Factory pattern with companion objects in Kotlin - Step-by-Step Execution

Concept Flow - Factory pattern with companion objects

Call factory method

↓

Companion object method

↓

Decide which class to create

↓

Create instance

↓

Return instance to caller

The factory method inside the companion object decides which class instance to create and returns it when called.

Execution Sample

Kotlin

class Animal private constructor(val type: String) {
    companion object {
        fun create(type: String): Animal = when(type) {
            "dog" -> Animal("dog")
            "cat" -> Animal("cat")
            else -> Animal("unknown")
        }
    }
}

val pet = Animal.create("dog")

This code uses a companion object factory method to create Animal instances based on a type string.

Execution Table

Step	Action	Input	Decision	Created Instance	Returned Value
1	Call factory method	type = "dog"	Check type value
2	Match type in when	"dog"	Matches "dog" case	Animal("dog")
3	Create Animal instance			Animal(type="dog")
4	Return instance				Animal(type="dog")
5	Assign to variable				pet = Animal(type="dog")
6	End				Factory method returns Animal instance

💡 Factory method returns the created Animal instance and execution ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
type	undefined	"dog"	"dog"	"dog"	"dog"	"dog"
instance	null	null	null	Animal(type="dog")	Animal(type="dog")	Animal(type="dog")
pet	null	null	null	null	Animal(type="dog")	Animal(type="dog")

Key Moments - 3 Insights

Why do we use a companion object for the factory method?

The companion object allows calling the factory method directly on the class without creating an instance first, as shown in step 1 of the execution_table.

What happens if the input type does not match any case?

The else branch creates an Animal with type "unknown", ensuring the factory always returns an instance, as implied in the when expression in step 2.

Why is the constructor private?

Making the constructor private forces users to create instances only through the factory method, controlling object creation as seen in step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what instance is created at step 3?

A Animal(type="unknown")

B Animal(type="cat")

C Animal(type="dog")

D No instance created

💡 Hint

Check the 'Created Instance' column at step 3 in the execution_table.

At which step is the factory method called?

A Step 1

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Action' column to find when the factory method is first called.

If the input type was "bird", what would be the returned instance type?

A "dog"

B "unknown"

C "cat"

D No instance returned

💡 Hint

Refer to the else branch in the when expression in the execution_sample code.

Concept Snapshot

Factory pattern with companion objects in Kotlin:
- Use companion object to hold factory methods.
- Factory method decides which instance to create.
- Private constructor restricts direct instantiation.
- Call factory method via ClassName.method() syntax.
- Ensures controlled and flexible object creation.

Full Transcript

This example shows how Kotlin's companion object can hold a factory method to create instances of a class. The class Animal has a private constructor, so it cannot be created directly. Instead, the companion object has a create() method that takes a type string. It uses a when expression to decide which Animal to create: dog, cat, or unknown. When we call Animal.create("dog"), the factory method runs, matches the "dog" case, creates an Animal with type "dog", and returns it. The variable pet then holds this instance. This pattern helps control how objects are made and keeps creation logic in one place.

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