tdelev

I was mostly referring to the Kotlin calling side of Java code and libraries, and yes you're technically right that would mean only 100% compatibility with existing Java code and not interoperability. But, when moving forward to using exclusively Kotlin and not building libraries that will be Java compatible that is totally ok.

For sure, declaring something as `val someReferenceOfMutableObject` will not make the object itself immutable, only the reference. The case I had in mind was when creating class with only `val` fields, and that would certainly make that class immutable.

I have been extensively programming in Kotlin in the last 2 years coming from a 10 years Java experience. Even Java advanced a lot from from version 8, still it is very far behind in reaching the pure joy of programing in Kotlin, especially in functional style. Immutability with 'val', data classes, functions as first class citizens, destructuring, if/when expressions, just to name a few of the features than enables this.

Also, the great 100% interoperability with Java makes possible to migrate part of your codebase to Kotlin and using tons of already proven and great libraries in the Java ecosystem. On top of the Kotlin standard library includes many improvements of the existing Java classes from collections to String, and all of this using extension functions, one very usefull and powerful feature.

I can keep going on and on, but to conclude Kotlin is not just Java with improved syntax, but completely new modern programming language incorporating or "borrowing" some of the best features from Scala, C# and other popular programming languages. So in my opinion it is a great language which has a lot more funtional features than Java, but still pragmatic and easier to learn than Scala.

The author here, if you have any questions happy to discuss.