Seamless Kotlin/Java Interop

Kotlin is fully interoperable with Java: you can call Java from Kotlin, and Kotlin from Java. While the former is straightforward, the latter can lead to more convoluted code as one has to incorporate Kotlin's new features. This talk will focus on making interop as seamless as possible, by highlighting common problems and give tips on how to handle them.

Transcript

1. Seamless Kotlin / Java Interop
2. HELLO! I am Pamela Hill Android Engineer @ Luno You can find me at @pamelaahill or pamelaahill.com 2
3. Topics ▹ Quick Kotlin 101 ▹ Why interop matters ▹ Kotlin → Java ▹ Java → Kotlin ▹ Q&A 3
4. 1. Quick Kotlin 101
5. We’re all still learning something 5
6. 6 Declaring a variable val PI = 3.14 var radius = 13.0 • Forced to think of immutability vs mutability • Nudge toward favouring immutability
7. 7 Type inference val isSquare = false val area = 12.5 • The compiler is smart! • Type declarations only when cannot infer
8. 8 Declaring a function fun plus(i: Int, j: Int): Int { return i + j } fun plus(i: Int, j: Int) = i + j • Expression body- one statement • Block bodies - more than one
9. 9 Top-level functions and variables val PI = 3.14 fun plus(i: Int, j: Int) = i + j • Removes need for utilities classes
10. 10 Named arguments fun plus(i: Int, j: Int) = i + j … plus(i = 1, j = 2) • Improves readability for long argument lists • Adds context
11. 11 Nullability val width : Double = 4.0 val length : Double = 5.0 val height : Double? = null • Once possibly null, always possibly null • Guards against dreaded NPE
12. 12 Data classes data class Rectangle (val width: Double, val height: Double) • Accessors, mutators, equals, hashCode, toString, copy implied • Concise, readable
13. 13 Inheritance open class Rectangle (val width: Double, val height: Double) class Square (width: Double): Rectangle(width, width) • Need to open classes/functions for inheritance/overriding • Helps with fragile base class problem
14. 14 Lambdas & higher-order functions val calculator = { i: Int, j: Int -> i + j} fun printCalculation (calc: (Int, Int) -> Int){ println("${calc(1, 2)}") } • Arrow library for more advanced features
15. 15 Extension functions class Rectangle (val width: Double, val height: Double) … fun Rectangle.calcArea () = width * height … Rectangle(1,2).calcArea() • Alter existing classes without changing code
16. 2. Why interop matters
17. Things can get a little freaky… 17
18. 18 To whom should it matter? • Java library developers • Kotlin library developers • Mixed/migrating codebase developers
19. 19 What should we aim for? • Safety • Readability • Performance
20. 3. Kotlin → Java
21. Because the ecosystem is like Christmas 21
22. 22 Don’t use hard keywords • object, in, is are keywords in Kotlin, but not Java • Kotlin will be forced to back-tick eg. `is`
23. 23 Use nullability annotations • Every parameter, return, field should have a nullability annotation • Eg. @NotNull or @Nullable • Different annotation sets supported
24. 24 Lambda parameters last fun calculation(x: Int, y: Int, calc: (Int, Int) -> Int) = calc(x,y) ... calculation( 3, 4) { i, j -> i * j } • Natural reading style, great for DSLs
25. 25 Bean standards for properties class Rectangle { private int width = 0; public int getWidth() {…} public void setWidth(int width){…} } rectangle.width = 2; println("${rectangle.width}")
26. 4. Java → Kotlin
27. Mixed codebases happen 27
28. 28 Files with top-level functions and variables In file CalcUtils.kt: fun calculation(x: Int, y: Int) = x * y CalcUtilsKt.calculation(3, 4)
29. 29 Files with top-level functions and variables In file CalcUtils.kt: @file:JvmName("CalcUtils") fun calculation(x: Int, y: Int) = x * y CalcUtils.calculation(3, 4)
30. 30 Exceptions fun parseInt(string: String) : Int { return string.toInt() } try { parseInt("abc") } catch (NumberFormatException nfe){…}
31. 31 Exceptions @Throws(NumberFormatException::class) fun parseInt(string: String) : Int { return string.toInt() } try { parseInt("abc") } catch (NumberFormatException nfe){…}
32. 32 Functions with defaults fun funWithDefaults(x: Int, y: Int = 0, z: Int = 0) { ... } funWithDefaults(x,0,0)
33. 33 Functions with defaults @JvmOverloads fun funWithDefaults(x: Int, y: Int = 0, z: Int = 0) { ... } funWithDefaults(x) funWithDefaults(x,y) funWithDefaults(x,y,z)
34. 34 Unit returns fun sayHi(callback: (String) -> Unit) = {…} greeter.sayHi(name -> { System.out.println("Hello, " + name + "!"); return Unit.INSTANCE; });
35. 35 Unit returns interface GreeterCallback { fun greetName(name: String): Unit } fun sayHi(callback: GreeterCallback) = {…} greeter.sayHi(name -> System.out.println("Greeting", "Hello, " + name + “!”));
36. 5. Q&A
37. I’m nice! →☝ Ask me anything about Kotlin 37
38. THANKS! You can find me at: @pamelaahill or pamelaahill.com 38
39. CREDITS Special thanks to all the people who made and released these awesome resources for free: ▹ Presentation template by SlidesCarnival ▹ Photographs by Unsplash and Pexels 39