A comparison of Java interoperability in Scala and Kotlin just for educational purposes

Quickstart

Run with ./gradlew build. Start reading from Runner.java.

Features

Good in both

• no problems implementing K/S interfaces in Java or Java interfaces in K/S
• easy to write a lambda implementing Java functional interface

Good in Scala

• good translation to static methods when used in Java
• can use named parameters on Java code compiled with -parameters
• sometimes you can use a lambda to create an anonymous class instance

Bad in Scala

• calling Java varargs from Scala is sometimes impossible
• calling Scala varargs from Java is very inconvenient if Scala API does not use a special annotation
• using collections from the other language: need to Google for complicated conversions in both directions
• when overriding methods, Scala Int is not Java Integer; same for Long
• some very confusing syntactic magic: foo() can be understood as foo(null)
• cannot use function instances as Java functional interfaces (but inline functions are fine) - see ScalaLambdas.passLambdasToJava; conversions will help

Good in Kotlin

• calling varargs in both directions
• using lists from Java in Kotlin: you can use Kotlin syntax sugar as with Kotlin types
• using overloaded operators on Java classes if Java methods use a convention
• null safety when using properly annotated Java classes
• null warnings in Java when calling nullable Kotlin code
• default method parameters used in Java: work great if you use a proper annotation, if you don't use - work as in Scala
• POJO-compatible data classes
• smart casts
• can use function instances as Java functional interfaces - see KotlinLambdas.passLambdasToJava

Bad in Kotlin

• default methods in Kotlin interfaces need to be implemented in Java
• immutable Kotlin collections seen as native Java collections in Java, compiler won't warn when you try to modify them (as in pure Java)
• wrong name of getter for boolean fields (does not conform to JavaBean spec)

Bad in both

• when implementing lambdas returning Unit ('void') in Java you need to return null
• cannot use Java functional interface instances as K/S functions (but inline lambdas are fine) - see Lambdas.passLambdasFromJava

Compilation time and result size

java-module 1.5s Kotlin 4s Scala 16s
0.25  0.9   7.8
0.2   1.5  14
5    15   102

More classes in Kotlin (it generate anonymous classes). Compare size with dependencies: gradle dependenciesSize

Testing refactoring

• from Java: rename MapCalculator.Result.status
• from Kotlin: rename KotlinButton.caption()
• from Scala: rename ScalaButton.caption()

Further reading

• https://kotlinlang.org/docs/reference/comparison-to-scala.html
• http://engineering.pivotal.io/post/spring-boot-application-with-kotlin/
• https://spring.io/blog/2017/01/04/introducing-kotlin-support-in-spring-framework-5-0
• https://www.slideshare.net/Odersky/scala-days-nyc-2016