Kotlin Help

Basic syntax

This is a collection of basic syntax elements with examples. At the end of every section, you'll find a link to a detailed description of the related topic.

You can also learn all the Kotlin essentials with the free Kotlin Basics track on JetBrains Academy.

Package definition and imports

Package specification should be at the top of the source file.

package my.demo import kotlin.text.* // ...

It is not required to match directories and packages: source files can be placed arbitrarily in the file system.

See Packages.

Program entry point

An entry point of a Kotlin application is the main function.

fun main() { println("Hello world!") }

Another form of main accepts a variable number of String arguments.

fun main(args: Array<String>) { println(args.contentToString()) }

print prints its argument to the standard output.

fun main() { //sampleStart print("Hello ") print("world!") //sampleEnd }

println prints its arguments and adds a line break, so that the next thing you print appears on the next line.

fun main() { //sampleStart println("Hello world!") println(42) //sampleEnd }

Functions

A function with two Int parameters and Int return type.

//sampleStart fun sum(a: Int, b: Int): Int { return a + b } //sampleEnd fun main() { print("sum of 3 and 5 is ") println(sum(3, 5)) }

A function body can be an expression. Its return type is inferred.

//sampleStart fun sum(a: Int, b: Int) = a + b //sampleEnd fun main() { println("sum of 19 and 23 is ${sum(19, 23)}") }

A function that returns no meaningful value.

//sampleStart fun printSum(a: Int, b: Int): Unit { println("sum of $a and $b is ${a + b}") } //sampleEnd fun main() { printSum(-1, 8) }

Unit return type can be omitted.

//sampleStart fun printSum(a: Int, b: Int) { println("sum of $a and $b is ${a + b}") } //sampleEnd fun main() { printSum(-1, 8) }

See Functions.

Variables

Read-only local variables are defined using the keyword val. They can be assigned a value only once.

fun main() { //sampleStart val a: Int = 1 // immediate assignment val b = 2 // `Int` type is inferred val c: Int // Type required when no initializer is provided c = 3 // deferred assignment //sampleEnd println("a = $a, b = $b, c = $c") }

Variables that can be reassigned use the var keyword.

fun main() { //sampleStart var x = 5 // `Int` type is inferred x += 1 //sampleEnd println("x = $x") }

You can declare variables at the top level.

//sampleStart val PI = 3.14 var x = 0 fun incrementX() { x += 1 } //sampleEnd fun main() { println("x = $x; PI = $PI") incrementX() println("incrementX()") println("x = $x; PI = $PI") }

See also Properties.

Creating classes and instances

To define a class, use the class keyword.

class Shape

Properties of a class can be listed in its declaration or body.

class Rectangle(var height: Double, var length: Double) { var perimeter = (height + length) * 2 }

The default constructor with parameters listed in the class declaration is available automatically.

class Rectangle(var height: Double, var length: Double) { var perimeter = (height + length) * 2 } fun main() { //sampleStart val rectangle = Rectangle(5.0, 2.0) println("The perimeter is ${rectangle.perimeter}") //sampleEnd }

Inheritance between classes is declared by a colon (:). Classes are final by default; to make a class inheritable, mark it as open.

open class Shape class Rectangle(var height: Double, var length: Double): Shape() { var perimeter = (height + length) * 2 }

See classes and objects and instances.

Comments

Just like most modern languages, Kotlin supports single-line (or end-of-line) and multi-line (block) comments.

// This is an end-of-line comment /* This is a block comment on multiple lines. */

Block comments in Kotlin can be nested.

/* The comment starts here /* contains a nested comment *⁠/ and ends here. */

See Documenting Kotlin Code for information on the documentation comment syntax.

String templates

fun main() { //sampleStart var a = 1 // simple name in template: val s1 = "a is $a" a = 2 // arbitrary expression in template: val s2 = "${s1.replace("is", "was")}, but now is $a" //sampleEnd println(s2) }

See String templates for details.

Conditional expressions

//sampleStart fun maxOf(a: Int, b: Int): Int { if (a > b) { return a } else { return b } } //sampleEnd fun main() { println("max of 0 and 42 is ${maxOf(0, 42)}") }

In Kotlin, if can also be used as an expression.

//sampleStart fun maxOf(a: Int, b: Int) = if (a > b) a else b //sampleEnd fun main() { println("max of 0 and 42 is ${maxOf(0, 42)}") }

See if -expressions.

for loop

fun main() { //sampleStart val items = listOf("apple", "banana", "kiwifruit") for (item in items) { println(item) } //sampleEnd }

or

fun main() { //sampleStart val items = listOf("apple", "banana", "kiwifruit") for (index in items.indices) { println("item at $index is ${items[index]}") } //sampleEnd }

See for loop.

while loop

fun main() { //sampleStart val items = listOf("apple", "banana", "kiwifruit") var index = 0 while (index < items.size) { println("item at $index is ${items[index]}") index++ } //sampleEnd }

See while loop.

when expression

//sampleStart fun describe(obj: Any): String = when (obj) { 1 -> "One" "Hello" -> "Greeting" is Long -> "Long" !is String -> "Not a string" else -> "Unknown" } //sampleEnd fun main() { println(describe(1)) println(describe("Hello")) println(describe(1000L)) println(describe(2)) println(describe("other")) }

See when expression.

Ranges

Check if a number is within a range using in operator.

fun main() { //sampleStart val x = 10 val y = 9 if (x in 1..y+1) { println("fits in range") } //sampleEnd }

Check if a number is out of range.

fun main() { //sampleStart val list = listOf("a", "b", "c") if (-1 !in 0..list.lastIndex) { println("-1 is out of range") } if (list.size !in list.indices) { println("list size is out of valid list indices range, too") } //sampleEnd }

Iterate over a range.

fun main() { //sampleStart for (x in 1..5) { print(x) } //sampleEnd }

Or over a progression.

fun main() { //sampleStart for (x in 1..10 step 2) { print(x) } println() for (x in 9 downTo 0 step 3) { print(x) } //sampleEnd }

See Ranges and progressions.

Collections

Iterate over a collection.

fun main() { val items = listOf("apple", "banana", "kiwifruit") //sampleStart for (item in items) { println(item) } //sampleEnd }

Check if a collection contains an object using in operator.

fun main() { val items = setOf("apple", "banana", "kiwifruit") //sampleStart when { "orange" in items -> println("juicy") "apple" in items -> println("apple is fine too") } //sampleEnd }

Using lambda expressions to filter and map collections:

fun main() { //sampleStart val fruits = listOf("banana", "avocado", "apple", "kiwifruit") fruits .filter { it.startsWith("a") } .sortedBy { it } .map { it.uppercase() } .forEach { println(it) } //sampleEnd }

See Collections overview.

Nullable values and null checks

A reference must be explicitly marked as nullable when null value is possible. Nullable type names have ? at the end.

Return null if str does not hold an integer:

fun parseInt(str: String): Int? { // ... }

Use a function returning nullable value:

fun parseInt(str: String): Int? { return str.toIntOrNull() } //sampleStart fun printProduct(arg1: String, arg2: String) { val x = parseInt(arg1) val y = parseInt(arg2) // Using `x * y` yields error because they may hold nulls. if (x != null && y != null) { // x and y are automatically cast to non-nullable after null check println(x * y) } else { println("'$arg1' or '$arg2' is not a number") } } //sampleEnd fun main() { printProduct("6", "7") printProduct("a", "7") printProduct("a", "b") }

or

fun parseInt(str: String): Int? { return str.toIntOrNull() } fun printProduct(arg1: String, arg2: String) { val x = parseInt(arg1) val y = parseInt(arg2) //sampleStart // ... if (x == null) { println("Wrong number format in arg1: '$arg1'") return } if (y == null) { println("Wrong number format in arg2: '$arg2'") return } // x and y are automatically cast to non-nullable after null check println(x * y) //sampleEnd } fun main() { printProduct("6", "7") printProduct("a", "7") printProduct("99", "b") }

See Null-safety.

Type checks and automatic casts

The is operator checks if an expression is an instance of a type. If an immutable local variable or property is checked for a specific type, there's no need to cast it explicitly:

//sampleStart fun getStringLength(obj: Any): Int? { if (obj is String) { // `obj` is automatically cast to `String` in this branch return obj.length } // `obj` is still of type `Any` outside of the type-checked branch return null } //sampleEnd fun main() { fun printLength(obj: Any) { println("Getting the length of '$obj'. Result: ${getStringLength(obj) ?: "Error: The object is not a string"} ") } printLength("Incomprehensibilities") printLength(1000) printLength(listOf(Any())) }

or

//sampleStart fun getStringLength(obj: Any): Int? { if (obj !is String) return null // `obj` is automatically cast to `String` in this branch return obj.length } //sampleEnd fun main() { fun printLength(obj: Any) { println("Getting the length of '$obj'. Result: ${getStringLength(obj) ?: "Error: The object is not a string"} ") } printLength("Incomprehensibilities") printLength(1000) printLength(listOf(Any())) }

or even

//sampleStart fun getStringLength(obj: Any): Int? { // `obj` is automatically cast to `String` on the right-hand side of `&&` if (obj is String && obj.length > 0) { return obj.length } return null } //sampleEnd fun main() { fun printLength(obj: Any) { println("Getting the length of '$obj'. Result: ${getStringLength(obj) ?: "Error: The object is not a string"} ") } printLength("Incomprehensibilities") printLength("") printLength(1000) }

See Classes and Type casts.

Last modified: 13 September 2021