Basic Syntax

    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.

    1. fun main() {
    2.     println("Hello world!")
    3. }

    Functions

    Function having two Int parameters with Int return type:

    1. //sampleStart
    2. fun sum(a: Int, b: Int): Int {
    3.     return a + b
    4. }
    5. //sampleEnd
    7. fun main() {
    8.     print("sum of 3 and 5 is ")
    9.     println(sum(3, 5))
    10. }

    Function with an expression body and inferred return type:

    1. //sampleStart
    2. fun sum(a: Int, b: Int) = a + b
    3. //sampleEnd
    5. fun main() {
    6.     println("sum of 19 and 23 is ${sum(19, 23)}")
    7. }

    Function returning no meaningful value:

    1. //sampleStart
    2. fun printSum(a: Int, b: Int): Unit {
    3.     println("sum of $a and $b is ${a + b}")
    4. }
    5. //sampleEnd
    7. fun main() {
    8.     printSum(-1, 8)
    9. }

    Unit return type can be omitted:

    1. //sampleStart
    2. fun printSum(a: Int, b: Int) {
    3.     println("sum of $a and $b is ${a + b}")
    4. }
    5. //sampleEnd
    7. fun main() {
    8.     printSum(-1, 8)
    9. }

    See .

    Variables

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

    1. fun main() {
    2. //sampleStart
    3.     val a: Int = 1  // immediate assignment
    4.     val b = 2   // `Int` type is inferred
    5.     val c: Int  // Type required when no initializer is provided
    6.     c = 3       // deferred assignment
    7. //sampleEnd
    8.     println("a = $a, b = $b, c = $c")
    9. }

    Variables that can be reassigned use the var keyword:

    1. fun main() {
    2. //sampleStart
    3.     var x = 5 // `Int` type is inferred
    4.     x += 1
    5. //sampleEnd
    6.     println("x = $x")
    7. }

    Top-level variables:

    1. //sampleStart
    2. val PI = 3.14
    3. var x = 0
    5. fun incrementX() { 
    6.     x += 1 
    7. }
    8. //sampleEnd
    10. fun main() {
    11.     println("x = $x; PI = $PI")
    12.     incrementX()
    13.     println("incrementX()")
    14.     println("x = $x; PI = $PI")
    15. }

    See also .

    Comments

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

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

    See for information on the documentation comment syntax.

    1. fun main() {
    2. //sampleStart
    3.     var a = 1
    4.     // simple name in template:
    5.     val s1 = "a is $a" 
    7.     a = 2
    8.     // arbitrary expression in template:
    9.     val s2 = "${s1.replace("is", "was")}, but now is $a"
    10. //sampleEnd
    11.     println(s2)
    12. }

    See String templates for details.

    Conditional expressions

    1. //sampleStart
    2. fun maxOf(a: Int, b: Int): Int {
    3.     if (a > b) {
    4.         return a
    5.     } else {
    6.         return b
    7.     }
    8. }
    9. //sampleEnd
    11. fun main() {
    12.     println("max of 0 and 42 is ${maxOf(0, 42)}")
    13. }

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

    1. //sampleStart
    2. fun maxOf(a: Int, b: Int) = if (a > b) a else b
    3. //sampleEnd
    5. fun main() {
    6.     println("max of 0 and 42 is ${maxOf(0, 42)}")
    7. }

    See if-expressions.

    Nullable values and null checks

    A reference must be explicitly marked as nullable when null value is possible.

    Return null if str does not hold an integer:

    1. fun parseInt(str: String): Int? {
    2. }

    Use a function returning nullable value:

    1. fun parseInt(str: String): Int? {
    2.     return str.toIntOrNull()
    3. }
    5. //sampleStart
    6. fun printProduct(arg1: String, arg2: String) {
    7.     val x = parseInt(arg1)
    8.     val y = parseInt(arg2)
    10.     // Using `x * y` yields error because they may hold nulls.
    11.     if (x != null && y != null) {
    12.         // x and y are automatically cast to non-nullable after null check
    13.         println(x * y)
    14.     }
    15.     else {
    16.         println("'$arg1' or '$arg2' is not a number")
    17.     }    
    18. }
    19. //sampleEnd
    22. fun main() {
    23.     printProduct("6", "7")
    24.     printProduct("a", "7")
    25.     printProduct("a", "b")
    26. }

    or

    1. fun parseInt(str: String): Int? {
    2.     return str.toIntOrNull()
    3. }
    5. fun printProduct(arg1: String, arg2: String) {
    6.     val x = parseInt(arg1)
    7.     val y = parseInt(arg2)
    9. //sampleStart
    10.     // ...
    11.     if (x == null) {
    12.         println("Wrong number format in arg1: '$arg1'")
    13.         return
    14.     }
    15.     if (y == null) {
    16.         println("Wrong number format in arg2: '$arg2'")
    17.         return
    18.     }
    20.     // x and y are automatically cast to non-nullable after null check
    21.     println(x * y)
    22. //sampleEnd
    23. }
    25. fun main() {
    26.     printProduct("6", "7")
    27.     printProduct("a", "7")
    28.     printProduct("99", "b")
    29. }

    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:

    1. //sampleStart
    2. fun getStringLength(obj: Any): Int? {
    3.     if (obj is String) {
    4.         // `obj` is automatically cast to `String` in this branch
    5.         return obj.length
    6.     }
    8.     // `obj` is still of type `Any` outside of the type-checked branch
    9.     return null
    10. }
    11. //sampleEnd
    14. fun main() {
    15.     fun printLength(obj: Any) {
    16.         println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
    17.     }
    18.     printLength("Incomprehensibilities")
    19.     printLength(1000)
    20.     printLength(listOf(Any()))
    21. }

    or

    1. //sampleStart
    2. fun getStringLength(obj: Any): Int? {
    3.     if (obj !is String) return null
    5.     // `obj` is automatically cast to `String` in this branch
    6.     return obj.length
    7. }
    8. //sampleEnd
    11. fun main() {
    12.     fun printLength(obj: Any) {
    13.         println("'$obj' string length is ${getStringLength(obj) ?: "... err, not a string"} ")
    14.     }
    15.     printLength("Incomprehensibilities")
    16.     printLength(1000)
    17.     printLength(listOf(Any()))
    18. }

    or even

    1. //sampleStart
    2. fun getStringLength(obj: Any): Int? {
    3.     // `obj` is automatically cast to `String` on the right-hand side of `&&`
    4.     if (obj is String && obj.length > 0) {
    5.         return obj.length
    6.     }
    8.     return null
    9. }
    10. //sampleEnd
    13. fun main() {
    14.     fun printLength(obj: Any) {
    15.         println("'$obj' string length is ${getStringLength(obj) ?: "... err, is empty or not a string at all"} ")
    16.     }
    17.     printLength("Incomprehensibilities")
    18.     printLength("")
    19.     printLength(1000)
    20. }

    See Classes and .

    for loop

    1. fun main() {
    2. //sampleStart
    3.     val items = listOf("apple", "banana", "kiwifruit")
    4.     for (index in items.indices) {
    5.         println("item at $index is ${items[index]}")
    6.     }
    7. }

    See .

    2. //sampleStart
    3.     val items = listOf("apple", "banana", "kiwifruit")
    4.     var index = 0
    5.     while (index < items.size) {
    6.         println("item at $index is ${items[index]}")
    7.         index++
    8.     }
    9. //sampleEnd
    10. }

    See while loop.

    when expression

    1. //sampleStart
    2. fun describe(obj: Any): String =
    3.     when (obj) {
    4.         1          -> "One"
    5.         "Hello"    -> "Greeting"
    6.         is Long    -> "Long"
    7.         !is String -> "Not a string"
    8.         else       -> "Unknown"
    9.     }
    10. //sampleEnd
    12. fun main() {
    13.     println(describe(1))
    14.     println(describe("Hello"))
    15.     println(describe(1000L))
    16.     println(describe(2))
    17.     println(describe("other"))
    18. }

    See when expression.

    Ranges

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

    1. fun main() {
    2. //sampleStart
    3.     val x = 10
    4.     val y = 9
    5.     if (x in 1..y+1) {
    6.         println("fits in range")
    7.     }
    8. //sampleEnd
    9. }

    Check if a number is out of range:

    1. fun main() {
    2. //sampleStart
    3.     val list = listOf("a", "b", "c")
    5.     if (-1 !in 0..list.lastIndex) {
    6.         println("-1 is out of range")
    7.     }
    8.     if (list.size !in list.indices) {
    9.         println("list size is out of valid list indices range, too")
    10.     }
    11. //sampleEnd
    12. }

    Iterating over a range:

    1. fun main() {
    2. //sampleStart
    3.     for (x in 1..5) {
    4.         print(x)
    5.     }
    6. //sampleEnd
    7. }

    or over a progression:

    1. fun main() {
    2. //sampleStart
    3.     for (x in 1..10 step 2) {
    4.         print(x)
    5.     }
    6.     println()
    7.     for (x in 9 downTo 0 step 3) {
    8.         print(x)
    9.     }
    10. //sampleEnd
    11. }

    See Ranges.

    Collections

    Iterating over a collection:

    1. fun main() {
    2.     val items = listOf("apple", "banana", "kiwifruit")
    3. //sampleStart
    4.     for (item in items) {
    5.         println(item)
    6.     }
    7. //sampleEnd
    8. }

    Checking if a collection contains an object using in operator:

    1. fun main() {
    2.     val items = setOf("apple", "banana", "kiwifruit")
    3. //sampleStart
    4.     when {
    5.         "orange" in items -> println("juicy")
    6.         "apple" in items -> println("apple is fine too")
    7.     }
    8. //sampleEnd
    9. }

    Using lambda expressions to filter and map collections:

    See Collections overview.

    Creating basic classes and their instances

    1. fun main() {
    2. //sampleStart
    3.     val rectangle = Rectangle(5.0, 2.0)
    4.     val triangle = Triangle(3.0, 4.0, 5.0)
    5. //sampleEnd
    6.     println("Area of rectangle is ${rectangle.calculateArea()}, its perimeter is ${rectangle.perimeter}")
    7.     println("Area of triangle is ${triangle.calculateArea()}, its perimeter is ${triangle.perimeter}")
    8. }
    10. abstract class Shape(val sides: List<Double>) {
    11.     val perimeter: Double get() = sides.sum()
    12.     abstract fun calculateArea(): Double
    13. }
    15. interface RectangleProperties {
    16.     val isSquare: Boolean
    17. }
    19. class Rectangle(
    20.     var height: Double,
    21.     var length: Double
    22. ) : Shape(listOf(height, length, height, length)), RectangleProperties {
    23.     override val isSquare: Boolean get() = length == height
    24.     override fun calculateArea(): Double = height * length
    25. }
    27. class Triangle(
    28.     var sideA: Double,
    29.     var sideB: Double,
    30.     var sideC: Double
    31. ) : Shape(listOf(sideA, sideB, sideC)) {
    32.     override fun calculateArea(): Double {
    33.         val s = perimeter / 2
    34.         return Math.sqrt(s * (s - sideA) * (s - sideB) * (s - sideC))
    35.     }
    36. }

    See classes and .