In Kotlin, the HelloWorld.kt program can be written as a simple function like the following snippet.

fun main(args: Array<String>) {
    println("Hello, World!")
}

From this little code snippet we can learn the following features of the Kotlin programming language:

• Kotlin program saved in a file with .kt extension.
• The fun keyword to declare a function to show you that programming can be fun again 😉
• To declare variables we start with the variable name followed by its type separated by a colon.
• We don’t have to end a statement with a semicolons.
• To print we can use the println function, which is a wrapper to Java System.out.println.
• The main function is the execution entry point of our HelloWorld program.
• We can create functions at the top level file without creating a class.

To compile the HelloWorld.kt program we run the following command:

kotlinc HelloWorld.kt

The compiler creates a class file called HelloWorldKt.class. To run it type the following command, assumed that you’ve set up the $KOTLIN_LIB environment variable. In my case the variable is set to /usr/local/Cellar/kotlin/1.2.40/libexec/lib.

java -cp $KOTLIN_LIB/kotlin-stdlib.jar:. HelloWorldKt