Are Kotlin lambdas the same as Java lambdas under the hood?

Question

This is a follow up question of this answer.

But when the application hasn’t used lambda expressions before¹, even the framework for generating the lambda classes has to be loaded (Oracle’s current implementation uses ASM under the hood). This is the actual cause of the slowdown, loading and initialization of a dozen internally used classes, not the lambda expression itself

Ok, Java uses ASM to generate the classes on runtime. I found this and if I understood correctly, it is basically saying that Kotlin lambdas are compiled to pre-existing anonymous classes being loaded at runtime (instead of generated).

If I'm correct, Kotlin lambdas aren't the same thing as Java and shouldn't have the same performance impact. Can someone confirm?

Answer 1

Of course, Kotlin has built-in support for inlining lambdas, where Java doesn't. So many lambdas in Kotlin code don't correspond to any objects at runtime at all.

But for those that can't be inlined, yes, according to https://medium.com/@christian.c.carroll/exploring-kotlin-lambda-bytecode-8c2d15afd490 the anonymous class translation seems to be always used. Unfortunately the post doesn't specify the Kotlin version (1.3.30 was the latest available at that time).

I would also consider this an implementation detail which could change depending on Kotlin version at least when jvmTarget is set to "1.8" or greater; so there is no substitute to actually checking your own bytecode.