Published Apr 18, 2023Updated Apr 28, 2023
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Functions are an essential part of any Kotlin program. Functions are a reusable part of code that can be called in any part of the program. A Kotlin program has to start with the main() function, or there will be an error. The fun keyword is used to define a function in Kotlin, but there are also predefined functions like println and print.

The Main Function

Function main() is the entry point to a Kotlin program and is required. The main() function is meant to include the code of the program and is not called anywhere. The code should be between the main() function curly brackets {}.

fun main() {
println("Hello World")

Creating Functions in Kotlin

User defined functions start with the fun keyword, followed by the function name, the parameters (if there are any), and the function body which is enclosed in curly braces.

fun functionName(arguments): return type {
  return statement
  • fun is used to define a function.
  • functionName is used to call the function in the program.
  • arguments have to be in the parentheses. Arguments are pieces of information fed to the function.
  • return type declares the type of a returning value. It is optional.
  • body contains the code to be executed.
  • return statement returns the value of the function.


Arguments are data passed to the function. They must have a name and data type. Between the argument name and data type must be a colon :. The arguments are separated by comma , .

fun functionName(argumentName: dataType, argumentName2 : datatype){
fun sample(num1: Int,num2: Int){
println(num1 + num2)

When calling the function, arguments can be put in their assigned spot or specified by name. If the arguments are specified by name they do not need to be in the same order as in the function header.

fun main(){
println(sample(num2=5, num1=3))

Arguments can have a default value. It is assigned with the equals sign =.

fun default(name: String = "Kotlin"){
print("Hello $name")

Default arguments can be changed when calling a function.

fun main(){
// Output
// Hello Java

Return Type

The return type expresses the returning value type. Including the return type is optional. The return type is entered after the parentheses and a colon :. By default the return value is Unit. Unit means there is no returning value.

fun functionName(argument): return type {
fun sample(name: String): String {
return "Hello " + name
fun main(){
// Output
// Hello Tom

Function Body

The function body contains the code for the function. It must be between curly brackets {}. The function body is indented, except for a single line function. The opening curly bracket { is at the end of the function header and the closing curly bracket } is below the last line of the function body.

fun functionName(argument) {

Return Statement

If there is a return statement in the function, then there must be also a return type. The return statement is defined by the keyword return. Lines after the return statement will not be executed.

fun adding(num1: Int,num2: Int): Int{
var sum = num1 + num2
// Return statement, returns the variable sum
return sum
// println will not execute
println("Nice job")

Single Line Function

The single line function can be written only if the function has a single expression.

fun area(side1: Int, side2: Int): Int{
return side1 * side2

Curly brackets {} and return can be removed and by adding an equals sign =, we get the single line function.

fun area(side1: Int, side2: Int): Int = side1 * side2

The compiler can infer the return type so we can also remove the return type declaration.

fun area(side1: Int, side2: Int) = side1 * side2

Function Literals

A function becomes a function literal when it is assigned as a value. There are two types of function literals: “lambda expressions” and “anonymous functions”.

Lambda Expressions

A lambda expression is a short function between curly braces {}. The return is replaced by ->. Adding return type is optional because the compiler can infer it from the type interface.

variable name = {Lambda Expression}
fun main() {
// Lambda Expression
val adding = {num1: Int, num2: Int -> num1 + num2 }
// Executing
// Output
// 3

Anonymous Function

An anonymous function is assigned as a variable, fun comes after equals sign =. It has no name, which is why it is called anonymous.

  • Option One: After the variable name is the argument types in parentheses () then -> and the return type. This is followed by the equals sign = and the fun keyword, arguments in parenthesis, the return type, and the single return statement in curly braces {} .
  • Option Two: After the variable name is an equals sign =, then the fun keyword, followed by the arguments with argument types in parentheses (), followed by : and the return type, followed by a single return statement in curly brackets {}.
fun main() {
// Option One
var addingOne: (Int, Int) -> Int = fun(num1, num2): Int { return num1 + num2 }
// Option Two
var addingTwo = fun(num1: Int, num2: Int): Int { return num1 + num2 }
println(addingOne(1, 2))
println(addingTwo(1, 2))
// Output
// 3
// 3

Video Walkthrough

Watch this video to learn about functions and their use in Kotlin programming.

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