Assuming it's really gaining momentum over Scala I won't understand why. What Kotlin has over Scala? Compile times? After coding for 1 hour I compile once to check if Intellij and I missed something. Are those 3-4 minutes too much time?
Well, that's the thing. If you have a small amount of code, the compile times are never a problem. Try compiling something like Apache Spark. It will be somewhere between 30-60 minutes depending on your disk speed, CPUs, etc. etc. The incremental builds are faster, but never less than 2 or 3 minutes for any change. You could potentially avoid this with the REPL, or use inotify to trigger a background compilation, but none of those really works all the time.
At the time Spark was started, Java didn't have lambdas or closures, making the equivalent Java code for functional-programming-style ML code much more verbose. Whether the same decision would be made today is an open question (and probably depends on who you ask). A lot of other frameworks have made different decisions, such as keeping the core of the project in Java and supporting other languages as shims on top (in my opinion a more sensible decision...)
From what I observed it's close enough to Java that people can easily switch and are willing to. There's also enough brevity with the language to be at ease with it daily (not as much as Scala) where it may be enough that perhaps people might not want to invest time to go all the way to Scala.
One of the things I love about Scala is that you don't have to "go all the way" though - you can start by writing Java-like code using Java libraries, and move on to writing Kotlin-like code as and when you want to, and move on to more advanced features than that as and when you want to.
> One of the things I love about Scala is that you don't have to "go all the way" though
This is true of any language so it doesn't mean much.
Still, we went through the same thing with C++ 20 years ago. Google even used to have (still does?) a style guide saying which features to use and which ones not to use.
Instead of having to use a guide that tells me which features of a language are dangerous, I prefer a language that doesn't require such a guide in the first place.
Unfortunately it's impossible to combine that with seamless Java interoperability, because so many Java features are dangerous (null, exceptions, pervasive mutability, all of the methods on Object, finalizers, low-level thread manipulation) or at least cumbersome (use-site variance and the resulting existential types, control flow constructs built into the language syntax).
I don't think there are any dangerous things you can do in Scala that you can't do in Kotlin. Only safe alternatives that exist in Scala but not in Kotlin.
2.12 doesn't include any (non-gated) changes though, so you can keep the same codebase and cross-build for 2.12 (for performance) and 2.11. So we're talking what, at least 3 years ("Aida", either 2.13 or 3.0) before Java 8 becomes a blocker for source-level features, at which point I imagine Android will have an answer.
ever is a strong word. I know that at the moment they use some parts of OpenJDK 7, but If they need to comply the Java (tm) caused by licensing, they will be forced (sooner or later) to actually don't use a deprecated JVM. As I know OpenJDK will be depracted on 2024. so it will take some time, but it will come.
