You can use Java collections in Scala just as you do in Kotlin, and in fact interop is easier in Scala since you can write typeclass instances for the Java types whereas there's no equivalent for that in Kotlin.
The real difference is that more of the Kotlin ecosystem uses Java's fundamentally mutable collections compared to the Scala ecosystem, and using actually immutable collections in Kotlin is extremely difficult to the point that essentially no-one does it. IMO that's a bad tradeoff in the long term, but you can absolutely take the same approach in Scala if you really want to.
Kotlin doesn't have immutable collection types in the standard library (it lets you use a read-only interface but the collection is really still mutable and will be seen as such by any Java code), so if you want actually immutable collections you have to use non-standard collections, and since Kotlin doesn't have typeclasses or implicit conversions it's difficult to interoperate between any non-standard collection library and other Kotlin code.
Yeah, if all your code and libraries are Kotlin then there's less risk of a collection being mutated under you (though even then, Kotlin gives you no way to write a method that only accepts an immutable collection - you can write a method that accepts a view-only interface, but it will always be possible to pass a mutable collection and mutate it in parallel while that method is running). But of course in that scenario there's also zero benefit from having compatibility with the Java collections.
Kotlin's STDLIB collection types are essentially aliases for the Java types. So while the Scala adapters are low-cost, in Kotlin everything's zero-cost.
One other benefit of that is you maintain object identity. I don't think that Scala's wrappers do that.
Scala has the full suite of Java collections without any conversion or overhead whatsoever.
But it also has Scala collections. With scala collections you get the full power of the Scala type system, as well as a much richer and full featured collections api. So most scala programmers won't bother with java collections unless they have specific java interop requirements.
The Scala adapters are merely ways of converting java collections to scala collections and vice versa.
I think it's a fair complaint that in Scala it is a pain in the ass to deal with Java collections. You have to litter your whole code with `.asScala` and `.asJava`, the java collections don't work with for comprehensions, etc.
No, it's not hard. In Scala, you are completely free to use those java collections and you can do it exactly how kotlin programmers do it. You want a java list, without any need to convert back and forth between Scala and Java? Import `java.util.List`. All of the same methods and iterators and expressions and constructs are still there. You get all of the lack of capabilities and grace that the java collections provide. Literally no different from using the same collections in Kotlin.
The problem is that those java collections suck in comparison to the Scala collections. So Scala programmers prefer to use Scala collections. Scala programmers would never willfully use java collections if they don't have to, and if they absolutely have to, they have minimal overhead conversions back and forth. The minimal conversion overhead is the price they're willing to pay to use better collections while maintaining java interoperability.
Except Kotlin enjoys the compatibility with Java, plus it outfits those java collections with extension methods and some compiler tricks to achieve all the same functionality as the Scala collections (in fact, I would say even more self-consistent and useably than in Scala). Just as in Scala, in Kotlin you can use functional transforations that Scala users are so accustomed to:
Kotlin:
listOf(1,2,3,4)
.map{i -> i + 1}
.filter { it % 2 == 0 }
.flatMap { listOf(it, it * 2, it * 3) }
Kotlin even has a similar take to Scala's views, which they call sequences:
listOf(1,2,3,4)
.asSequence()
.map{i -> i + 1}
.filter { it % 2 == 0 }
.flatMap { listOf(it, it * 2, it * 3) }
.toList()
Is this really so "lacking in capability and grace" compared to Scala? The only think missing is persistent immutable collections, which are implemented in kotlinx.
It's only a problem if you can't decide which part of your project to write with which language.
If I have to use Java, I put it into a separate sub-project and make sure that I have a nice API to interface between the subprojects. `.asJava` and `.asScala` then only appear at very specific places where the interop happens.
What you're describing is in and of itself a severe cost and barrier to the very thing we're describing: interoperability between Java and Scala collections. On the one hand you have Scala where you either have to constantly `.asScala` and `.asJava`, or go your route of ensuring that in any given module only deals with either Scala or Java collections. This may involve additional abstractions or classes to achieve. On the other hand you have Kotlin which just directly, frictionlessly deals with Kotlin/Java collections the same way (in fact, they literally are the same). We're comparing "some small-to-medium cost" against "zero-cost".
Or an other way of saying it is that your approach of walling off modules as either java-collection or scala-collection is kind of like saying "Java and Scala collections work well together, so long as you don't have to use them together too much. If you minimize how much they need to interoperate, the problem is not so bad."
The real difference is that more of the Kotlin ecosystem uses Java's fundamentally mutable collections compared to the Scala ecosystem, and using actually immutable collections in Kotlin is extremely difficult to the point that essentially no-one does it. IMO that's a bad tradeoff in the long term, but you can absolutely take the same approach in Scala if you really want to.