[jvmls 12] kotlin / java interop

Kotlin / Java Interop

Andrey Breslav

A Friend In Need Is a Friend Indeed

1Wednesday, August 1, 12

Kotlin

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Modern Language for Industry

• Static typing

• Readability

• Tool support

• Interoperability

• Smart compiler

➡ Less boilerplate

• Flexible abstractions

➡ Powerful libraries

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Stuff

• Apache 2

• Target➡ JVM (1.6)

➡ JavaScript

• Build➡ Ant

➡ Maven

• IDE➡ IntelliJ

➡ Web Demo

3* Planned

*

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Outline

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Outline• Enhancing Existing APIs

➡ Extensions

➡ Splitting Collections Interfaces

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Outline• Enhancing Existing APIs

➡ Extensions

➡ Splitting Collections Interfaces

• Fighting for Safety➡ Nullable types

➡ Interaction with generics

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4Wednesday, August 1, 12

Outline• Enhancing Existing APIs

➡ Extensions

➡ Splitting Collections Interfaces

• Fighting for Safety➡ Nullable types

➡ Interaction with generics

• Statics➡ Packages

➡ Class objects4

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Making Existing Things Nicer

ExtensionsVariance Tricks

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Extensions

• Demo

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Collections & Variance

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Java:

Kotlin:

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Declaration-Site Variance

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Translation

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Øjava.util.List

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Translation: Inheritance

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Translation: Variance

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Calling Java From Kotlin

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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

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Fighting NPEsNullable Types

Annotating Java Code

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Nullable Types

• Demo

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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()!!

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Annotations to The Rescue

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JSR 305: Annotations for Software Defect Detection

String?String

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Generics?!

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@NotNulldoesn't

help

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More Use-Cases of @Kotlin

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External Annotations

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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

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StaticsPackages

Class Objects

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Package-Level Code

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file1.kt

file2.kt

foo/namespace.class

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Package-Level Code

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file1.kt

file2.kt

foo/namespace.class

Compiled From: file1.kt

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Package-Level Code

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file1.kt

file2.kt

foo/namespace.class

foo/namespace$file1.class

foo/namespace$file2.class

from:file1.kt

from:file2.kt

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Package-Level Code

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file1.kt

file2.kt

foo/namespace.class

foo/namespace$file1.class

foo/namespace$file2.class

from:file1.kt

from:file2.kt

Initialization Order?!

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Kotlin ClassesHave No Statics

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Class Object Translation

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Class Object Translation

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Only fields!

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Resources

• Docs: http://kotlin.jetbrains.org

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

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

• Twitter:➡ @project_kotlin

➡ @abreslav

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