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 Core track by 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 to the standard output

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

In Kotlin, you declare a variable starting with a keyword, val or var, followed by the name of the variable.

Use the val keyword to declare variables that are assigned a value only once. These are immutable, read-only local variables that can’t be reassigned a different value after initialization:

fun main() {
//sampleStart
    // Declares the variable x and initializes it with the value of 5
    val x: Int = 5
    // 5
//sampleEnd
    println(x)
}

Use the var keyword to declare variables that can be reassigned. These are mutable variables, and you can change their values after initialization:

fun main() {
//sampleStart
    // Declares the variable x and initializes it with the value of 5
    var x: Int = 5
    // Reassigns a new value of 6 to the variable x
    x += 1
    // 6
//sampleEnd
    println(x)
}

Kotlin supports type inference and automatically identifies the data type of a declared variable. When declaring a variable, you can omit the type after the variable name:

fun main() {
//sampleStart
    // Declares the variable x with the value of 5;`Int` type is inferred
    val x = 5
    // 5
//sampleEnd
    println(x)
}

You can use variables only after initializing them. You can either initialize a variable at the moment of declaration or declare a variable first and initialize it later. In the second case, you must specify the data type:

fun main() {
//sampleStart
    // Initializes the variable x at the moment of declaration; type is not required
    val x = 5
    // Declares the variable c without initialization; type is required
    val c: Int
    // Initializes the variable c after declaration 
    c = 3
    // 5 
    // 3
//sampleEnd
    println(x)
    println(c)
}

You can declare variables at the top level:

//sampleStart
val PI = 3.14
var x = 0

fun incrementX() {
    x += 1
}
// x = 0; PI = 3.14
// incrementX()
// x = 1; PI = 3.14
//sampleEnd

fun main() {
    println("x = $x; PI = $PI")
    incrementX()
    println("incrementX()")
    println("x = $x; PI = $PI")
}

For information about declaring properties, see 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(val height: Double, val length: Double) {
    val perimeter = (height + length) * 2 
}

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

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

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(val height: Double, val length: Double): Shape() {
    val perimeter = (height + length) * 2 
}

For more information about constructors and inheritance, 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
}

Use 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: 16 April 2024