# Kotlin — Modern Statically Typed Language for JVM and Beyond

> Kotlin is a statically typed, general-purpose programming language developed by JetBrains. Runs on the JVM, Android, native, WebAssembly, and JS targets. Officially endorsed by Google as the preferred language for Android development.

## Install

Save the content below to `.claude/skills/` or append to your `CLAUDE.md`:

## Quick Use

```bash
# Install via SDKMAN
curl -s https://get.sdkman.io | bash
sdk install kotlin

# Or via Homebrew
brew install kotlin

kotlinc -version
kotlin -version
```

Hello world:
```kotlin
// Main.kt
fun main() {
    val users = listOf(
        User("Alice", 95),
        User("Bob",   87),
        User("Carol", 91),
    )

    val topUser = users.maxByOrNull { it.score }
    println("Top: ${topUser?.name}")

    users
        .sortedByDescending { it.score }
        .forEach { println("${it.name}: ${it.score}") }
}

data class User(val name: String, val score: Int)
```

```bash
kotlinc Main.kt -include-runtime -d main.jar
java -jar main.jar
```

Gradle (most common):
```kotlin
// build.gradle.kts
plugins {
    kotlin("jvm") version "2.0.0"
    application
}

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.8.0")
}

application {
    mainClass.set("MainKt")
}
```

## Intro

Kotlin is a statically typed, general-purpose programming language developed by JetBrains. Kotlin compiles to Java bytecode (JVM), JavaScript, WebAssembly, and native binaries. Officially endorsed by Google as a preferred language for Android development in 2017. Now the #1 language for new Android apps and widely used on the server (Spring Boot + Kotlin, Ktor) and multiplatform (Kotlin Multiplatform).

- **Repo**: https://github.com/JetBrains/kotlin
- **Stars**: 52K+
- **License**: Apache 2.0

## What Kotlin Does

- **Null safety** — types distinguish nullable (`String?`) from non-null (`String`)
- **Data classes** — auto-generate equals, hashCode, copy
- **Extension functions** — add methods to existing classes
- **Coroutines** — structured concurrency with suspend functions
- **Smart casts** — compiler narrows types after checks
- **Type inference** — minimal explicit typing
- **Functional** — higher-order, lambdas, collections DSL
- **Multiplatform** — share code across JVM, JS, Native, iOS, Android
- **Compose** — Jetpack Compose (Android) and Compose Multiplatform (desktop/web/iOS)
- **Full Java interop** — call Java from Kotlin and vice versa

## Architecture

Kotlin compiler targets JVM bytecode by default (also JS, Native via LLVM, WASM). The Kotlin Multiplatform toolchain compiles shared code for multiple targets. Coroutines are implemented as a compiler transform + runtime library — no OS thread per coroutine.

## Self-Hosting

Language toolchain.

## Key Features

- Null safety in the type system
- Data classes
- Extension functions
- Coroutines (structured concurrency)
- Higher-order functions and lambdas
- Operator overloading
- Sealed classes + when expressions
- Kotlin Multiplatform
- Jetpack Compose (Android UI)
- Compose Multiplatform (desktop + iOS)
- Gradle Kotlin DSL

## Comparison

| Language | Target | Null Safety | Coroutines |
|---|---|---|---|
| Kotlin | JVM/JS/Native | Yes | Yes |
| Java | JVM | Optional | Project Loom |
| Scala | JVM | Option type | Yes |
| Swift | Native | Yes | async/await + actors |
| C# | CLR | Nullable ref types | async/await |
| Go | Native | No | Goroutines |

## FAQ

**Q: Compared to Java?**
A: Kotlin is Java's improved version: null safety, data classes, lambdas, extension functions, coroutines, and less boilerplate. Most Java projects can gradually migrate to Kotlin, and the two languages can coexist in the same project.

**Q: Must I use Kotlin for Android?**
A: Not required, but Google strongly recommends it. Since 2019, new APIs, official docs, and Android KTX are Kotlin-first.

**Q: Is Compose Multiplatform mature?**
A: In 2024, Desktop and Android are stable, iOS is gradually stabilizing, and Web is still maturing. Usable for production but evaluate against your own needs.

## Sources

- Docs: https://kotlinlang.org/docs
- GitHub: https://github.com/JetBrains/kotlin
- License: Apache 2.0

---
Source: https://tokrepo.com/en/workflows/kotlin-modern-statically-typed-language-jvm-beyond-7cce7cf5
Author: Script Depot