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.