Factory pattern with companion objects in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a companion object inside the class.

Kotlin

class Car {
    [1] object {
        fun create() = Car()
    }
}

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

Acompanion

BFactory

Cobject

DCompanion

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Companion' instead of lowercase 'companion'.

Forgetting the keyword 'companion' before 'object'.

In Kotlin, the keyword companion is used to declare a companion object inside a class.

2fill in blank

medium

Complete the factory method to create a Car instance.

Kotlin

class Car {
    companion object {
        fun [1](): Car {
            return Car()
        }
    }
}

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

Acreate

Bnew

Cbuild

Dmake

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'new' which is not a function name in Kotlin.

Using unrelated method names like 'build' or 'make' without context.

The factory method is commonly named create to indicate it returns a new instance.

3fill in blank

hard

Fix the error in the factory method call to create a Car instance.

Kotlin

fun main() {
    val car = Car.[1]()
    println("Car created: $car")
}

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

AnewCar

Bcreate

Cbuild

Dmake

Attempts:

3 left

💡 Hint

Common Mistakes

Using method names that do not exist in the companion object.

Trying to call a constructor directly instead of using the factory method.

The correct factory method name is create as defined in the companion object.

4fill in blank

hard

Fill both blanks to complete the factory pattern with a companion object and a private constructor.

Kotlin

class Bike private constructor() {
    companion object {
        fun [1](): Bike {
            return Bike()
        }
    }
}

fun main() {
    val bike = Bike.[2]()
    println("Bike created: $bike")
}

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

Acreate

Bbuild

Cmake

Dnew

Attempts:

3 left

💡 Hint

Common Mistakes

Using different method names for declaration and call.

Trying to call the constructor directly which is private.

The factory method is named create and is called as Bike.create() to create a new instance.

5fill in blank

hard

Fill all three blanks to implement a factory pattern with companion object, private constructor, and a property.

Kotlin

class Truck private constructor(val model: String) {
    companion object {
        fun [1](model: String): Truck {
            return Truck([2])
        }
    }
}

fun main() {
    val truck = Truck.[3]("BigTruck")
    println("Truck model: ${truck.model}")
}

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

Acreate

Bmodel

C"model"

DmodelName

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the parameter as a string literal instead of the variable.

Using inconsistent method names for factory method declaration and call.

The factory method is named create. It takes a parameter model and passes it to the private constructor. The call uses Truck.create("BigTruck").

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