Factory pattern with companion objects in Kotlin

Introduction

The factory pattern helps create objects without exposing the creation logic. Companion objects in Kotlin let us put this logic inside the class itself.

When you want to control how objects of a class are created.

When you want to return different types of objects from the same method.

When you want to hide complex creation steps from the user.

When you want to create objects with some default or custom settings.

When you want to avoid calling constructors directly.

Syntax

Kotlin

class ClassName private constructor(val property: Type) {
    companion object {
        fun create(param: Type): ClassName {
            // creation logic
            return ClassName(param)
        }
    }
}

The constructor is private to prevent direct creation from outside.

The companion object acts like a static factory inside the class.

Examples

Simple factory method inside companion object to create a User.

Kotlin

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

Factory methods to create different shapes without exposing constructor details.

Kotlin

class Shape private constructor(val type: String) {
    companion object {
        fun createCircle(): Shape = Shape("Circle")
        fun createSquare(): Shape = Shape("Square")
    }
}

Sample Program

This program uses the factory pattern with companion objects to create Vehicle objects. The constructor is private, so you must use the factory methods to create vehicles.

Kotlin

class Vehicle private constructor(val type: String) {
    companion object {
        fun createCar(): Vehicle = Vehicle("Car")
        fun createBike(): Vehicle = Vehicle("Bike")
    }
}

fun main() {
    val car = Vehicle.createCar()
    val bike = Vehicle.createBike()
    println("Vehicle 1 is a ${car.type}")
    println("Vehicle 2 is a ${bike.type}")
}

Important Notes

Using private constructors ensures objects are only created through factory methods.

Companion objects let you group factory methods inside the class for better organization.

This pattern helps keep object creation flexible and easy to change later.

Summary

The factory pattern controls object creation.

Companion objects in Kotlin provide a neat place for factory methods.

Private constructors prevent direct object creation, enforcing use of factories.

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