Functions

Kotlin functions are declared using the fun keyword:

fun double(x: Int): Int {
    return 2 * x
}

Function usage

Functions are called using the standard approach:

val result = double(2)

Calling member functions uses dot notation:

Stream().read() // create instance of class Stream and call read()

Parameters

Function parameters are defined using Pascal notation - name: type. Parameters are separated using commas, and each parameter must be explicitly typed:

fun powerOf(number: Int, exponent: Int): Int { /*...*/ }

You can use a trailing comma when you declare function parameters:

fun powerOf(
    number: Int,
    exponent: Int, // trailing comma
) { /*...*/ }

Default arguments

Function parameters can have default values, which are used when you skip the corresponding argument. This reduces the number of overloads:

fun read(
    b: ByteArray,
    off: Int = 0,
    len: Int = b.size,
) { /*...*/ }

A default value is set by appending = to the type.

Overriding methods always use the base method's default parameter values. When overriding a method that has default parameter values, the default parameter values must be omitted from the signature:

open class A {
    open fun foo(i: Int = 10) { /*...*/ }
}

class B : A() {
    override fun foo(i: Int) { /*...*/ }  // No default value is allowed.
}

If a default parameter precedes a parameter with no default value, the default value can only be used by calling the function with named arguments:

fun foo(
    bar: Int = 0,
    baz: Int,
) { /*...*/ }

foo(baz = 1) // The default value bar = 0 is used

If the last argument after default parameters is a lambda, you can pass it either as a named argument or outside the parentheses:

fun foo(
    bar: Int = 0,
    baz: Int = 1,
    qux: () -> Unit,
) { /*...*/ }

foo(1) { println("hello") }     // Uses the default value baz = 1
foo(qux = { println("hello") }) // Uses both default values bar = 0 and baz = 1
foo { println("hello") }        // Uses both default values bar = 0 and baz = 1

Named arguments

You can name one or more of a function's arguments when calling it. This can be helpful when a function has many arguments and it's difficult to associate a value with an argument, especially if it's a boolean or null value.

When you use named arguments in a function call, you can freely change the order that they are listed in. If you want to use their default values, you can just leave these arguments out altogether.

Consider the reformat() function, which has 4 arguments with default values.

fun reformat(
    str: String,
    normalizeCase: Boolean = true,
    upperCaseFirstLetter: Boolean = true,
    divideByCamelHumps: Boolean = false,
    wordSeparator: Char = ' ',
) { /*...*/ }

When calling this function, you don't have to name all its arguments:

reformat(
    "String!",
    false,
    upperCaseFirstLetter = false,
    divideByCamelHumps = true,
    '_'
)

You can skip all the ones with default values:

reformat("This is a long String!")

You are also able to skip specific arguments with default values, rather than omitting them all. However, after the first skipped argument, you must name all subsequent arguments:

reformat("This is a short String!", upperCaseFirstLetter = false, wordSeparator = '_')

You can pass a variable number of arguments (vararg) with names using the spread operator:

fun foo(vararg strings: String) { /*...*/ }

foo(strings = *arrayOf("a", "b", "c"))

Unit-returning functions

If a function does not return a useful value, its return type is Unit. Unit is a type with only one value - Unit. This value does not have to be returned explicitly:

fun printHello(name: String?): Unit {
    if (name != null)
        println("Hello $name")
    else
        println("Hi there!")
    // `return Unit` or `return` is optional
}

The Unit return type declaration is also optional. The above code is equivalent to:

fun printHello(name: String?) { ... }

Single-expression functions

When the function body consists of a single expression, the curly braces can be omitted and the body specified after an = symbol:

fun double(x: Int): Int = x * 2

Explicitly declaring the return type is optional when this can be inferred by the compiler:

fun double(x: Int) = x * 2

Explicit return types

Functions with block body must always specify return types explicitly, unless it's intended for them to return Unit, in which case specifying the return type is optional.

Kotlin does not infer return types for functions with block bodies because such functions may have complex control flow in the body, and the return type will be non-obvious to the reader (and sometimes even for the compiler).

Variable number of arguments (varargs)

You can mark a parameter of a function (usually the last one) with the vararg modifier:

fun <T> asList(vararg ts: T): List<T> {
    val result = ArrayList<T>()
    for (t in ts) // ts is an Array
        result.add(t)
    return result
}

In this case, you can pass a variable number of arguments to the function:

val list = asList(1, 2, 3)

Inside a function, a vararg-parameter of type T is visible as an array of T, as in the example above, where the ts variable has type Array<out T>.

Only one parameter can be marked as vararg. If a vararg parameter is not the last one in the list, values for the subsequent parameters can be passed using named argument syntax, or, if the parameter has a function type, by passing a lambda outside the parentheses.

When you call a vararg-function, you can pass arguments individually, for example asList(1, 2, 3). If you already have an array and want to pass its contents to the function, use the spread operator (prefix the array with *):

val a = arrayOf(1, 2, 3)
val list = asList(-1, 0, *a, 4)

If you want to pass a primitive type array into vararg, you need to convert it to a regular (typed) array using the toTypedArray() function:

val a = intArrayOf(1, 2, 3) // IntArray is a primitive type array
val list = asList(-1, 0, *a.toTypedArray(), 4)

Infix notation

Functions marked with the infix keyword can also be called using the infix notation (omitting the dot and the parentheses for the call). Infix functions must meet the following requirements:

They must be member functions or extension functions.
They must have a single parameter.
The parameter must not accept variable number of arguments and must have no default value.

infix fun Int.shl(x: Int): Int { ... }

// calling the function using the infix notation
1 shl 2

// is the same as
1.shl(2)

Note that infix functions always require both the receiver and the parameter to be specified. When you're calling a method on the current receiver using the infix notation, use this explicitly. This is required to ensure unambiguous parsing.

class MyStringCollection {
    infix fun add(s: String) { /*...*/ }
    
    fun build() {
        this add "abc"   // Correct
        add("abc")       // Correct
        //add "abc"        // Incorrect: the receiver must be specified
    }
}

Function scope

Kotlin functions can be declared at the top level in a file, meaning you do not need to create a class to hold a function, which you are required to do in languages such as Java, C#, and Scala (top level definition is available since Scala 3). In addition to top level functions, Kotlin functions can also be declared locally as member functions and extension functions.

Local functions

Kotlin supports local functions, which are functions inside other functions:

fun dfs(graph: Graph) {
    fun dfs(current: Vertex, visited: MutableSet<Vertex>) {
        if (!visited.add(current)) return
        for (v in current.neighbors)
            dfs(v, visited)
    }

    dfs(graph.vertices[0], HashSet())
}

A local function can access local variables of outer functions (the closure). In the case above, visited can be a local variable:

fun dfs(graph: Graph) {
    val visited = HashSet<Vertex>()
    fun dfs(current: Vertex) {
        if (!visited.add(current)) return
        for (v in current.neighbors)
            dfs(v)
    }

    dfs(graph.vertices[0])
}

Member functions

A member function is a function that is defined inside a class or object:

class Sample {
    fun foo() { print("Foo") }
}

Member functions are called with dot notation:

Sample().foo() // creates instance of class Sample and calls foo

For more information on classes and overriding members see Classes and Inheritance.

Generic functions

Functions can have generic parameters, which are specified using angle brackets before the function name:

fun <T> singletonList(item: T): List<T> { /*...*/ }

For more information on generic functions, see Generics.

Tail recursive functions

Kotlin supports a style of functional programming known as tail recursion. For some algorithms that would normally use loops, you can use a recursive function instead without the risk of stack overflow. When a function is marked with the tailrec modifier and meets the required formal conditions, the compiler optimizes out the recursion, leaving behind a fast and efficient loop based version instead:

val eps = 1E-10 // "good enough", could be 10^-15

tailrec fun findFixPoint(x: Double = 1.0): Double =
    if (Math.abs(x - Math.cos(x)) < eps) x else findFixPoint(Math.cos(x))

This code calculates the fixpoint of cosine, which is a mathematical constant. It simply calls Math.cos repeatedly starting at 1.0 until the result no longer changes, yielding a result of 0.7390851332151611 for the specified eps precision. The resulting code is equivalent to this more traditional style:

val eps = 1E-10 // "good enough", could be 10^-15

private fun findFixPoint(): Double {
    var x = 1.0
    while (true) {
        val y = Math.cos(x)
        if (Math.abs(x - y) < eps) return x
        x = Math.cos(x)
    }
}

To be eligible for the tailrec modifier, a function must call itself as the last operation it performs. You cannot use tail recursion when there is more code after the recursive call, within try/catch/finally blocks, or on open functions. Currently, tail recursion is supported by Kotlin for the JVM and Kotlin/Native.