TopolearnGetting a program running is the first step, and Kotlin projects are almost always built with Gradle.
A Kotlin program starts at a top-level main function; no class is required:
fun main() { println("Hello, Kotlin!") }If you need command-line arguments, declare fun main(args: Array<String>). Statements don't need semicolons, and println / print write to standard output; readln() reads a line from standard input.
Ways to run Kotlin:
- IntelliJ IDEA (made by JetBrains, Kotlin's creator) or Android Studio: click the run gutter icon next to
main. - Gradle with the Kotlin JVM plugin, the standard for real projects:
./gradlew run. Build scripts themselves are often written in Kotlin (build.gradle.kts). - The command-line compiler:
kotlinc hello.kt -include-runtime -d hello.jarthenjava -jar hello.jar. - Scripts (
.main.kts) and the online Kotlin Playground for quick experiments.
Kotlin compiles to JVM bytecode, so a Kotlin app is a normal Java application and can use any Java library. The same language also targets JavaScript, WebAssembly and native binaries through Kotlin Multiplatform.
Files contain top-level functions, properties and classes, optionally in a package. A file Main.kt with top-level functions compiles to a JVM class named MainKt.
Example
// Main.kt - run with: kotlinc Main.kt -include-runtime -d app.jar && java -jar app.jar
fun main(args: Array<String>) {
println("Hello, Kotlin!") // Hello, Kotlin!
println("Args: ${args.size}") // Args: 0
val name = "Ada"
print("Hi, ")
println(name) // Hi, Ada
}Remember
- Programs start at a top-level fun main()
- Real projects build with Gradle; IntelliJ IDEA is the main IDE
- Kotlin compiles to JVM bytecode and uses Java libraries directly