Parking strategy pattern in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Parking Strategy Pattern?

It is a design approach that helps decide how to park vehicles in a parking lot using different strategies, like parking nearest to the entrance or in the largest free space.

intermediate

Why use the Strategy Pattern in parking systems?

It allows changing parking rules easily without changing the main system. For example, switching from first-come-first-served to reserved parking without big code changes.

beginner

Name two common parking strategies.

1. Nearest Spot First: Park the vehicle in the closest available spot to the entrance.
2. Largest Spot First: Park the vehicle in the largest available spot to fit bigger vehicles.

intermediate

How does the Parking Strategy Pattern improve scalability?

By separating parking logic into strategies, the system can add new parking rules without affecting existing code, making it easier to grow and maintain.

intermediate

What role does the Context play in the Parking Strategy Pattern?

The Context is the parking lot system that uses a chosen strategy to decide where to park vehicles. It delegates the decision to the strategy object.

What does the Parking Strategy Pattern help with?

A Fixing broken cars

B Building physical parking lots

C Choosing different ways to park vehicles

D Calculating parking fees

The pattern focuses on how to decide parking spots using different strategies.

Which of these is NOT a benefit of using the Parking Strategy Pattern?

A Automatically cleans the parking lot

B Easier to add new parking rules

C Improves system flexibility

D Separates parking logic from main system

Cleaning the parking lot is unrelated to the strategy pattern.

In the Parking Strategy Pattern, what is the 'Context'?

A The parking lot entrance

B A vehicle looking for parking

C A parking ticket machine

D The parking lot system using a strategy

The Context is the system that applies the chosen parking strategy.

Which strategy might park a vehicle in the closest spot to the entrance?

A Last Spot First

B Nearest Spot First

C Random Spot

D Largest Spot First

Nearest Spot First chooses the closest available spot.

How does the Parking Strategy Pattern help when parking rules change?

A By changing the strategy without rewriting the whole system

B By rebuilding the parking lot

C By hiring more parking attendants

D By increasing parking fees

The pattern allows swapping strategies easily to adapt to new rules.

Explain the Parking Strategy Pattern and how it helps manage parking decisions.

Think about how different parking rules can be swapped easily.

Describe the roles of Context and Strategy in the Parking Strategy Pattern with an example.

Imagine the system choosing where to park based on a rule.

Practice

1. What is the main purpose of using the Parking Strategy Pattern in a parking lot system?

easy

A. To store vehicle details in a database

B. To manage payment processing for parking fees

C. To allow different algorithms for finding parking spots without changing the main system

D. To control the physical gates of the parking lot

Solution

1. Step 1: Understand the role of strategy pattern

   The strategy pattern lets you swap different algorithms easily without changing the main code.

2. Step 2: Apply this to parking system context

   In parking, it means you can change how spots are found without rewriting the whole system.

3. Final Answer:

   To allow different algorithms for finding parking spots without changing the main system -> Option C

4. Quick Check:

   Strategy pattern = flexible parking spot finding [OK]

Hint: Strategy pattern = flexible algorithm swapping [OK]

Common Mistakes:

• Confusing strategy pattern with data storage
• Thinking it controls hardware like gates
• Mixing it with payment processing logic

2. Which of the following is the correct way to define a parking strategy interface in a typical object-oriented design?

easy

A. class ParkingStrategy { findSpot(vehicle) {} }

B. function findSpot(vehicle) { return spot; }

C. var ParkingStrategy = new Object();

D. interface ParkingStrategy { findSpot(vehicle): Spot; }

Solution

1. Step 1: Identify interface syntax in OOP

   Interfaces define method signatures without implementation, e.g., interface ParkingStrategy { findSpot(vehicle): Spot; }.

2. Step 2: Compare options

   interface ParkingStrategy { findSpot(vehicle): Spot; } correctly uses interface with method signature. Others are class, function, or object, not interface.

3. Final Answer:

   interface ParkingStrategy { findSpot(vehicle): Spot; } -> Option D

4. Quick Check:

   Interface = method signature only [OK]

Hint: Interface defines method signatures only [OK]

Common Mistakes:

• Using class instead of interface for strategy
• Defining functions outside interface context
• Confusing object creation with interface definition

3. Given the following code snippet for two parking strategies, what will be the output when findSpot(vehicle) is called using NearestParkingStrategy?

class NearestParkingStrategy {
  findSpot(vehicle) {
    return "Nearest spot found";
  }
}
class RandomParkingStrategy {
  findSpot(vehicle) {
    return "Random spot found";
  }
}
const strategy = new NearestParkingStrategy();
console.log(strategy.findSpot('car'));

medium

A. "Nearest spot found"

B. "Random spot found"

C. undefined

D. Error: findSpot not defined

Solution

1. Step 1: Identify which strategy instance is used

   The code creates an instance of NearestParkingStrategy and calls findSpot.

2. Step 2: Check method output for that class

   NearestParkingStrategy.findSpot returns "Nearest spot found".

3. Final Answer:

   "Nearest spot found" -> Option A

4. Quick Check:

   Instance method output = "Nearest spot found" [OK]

Hint: Instance method output matches class used [OK]

Common Mistakes:

• Confusing which strategy instance is created
• Assuming random strategy is used
• Expecting undefined or error without reason

4. In the following code, what is the main issue that prevents the ParkingLot from using different parking strategies correctly?

class ParkingLot {
  constructor() {
    this.strategy = null;
  }
  setStrategy(strategy) {
    this.strategy = strategy;
  }
  park(vehicle) {
    return this.strategy.findSpot(vehicle);
  }
}
const lot = new ParkingLot();
lot.park('car');

medium

A. No strategy is set before calling park, causing an error

B. The park method should not call findSpot

C. The strategy property should be a list, not a single object

D. The constructor should initialize strategy with a default value

Solution

1. Step 1: Analyze object initialization and method calls

   The ParkingLot is created with strategy = null. No strategy is set before calling park.

2. Step 2: Understand consequence of null strategy

   Calling this.strategy.findSpot(vehicle) when strategy is null causes an error.

3. Final Answer:

   No strategy is set before calling park, causing an error -> Option A

4. Quick Check:

   Null strategy causes error on method call [OK]

Hint: Always set strategy before use [OK]

Common Mistakes:

• Thinking park method logic is wrong
• Assuming strategy should be a list
• Ignoring null initialization problem

5. You want to design a parking system that supports multiple vehicle types (car, bike, truck) and different parking strategies (nearest, random, reserved). Which design approach best uses the Parking Strategy Pattern to handle this complexity?

hard

A. Create separate parking lot classes for each vehicle type and hardcode the strategy inside each

B. Use a single ParkingLot class with a strategy interface; implement different strategies and select based on vehicle type at runtime

C. Store all vehicles in one list and assign spots randomly without strategy pattern

D. Use global variables to track vehicle types and apply strategies in procedural code

Solution

1. Step 1: Identify need for flexibility and scalability

   Supporting multiple vehicle types and strategies requires flexible design without code duplication.

2. Step 2: Apply strategy pattern with runtime selection

   Using one ParkingLot class with strategy interface allows swapping strategies dynamically based on vehicle type.

3. Step 3: Evaluate other options

   Create separate parking lot classes for each vehicle type and hardcode the strategy inside each duplicates code, C ignores strategy pattern, D uses poor global state management.

4. Final Answer:

   Use a single ParkingLot class with a strategy interface; implement different strategies and select based on vehicle type at runtime -> Option B

5. Quick Check:

   Strategy pattern + runtime selection = best design [OK]

Hint: Use one class + strategy interface + runtime choice [OK]

Common Mistakes:

• Duplicating classes per vehicle type
• Ignoring strategy pattern benefits
• Using global variables instead of OOP