vararg Parameters

A Kotlin function parameter can be defined as a vararg parameter so it can be called with zero or more values.

Key points

  • Declare function parameters with the vararg keyword
  • When arrays are passed in, handle them with the * character
  • Note: “In Kotlin, a vararg parameter of type T is internally represented as an array of type T (Array<T>) inside the function body.”

Basic example

Here’s a function that takes a vararg parameter:

fun printAll(vararg ints: Int) {
    for (i in ints) println(i)

The REPL shows how it works:

> printAll(1)

> printAll(1,2,3)

> printAll()

Passing in an array

Passing in an array won’t work:

val arr = intArrayOf(1,2,3)

> printAll(arr)
error: type mismatch: inferred type is Array<Int> but Int was expected

The solution is that you need to pass the array into your vararg parameter with this syntax:


The * character in this use is known as the “spread operator.” The spread operator lets you convert arrays — well, most arrays — so they can be passed into a vararg parameter.

Not all arrays work (TODO)

The spread operator only works with certain types of arrays with vararg parameters. For example, I just showed that intArrayOf works:

> val intArray = intArrayOf(1,2,3)

> printAll(*intArray)

However, arrayOf does not work when it’s given a list of integers:

> val arrayOfInt = arrayOf(1,2,3)

> printAll(*arrayOfInt)
error: type mismatch: inferred type is Array<Int> but IntArray was expected

To understand this it helps to look at the data types in the Kotlin REPL. Here are some details about the result from intArrayOf:

> val x = intArrayOf(1,2,3)

> println(

> println(x.javaClass.kotlin)
class kotlin.IntArray

> println(x.javaClass.kotlin.qualifiedName)

Similarly, here are some results for arrayOf:

> val x = arrayOf(1,2,3)

> println(

> println(x.javaClass.kotlin)
class kotlin.Array

> println(x.javaClass.kotlin.qualifiedName)

As shown by those results, intArrayOf creates a type of kotlin.IntArray, which works with a vararg parameter of type Int, but arrayOf creates a type of kotlin.Array, which does not work with the same vararg parameter. (This is true as of Kotlin version 1.2.51.)

Bonus: Make it generic (TODO: NOT WORKING)

fun <T> printAll(vararg elems: T) {
    for (e in elems) println(e)

fun Int.toString() = this printAll(arrayOf(1,2,3)) printAll(intArrayOf(1,2,3))

val x = arrayOf(1,2,3) val x = intArrayOf(1,2,3)

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