try to occupy the same spot

Tracking availability in real-time for large parking lots

Solutions

Use efficient data structures (e.g., priority queues) to quickly find free spots

Implement locking or atomic operations to prevent double allocation

Cache availability status and update asynchronously to reduce load

Interview Tips

Time: 10 minutes to clarify requirements and ask questions, 20 minutes to design classes and relationships, 10 minutes to discuss scaling and improvements, 5 minutes for summary

Explain class responsibilities clearly

Show understanding of relationships and associations

Discuss how to handle different vehicle and spot types

Mention concurrency and consistency considerations

Talk about extensibility for future features

Practice

1. Which class in a parking system is responsible for managing multiple floors?

easy

A. ParkingLot

B. Floor

C. Spot

D. Vehicle

Solution

1. Step 1: Understand the role of ParkingLot

   The ParkingLot class represents the entire parking area and manages multiple floors within it.

2. Step 2: Compare with other classes

   Floor manages spots on a single level, Spot represents a single parking space, and Vehicle represents the car or bike.

3. Final Answer:

   ParkingLot -> Option A

4. Quick Check:

   ParkingLot manages floors = C [OK]

Hint: ParkingLot holds floors; floors hold spots [OK]

Common Mistakes:

• Confusing Floor as managing multiple floors
• Thinking Spot manages floors
• Assigning Vehicle to manage floors

2. Which of the following is the correct way to represent a parking spot in a class diagram?

easy

A. class Vehicle { int spotNumber; boolean isOccupied; }

B. class Spot { int spotNumber; boolean isOccupied; }

C. class Floor { int spotNumber; boolean isOccupied; }

D. class ParkingLot { int spotNumber; boolean isOccupied; }

Solution

1. Step 1: Identify the class representing a parking spot

   The Spot class should have attributes like spotNumber and isOccupied to represent a parking space.

2. Step 2: Check other classes for correctness

   Vehicle represents cars, Floor represents a level, and ParkingLot represents the whole area, so they should not have spotNumber or isOccupied attributes.

3. Final Answer:

   class Spot { int spotNumber; boolean isOccupied; } -> Option B

4. Quick Check:

   Spot class holds spot info = A [OK]

Hint: Spot class holds spot details like number and occupancy [OK]

Common Mistakes:

• Assigning spot attributes to Vehicle
• Putting spotNumber in Floor or ParkingLot
• Confusing class roles in diagram

3. Given the following code snippet, what will be the output?

class Vehicle {
  String licensePlate;
  Vehicle(String plate) { licensePlate = plate; }
}
class Spot {
  Vehicle parkedVehicle;
  boolean isOccupied() { return parkedVehicle != null; }
}
Spot spot = new Spot();
System.out.println(spot.isOccupied());

medium

A. true

B. Compilation error

C. null

D. false

Solution

1. Step 1: Analyze Spot initialization

   The Spot object is created but parkedVehicle is not assigned, so it defaults to null.

2. Step 2: Evaluate isOccupied method

   isOccupied returns true if parkedVehicle is not null; here it is null, so it returns false.

3. Final Answer:

   false -> Option D

4. Quick Check:

   parkedVehicle is null, so isOccupied() = false [OK]

Hint: Unassigned vehicle means spot is free (false) [OK]

Common Mistakes:

• Assuming default boolean is true
• Confusing null with false
• Expecting compilation error due to missing constructor

4. Identify the error in this class design snippet:

class Floor {
  List<Spot> spots;
  void addSpot(Spot s) {
    spots.add(s);
  }
}

medium

A. Spot class should be inside Floor class

B. Method addSpot should return boolean

C. spots list is not initialized before adding

D. Floor class should not have spots list

Solution

1. Step 1: Check initialization of spots list

   The spots list is declared but not initialized, so calling add on it will cause a runtime error.

2. Step 2: Validate other options

   Returning boolean is optional, Spot class can be separate, and Floor should have spots list to manage spots.

3. Final Answer:

   spots list is not initialized before adding -> Option C

4. Quick Check:

   Uninitialized list causes error = A [OK]

Hint: Always initialize lists before use [OK]

Common Mistakes:

• Ignoring list initialization
• Thinking method return type matters here
• Believing Spot must be nested class

5. You want to design a system where each Vehicle can only park in a Spot that matches its size (e.g., small, medium, large). Which class design change best supports this requirement?

hard

A. Add a size attribute to both Vehicle and Spot classes and check compatibility before parking

B. Add a size attribute only to Vehicle class and ignore Spot size

C. Add a size attribute only to Spot class and ignore Vehicle size

D. Remove size attributes and allow any Vehicle to park anywhere

Solution

1. Step 1: Understand size matching requirement

   Both Vehicle and Spot need size attributes to compare and ensure compatibility.

2. Step 2: Evaluate options

   Ignoring size in either class prevents proper matching; removing size ignores requirement.

3. Final Answer:

   Add a size attribute to both Vehicle and Spot classes and check compatibility before parking -> Option A

4. Quick Check:

   Size match needs attributes in both classes = B [OK]

Hint: Both Vehicle and Spot need size info to match [OK]

Common Mistakes:

• Adding size to only one class
• Ignoring size and allowing any parking
• Confusing attribute placement