The main() Function

Declaring main()

The simplest entry point is fun main() { ... }, which takes no parameters. This is enough for most command-line programs, including everything covered in this course.

Example: Declaring main()

fun main() {
    println("main() with no parameters")
}

Accepting Command-Line Arguments

Kotlin also supports fun main(args: Array<String>), where args holds any arguments passed on the command line when the program is launched, such as java -jar app.jar hello world.

Note: If no arguments are passed, args is simply an empty array, not null.

Example: Accepting Command-Line Arguments

fun main(args: Array<String>) {
    println("Received ${args.size} arguments")
    for (arg in args) {
        println("Argument: $arg")
    }
}

Calling Other Functions from main()

Real programs break work into smaller functions and call them from main. This keeps main short and readable, acting as a high-level outline of what the program does.

Example: Calling Other Functions from main()

fun greet(name: String) {
    println("Hello, $name!")
}

fun main() {
    greet("Kotlin")
    println("Back in main() after calling greet()")
}

Program Exit

A Kotlin program ends naturally when main finishes running its last statement. Using return inside main exits it early, skipping any code written after that line.

Note: Avoid unreachable code after return -- the compiler will warn you about it.

Example: Program Exit

fun main() {
    println("Before return")
    return
    // The next line would never run
}