[jvmls 12] kotlin / java interop

Kotlin / Java Interop

Andrey Breslav

A Friend In Need Is a Friend Indeed

1Wednesday, August 1, 12

Kotlin

2

Modern Language for Industry

• Static typing

• Readability

• Tool support

• Interoperability

• Smart compiler

➡ Less boilerplate

• Flexible abstractions

➡ Powerful libraries


Stuff

• Apache 2

• Target➡ JVM (1.6)

➡ JavaScript

• Build➡ Ant

➡ Maven

• IDE➡ IntelliJ

➡ Web Demo

3* Planned

*


Outline

4


Outline• Enhancing Existing APIs

➡ Extensions

➡ Splitting Collections Interfaces

4



➡ Extensions


• Fighting for Safety➡ Nullable types

➡ Interaction with generics

4



➡ Extensions


• Fighting for Safety➡ Nullable types

➡ Interaction with generics

• Statics➡ Packages

➡ Class objects4


Making Existing Things Nicer

ExtensionsVariance Tricks


Extensions

• Demo

6


Collections & Variance

7

Java:

Kotlin:


Declaration-Site Variance

8


Translation

9

Øjava.util.List


Translation: Inheritance

10


Translation: Variance

11


Calling Java From Kotlin

12


Summary

• Use JDK collections in Kotlin

• Extend them with useful operations

• Make them variant for Kotlin

• At the expense of some unsoundness around the edges

13


Fighting NPEsNullable Types

Annotating Java Code


Nullable Types

• Demo

15


Nullable Types Summary• null is a legitimate value, not a sin

• No overhead (as opposed to Option)

• Proper subtyping:➡ List<String> is a List<String?>

• At the expense of:

file.getParent()!!.getName()!!

16


Annotations to The Rescue

17

JSR 305: Annotations for Software Defect Detection

String?String


Generics?!

18

@NotNulldoesn't

help


More Use-Cases of @Kotlin

19


External Annotations

20


It Is Not So Bad

• IDE aids annotating with quick fixes

• Some annotations can be inferred automatically

• Alternative: have a special kind of types for Java things

21


StaticsPackages

Class Objects


Package-Level Code

23

file1.kt

file2.kt

foo/namespace.class


Package-Level Code

23

file1.kt

file2.kt

foo/namespace.class

Compiled From: file1.kt


Package-Level Code

24

file1.kt

file2.kt

foo/namespace.class

foo/namespace$file1.class


from:file1.kt

from:file2.kt


Package-Level Code

24

file1.kt

file2.kt

foo/namespace.class



from:file1.kt

from:file2.kt

Initialization Order?!


Kotlin ClassesHave No Statics


Class Object Translation

26


Class Object Translation

26

Only fields!


Resources

• Docs: http://kotlin.jetbrains.org

• Demo: http://kotlin-demo.jetbrains.com

• Code: http://github.com/jetbrains/kotlin

• Twitter:➡ @project_kotlin

➡ @abreslav

27


[jvmls 12] kotlin / java interop

Technology

kt foonamespace

class foonamespace

collections interfaces

packagelevel codele1

existing apis extensions

nullable types demo15wednesday

nullable types summary

collections variancejava