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Kotlin: Why Do You Care?

Dmitry Jemerov <yole@jetbrains.com>

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• Statically typed programming language targeting the JVM

• Support for functional and OO paradigms

• Pragmatic, safe, concise, great Java interop

• Free and open-source

• In public beta, 1.0 coming Real Soon Now

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Outline• Why Kotlin was created

• Kotlin feature highlights

• How and why we’re using Kotlin

• Where Kotlin works best

• How you can benefit from Kotlin in your projects

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Why was Kotlin created?

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“We’ve built tools to support so many nice languages, and we’re still using Java”

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JetBrains, 2010

Developer Productivity

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Develop with Pleasure

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Thought Leadership

• Programming languages are a popular discussion topic

• IntelliJ IDEA is always going to have best support for Kotlin

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Good chance of success

• Technical: experience building intelligent tools to support other languages

• Marketing: company reputation and user trust

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Kotlin Feature Highlights

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Primary Constructorclass Person(val firstName: String, val lastName: String) {

}

Propertiesclass Person(val firstName: String, val lastName: String) { val fullName: String get() = "$firstName $lastName" }

Propertiesclass Person(val firstName: String, val lastName: String) { val fullName: String get() = "$firstName $lastName" }

Smart Castsinterface Exprclass Constant(val value: Double) : Exprclass Sum(val op1: Expr, val op2: Expr) : Expr

Smart Castsfun eval(e: Expr): Double = when(e) {

}

Smart Castsfun eval(e: Expr): Double = when(e) {

}

Smart Castsfun eval(e: Expr): Double = when(e) { is Constant -> e.value

}

Smart Castsfun eval(e: Expr): Double = when(e) { is Constant -> e.value

}

class Constant(val value: Double) : Expr

Smart Castsfun eval(e: Expr): Double = when(e) { is Constant -> e.value is Sum -> eval(e.op1) + eval(e.op2) else -> throw IllegalArgumentException()}

class Sum(val op1: Expr, val op2: Expr) : Expr

Extension Functions

fun String.lastChar() = this.get(this.length - 1) "abc".lastChar()

Extension Functions

fun String.lastChar() = this.get(this.length - 1) "abc".lastChar()

Extension Functions

fun String.lastChar() = this.get(this.length - 1) "abc".lastChar()

Nullability

Nullabilityclass TreeNode(val parent: TreeNode?, val left: TreeNode?, val right: TreeNode?) {

}

Nullabilityclass TreeNode(val parent: TreeNode?, val left: TreeNode?, val right: TreeNode?) { fun depth(): Int { val parentDepth = if (parent != null) parent.depth() else 0 return parentDepth + 1 } }

Nullabilityclass TreeNode(val parent: TreeNode?, val left: TreeNode?, val right: TreeNode?) { fun depth(): Int { return (parent?.depth() ?: 0) + 1 } }

Nullabilityclass TreeNode(val parent: TreeNode?, val left: TreeNode?, val right: TreeNode?) { fun depth(): Int { return (parent?.depth() ?: 0) + 1 } }

Collections and Lambdas

fun nobleNames(persons: List<Person>) = persons .filter { "von " in it.lastName }

Collections and Lambdas

fun nobleNames(persons: List<Person>) = persons .filter { "von " in it.lastName }

Collections and Lambdas

fun nobleNames(persons: List<Person>) = persons .filter { "von " in it.lastName } .sortBy { it.firstName }

Collections and Lambdas

fun nobleNames(persons: List<Person>) = persons .filter { "von " in it.lastName } .sortBy { it.firstName } .map { "${it.firstName} ${it.lastName}” }

How JetBrains Uses Kotlin

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Projects Using Kotlin• Kotlin

• JetProfile (CRM, sales and license management)

• IntelliJ IDEA 15

• YouTrack v.next

• 2 new unannounced projects

• Research projects related to bioinformatics

• Plugins, Intranet services etc.

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JetProfile

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Проект JetProfile

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JetProfile Project

• Started in July 2013

• Replaces old Spring-based system

• 100% Kotlin, 100k LOC

• Team of 6 developers

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IntelliJ IDEA

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Kotlin in IntelliJ IDEA

• In tests since version 14, in production since 15

• 74k LOC in Kotlin, out of which 18k in tests

• A few developers actively using Kotlin

• Kotlin team provides support with updating to new Kotlin versions

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Where Kotlin Works Best

The Strengths of Kotlin

• Modeling the data of your application concisely and expressively

• Creating reusable abstractions using functional programming techniques

• Creating expressive domain-specific languages

Kotlin on the Server Side

• Java interop ensures that all existing Java frameworks can be used

• No rewrite required to start using Kotlin in existing codebase

• Existing investment is fully preserved

Server-side Kotlin Frameworks

• Kara: Web framework powering JetProfile https://github.com/shafirov/kara

• Exposed: Database access framework https://github.com/JetBrains/exposed

• Ktor: Experimental next generation Web framework https://github.com/kotlin/ktor

Kara: HTML Buildersfun HtmlBodyTag.renderPersonList( persons: Collection<Person>) { table { for (person in persons) { tr { td { +person.name } td { +person.age } } } } }

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Kara: HTML Buildersfun HtmlBodyTag.renderPersonList( persons: Collection<Person>) { table { for (person in persons) { tr { td { +person.name } td { +person.age } } } } }

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Exposed: Table structure

object CountryTable : IdTable() { val name = varchar("name", 250) .uniqueIndex()

val iso = varchar("iso", 3) .uniqueIndex() val currency = varchar("currency", 3) }

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Exposed: Entities

class Country(id: EntityID) : Entity(id) { var name: String by CountryTable.name val allCustomers by Customer.referrersOn( CustomerTable.country_id) }

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Exposed: Queries

val germany = Country .find { CountryTable.iso eq "de" } .first()

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Kotlin for Android• Compatible with Java 6

• Small runtime library

• Great tooling in Android Studio

• Compatible with existing Java libraries and tools

• 100% Java performance

Kotlin Tools for Android

• Kotlin Android Extensions: no more findViewById()

• Anko: DSL for UI, extension functions https://github.com/jetbrains/anko

• KApt: annotation processing

Kotlin Android Extensionsimport kotlinx.android.synthetic.activity_main.click_me_buttonpublic class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) click_me_button.onClick { toast("Thank you!") } } }

Kotlin Android Extensionsimport kotlinx.android.synthetic.activity_main.click_me_buttonpublic class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) click_me_button.onClick { toast("Thank you!") } } }

Kotlin Android Extensionsimport kotlinx.android.synthetic.activity_main.click_me_buttonpublic class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) click_me_button.onClick { toast("Thank you!") } } }

Anko: Android UI Builders

tableLayout { for (location in locations) { tableRow { textView(text = location.name) .lparams(column = 1) } }}

Kotlin in the Community

How to succeed with Kotlin

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Take initiative

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Take initiative

• Kotlin is not a silver bullet and not a magical solution for the problems of your project

• Kotlin makes you more productive and your work more enjoyable

• Try it and share the results with your teammates

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Don’t wait for a new project

• Kotlin can be easily integrated into existing Java codebases

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Where to start using Kotlin

• Tests - OK not informative

• Plugins and utilities - better

• Core and frameworks - best

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Don’t fear the learning curve

• At JetBrains, summer interns become productive with Kotlin very quickly and deliver great results

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Use the Java to Kotlin converter

• Convert only the classes that you’re going to modify

• Cleanup and refactor the code produced by the converter

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Use the Java to Kotlin converter

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Summary

• JetBrains entrusts the most business-critical tasks to Kotlin

• Kotlin works great for both server-side and Android development

• Take initiative and start using Kotlin today!

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Q&A

https://kotlinlang.org/ https://try.kotlinlang.org/

@kotlin

yole@jetbrains.com @intelliyole

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