Writing Java-friendly Kotlin code (Mobius 2017)

While Kotlin becomes more and more popular, lots of Java libraries are getting Kotlin helpers to make their usage in Kotlin code more idiomatic and clean. People, who’ve already tried Kotlin, understand that this language is much more pleasant to write code in than Java. Sure, one of the main selling sides of the language is its stunning Java interoperability. It’s really easy to call Java code from Kotlin, however, the opposite has its pitfalls.

In this talk we'll discuss, what tools Kotlin creators have prepared for such cases and how they will affect the resulting JVM bytecode. This talk will be useful not only for library creators, but also for developers who are hesitant to migrate their Java codebase.

Video: https://youtu.be/1L0q5VKx_-s

Transcript

1. Writing Java-friendly Kotlin code Sergey Ryabov
2. compile 'rxbinding-appcompat-v7:x.y.x' compile 'autodispose:x.y.z' compile 'autodispose-android-archcomponents:x.y.z' compile 'rxbinding:x.y.x' compile 'rxbinding-design:x.y.x' compile 'autodispose-android:x.y.z'
3. compile 'rxbinding-kotlin:x.y.x' compile 'rxbinding-design:x.y.x' compile 'autodispose-android-archcomponents-kotlin:x.y.z' compile 'autodispose-kotlin:x.y.z' compile 'autodispose-android-kotlin:x.y.z' compile 'autodispose:x.y.z' compile 'autodispose-android:x.y.z' compile 'autodispose-android-archcomponents:x.y.z' compile 'rxbinding:x.y.x' compile 'rxbinding-appcompat-v7:x.y.x' compile 'rxbinding-design-kotlin:x.y.x' compile 'rxbinding-appcompat-v7-kotlin:x.y.x'
4. JAVA
5. JAVA •volatile •synchronized •strictfp •transient
6. KOTLIN •@Volatile •@Synchronized •@Strictfp •@Transient
7. KOTLIN •@Volatile •@Synchronized •@Strictfp •@Transient package kotlin.jvm
8. ANALYTICS ABSTRACTION LIBRARY
9. ANALYTICS ABSTRACTION LIBRARY object Analytics { fun send(event: Event) {} fun addPlugins(plugs: List<Plugin>) {} fun getPlugins(): List<Plugin> {} }
10. ANALYTICS ABSTRACTION LIBRARY object Analytics { fun send(event: Event) {} fun addPlugins(plugs: List<Plugin>) {} fun getPlugins(): List<Plugin> {} } interface Plugin { fun init() fun send(event: Event) fun close() }
11. ANALYTICS ABSTRACTION LIBRARY object Analytics { fun send(event: Event) {} fun addPlugins(plugs: List<Plugin>) {} fun getPlugins(): List<Plugin> {} } data class Event( val name: String, val context: Map<String, Any> = emptyMap() ) interface Plugin { fun init() fun send(event: Event) fun close() }
12. TIME FOR SOME CODE!
13. 8 Generics
14. VARIANCE
15. VARIANCE Animal Dog
16. VARIANCE Animal Dog
17. VARIANCE Iterator<Animal> Iterator<Dog>
18. VARIANCE Iterator<Animal> Iterator<Dog>
19. VARIANCE Iterator<Animal> Iterator<Dog> COVARIANT
20. VARIANCE Iterator<Animal> Iterator<Dog> Action<Animal> Action<Dog> COVARIANT
21. VARIANCE Iterator<Animal> Iterator<Dog> Action<Animal> Action<Dog> COVARIANT
22. VARIANCE Iterator<Animal> Iterator<Dog> Action<Animal> Action<Dog> COVARIANT CONTRAVARIANT
23. VARIANCE Iterator<Animal> Iterator<Dog> Action<Animal> Action<Dog> ArrayList<Animal> ArrayList<Dog> COVARIANT CONTRAVARIANT
24. VARIANCE Iterator<Animal> Iterator<Dog> Action<Animal> Action<Dog> ArrayList<Animal> ArrayList<Dog> COVARIANT CONTRAVARIANT INVARIANT
25. VARIANCE // Java List<Dog> dogs = new ArrayList <>(); List<Animal>kanimals = dogs;
26. VARIANCE // Java List<Dog> dogs = new ArrayList <>(); List<Animal>kanimals = dogs;
27. USE-SITE VS DECLARATION-SITE VARIANCE // Java List<Dog> dogs = new ArrayList <>(); List<? extends Animal> animals = dogs;
28. USE-SITE VS DECLARATION-SITE VARIANCE // Java List<Dog> dogs = new ArrayList <>(); List<? extends Animal> animals = dogs; // Kotlin val dogs: List<Dog> = ArrayList() val animals: List<Animal> = dogs
29. USE-SITE VS DECLARATION-SITE VARIANCE // Java List<Dog> dogs = new ArrayList <>(); List<? extends Animal> animals = dogs; interface List<out E> : Collection<E> // Kotlin val dogs: List<Dog> = ArrayList() val animals: List<Animal> = dogs
30. USE-SITE VS DECLARATION-SITE VARIANCE // Java List<Dog> dogs = new ArrayList <>(); List<? extends Animal> animals = dogs; interface List<out E> : Collection<E> // Kotlin val dogs: List<Dog> = ArrayList() val animals: List<Animal> = dogs
31. 14 NOW BACK TO THE CODE
32. MAY THE ANNOTATIONS BE WITH YOU • @JvmOverloads • @JvmStatic • @JvmField • @JvmWildcard @JvmSuppressWildcards • @Throws • @JvmName • @JvmMultifileClass
33. WORTH NOTING
34. WORTH NOTING • Inline functions
35. WORTH NOTING • Inline functions • Reified type params
36. WORTH NOTING • Inline functions • Reified type params • java.lang.Class
37. WORTH NOTING • Inline functions • Reified type params • java.lang.Class • Unit
38. WORTH NOTING • Inline functions • Reified type params • java.lang.Class • Unit • Typealiases
39. WORTH NOTING • Inline functions • Reified type params • java.lang.Class • Unit • Typealiases • Visibility
40. WORTH NOTING • Inline functions • Reified type params • java.lang.Class • Unit • Typealiases • internal visibility (!)
41. WANT MORE? • Code from the talk https://goo.gl/ZWneG9 • Sinking Your Teeth Into Bytecode by Jake Wharton https://goo.gl/JLqFHr • Exploring Kotlin’s hidden costs  by Christophe Beyls https://goo.gl/xxLvWE
42. Writing Java-friendly Kotlin code @colriot