Factory pattern with companion objects in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Factory pattern in Kotlin?

The Factory pattern is a way to create objects without exposing the creation logic to the client. It uses a method to return instances, making object creation flexible and centralized.

beginner

What is a companion object in Kotlin?

A companion object is a special object inside a class that allows you to define methods and properties tied to the class itself, similar to static members in other languages.

intermediate

How does the Factory pattern use companion objects in Kotlin?

In Kotlin, the Factory pattern often uses companion objects to hold factory methods. This lets you create instances by calling methods on the class itself, keeping creation logic inside the class.

intermediate

Example: What does this Kotlin code do?
<pre>class Car private constructor(val type: String) {
  companion object {
    fun createSedan() = Car("Sedan")
    fun createSUV() = Car("SUV")
  }
}</pre>

This code defines a Car class with a private constructor, so it can't be created directly. The companion object has factory methods createSedan() and createSUV() that create Car instances of specific types.

intermediate

Why use a private constructor with a companion object factory in Kotlin?

Using a private constructor prevents direct creation of objects. The companion object factory methods control how objects are made, ensuring consistent and safe creation.

What keyword makes a constructor private in Kotlin to support the Factory pattern?

A private

B protected

C internal

D public

The 'private' keyword restricts constructor access, so objects can only be created through factory methods.

Where are factory methods typically placed in Kotlin when using the Factory pattern?

A In interface declarations

B In separate utility classes

C Inside companion objects

D In the main function

Companion objects hold factory methods to create instances without exposing constructors.

What is the main benefit of using the Factory pattern with companion objects?

A Makes all properties mutable

B Automatically generates getters and setters

C Allows multiple inheritance

D Centralizes object creation logic

Factory pattern centralizes and controls how objects are created.

How do you call a factory method named createSedan in a companion object of class Car?

A new Car.createSedan()

B Car.createSedan()

C createSedan.Car()

D Car().createSedan()

Factory methods in companion objects are called on the class name directly.

What happens if the constructor is not private in a Factory pattern?

A Objects can be created directly, bypassing factory methods

B Factory methods won't work

C The class becomes abstract

D The compiler throws an error

Without a private constructor, clients can create objects directly, which may break the pattern.

Explain how the Factory pattern works with companion objects in Kotlin.

Think about how you hide the constructor and provide special methods to create objects.

Describe the benefits of using a private constructor with companion object factory methods.

Why would you want to stop people from making objects any way they want?

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