Kotlin 1.5.0 Help

Basic syntax

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: 05 May 2021