Class identification (ParkingLot, Floor, Spot, Vehicle) in LLD - Architecture Diagram

System Overview - Class identification (ParkingLot, Floor, Spot, Vehicle)

This system models a parking lot with multiple floors and parking spots. It manages vehicles entering and leaving, assigning spots based on availability and type. The key requirement is to efficiently track spot occupancy and vehicle location.

Architecture Diagram

User
  |
  v
ParkingLot
  |
  v
Floor(s)
  |
  v
Spot(s) <--> Vehicle(s)

Components

User

actor

Person who parks or retrieves vehicles

ParkingLot

class

Manages floors and overall parking operations

Floor

class

Represents a single floor with multiple parking spots

Spot

class

Represents a parking spot that can be occupied by a vehicle

Vehicle

class

Represents a vehicle that needs parking

Request Flow - 6 Hops

User → ParkingLot

ParkingLot → Floor

Floor → Spot

Spot → Floor

Floor → ParkingLot

ParkingLot → User

Failure Scenario

Component Fails:Spot

Impact:Spot incorrectly marked as available or occupied, causing parking conflicts or failures

Mitigation:Implement spot status validation and periodic synchronization with sensors or manual checks

Architecture Quiz - 3 Questions

Which class is responsible for managing multiple floors in the parking system?

A Spot

B ParkingLot

C Floor

D Vehicle

Design Principle

This design uses clear class responsibilities to separate concerns: ParkingLot manages floors, Floor manages spots, and Spot manages vehicle occupancy. This modular approach simplifies maintenance and scaling.

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