[pron]

Kotlin is great, but I would like to point out a few things:

> Lambdas can be inlined!

Kotlin's inline functions are meant to serve as a poor man's macro system (to implement simple control structures) and/or for argument type specialization -- not to save the "inner class overhead" because, guess what? There isn't one! While HotSpot's JIT will certainly not inline lambdas in every circumstance (though it's getting better), it will inline them in all cases where Kotlin's inline functions can be used (and many others where they can't). It is possible, however, that inner classes do have some overhead on Android.

> Reified generics!

Kotlin does not reify generics, and like in Java, that's a good thing! Java's erased generics are precisely what makes polyglot interop easy.

[bkase]

I haven't looked into it much in the past few years so it is possible the inner class overhead on Android is not significant anymore, but a few years ago it was definitely something to watch out for on Dalvik.

Again I wasn't being very clear, sorry about that: Kotlin doesn't always reify generics, but it can for functions you mark as inline if you want (it's useful!). Check out the inline function docs!

[mike_hearn]

Inlining is also about performance. Classes impose significant memory and code size overhead even if HotSpot does inline the code eventually .... but much code is still interpreted. And Kotlin also targets Android. Finding documentation about what optimisations ART does is difficult, but I imagine it's got a weaker compiler than HotSpot if only because it's newer.

[pron]

Not directly related, but for completeness sake: Java 8 lambdas only generate classes for capturing lambdas, and even then, only at runtime (a class file isn't generated). Kotlin can't quite do that because it relies on method handles and invokedynamic, which are missing from Android.

Also, there are few compilers anywhere with optimizations as powerful as HotSpot :) And HotSpot compilation is only getting better and better. Java 9 will even let us use Graal as the optimizing compiler (i.e. instead of C2), which can do this: https://twitter.com/ChrisGSeaton/status/619885182104043520