When to use which creational pattern in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the Singleton pattern?

The Singleton pattern ensures a class has only one instance and provides a global point of access to it.

beginner

When should you use the Factory Method pattern?

Use Factory Method when you want to create objects without specifying the exact class, allowing subclasses to decide which class to instantiate.

intermediate

What problem does the Abstract Factory pattern solve?

Abstract Factory provides an interface to create families of related or dependent objects without specifying their concrete classes.

intermediate

When is the Builder pattern most useful?

Builder is useful when constructing complex objects step-by-step, especially when the object has many optional parts or configurations.

intermediate

Why use the Prototype pattern?

Prototype is used to create new objects by copying existing ones, which is helpful when object creation is costly or complex.

Which creational pattern ensures only one instance of a class exists?

A Builder

B Factory Method

C Singleton

D Prototype

Singleton pattern restricts instantiation of a class to one object.

Which pattern is best when you need to create families of related objects without specifying their concrete classes?

A Abstract Factory

B Builder

C Prototype

D Singleton

Abstract Factory provides interfaces for creating related objects without specifying their concrete classes.

When should you use the Builder pattern?

A To construct complex objects step-by-step

B To create a single instance globally

C To clone existing objects

D To decide which class to instantiate

Builder pattern helps build complex objects step-by-step, especially with many optional parts.

Which pattern allows subclasses to decide which class to instantiate?

A Singleton

B Prototype

C Builder

D Factory Method

Factory Method lets subclasses choose the class to instantiate.

What is a key benefit of the Prototype pattern?

A Ensures only one instance

B Creates objects by cloning existing ones

C Builds complex objects step-by-step

D Creates families of related objects

Prototype creates new objects by copying existing ones, saving creation cost.

Explain when to use the Singleton, Factory Method, and Builder patterns in simple terms.

Think about controlling instance count, choosing object types, and building complex objects.

Describe the difference between Abstract Factory and Prototype patterns and when each is appropriate.

Focus on object families vs cloning.

Practice

1. Which creational pattern is best when you need only one instance of a class throughout the application?

easy

A. Singleton pattern

B. Factory pattern

C. Builder pattern

D. Prototype pattern

Solution

1. Step 1: Understand the requirement for a single instance

   The question asks for a pattern that ensures only one instance of a class exists.

2. Step 2: Match the pattern to the requirement

   Singleton pattern restricts instantiation to one object and provides a global access point.

3. Final Answer:

   Singleton pattern -> Option A

4. Quick Check:

   Single instance = Singleton [OK]

Hint: Single instance? Think Singleton pattern [OK]

Common Mistakes:

• Confusing Factory with Singleton
• Using Builder for single instance
• Thinking Prototype creates single instance

2. Which creational pattern allows creating different types of objects without specifying their concrete classes directly?

easy

A. Singleton pattern

B. Factory pattern

C. Builder pattern

D. Adapter pattern

Solution

1. Step 1: Identify the pattern for flexible object creation

   The question wants a pattern that creates objects without exposing the creation logic.

2. Step 2: Match the pattern to the description

   Factory pattern provides an interface to create objects but lets subclasses decide which class to instantiate.

3. Final Answer:

   Factory pattern -> Option B

4. Quick Check:

   Flexible object creation = Factory [OK]

Hint: Flexible creation? Use Factory pattern [OK]

Common Mistakes:

• Choosing Singleton for flexible creation
• Confusing Builder with Factory
• Selecting Adapter which is not creational

3. Consider this code snippet using the Builder pattern:

class CarBuilder {
  constructor() {
    this.car = new Car();
  }
  setColor(color) {
    this.car.color = color;
    return this;
  }
  setWheels(wheels) {
    this.car.wheels = wheels;
    return this;
  }
  build() {
    return this.car;
  }
}

const car = new CarBuilder().setColor('red').setWheels(4).build();
console.log(car.color, car.wheels);

What will be the output?

medium

A. red 4

B. undefined undefined

C. red undefined

D. Error: Car is not defined

Solution

1. Step 1: Check if Car class is defined

   The code uses new Car() but Car class is not defined anywhere, causing a runtime error.

2. Step 2: Understand the consequence of missing class

   Since Car is undefined, the code throws a ReferenceError before any properties are set.

3. Final Answer:

   Error: Car is not defined -> Option D

4. Quick Check:

   Missing class definition = ReferenceError [OK]

Hint: Check if all classes are defined before use [OK]

Common Mistakes:

• Assuming output is 'red 4' without Car class
• Ignoring missing class error
• Confusing Builder pattern with Factory output

4. You have a Singleton class implemented as below:

class Logger {
  constructor() {
    if (Logger.instance) {
      return Logger.instance;
    }
    this.logs = [];
    Logger.instance = this;
  }
  log(message) {
    this.logs.push(message);
  }
}

What is the main issue with this implementation?

medium

A. It creates a new instance every time

B. It does not store logs properly

C. It does not prevent multiple instances

D. It lacks a method to retrieve logs

Solution

1. Step 1: Analyze Singleton enforcement

   The constructor returns existing instance if present, but the implementation does not fully prevent multiple instances.

2. Step 2: Identify the issue in JavaScript context

   In JavaScript, this pattern works but can be bypassed by other means; better to use static method to get instance.

3. Final Answer:

   It does not prevent multiple instances -> Option C

4. Quick Check:

   Singleton enforcement flaw = It does not prevent multiple instances [OK]

Hint: Singleton needs controlled instance creation [OK]

Common Mistakes:

• Thinking logs are not stored
• Assuming new instance is always created
• Ignoring missing retrieval method

5. You are designing a system to create complex meal orders with many optional parts (drink, main dish, dessert). Which creational pattern is most suitable and why?

hard

A. Builder pattern, because it simplifies creating complex objects step-by-step

B. Singleton pattern, because only one meal order should exist

C. Factory pattern, because it creates different meal types without specifying details

D. Prototype pattern, because it clones existing meal orders

Solution

1. Step 1: Understand the complexity of the object

   Meal orders have many optional parts and require step-by-step construction.

2. Step 2: Match pattern to complex object creation

   Builder pattern is designed to build complex objects step-by-step, allowing optional parts to be added flexibly.

3. Final Answer:

   Builder pattern, because it simplifies creating complex objects step-by-step -> Option A

4. Quick Check:

   Complex object with options = Builder [OK]

Hint: Complex object? Use Builder pattern [OK]

Common Mistakes:

• Choosing Singleton for multiple orders
• Confusing Factory with Builder for complex objects
• Using Prototype without cloning need