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
}
The readln() function reads from the standard input. This function reads the entire line the user enters as a string.
You can use the println(), readln(), and print() functions together to print messages requesting and showing user input:
// Prints a message to request input
println("Enter any word: ")
// Reads and stores the user input. For example: Happiness
val yourWord = readln()
// Prints a message with the input
print("You entered the word: ")
print(yourWord)
// You entered the word: Happiness
For more information, see Read standard input.
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.
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 expressions and statements.
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: 06 November 2024