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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 :

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 , you can pass it either as a named argument or :

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 with names using the spread operator:

fun foo(vararg strings: String) { /*...*/ } foo(strings = *arrayOf("a", "b", "c"))

When calling Java functions on the JVM, you can't use the named argument syntax because Java bytecode does not always preserve the names of function parameters.

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 a function returns a single expression, the curly braces can be omitted and the body is specified after a = symbol:

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

Explicitly declaring the return type is 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, .

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 =4 is visible as an array of =4, as in the example above, where the =6 variable has type =7.

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 null1. If you already have an array and want to pass its contents to the function, use the spread operator (prefix the array with null2):

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

If you want to pass a into vararg, you need to convert it to a regular (typed) array using the null4 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 null5 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 and must have no .

infix fun Int.shl(x: Int): Int { ... } // calling the function using the infix notation 1 shl 2 // is the same as 1.shl(2)

Infix function calls have lower precedence than arithmetic operators, type casts, and the null6 operator. The following expressions are equivalent:

• null7 is equivalent to null8

• null9 is equivalent to reformat()0

• reformat()1 is equivalent to reformat()2

On the other hand, an infix function call's precedence is higher than that of the boolean operators reformat()3 and reformat()4, reformat()5- and reformat()6-checks, and some other operators. These expressions are equivalent as well:

• reformat()7 is equivalent to reformat()8

• reformat()9 is equivalent to vararg0

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 vararg1 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 (). In addition to top level functions, Kotlin functions can also be declared locally as member functions and extension functions.

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 vararg3 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 vararg4 of cosine, which is a mathematical constant. It simply calls vararg5 repeatedly starting at vararg6 until the result no longer changes, yielding a result of vararg7 for the specified vararg8 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 vararg3 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 spread0/spread1/spread2 blocks, or on open functions. Currently, tail recursion is supported by Kotlin for the JVM and Kotlin/Native.