I really love working in Kotlin, but it seems like the stars never quite align to where I can use it as my primary language at work. It's usually my default for new backend stuff for personal projects now. I think my favorite thing is how modern it feels but you also get the power of the very mature Java ecosystem.
For more ad-hoc stuff I love python, I can cobble together something workable extremely quickly. Also, comprehensions are a wonderful quality of life feature.
I'm in the same boat. I use Kotlin almost exclusively for my side projects, both JVM and JS stuff, but my work projects are mainly still Java. Some projects are slowly adding a bit of Kotlin, but its just hard to start integrating new Kotlin while most of the app is plain Java. It's just so natural, when fixing a bug in Java code, to just keep writing new Java for it.
I think it's just a matter of time. Kotlin has really only been a "very mainstream" language for a year or so now. As more greenfield projects get started with it, it will get more acceptable to migrate existing projects to it as well.
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