Why Class identification (ParkingLot, Floor, Spot, Vehicle) in LLD? - Purpose & Use Cases

The Big Idea

What if you could never lose a car in a huge parking lot again, thanks to smart class design?

The Scenario

Imagine managing a large parking garage by writing down every car's location on paper. You try to remember which floor and spot each vehicle is in, but it quickly becomes confusing and chaotic.

The Problem

Using manual notes or simple lists to track cars and spots is slow and error-prone. You might lose track of spots, double-book a parking space, or waste time searching for a vehicle. It's hard to scale as the garage grows.

The Solution

By identifying clear classes like ParkingLot, Floor, Spot, and Vehicle, you organize the system logically. Each class has its own role and data, making it easy to track and manage parking efficiently and without confusion.

Before vs After

✗ Before

parking_list = [("Floor1", "Spot5", "CarA"), ("Floor2", "Spot3", "CarB")]

✓ After

class Vehicle: pass
class Spot: pass
class Floor: pass
class ParkingLot: pass

What It Enables

This approach enables building a clear, scalable system that can easily handle thousands of vehicles and spots without errors or confusion.

Real Life Example

Think of a shopping mall parking system that shows you exactly where your car is parked on a map app, updating in real-time as cars come and go.

Key Takeaways

Manual tracking is confusing and error-prone.

Classes organize the system into clear parts.

Class identification makes scaling and managing easy.

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