Functions Introduction

Functions are the building blocks of any Kotlin program. You've already seen fun main() — now let's learn how to create your own.

Declaring a function

Use the fun keyword, followed by the name, parentheses, and the body in curly braces.

fun greet() {
  println("Hello!")
}

fun main() {
  greet()
  greet()
}
    

You call a function by its name followed by parentheses.

Return types

If a function returns a value, specify its type after :.

fun add(a: Int, b: Int): Int {
  return a + b
}

fun main() {
  val result = add(3, 7)
  println(result)
}
    

The return keyword gives back a value to the caller.

Unit return type

When a function doesn't return a meaningful value, its return type is Unit — just like void in Java. You can omit it.

fun sayHello(name: String): Unit {
  println("Hello, $name!")
}

fun main() {
  sayHello("Kotlin")
}
    

In practice, you rarely write : Unit. Kotlin assumes it when there's no return type declared.

Single-expression functions

For a function that's just one expression, skip the curly braces and use =.

fun square(x: Int) = x * x

fun main() {
  println(square(5))
}
    

Kotlin infers the return type automatically. This is concise and elegant.