To me the question "Why would I use X over Y?" in programming space is a bit redundant. Why would I use Kotlin over Closure? Why would I use Groovy over Scala?
Even only considering JVM languages you run in to way too many options for the question in itself to be worth your time as a serious discussion.
IMO - use whatever you and your team like best and that suits your needs. Java is great for hiring new devs and is arguably the most well established in an enterprise settings.
If you want an actual answer: Personal preference.
Android OS layer has zero Kotlin, and Google had to backtrack on their decision to let Java languish, as the Android ecosystem started to lose out on Java ecosystem, thus ART is now updatable via Play Store since Android 12, and Java 17 LTS is the latest supported version.
Kotlin is only used on Jetpack Compose and a couple of AndroidX libraries, that you have to ship alongside the application. And as alternative to Groovy on Gradle build scripts.
I expect this year's GDC to announce Java 21 LTS support.
Interesting, I don't develop for Android so I don't know the details in this space. Surprising to learn that the Android OS layer is 100% Java. In terms of a platform, I was more referring to the mass adoption of Kotlin for Android app development. 95% of the top 1,000 Android apps are written in Kotlin and adoption overall is > 50% according to their overview page https://kotlinlang.org/docs/android-overview.html
IMO those numbers create a lot of staying power for the language.
More like 70%, the remaining 30% are split between Linux kernel (C), Treble drivers written in C++ (Java is also supported), SQLlite (C), and naturally Skia, ART are written in C++. With a couple of newer drivers like the Bluetooth stack, adopting Rust.
However zero Kotlin.
Naturally Kotlin has such an adoption on userspace, Jetpack Compose the new UI framework is written in Kotlin, so already adopting that jumps up the adotption numbers.
It is like measuring Swift adoption by the use of SwiftUI, even if the business logic is still written in Objective-C.
I do agree that Google being Kotlin's godfather, pushing it for new projects, does create a lot of staying power.
However it is also the reason why on JVM, it is only yet another guest language, as there is no one telling Java developers they should not write Java to use framework XYZ.
All those that tried, Grails, Spark, Akka,... eventually lose adoption speed, and got replaced by Java alternatives.
Which Google had to accept, Kotlin's value without the breath of Java written libraries isn't that great any longer, and they aren't going to rewrite the whole Maven Central into Kotlin, as the Java community moves beyond Java 8.
The downvotes are unfortunate. To clarify I wasn't trying to start a flame war.
I was hoping someone would answer with concrete technical tradeoffs and comparisons e.g. compile times, null safety, common footguns, personal experiences, etc.
I was recently asked for a comparison between Kotlin and modern Java by a team that's considering a migration of their legacy Java service. I haven't kept up with Java's changes, so didn't have an offhand unbiased comparison to share.
I've worked on backend Kotlin and Java codebases for the last five years and have introduced Kotlin to dozens of teams across my company. Nobody I've introduced Kotlin to has ever ended up preferring Java or switching back to it. There's been initial skepticism at first (reasonable), but after working in Kotlin for a week or two, I've seen even the staunchest Java devs proclaim they're never going back.
To the point on personal preference, I think it's a factor but I don't think it's _the answer_. It's a blanket statement that ignores important details. Languages have different affordances, features, paradigms, and patterns that lead you to develop and think in different ways. Some differences like null safe types are obvious improvements for a large codebase with many contributors. The feature catches real bugs and makes the system easier to build and maintain.
I think there's a lot of warranted skepticism of JVM languages that aren't Java, but something about Kotlin feels very different. For comparison, I've worked with Scala (Spark) and played around with Clojure in my free time.
I'm not sure what feels so much better compared to other JVM languages, but if I had to guess I'd say it's an emergent quality from a few things JetBrains has done really well. In particular:
- The 100% Java interop claim is real. I've only had minor hiccups with Lombok
- The ability to incrementally adopt Kotlin in an existing Java codebase
- The IDE support in IntelliJ IDEA is unbelievable. The suggested refactorings subtly teach you the language
- Null safety lets you focus on domain modeling and business logic instead of error handling and validation
- Kotlin's backwards compatibility. New versions never seem to break anything. And they even provide refactoring actions in IntelliJ to automatically update deprecated usages
- Very few significant paradigm shifts. Kotlin doesn't force you to totally change your Java code to fit a functional style or anything like that. It just tries to make safer code easier to write
- The political stability and longevity gained from Android's blessing as a first-class language makes Kotlin feel like more than just another JVM language. There's Kotlin running on billions of devices. That's a pretty big ship to turn around, sink, or replace
To summarize, Kotlin just feels like the elephant in the room. Aside from low-level changes like new FFI possibilities and GC improvements, it's not clear to me what value Java upgrades add in comparison to Kotlin—especially in regards to new language features and syntax.
Anyways, hopefully this followup post starts a more fruitful discussion. I asked this question in earnest based on my personal experiences and observations. If you haven't tried Kotlin yet, I'd suggest cracking open an IntelliJ scratch file and test driving it yourself.
Even only considering JVM languages you run in to way too many options for the question in itself to be worth your time as a serious discussion.
IMO - use whatever you and your team like best and that suits your needs. Java is great for hiring new devs and is arguably the most well established in an enterprise settings.
If you want an actual answer: Personal preference.