Factory pattern with companion objects in Kotlin - Mini Project: Build & Apply

Factory pattern with companion objects

📖 Scenario: You are building a simple app that creates different types of vehicles. Each vehicle has a name and a type. You want to use the factory pattern with companion objects to create vehicles easily.

🎯 Goal: Build a Kotlin program that uses a companion object as a factory to create Vehicle objects with different types.

📋 What You'll Learn

Create a Vehicle class with name and type properties

Add a companion object inside Vehicle with a factory method create

The factory method should take name and type as parameters and return a Vehicle object

Use the factory method to create a vehicle

Print the vehicle's details

💡 Why This Matters

🌍 Real World

Factories help create objects in a controlled way, useful in apps that manage many similar objects like vehicles, users, or products.

💼 Career

Understanding factory patterns and companion objects is important for Kotlin developers to write clean, maintainable, and scalable code.

1

Create the Vehicle class

Create a Kotlin class called Vehicle with two properties: name of type String and type of type String. Use a primary constructor to set these properties.

Kotlin

class Vehicle(val name: String, val type: String) {
    // Your code here
}

Hint

Use class Vehicle(val name: String, val type: String) to define the class and its properties.

2

Add a companion object with a factory method

Inside the Vehicle class, add a companion object. Inside it, create a function called create that takes two parameters: name and type, both of type String. The function should return a new Vehicle object using these parameters.

Kotlin

class Vehicle(val name: String, val type: String) {
    companion object {
        // Your code here
    }
}

Hint

Use companion object and define fun create(name: String, type: String): Vehicle inside it.

3

Use the factory method to create a Vehicle

Outside the Vehicle class, create a variable called car and assign it the result of calling Vehicle.create with "Car" as the name and "Sedan" as the type.

Kotlin

class Vehicle(val name: String, val type: String) {
    companion object {
        fun create(name: String, type: String): Vehicle {
            return Vehicle(name, type)
        }
    }
}

// Your code here

Hint

Call Vehicle.create("Car", "Sedan") and assign it to car.

4

Print the vehicle details

Write a println statement to print the text: "Vehicle: [name], Type: [type]" using the car object's name and type properties. Use a Kotlin string template.

Kotlin

class Vehicle(val name: String, val type: String) {
    companion object {
        fun create(name: String, type: String): Vehicle {
            return Vehicle(name, type)
        }
    }
}

val car = Vehicle.create("Car", "Sedan")

// Your code here

Hint

Use println("Vehicle: ${car.name}, Type: ${car.type}") to print the details.

Practice

1. What is the main purpose of using a companion object in Kotlin's factory pattern?

easy

A. To provide a place for factory methods that create instances

B. To allow multiple constructors in a class

C. To make the class inherit from another class

D. To store instance variables for each object

Solution

1. Step 1: Understand companion objects

   Companion objects in Kotlin are used to hold methods and properties related to the class but not tied to an instance.

2. Step 2: Role in factory pattern

   In the factory pattern, companion objects provide factory methods to create instances, especially when constructors are private.

3. Final Answer:

   To provide a place for factory methods that create instances -> Option A

4. Quick Check:

   Companion object = factory methods [OK]

Hint: Companion objects hold factory methods for instance creation [OK]

Common Mistakes:

• Thinking companion objects store instance variables
• Confusing companion objects with inheritance
• Believing companion objects allow multiple constructors

2. Which of the following is the correct syntax to declare a private constructor in a Kotlin class?

easy

A. private class MyClass constructor() {}

B. class MyClass private constructor() {}

C. class MyClass constructor private() {}

D. class MyClass constructor() private {}

Solution

1. Step 1: Recall Kotlin constructor syntax

   In Kotlin, the constructor keyword follows the class name, and modifiers like private come before constructor.

2. Step 2: Identify correct placement

   The correct syntax places 'private' before 'constructor()' right after the class name.

3. Final Answer:

   class MyClass private constructor() {} -> Option B

4. Quick Check:

   Private constructor syntax = class MyClass private constructor() {} [OK]

Hint: Put 'private' before 'constructor()' after class name [OK]

Common Mistakes:

• Placing 'private' after constructor parentheses
• Putting 'private' before class keyword
• Misordering 'constructor' and 'private'

3. What will be the output of the following Kotlin code?

class Product private constructor(val name: String) {
    companion object {
        fun create(name: String): Product {
            return Product(name)
        }
    }
}

fun main() {
    val p = Product.create("Book")
    println(p.name)
}

medium

A. Book

B. null

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand private constructor usage

   The class Product has a private constructor, so instances cannot be created directly outside the class.

2. Step 2: Use of companion object factory method

   The companion object provides a create() method that calls the private constructor and returns a Product instance.

3. Step 3: Output of main function

   In main(), Product.create("Book") creates an instance with name "Book". Printing p.name outputs "Book".

4. Final Answer:

   Book -> Option A

5. Quick Check:

   Factory method returns instance with name "Book" [OK]

Hint: Factory method returns instance; print property directly [OK]

Common Mistakes:

• Expecting compilation error due to private constructor
• Thinking create() returns null
• Confusing runtime exception with private constructor

4. Identify the error in this Kotlin factory pattern code:

class User private constructor(val id: Int) {
    companion object {
        fun create(id: Int): User {
            User(id)
        }
    }
}

medium

A. Companion object cannot access private constructor

B. Constructor should not be private

C. Function create() should be outside companion object

D. Missing return statement in create() method

Solution

1. Step 1: Check create() method body

   The create() method calls User(id) but does not return the created object.

2. Step 2: Understand Kotlin function return rules

   Functions must explicitly return a value unless they are Unit (void). Here, create() should return the new User instance.

3. Final Answer:

   Missing return statement in create() method -> Option D

4. Quick Check:

   Factory method must return created object [OK]

Hint: Always return the new instance in factory methods [OK]

Common Mistakes:

• Assuming constructor can't be private for companion access
• Placing factory method outside companion object
• Forgetting return keyword in factory method

5. Given a Kotlin class with a private constructor and a companion object factory, how can you modify the factory method to return a default instance when input is invalid (e.g., empty string)?

class Device private constructor(val model: String) {
    companion object {
        fun create(model: String): Device {
            return Device(model)
        }
    }
}

hard

A. Throw an exception inside create() if model is empty

B. Make constructor public and handle validation outside

C. Add a condition in create() to return Device("Unknown") if model is empty

D. Remove companion object and use secondary constructor

Solution

1. Step 1: Understand the problem

   The factory method should handle invalid input by returning a default instance instead of failing.

2. Step 2: Modify create() method

   Add a check: if model is empty, return Device("Unknown"), else return Device(model).

3. Step 3: Why this works

   This approach keeps constructor private, centralizes instance creation, and handles invalid input gracefully.

4. Final Answer:

   Add a condition in create() to return Device("Unknown") if model is empty -> Option C

5. Quick Check:

   Factory method handles invalid input with default instance [OK]

Hint: Use if-check in factory to return default instance on invalid input [OK]

Common Mistakes:

• Making constructor public defeats factory pattern
• Throwing exception instead of default instance
• Removing companion object loses factory benefits