Builder pattern in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of the Builder Pattern

Which of the following best describes the main purpose of the Builder pattern in system design?

A To allow an object to alter its behavior when its internal state changes.

B To provide a global point of access to a single instance of a class.

C To define a family of algorithms, encapsulate each one, and make them interchangeable.

D To separate the construction of a complex object from its representation, allowing the same construction process to create different representations.

💡 Hint

Think about how the Builder pattern helps when creating complex objects step by step.

The Builder pattern separates the construction of a complex object from its representation. This allows the same construction process to create different representations of the object.

❓ Architecture

intermediate

Components of the Builder Pattern

In a typical Builder pattern implementation, which component is responsible for defining the steps to build the product?

A Director

B Concrete Builder

C Product

D Client

💡 Hint

This component controls the building process but does not create the product itself.

The Director defines the order and steps to build the product. It uses the Builder interface but does not create the product directly.

❓ scaling

advanced

Scaling Builder Pattern for Multiple Product Variants

You need to design a system using the Builder pattern to create multiple variants of a product with many optional parts. Which approach best supports scalability and maintainability?

A Implement separate Concrete Builders for each product variant, each handling its own construction logic.

B Use the Director to handle all variant logic and keep the Builder simple.

C Create a single Concrete Builder with many conditional statements to handle all variants.

D Avoid the Builder pattern and use a simple constructor with many parameters instead.

💡 Hint

Think about how to keep code clean and easy to extend when adding new product variants.

Using separate Concrete Builders for each variant keeps construction logic isolated and makes the system easier to maintain and extend.

❓ tradeoff

advanced

Tradeoffs of Using Builder Pattern

What is a common tradeoff when using the Builder pattern in system design?

A It reduces code complexity but limits the ability to create different product representations.

B It increases the complexity of code by adding more classes but improves flexibility in object creation.

C It eliminates the need for a Director, simplifying the design.

D It forces all product parts to be mandatory, reducing optionality.

💡 Hint

Consider the balance between code complexity and flexibility.

The Builder pattern adds extra classes which can increase complexity, but this tradeoff allows flexible and clear construction of complex objects.

❓ component

expert

Request Flow in Builder Pattern

Consider a system where a client requests a complex object via a Director and a Concrete Builder. Which sequence correctly describes the request flow?

A 1, 3, 2, 4

B 2, 1, 3, 4

C 1, 2, 3, 4

D 3, 2, 1, 4

💡 Hint

Think about who initiates the build and the order of method calls.

The client starts the process by calling the Director. The Director then calls Builder methods in order. The Builder assembles parts and stores state. Finally, the Director gets the product and returns it to the client.

Practice

1. What is the main purpose of the Builder pattern in system design?

easy

A. To construct complex objects step by step, separating construction from representation

B. To enforce a single instance of a class throughout the system

C. To provide a way to access elements of a collection sequentially

D. To define a family of algorithms and make them interchangeable

Solution

1. Step 1: Understand Builder pattern purpose

   The Builder pattern is designed to create complex objects step by step, allowing different representations.

2. Step 2: Compare with other patterns

   Options A, C, and D describe Singleton, Iterator, and Strategy patterns respectively, not Builder.

3. Final Answer:

   To construct complex objects step by step, separating construction from representation -> Option A

4. Quick Check:

   Builder pattern = stepwise construction [OK]

Hint: Builder pattern builds complex objects stepwise [OK]

Common Mistakes:

• Confusing Builder with Singleton pattern
• Thinking Builder manages object access, not construction
• Mixing Builder with Strategy or Iterator patterns

2. Which of the following is the correct way to implement method chaining in a Builder class?

easy

A. Each setter method returns a new Builder instance every time

B. Each setter method returns void and does not return anything

C. Each setter method returns the final built object immediately

D. Each setter method returns this (the builder instance) to allow chaining

Solution

1. Step 1: Understand method chaining in Builder

   Method chaining requires each setter to return the builder instance (this) so calls can be chained.

2. Step 2: Analyze other options

   Returning void breaks chaining; returning new builder instances is inefficient; returning final object ends chain prematurely.

3. Final Answer:

   Each setter method returns this (the builder instance) to allow chaining -> Option D

4. Quick Check:

   Method chaining = return this [OK]

Hint: Return 'this' in setters for chaining [OK]

Common Mistakes:

• Returning void and expecting chaining
• Creating new builder instances in setters
• Returning final object too early

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

class House {
  constructor(walls, doors) {
    this.walls = walls;
    this.doors = doors;
  }
  describe() {
    return `House with ${this.walls} walls and ${this.doors} doors.`;
  }
}

class HouseBuilder {
  constructor() {
    this.walls = 0;
    this.doors = 0;
  }
  setWalls(count) {
    this.walls = count;
    return this;
  }
  setDoors(count) {
    this.doors = count;
    return this;
  }
  build() {
    return new House(this.walls, this.doors);
  }
}

const house = new HouseBuilder().setWalls(4).setDoors(2).build();
console.log(house.describe());

medium

A. House with 4 walls and 2 doors.

B. House with 0 walls and 0 doors.

C. House with 2 walls and 4 doors.

D. Error: setWalls and setDoors do not return builder

Solution

1. Step 1: Trace method calls and values

   The builder sets walls to 4 and doors to 2, then builds a House with these values.

2. Step 2: Check describe output

   The describe method returns a string with walls and doors values, so output is 'House with 4 walls and 2 doors.'

3. Final Answer:

   House with 4 walls and 2 doors. -> Option A

4. Quick Check:

   Builder sets walls=4, doors=2 [OK]

Hint: Follow method chain values stepwise [OK]

Common Mistakes:

• Mixing walls and doors values
• Assuming setters don't return builder
• Ignoring build() method output

4. Identify the error in this Builder pattern implementation:

class CarBuilder {
  constructor() {
    this.color = '';
  }
  setColor(color) {
    this.color = color;
  }
  build() {
    return { color: this.color };
  }
}

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

medium

A. build method returns wrong object type

B. setColor does not return the builder instance, so chaining fails

C. Constructor does not initialize color properly

D. No error, code works fine

Solution

1. Step 1: Check method chaining in setColor

   setColor does not return this, so chaining to build() fails.

2. Step 2: Analyze consequences

   Because setColor returns undefined, calling build() immediately after causes an error or unexpected behavior.

3. Final Answer:

   setColor does not return the builder instance, so chaining fails -> Option B

4. Quick Check:

   Missing 'return this' breaks chaining [OK]

Hint: Setters must return 'this' for chaining [OK]

Common Mistakes:

• Ignoring missing return in setter
• Thinking build returns wrong type
• Assuming constructor error

5. You need to design a system to build customizable computer configurations with optional components like GPU, RAM, and Storage. Which Builder pattern approach best supports this requirement?

hard

A. Build the computer object directly without a builder, setting all fields in constructor

B. Use multiple Builder classes, each building a fixed configuration without chaining

C. Create a single Builder class with methods for each optional component, returning the builder for chaining

D. Use a factory method that returns pre-built fixed configurations only

Solution

1. Step 1: Analyze requirement for optional components

   Optional components require flexible, stepwise construction with chaining for readability.

2. Step 2: Evaluate options

   Create a single Builder class with methods for each optional component, returning the builder for chaining supports chaining and optional parts; B lacks chaining and flexibility; C is inflexible; D only returns fixed configs.

3. Final Answer:

   Create a single Builder class with methods for each optional component, returning the builder for chaining -> Option C

4. Quick Check:

   Flexible builder with chaining fits optional parts [OK]

Hint: Use one builder with chaining for optional parts [OK]

Common Mistakes:

• Using multiple builders without chaining
• Setting all options in constructor (inflexible)
• Relying on fixed pre-built configurations only