Function references (::functionName) in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function reference with map

What is the output of this Kotlin code using function references?

Kotlin

fun square(x: Int) = x * x

fun main() {
    val numbers = listOf(1, 2, 3, 4)
    val squares = numbers.map(::square)
    println(squares)
}

A Compilation error

B [1, 2, 3, 4]

C [2, 4, 6, 8]

D [1, 4, 9, 16]

💡 Hint

Remember that ::square passes the function itself to map, which applies it to each element.

The function reference ::square is passed to map, which calls square on each number, producing their squares.

❓ Predict Output

intermediate

Using bound function reference

What will this Kotlin code print?

Kotlin

class Greeter(val greeting: String) {
    fun greet(name: String) = "$greeting, $name!"
}

fun main() {
    val greeter = Greeter("Hello")
    val greetFunction = greeter::greet
    println(greetFunction("Alice"))
}

A Hello, greetFunction!

B Compilation error

C Hello, Alice!

D greetFunction

💡 Hint

Bound function references keep the instance context.

The bound reference greeter::greet keeps the greeter instance, so calling greetFunction("Alice") returns "Hello, Alice!".

🔧 Debug

advanced

Identify the error with function reference usage

What error does this Kotlin code produce?

Kotlin

fun add(a: Int, b: Int) = a + b

fun main() {
    val numbers = listOf(1, 2, 3)
    val result = numbers.map(::add)
    println(result)
}

A Type mismatch: Expected a function of type (Int) -> R but found (Int, Int) -> Int

B Compilation error: Unresolved reference 'add'

C [2, 3, 4]

D [1, 2, 3]

💡 Hint

Check the expected function type for map and the signature of add.

map expects a function taking one argument, but add takes two, causing a type mismatch error.

❓ Predict Output

advanced

Using function reference with filter and predicate

What is the output of this Kotlin code?

Kotlin

fun isEven(n: Int) = n % 2 == 0

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5, 6)
    val evens = numbers.filter(::isEven)
    println(evens)
}

A [2, 4, 6]

B [1, 3, 5]

C [1, 2, 3, 4, 5, 6]

D Compilation error

💡 Hint

filter uses the function reference as a predicate to select elements.

The function reference ::isEven is used as a predicate to filter even numbers from the list.

❓ Predict Output

expert

Output of function reference with overloaded functions

What will this Kotlin code print?

Kotlin

fun printValue(value: Int) = println("Int: $value")
fun printValue(value: String) = println("String: $value")

fun main() {
    val printer: (Any) -> Unit = ::printValue
    printer(42)
}

A Runtime error

B Compilation error: Overload resolution ambiguity

C String: 42

D Int: 42

💡 Hint

Consider how Kotlin resolves overloaded function references with a general type.

The compiler cannot decide which printValue to use for (Any) -> Unit, causing an overload resolution ambiguity error.