Companion objects as static alternatives in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of companion object property access

What is the output of this Kotlin code?

Kotlin

class Example {
    companion object {
        val greeting = "Hello, Kotlin!"
    }
}

fun main() {
    println(Example.greeting)
}

A null

B Compilation error: Cannot access companion object property directly

C Hello, Kotlin!

D Runtime exception

💡 Hint

Companion object properties can be accessed using the class name.

In Kotlin, companion object members can be accessed directly via the class name without creating an instance.

❓ Predict Output

intermediate

Calling companion object function

What will this Kotlin program print?

Kotlin

class Calculator {
    companion object {
        fun add(a: Int, b: Int) = a + b
    }
}

fun main() {
    println(Calculator.add(5, 7))
}

A 12

B Compilation error: Cannot call companion object function like this

C 0

D Runtime exception

💡 Hint

Companion object functions behave like static methods.

The add function inside the companion object can be called using the class name without creating an instance.

🔧 Debug

advanced

Why does this companion object code fail?

This Kotlin code produces a compilation error. What is the cause?

Kotlin

class Person {
    companion object {
        val name: String
    }

    init {
        name = "Alice"
    }
}

fun main() {
    println(Person.name)
}

A Cannot access companion object property from init block

B The init block cannot assign values to companion object properties

C Companion object cannot have properties

D Property 'name' in companion object must be initialized or declared abstract

💡 Hint

Companion object properties need initialization at declaration or in an init block inside the companion object.

The property 'name' in the companion object is declared but not initialized. Kotlin requires properties in companion objects to be initialized or abstract.

🧠 Conceptual

advanced

Companion object vs static in Kotlin

Which statement correctly describes the difference between Kotlin companion objects and Java static members?

A Companion objects are compiled as static members in Java bytecode with no object instance.

B Companion objects are real objects and can implement interfaces, unlike Java static members.

C Companion objects cannot have functions, only properties.

D Companion objects are only accessible through instances of the class.

💡 Hint

Think about how companion objects behave as objects themselves.

Companion objects are singleton objects inside a class and can implement interfaces, have their own state, and be passed around as objects, unlike static members in Java.

❓ Predict Output

expert

Output of companion object with private constructor

What is the output of this Kotlin program?

Kotlin

class Secret private constructor(val code: Int) {
    companion object {
        fun create(code: Int): Secret = Secret(code)
    }
}

fun main() {
    val secret = Secret.create(42)
    println(secret.code)
}

A 42

B Compilation error: Cannot access private constructor

C Runtime exception

D null

💡 Hint

Companion objects can access private constructors of their class.

The companion object is part of the class and can call private constructors. The create function returns an instance with code 42.