Named companion objects in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing a named companion object property

What is the output of this Kotlin code?

Kotlin

class Example {
    companion object Factory {
        val value = 42
    }
}

fun main() {
    println(Example.Factory.value)
}

A 42

B Factory

C null

D Compilation error

💡 Hint

Access the property using the named companion object.

The companion object named Factory holds the property 'value'. Accessing it via Example.Factory.value prints 42.

🧠 Conceptual

intermediate

Purpose of naming a companion object

Why would you name a companion object in Kotlin instead of leaving it unnamed?

A To provide a specific name for better code clarity and explicit access

B To allow multiple companion objects in the same class

C To make the companion object private

D To enable inheritance from the companion object

💡 Hint

Think about how naming affects code readability and access.

Naming a companion object gives it a clear identifier, improving readability and allowing explicit access like ClassName.CompanionName.

🔧 Debug

advanced

Identify the error when accessing an unnamed companion object with a name

What error occurs when you try to access an unnamed companion object using a name?

Kotlin

class Sample {
    companion object {
        val data = "Hello"
    }
}

fun main() {
    println(Sample.Factory.data)
}

A Compilation succeeds and prints 'Hello'

B Unresolved reference: Factory

C NullPointerException at runtime

D Type mismatch error

💡 Hint

Check if the companion object has the name you are using.

The companion object is unnamed, so accessing it as Sample.Factory causes a compilation error: Unresolved reference.

📝 Syntax

advanced

Correct syntax to declare a named companion object

Which of the following is the correct syntax to declare a named companion object in Kotlin?

A

class MyClass {
    object companion Factory {
        val x = 10
    }
}

B

class MyClass {
    companion Factory {
        val x = 10
    }
}

C

class MyClass {
    companion object Factory {
        val x = 10
    }
}

D

class MyClass {
    companion object {
        val x = 10
    } Factory
}

💡 Hint

Remember the keyword order for companion object naming.

The correct syntax is 'companion object Factory' inside the class to name the companion object Factory.

🚀 Application

expert

Using named companion object to implement a factory method

Given the class below, which option correctly uses the named companion object to create an instance with a factory method?

Kotlin

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

fun main() {
    val user = ???
    println(user.name)
}

A User.create("Alice")

B User.Creator("Alice")

C User.Companion.create("Alice")

D User.Creator.create("Alice")

💡 Hint

Use the named companion object to call the factory method.

Since the companion object is named Creator, you must call the factory method as User.Creator.create("Alice").