Function references (::functionName) in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a function reference in Kotlin?

A function reference in Kotlin is a way to refer to a function by its name using the :: operator, allowing you to pass the function as a value or assign it to a variable.

beginner

How do you create a function reference for a top-level function named printMessage?

You create it by writing ::printMessage. This refers to the function itself without calling it.

beginner

What is the difference between printMessage() and ::printMessage in Kotlin?

printMessage() calls the function immediately, while ::printMessage creates a reference to the function without calling it.

intermediate

Can you use function references to pass functions as arguments in Kotlin? Give an example.

Yes. For example, listOf(1, 2, 3).forEach(::println) passes the println function as a reference to forEach.

intermediate

How do you reference a member function of a class in Kotlin?

You use the class name followed by :: and the function name, like <code>String::toUpperCase</code>. This creates a reference to the member function.

What does the Kotlin expression ::myFunction represent?

A A reference to the function named myFunction

B A call to the function myFunction

C A variable named myFunction

D An error in syntax

Using :: before a function name creates a reference to that function without calling it.

Which of the following is a correct way to pass a function reference to a higher-order function in Kotlin?

A list.forEach(::print)

B list.forEach(print())

C list.forEach(print)

D list.forEach(:print)

The correct syntax to pass a function reference is ::functionName, so ::print is correct.

What is the output of this Kotlin code?

fun greet() = println("Hello")
val ref = ::greet
ref()

A No output

B Hello

C ref

D Compilation error

Calling ref() calls the function greet(), which prints "Hello".

How do you reference a member function length of the String class in Kotlin?

A ::String.length

B length::String

C String::length

D String.length::

The correct syntax for referencing a member function is ClassName::functionName.

Which statement is true about function references in Kotlin?

A They are not supported in Kotlin.

B They immediately execute the function.

C They can only reference top-level functions.

D They allow functions to be passed as values.

Function references let you treat functions as values to pass or store without calling them.

Explain what a function reference (::functionName) is in Kotlin and how it differs from calling a function.

Think about how you can pass a function itself, not its result.

Describe how to use function references with higher-order functions in Kotlin with an example.

Consider how you can pass println directly to forEach.