Extension function syntax in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple extension function call

What is the output of this Kotlin code using an extension function?

Kotlin

fun String.shout() = this.uppercase() + "!"

fun main() {
    val word = "hello"
    println(word.shout())
}

A hello!

B HELLO!

C Hello!

D HELLO

💡 Hint

Remember that the extension function converts the string to uppercase and adds an exclamation mark.

The extension function shout() converts the string to uppercase and appends an exclamation mark, so "hello" becomes "HELLO!".

🧠 Conceptual

intermediate

Understanding receiver type in extension functions

In Kotlin, what does the receiver type in an extension function represent?

A The type of the object the extension function is called on

B The type of the variable inside the extension function

C The type of the parameter passed to the extension function

D The return type of the extension function

💡 Hint

Think about what object you are extending when you write an extension function.

The receiver type is the type of the object that the extension function is called on, allowing you to add new functions to that type.

🔧 Debug

advanced

Identify the error in this extension function

What error will this Kotlin code produce?

Kotlin

fun Int.add(x: Int): Int {
    return this + x
}

fun main() {
    val result = 5.add()
    println(result)
}

A Error: No value passed for parameter 'x'

B Error: Extension function cannot have parameters

C Error: 'this' cannot be used in extension functions

D No error, output is 5

💡 Hint

Check how the extension function is called and what parameters it expects.

The extension function add requires one parameter 'x', but the call 5.add() provides none, causing a compilation error.

📝 Syntax

advanced

Correct syntax for extension function with nullable receiver

Which option shows the correct syntax for an extension function on a nullable String receiver that returns its length or zero if null?

A fun String?.lengthOrZero() = length ?: 0

B fun String.lengthOrZero() = this?.length ?: 0

C fun String?.lengthOrZero() = this?.length ?: 0

D fun String?.lengthOrZero() = this.length ?: 0

💡 Hint

Remember to use the safe call operator when the receiver can be null.

Option C correctly declares the receiver as nullable String? and uses safe call 'this?.length' to avoid null pointer exceptions.

🚀 Application

expert

Extension function to filter and transform a list

Given this extension function on List, what is the output of the code below?

Kotlin

fun List<Int>.filterAndDouble(predicate: (Int) -> Boolean): List<Int> {
    val result = mutableListOf<Int>()
    for (item in this) {
        if (predicate(item)) {
            result.add(item * 2)
        }
    }
    return result
}

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5)
    val filtered = numbers.filterAndDouble { it % 2 == 1 }
    println(filtered)
}

A [2, 4, 6, 8, 10]

B [1, 3, 5]

C [1, 2, 3, 4, 5]

D [2, 6, 10]

💡 Hint

The predicate filters odd numbers, then doubles them before adding to the result list.

The function filters odd numbers (1,3,5) and doubles them (2,6,10), so the output is [2, 6, 10].