Builder pattern in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the Builder interface method for setting the product's name.

LLD

interface Builder {
    Builder setName([1] name);
}

Drag options to blanks, or click blank then click option'

AString

Bint

Cvoid

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using int or boolean instead of String for the name parameter.

Using void as a parameter type.

The setName method should accept a String parameter representing the product's name.

2fill in blank

medium

Complete the code to return the built product from the Builder.

LLD

class ProductBuilder implements Builder {
    private Product product = new Product();

    public Product [1]() {
        return product;
    }
}

Drag options to blanks, or click blank then click option'

Acreate

Bbuild

CgetProduct

Dmake

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names like create or make which are less standard.

Using getProduct which is verbose and less idiomatic.

The standard method name to return the final product in the Builder pattern is build().

3fill in blank

hard

Fix the error in the Director class method to correctly use the Builder to construct a product.

LLD

class Director {
    public Product construct(Builder builder) {
        builder.setName("Toy");
        builder.setPrice(10);
        return builder.[1]();
    }
}

Drag options to blanks, or click blank then click option'

Amake

Bcreate

Cget

Dbuild

Attempts:

3 left

💡 Hint

Common Mistakes

Using create() or make() instead of build().

Forgetting to call the method to return the product.

The Director calls the build() method on the Builder to get the final product.

4fill in blank

hard

Fill both blanks to complete the fluent interface methods in the Builder implementation.

LLD

class ProductBuilder implements Builder {
    private Product product = new Product();

    public Builder setName([1] name) {
        product.name = name;
        return [2];
    }
}

Drag options to blanks, or click blank then click option'

AString

Bthis

Cproduct

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Returning void instead of this breaks chaining.

Using wrong parameter types like product or void.

The method parameter type is String for the name, and the method returns this to allow method chaining.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension that maps product names to prices for products costing more than 100.

LLD

expensive_products = {product.[1]: product.[2] for product in products if product.[3] > 100}

Drag options to blanks, or click blank then click option'

Aname

Bprice

Dquantity

Attempts:

3 left

💡 Hint

Common Mistakes

Using quantity instead of price in the condition.

Mixing up keys and values in the dictionary comprehension.

The dictionary maps product name to price for products where price is greater than 100.

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