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.

Practice

1. What does the Kotlin syntax ::functionName represent?

easy

A. A call to the function named functionName

B. A comment about the function named functionName

C. A variable declaration named functionName

D. A reference to the function named functionName

Solution

1. Step 1: Understand the :: operator in Kotlin

   The :: operator is used to get a reference to a function, not to call it.

2. Step 2: Differentiate between calling and referencing

   Calling a function uses parentheses like functionName(), while referencing uses ::functionName.

3. Final Answer:

   A reference to the function named functionName -> Option D

4. Quick Check:

   Function reference = ::functionName [OK]

Hint: Remember :: means 'point to function', not call it [OK]

Common Mistakes:

• Confusing function reference (::) with function call (())
• Thinking :: declares a variable
• Assuming :: is a comment or annotation

2. Which of the following is the correct syntax to pass a function reference to map in Kotlin?

easy

A. list.map(::functionName)

B. list.map(:functionName)

C. list.map(functionName())

D. list.map(functionName)

Solution

1. Step 1: Identify how to pass function references in Kotlin

   Function references use the :: operator before the function name when passed as arguments.

2. Step 2: Check each option for correct syntax

   list.map(::functionName) uses ::functionName, which is the correct way to pass a function reference to map.

3. Final Answer:

   list.map(::functionName) -> Option A

4. Quick Check:

   Pass function reference with :: [OK]

Hint: Use :: before function name when passing as argument [OK]

Common Mistakes:

• Passing function call instead of reference (using parentheses)
• Omitting :: operator
• Using incorrect single colon : instead of double ::

3. What is the output of this Kotlin code?

fun square(x: Int) = x * x
val numbers = listOf(1, 2, 3)
val squares = numbers.map(::square)
println(squares)

medium

A. [1, 4, 9]

B. [square, square, square]

C. [1, 2, 3]

D. Compilation error

Solution

1. Step 1: Understand the function reference usage

   The ::square passes the square function to map, which applies it to each element.

2. Step 2: Calculate the result of applying square to each number

   Applying square to 1, 2, 3 gives 1, 4, 9 respectively.

3. Final Answer:

   [1, 4, 9] -> Option A

4. Quick Check:

   map(::square) squares each element [OK]

Hint: map(::function) applies function to each list item [OK]

Common Mistakes:

• Expecting original list instead of transformed list
• Confusing function reference with function call
• Thinking output is function names instead of results

4. Identify the error in this Kotlin code snippet:

fun greet(name: String) = "Hello, $name"
val greeter = greet
println(greeter("Alice"))

medium

A. Missing parentheses when assigning function reference

B. Cannot assign function directly without :: operator

C. Function greet must return Unit

D. No error, code runs fine

Solution

1. Step 1: Check how function references are assigned

   Assigning a function to a variable requires the :: operator to get a reference.

2. Step 2: Identify the missing :: operator

   The code assigns greet instead of ::greet, causing a type mismatch error.

3. Final Answer:

   Cannot assign function directly without :: operator -> Option B

4. Quick Check:

   Use :: to assign function reference [OK]

Hint: Use :: when assigning function to variable [OK]

Common Mistakes:

• Assigning function name without ::
• Adding parentheses which calls function instead of referencing
• Assuming function must return Unit

5. Given this Kotlin code, what will be the output?

fun isEven(n: Int) = n % 2 == 0
val numbers = listOf(1, 2, 3, 4, 5, 6)
val evenNumbers = numbers.filter(::isEven)
println(evenNumbers)

Choose the correct output and explain why function references are useful here.

hard

A. [true, false, true, false, true, false]

B. [1, 3, 5]

C. [2, 4, 6]

D. Compilation error due to filter usage

Solution

1. Step 1: Understand the function reference with filter

   The ::isEven passes the isEven function to filter, which keeps elements where the function returns true.

2. Step 2: Apply isEven to each number and filter

   Numbers 2, 4, and 6 are even, so they remain in the filtered list.

3. Step 3: Explain usefulness of function references

   Function references let us pass existing functions easily without writing extra lambdas.

4. Final Answer:

   [2, 4, 6] -> Option C

5. Quick Check:

   filter(::isEven) keeps even numbers [OK]

Hint: filter(::function) keeps items where function returns true [OK]

Common Mistakes:

• Confusing filter with map output
• Expecting boolean list instead of filtered list
• Not using :: for function reference