Once you have installed the Kotlin Command-line Compiler, you may be wondering how to write your first code in Kotlin. In this article, we would also discuss how to create a .jar executable from it, which includes a class file as well as Kotlin run-time library.
The source code written in Kotlin is saved in a file with .kt extension.
Write your first “Hello World!” code in Kotlin
Open a text-editor and save the file as HelloWorld.kt. We have used nano text-editor, choose one which you are comfortable with.
nano HelloWorld.kt
Next, insert the following code –
fun main(args: Array<String>) {
println("Hello, World!")
}
where,
fun – it means function, just to let compiler know that we are going to call a function.
main – its the function main() which is to be called.
args – is the name of our variable.
Array<String> – tells the compiler that variable args is an Array of type String.
println() – print line function.
“Hello, World!” – the message we would like to print.
Save the HelloWorld.kt and exit.
So far, we have created a file with .kt extension and then inserted our code understood by the Kotlin Compiler. Now, we will compile the code with the help of following command –
kotlinc HelloWorld.kt
Notice that, it has created a file – HelloWorldKt.class and a folder META-INF with main.kotlin_module. Compiling the code puts the main function inside the HelloWorldKt.class file, which later could run inside a Java Virtual Machine (JVM).
If you would like to see byte-code inside the HelloWorldKt.class then you may have to use javap command. javap command is basically a command line utility which lets us see the code so generated inside the .class file.
javap -c HelloworldKt.class
Output should resemble –
Compiled from "HelloWorld.kt"
public final class HelloWorldKt {
....
}
Make Kotlin Compiler produce a .jar executable
The executable so produced will include Kotlin run-time library as well as HelloworldKt.class. Thus, eliminating the need to distribute Kotlin run-time along-with .jar executable. To make one, run the following in terminal –
kotlinc HelloWorld.kt -include-runtime -d HelloWorld.jar
where,
-include-runtime – asks the compiler to include Kotlin run-time library to make .jar executable.
-d – for output compiled as .jar executable
Lastly, run the following to execute .jar file –
java -jar HelloWorld.jar
This should get us expected output i.e. Hello, World!
In conclusion, we have discussed how to write our first Kotlin code and then compile it to a .jar executable.
