Hello, World!

Time to write and run your first real Kotlin program.

The program entry point

Every Kotlin application starts at the main function:

fun main() {
    println("Hello, World!")
}

• fun declares a function.
• main is the special name the runtime looks for to start the program.
• println(...) prints a line to standard output.
• No class, no public static void, no semicolons required.

Reading program arguments

main can optionally receive command-line arguments as an Array<String>:

fun main(args: Array<String>) {
    val name = if (args.isNotEmpty()) args[0] else "World"
    println("Hello, $name!")
}

Run it with an argument and it greets that name; run it with none and it falls back to "World". Note the string template "$name".

Extracting a function

Pulling the greeting into its own function makes it reusable and testable:

fun greeting(name: String): String = "Hello, $name!"

fun main() {
    println(greeting("Kotlin"))
}

greeting is a single-expression function: when a function just returns one expression, you can use = instead of a body with return.

The runnable example

This program is in the guide’s examples as the hello module:

examples/core/hello/
├── build.gradle.kts
└── src/
    ├── main/kotlin/examples/hello/Hello.kt
    └── test/kotlin/examples/hello/HelloTest.kt

Run it from the examples/core directory:

cd examples/core
./gradlew :hello:run

And run its test:

./gradlew :hello:test

You can also paste any of the snippets above directly into the Kotlin Playground.

Exercises

Try these yourself, then expand the solutions. The reference solutions live in examples/core/hello/ and are compiled and tested by CI.

1. Write a function shout(message: String): String that returns the message in uppercase with an exclamation mark appended. For example, shout("hello") should return "HELLO!".

   Solution

   fun shout(message: String): String = "${message.uppercase()}!"
   

</details>

1. Write a function greetAll(vararg names: String): String that returns each name greeted with greeting(...), one per line.

   Solution

   fun greetAll(vararg names: String): String =
       names.joinToString(separator = "\n") { greeting(it) }
   

</details>

1. Change main so it greets the name passed as the first command-line argument, defaulting to "World" when no argument is given. (Hint: see Reading program arguments above.)

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