Hello, World (Part 2)

To understand how Kotlin works, let’s take a look at that Hello.kt file again:

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

This time, rather than creating an executable jar file, just compile the code like this:

$ kotlinc Hello.kt

That command creates a file named HelloKt.class. Since this is a normal JVM class file, use the javap command to disassemble it and see what’s inside:

$ javap HelloKt.class 

Compiled from "Hello.kt"
public final class HelloKt {
  public static final void main(java.lang.String[]);
}

As shown, Kotlin creates a class named HelloKt, and it contains a normal Java public static void main method. Kotlin creates the class with the name HelloKt because we didn’t supply a class name. The class file name comes from the filename — Hello.kt becomes HelloKt.

A little more fun

Before we move on, let’s have a little more fun. Save this source code to a file named HelloYou.kt:

fun main(args: Array<String>) {
    if (args.size == 0)
        println("Hello, you")
    else
        println("Hello, ${args[0]}")
}

I hope you can see how it works:

• if/else statements work just like Java.
• Kotlin supports String Templates, which are similar to String Interpolation in languages like Scala, Groovy, and Ruby, so ${args[0]} prints the first command line argument.

Now compile that file and create a new executable jar file:

$ kotlinc HelloYou.kt -include-runtime -d HelloYou.jar

Then run it with and without a command line argument:

$ java -jar HelloYou.jar
Hello, you

$ java -jar HelloYou.jar Alvin
Hello, Alvin

Extra credit

To have a little more fun with this example, run this Java jar command on HelloYou.jar:

$ jar tvf HelloYou.jar

If you know Java and how the JVM works, you can guess that the initial output of that command looks like this:

  79 Wed Aug 01 13:54:12 MDT 2018 META-INF/MANIFEST.MF
1249 Wed Aug 01 13:54:12 MDT 2018 HelloYouKt.class

The rest of the output is too long to include here — 654 lines total, to be precise — but I encourage you to run that command to get an idea of what’s needed to create an executable jar file.