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Having a go at learning some Kotlin

What's this about? 

The year 2025 is almost over, so that means that it has been a bit over a decade since my old colleague Filippo gave a presentation to the development team of ScienceDirect covering the merits of the Kotlin programming language. So, it's about time that I had a proper go at using it.

This blog post is intended to trace what the experience has been like, covering surprises that I encounter along the way.

Getting started

The programming language that I am most experienced with is Java, so I have chosen to try out implementing some functionality in Kotlin from a recent hobby project that I developed in Java involving spinning up a database in a Docker container and running some queries.

JVM version support

IntelliJ IDEA includes some automation for creating a new project, so I selected the relevant options to use the latest LTS version of the Java virtual machine with Spring Boot, Kotlin, Postgresql and Test containers.

After a few seconds I had a new project in place ready for me to start development, but it didn't quite match up with what I had specified. In the auto-generated HELP.md file there was a mention of:

* The JVM level was changed from '25' to '24' as the Kotlin version does not support Java 25 yet.

Hmm, that's not a limitation that I was expecting to encounter. Java 25 has been out for a couple of months now, and as it is a long term support (LTS) release I would expect that providers of languages and frameworks would prioritise compatibility.

What does it actually mean?

After some brief checking it turns out that Kotlin won't currently compile to JVM 25 bytecode, but that doesn't mean that we can't deploy and run on JVM 25.

Some JUnit annotations are not a clean fit

Static methods?

This may be a bit subjective, but having to set up a companion object and an additional annotation leaves me feeling that Kotlin isn't a great fit for JUnit.

I wanted to specify some particular set up logic to run prior to the tests in my first test class. In Java it would just involve declaring a static method and annotating it, but static methods aren't a directly applicable concept in Kotlin, so it ends up looking like the following:

companion object {
    @BeforeAll
    @JvmStatic
    internal fun setUp()
    { 
// Here's where the magic happens
    }
}

So it seems like I should also look into what test support libraries are best for idiomatic Kotlin. 

In conclusion...

I'm going to hit "publish" on this post before I have any code to share, as I want it to still be relevant.

Kotlin 2.3.0-RC2 has just been released, that includes support for Java 25:

https://kotlinlang.org/docs/whatsnew-eap.html#kotlin-jvm-support-for-java-25 

"Starting with Kotlin 2.3.0-RC2, the compiler can generate classes containing Java 25 bytecode."

I may come back later with some follow up post(s) about how I get on with learning some more about Kotlin - preferably the language features rather than adoption rough edges.

 

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