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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