A Builder's Intro to Kotlin

A presentation at Gradle Summit in June 2017 in Palo Alto, CA, USA by Jake Wharton

Plugin Author Buildscript Author App Developer

val firstName: String = "Jake"   val lastName: String? = null

val firstName = "Jake"   val lastName: String? = null

class User {  
public String getName() {  
// ...  
}  
public void setName(String name) {  
// ...  
}   }
// ^^^ Java

class User {  
public String getName() {  
// ...  
} X  
public void setName(String name) {  
// ...  
} Y   } Z
// ^^^ Java vvv Kotlin
val user = User()   println ( "Name is "
user. name )

class User {  
public String getName() {  
// ...  
} X  
public void setName(String name) {  
// ...  
} Y   } Z
// ^^^ Java vvv Kotlin
val user = User()   println ( "Name is "
user. name ) X

class User {  
public String getName() {  
// ...  
} X  
public void setName(String name) {  
// ...  
} Y   } Z
// ^^^ Java vvv Kotlin
val user = User()   println ( "Name is $ user " ) X

class User {  
var name
"Jake"   }
// ^^^ Kotlin

class User {  
var name
"Jake"   }
// ^^^ Kotlin vvv Java User user = new User(); System. out .println( "Name is "
user.getName());

class User {  
var name
"Jake"   }
// ^^^ Kotlin vvv Java User user = new User(); System. out .println( "Name is "
user.getName());
user.setName( "Jane" );

val user = User()   user = User()  
  var currentUser = User()   currentUser = User()

fun Date.isTuesday(): Boolean {  
return day
2   }

fun Date.isTuesday(): Boolean {  
return day
2   }
val epoch = Date( 1970 , 0 , 0 )   if (epoch. isTuesday ()) {  
println ( "The epoch was a Tuesday." )   } else {  
println ( "The epoch was not a Tuesday." )   }

val executor = Executors.newSingleThreadExecutor();   executor. execute { B println ( "Background thread!" ) } X

val executor = Executors.newSingleThreadExecutor();
val foo = Foo()   executor. execute (foo::printIt)
class Foo {
fun printIt() { B
`println(`
"Background thread!" ) } X
}

fun < T
List< T .filter(predicate: ( T ) -> Boolean): List< T {  
// ...   }

fun < T
List< T .filter(predicate: ( T ) -> Boolean): List< T {  
// ...   } A
val items = listOf ( 1 , 2 , 3 ) val odds = items. filter ( { item -> item % 2 != 0 } ) B

fun < T
List< T .filter(predicate: ( T ) -> Boolean): List< T {  
// ...   } A
val items = listOf ( 1 , 2 , 3 ) val odds = items. filter ( { it % 2 != 0 } ) B

fun < T
List< T .filter(predicate: ( T ) -> Boolean): List< T {  
// ...   } A
val items = listOf ( 1 , 2 , 3 ) val odds = items. filter () B { it % 2 != 0 }

fun < T
List< T .filter(predicate: ( T ) -> Boolean): List< T {  
// ...   } A
val items = listOf ( 1 , 2 , 3 ) val odds = items. filter { it % 2 != 0 }

fun < T
List< T .filter(predicate: ( T ) -> Boolean): List< T {  
// ...   } A
val items = listOf ( 1 , 2 , 3 ) val oddList = items. filter { it % 2 != 0 }
val oddSet = items. filterTo(mutableListOf()) { it % 2 != 0 }

inline fun < T
List< T .filter(predicate: ( T ) -> Boolean): List< T {  
// ...   } A
val items = listOf ( 1 , 2 , 3 ) val odds = items. filter { it % 2 != 0 }

inline fun < T
List< T .filter(predicate: ( T ) -> Boolean): List< T {
val destination = mutableListOf < T
()
for (item in this ) {
if (predicate(item)) destination.add(item) } B
return destination } A
val items = listOf ( 1 , 2 , 3 ) val odds = items. filter { it % 2 != 0 } val destination = mutableListOf <
() for (item in
) {
if ( item ) destination.add(item) } G
destination

fun < T
List< T .filterIsInstance(c: Class< T ): List< T {  
val destination = mutableListOf < T
() for (item in this ) {
if (c.isInstance(item)) destination.add(item)
}
return destination   } A

fun < T
List< T .filterIsInstance(c: Class< T ): List< T {  
val destination = mutableListOf < T
() for (item in this ) {
if (c.isInstance(item)) destination.add(item) } G
return destination   } A

fun < T
List< T .filterIsInstance(): List< T {  
val destination = mutableListOf < T
() for (item in this ) {
if (item in
T ) destination.add(item) } G
return destination   } A
c: Class<T>  
c.isInstance(
)

fun < T
List< T .filterIsInstance(): List< T {  
val destination = mutableListOf < T
() for (item in this ) {
if (item in
T ) destination.add(item) } G
return destination   } A

inline fun < T
List< T .filterIsInstance(): List< T {  
val destination = mutableListOf < T
() for (item in this ) {
if (item in
T ) destination.add(item) } G
return destination   } A

inline fun < reified T
List< T .filterIsInstance(): List< T {  
val destination = mutableListOf < T
() for (item in this ) {
if (item in
T ) destination.add(item) } G
return destination   } A

inline fun < reified T
List< T .filterIsInstance(): List< T {  
val destination = mutableListOf < T
() for (item in this ) {
if (item in
T ) destination.add(item)
}
return destination   } A val list = listOf ( 1 , 2f , 3 , 4f ) val ints = list. filterIsInstance <Int>()

class User(name: String) { D  
val name = name   } A  
`"`
Jake"

class User( val
name : String)
val jake = User( "Jake" ) println ( "Hello, $ jake !" )

class User( val
name : String)
val jake = User( "Jake" ) println ( "Hello, $ jake !" )
Hello, User@3a71f4dd !

data class User( val
name : String)
val jake = User( "Jake" ) println ( "Hello, $ jake !" )
Hello, User@3a71f4dd !

@3a71f4dd data class User( val
name : String)
val jake = User( "Jake" ) println ( "Hello, $ jake !" )
Hello, User(name=Jake) !

data class User( val
name : String)
val jake = User( "Jake" ) println ( "Hello, $ jake !" )
Hello, User(name=Jake) !

class UserPersisence(db: SqlDatabase) {
private val deleteByName
= db.createStatement( "DELETE FROM user WHERE name = ?" )
fun delete(name: String) {
deleteByName .bind( 1 , name)
deleteByName .execute() } }

class UserPersisence(db: SqlDatabase) {
private val deleteByName by lazy {
db.createStatement( "DELETE FROM user WHERE name = ?" )
} C
fun delete(name: String) {
deleteByName .bind( 1 , name)
deleteByName .execute() } B
} A

val deleteByName by lazy {
db.createStatement( "DELETE FROM user WHERE name = ?" ) } C

val deleteByName by lazy {
db.createStatement( "DELETE FROM user WHERE name = ?" ) } C var name by Delegates.observable( "Jane" ) { prop, old, new ->
println ( "Name changed from $ old to $ new " ) }

fun main( vararg args: String) = runBlocking<Unit> {
val jobs = List ( 100_000 ) {
launch(CommonPool) {
delay( 1000L )
print ( "." )
} }
jobs.forEach { it.join() } }

App Developer
Plugin Author
Buildscript Author

apply plugin : 'org.jetbrains.kotlin.jvm'

apply plugin : 'org.jetbrains.kotlin.jvm' src/ main/ java/ Fizz.java Buzz.java

apply plugin : 'org.jetbrains.kotlin.jvm' src/ main/ java/ Fizz.java Buzz.kt

kotlin-stdlib *.java *.kt kotlinc javac
project(':some-library')

apply plugin : 'org.jetbrains.kotlin.jvm'
apply plugin : 'org.jetbrains.kotlin.android '

apply plugin : 'org.jetbrains.kotlin.jvm'
apply plugin : 'org.jetbrains.kotlin.android'
apply plugin : 'kotlin2js '

apply plugin : 'org.jetbrains.kotlin.jvm'
apply plugin : 'org.jetbrains.kotlin.android'
apply plugin : 'kotlin2js'
apply plugin : 'konan'
Different classpath dependency

Buildscript Author
Plugin Author
App Developer

apply plugin : 'org.jetbrains.kotlin.jvm' apply plugin : 'org.jetbrains.kotlin.kapt' sourceCompatibility = JavaVersion. VERSION_1_8 targetCompatibility = JavaVersion. VERSION_1_8 dependencies { compile deps. kotlin .stdLibJre8 compile deps. javaPoet
compile deps. autoCommon
compileOnly deps. autoService
kapt deps. autoService
testCompile deps. junit
testCompile deps. truth
testCompile deps. compileTesting
}

apply( plugin : 'org.jetbrains.kotlin.jvm' ) X
apply plugin : 'org.jetbrains.kotlin.kapt' sourceCompatibility = JavaVersion. VERSION_1_8 targetCompatibility = JavaVersion. VERSION_1_8 dependencies { compile deps. kotlin .stdLibJre8 compile deps. javaPoet
compile deps. autoCommon
compileOnly deps. autoService
kapt deps. autoService
testCompile deps. junit
testCompile deps. truth
testCompile deps. compileTesting
}

apply( singletonMap ( 'plugin' , 'org.jetbrains.kotlin.jvm' )) X
apply plugin : 'org.jetbrains.kotlin.kapt' sourceCompatibility = JavaVersion. VERSION_1_8 targetCompatibility = JavaVersion. VERSION_1_8 dependencies { compile deps. kotlin .stdLibJre8 compile deps. javaPoet
compile deps. autoCommon
compileOnly deps. autoService
kapt deps. autoService
testCompile deps. junit
testCompile deps. truth
testCompile deps. compileTesting
}

android { // ...
testOptions {
unitTests.all {
systemProperty( 'robolectric.dependency.repo.id' , 'example-nexus' ) systemProperty( 'robolectric.dependency.repo.url' ,
'https://nexus.example.com/content/groups/public' )
} }
}

apply {
plugin( 'org.jetbrains.kotlin.jvm' )
plugin( 'org.jetbrains.kotlin.kapt' )
} java {
sourceCompatibility = JavaVersion. VERSION_1_8
targetCompatibility = JavaVersion. VERSION_1_8
}
dependencies { compile(deps. kotlin .stdLibJre8)
compile(deps. javaPoet )
compile(deps. autoCommon )
compileOnly(deps. autoService )
kapt(deps. autoService )
testCompile(deps. junit )
testCompile(deps. truth )
testCompile(deps. compileTesting )
} A build.gradle.kts

class My Plugin : Plugin<Project> {
override fun apply(project: Project) {
// ...
} B
} A

import org.gradle.kotlin.dsl.* class My Plugin : Plugin<Project> {
override fun apply(project: Project) {
// ...
} B
} A

kotlin-stdlib *.java *.kt kotlinc javac
apply plugin : 'your-gradle-plugin'

Jake Wharton
@jakewharton

1 / 135

Kotlin’s popularity has been exploding in the past year. From last year’s announcement that Gradle would support it as a build language, to its adoption on large platforms like Android and Spring, to its general applicability for writing IDE and build system plugins, it’s hard to find an area that isn’t adopting the language in some form.

This talk will be an introduction to the language in the context of Gradle. After covering the language syntax and features and their benefits, we’ll look at why it’s great for authoring Gradle plugins. Finally, we’ll set the stage for Gradle’s Kotlin build script support for which there are other full talks at the conference.

A Builder's Intro to Kotlin

var name

var name

var name

var name

var name

return day

return day

return day

return day

item

destination

item

val name

Different classpath dependency

Different classpath dependency

Different classpath dependency

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