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.
An entry point of a Kotlin application is the main function.
fun main() {
println("Hello world!")
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
Another form of main accepts a variable number of String arguments.
fun main(args: Array<String>) {
println(args.contentToString())
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
print prints its argument to the standard output.
fun main() {
//sampleStart
print("Hello ")
print("world!")
//sampleEnd
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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))
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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)}")
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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)
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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)
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
See Functions.
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")
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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")
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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")
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
See also Properties.
To define a class, use the class keyword.
class ShapeProperties 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
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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.
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.
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)
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
See String templates for details.
//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)}")
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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)}")
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
See if-expressions.
fun main() {
//sampleStart
val items = listOf("apple", "banana", "kiwifruit")
for (item in items) {
println(item)
}
//sampleEnd
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
or
fun main() {
//sampleStart
val items = listOf("apple", "banana", "kiwifruit")
for (index in items.indices) {
println("item at $index is ${items[index]}")
}
//sampleEnd
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
See for 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
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
See while loop.
//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"))
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
See when expression.
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
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
Iterate over a range.
fun main() {
//sampleStart
for (x in 1..5) {
print(x)
}
//sampleEnd
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
Iterate over a collection.
fun main() {
val items = listOf("apple", "banana", "kiwifruit")
//sampleStart
for (item in items) {
println(item)
}
//sampleEnd
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
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
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
See Collections overview.
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 '$arg2' is not a number")
}
}
//sampleEnd
fun main() {
printProduct("6", "7")
printProduct("a", "7")
printProduct("a", "b")
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
or
fun parseInt(str: String): Int? {
return str.toIntOrNull()
}
fun printProduct(arg1: String, arg2: String) {
val x = parseInt(arg1)
val y = parseInt(arg2)
//sampleStart
// ...
if (x == null) {
println("Wrong number format in arg1: '$arg1'")
return
}
if (y == null) {
println("Wrong number format in arg2: '$arg2'")
return
}
// x and y are automatically cast to non-nullable after null check
println(x * y)
//sampleEnd
}
fun main() {
printProduct("6", "7")
printProduct("a", "7")
printProduct("99", "b")
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
See Null-safety.
The is operator checks if an expression is an instance of a type.
If an immutable local variable or property is checked for a specific type, there's no need to cast it explicitly:
//sampleStart
fun getStringLength(obj: Any): Int? {
if (obj is String) {
// `obj` is automatically cast to `String` in this branch
return obj.length
}
// `obj` is still of type `Any` outside of the type-checked branch
return null
}
//sampleEnd
fun main() {
fun printLength(obj: Any) {
println("Getting the length of '$obj'. Result: ${getStringLength(obj) ?: "Error: The object is not a string"} ")
}
printLength("Incomprehensibilities")
printLength(1000)
printLength(listOf(Any()))
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
or
//sampleStart
fun getStringLength(obj: Any): Int? {
if (obj !is String) return null
// `obj` is automatically cast to `String` in this branch
return obj.length
}
//sampleEnd
fun main() {
fun printLength(obj: Any) {
println("Getting the length of '$obj'. Result: ${getStringLength(obj) ?: "Error: The object is not a string"} ")
}
printLength("Incomprehensibilities")
printLength(1000)
printLength(listOf(Any()))
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
or even
//sampleStart
fun getStringLength(obj: Any): Int? {
// `obj` is automatically cast to `String` on the right-hand side of `&&`
if (obj is String && obj.length > 0) {
return obj.length
}
return null
}
//sampleEnd
fun main() {
fun printLength(obj: Any) {
println("Getting the length of '$obj'. Result: ${getStringLength(obj) ?: "Error: The object is not a string"} ")
}
printLength("Incomprehensibilities")
printLength("")
printLength(1000)
}{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
See Classes and Type casts.