Hello, World!
fun main() {
println("Hello, World!")
}1 file
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Kotlin 2.4.10ReadyMain.ktLn 1, Col 1Spaces: 4UTF-8
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Main.kt · output appears here
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Output streams here as your program runs.
Kotlin is a statically typed, general-purpose programming language developed by JetBrains, with its first stable release (1.0) in 2016. It runs on the Java Virtual Machine and is fully interoperable with Java, while offering concise syntax, null safety, and support for both object-oriented and functional programming paradigms.
Kotlin is Google's preferred language for Android app development and is also used for server-side applications, web (via Kotlin/JS), and multiplatform projects. Its expressive syntax, coroutines for asynchronous programming, and strong tooling make it popular for building modern, maintainable applications across mobile and backend.
Main.kt is the entry point. Add helper files from the explorer and import them into your project.
Use the Input panel to pipe standard input to your program when it needs data.
Run sends your code to a fresh hosted runtime and streams the result into the output panel.
Your draft stays in this browser, and Share creates a separate snapshot link.
Kotlin tabs compile together, so Main.kt can call a top-level helper declaration.
fun main() {
println(answer())
}
fun answer(): Int = 42
fun main() {
println("Hello, World!")
}fun main() {
print("Enter your name: ")
val name = readLine()
println("Hello, $name!")
}fun main() {
val fruits = listOf("apple", "banana", "cherry")
for ((i, fruit) in fruits.withIndex()) {
println("${i + 1}: $fruit")
}
}fun factorial(n: Int): Long =
if (n <= 1) 1 else n * factorial(n - 1)
fun main() {
println(factorial(5))
}fun main() {
try {
val n = "abc".toInt()
println(n)
} catch (e: NumberFormatException) {
println("Invalid number: ${e.message}")
} finally {
println("Done")
}
}fun main() {
val counts = mutableMapOf<Char, Int>()
for (c in "banana") {
counts[c] = (counts[c] ?: 0) + 1
}
for ((ch, n) in counts) {
println("$ch: $n")
}
}fun main() {
val nums = listOf(3, 1, 4, 1, 5)
val doubled = nums.sorted().map { it * 2 }
println(doubled)
println("Sum: ${nums.sum()}")
}data class Person(val name: String, val age: Int) {
fun describe() = "$name ($age)"
}
fun main() {
val p = Person("Ada", 36)
println(p.describe())
}sealed class Shape
class Circle(val r: Double) : Shape()
class Square(val side: Double) : Shape()
fun area(s: Shape): Double = when (s) {
is Circle -> 3.14159 * s.r * s.r
is Square -> s.side * s.side
}
fun main() {
println(area(Square(3.0)))
}fun String.shout() = this.uppercase() + "!"
fun main() {
println("hello".shout())
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Kotlin is one of 23 linked-project compilers. Run always starts Main.kt; selecting a helper tab changes what you edit, not the entry file.
Kotlin tabs compile together, so Main.kt can call a top-level helper declaration.
Linked-project languages (23): Node.js, TypeScript, Deno, Python, C, Go, Dart, Swift, Rust, Ruby, PHP, Java, Kotlin, C++, C#, Nim, Crystal, Haxe, Zig, Scala, D, Bun, Objective-C.
Single-file scratchpads (46): Matplotlib, NumPy, pandas, Haskell, Clojure, Perl, Lua, Tcl, jq, Raku, Elixir, Bash, Zsh, Nushell, R, Assembly (NASM), Julia, Octave, Groovy, PowerShell, Visual Basic .NET, Odin, Pascal, Lisp, Erlang, CoffeeScript, COBOL, BASIC, V, Racket, OCaml, Prolog, AWK, Emacs Lisp, Emojicode, Forth, F#, Fortran, Assembly (NASM64), Solidity, Mojo, Ada, WASM (WAT), Gleam, Shell Script (sh), Scheme. Their extra project files are not submitted to the runtime.
Download project exports every Kotlin tab and a README in one ZIP archive.