Extensions on built-in types in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of extension function on String

What is the output of this Kotlin code that uses an extension function on a String?

Kotlin

fun String.addExclamation(): String {
    return this + "!"
}

fun main() {
    val greeting = "Hello"
    println(greeting.addExclamation())
}

A Compilation error

B Hello

C HELLO!

D Hello!

💡 Hint

Look at how the extension function adds an exclamation mark to the original string.

The extension function addExclamation appends an exclamation mark to the original string. So "Hello" becomes "Hello!".

❓ Predict Output

intermediate

Extension function on List with condition

What is the output of this Kotlin code that defines an extension function on List to sum only even numbers?

Kotlin

fun List<Int>.sumEven(): Int {
    return this.filter { it % 2 == 0 }.sum()
}

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5)
    println(numbers.sumEven())
}

A 9

B 10

C 6

D Compilation error

💡 Hint

Sum only the even numbers: 2 and 4.

The extension function filters the list to keep only even numbers (2 and 4) and sums them: 2 + 4 = 6.

🔧 Debug

advanced

Why does this extension function cause a compilation error?

This Kotlin code tries to add an extension function to Int to check if it is prime. Why does it cause a compilation error?

Kotlin

fun Int.isPrime(): Boolean {
    if (this < 2) return false
    for (i in 2 until this) {
        if (this % i == 0) return false
    }
    return true
}

fun main() {
    val number = 7
    println(number.isPrime())
}

A No error, code compiles and prints true

B Error because extension functions cannot be defined on Int

C Error because 'until' is not imported

D Error because 'this' cannot be used in extension functions

💡 Hint

Extension functions can be defined on any type including Int.

Extension functions can be defined on Int. The code compiles and correctly prints true for 7 being prime.

📝 Syntax

advanced

Identify the syntax error in this extension function

Which option shows the correct syntax for an extension function on String that returns the first character?

Kotlin

fun String.firstChar(): Char {
    return this[0]
}

A fun String.firstChar: Char { return this[0] }

B fun String.firstChar() = this[0]

C fun firstChar(this: String): Char = this[0]

D fun String.firstChar() { return this[0] }

💡 Hint

Extension functions must have the receiver type before the function name.

Option B uses correct syntax with expression body. Option B is a normal function, not extension. Option B misses parentheses. Option B has a block but no return type declared.

🚀 Application

expert

Using extension functions to add behavior to built-in types

You want to add an extension function to List that returns a single string with all elements joined by a comma and space. Which option produces the correct output for listOf("apple", "banana", "cherry")?

Kotlin

fun List<String>.toCommaSeparated(): String {
    // implementation
}

fun main() {
    val fruits = listOf("apple", "banana", "cherry")
    println(fruits.toCommaSeparated())
}

A return this.joinToString(", ")

B return this.reduce { acc, s -> acc + ", " + s }

C return this.joinToString(separator = ",")

D return this.fold("") { acc, s -> acc + s + ", " }

💡 Hint

Use the built-in joinToString function with proper separator.

Option A uses joinToString with ", " separator correctly. Option A works for non-empty lists but throws an exception on empty lists. Option A uses "," without space. Option A adds extra comma at end.