A Builder's Intro to Kotlin (Gradle Summit 2017)

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.

Video: https://youtu.be/UAec6OpJiA8

Transcript

1. Jake Wharton A Builder's Intro to Kotlin
2. Kotlin?
15. Why Kotlin?
16. Plugin Author
17. Plugin Author Buildscript Author
18. Plugin Author Buildscript Author App Developer
19. val firstName: String = "Jake"
 val lastName: String? = null
20. val firstName: String = "Jake"
 val lastName: String? = null
21. val firstName: String = "Jake"
 val lastName: String? = null
22. val firstName: String = "Jake"
 val lastName: String? = null
23. val firstName: String = "Jake"
 val lastName: String? = null
24. val firstName = "Jake"
 val lastName: String? = null
25. class User {
 public String getName() {
 // ...
 }
 public void setName(String name) {
 // ...
 }
 } // ^^^ Java
26. class User {
 public String getName() {
 // ...
 }X
 public void setName(String name) {
 // ...
 }Y
 }Z // ^^^ Java vvv Kotlin val user = User()
 println("Name is " + user.name)
27. 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
28. 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
29. class User {
 public String getName() {
 // ...
 }X
 public void setName(String name) {
 // ...
 }Y
 }Z // ^^^ Java vvv Kotlin val user = User()
 println("Name is $user")X
30. class User {
 public String getName() {
 // ...
 }X
 public void setName(String name) {
 // ...
 }Y
 }Z // ^^^ Java vvv Kotlin val user = User()
 println("Name is $user")X
31. class User {
 var name = "Jake"
 } // ^^^ Kotlin
32. class User {
 var name = "Jake"
 } // ^^^ Kotlin vvv Java User user = new User(); System.out.println("Name is " + user.getName());
33. class User {
 var name = "Jake"
 } // ^^^ Kotlin vvv Java User user = new User(); System.out.println("Name is " + user.getName());
34. class User {
 var name = "Jake"
 } // ^^^ Kotlin vvv Java User user = new User(); System.out.println("Name is " + user.getName()); user.setName("Jane");
35. class User {
 var name = "Jake"
 } // ^^^ Kotlin vvv Java User user = new User(); System.out.println("Name is " + user.getName()); user.setName("Jane");
36. val user = User()
37. val user = User()
 user = User()
38. val user = User()
 user = User()
 
 var currentUser = User()
 currentUser = User()
39. fun Date.isTuesday(): Boolean {
 return day == 2
 }
40. 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.")
 }
41. 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.")
 }
42. 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.")
 } // ^^^ Kotlin vvv Java DateKt.isTuesday(date)
43. val executor = Executors.newSingleThreadExecutor();
 executor.execute {Bprintln("Background thread!") }X
44. val executor = Executors.newSingleThreadExecutor(); val foo = Foo()
 executor.execute(foo::printIt) class Foo { fun printIt() {B println("Background thread!") }X }
45. val executor = Executors.newSingleThreadExecutor(); val foo = Foo()
 executor.execute(foo::printIt) class Foo { fun printIt() {B println("Background thread!") }X }
46. fun <T> List<T>.filter(predicate: (T) -> Boolean): List<T> {
 // ...
 }
47. fun <T> List<T>.filter(predicate: (T) -> Boolean): List<T> {
 // ...
 }
48. 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
49. 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
50. 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
51. 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 }
52. fun <T> List<T>.filter(predicate: (T) -> Boolean): List<T> {
 // ...
 }A val items = listOf(1, 2, 3) val odds = items.filter { it % 2 != 0 }
53. 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 }
54. fun <T> List<T>.filter(predicate: (T) -> Boolean): List<T> {
 // ...
 }A val items = listOf(1, 2, 3) val odds = items.filter { it % 2 != 0 }
55. 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 }
56. 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
57. 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 destination = mutableListOf<Int>() for (item in items) { if (item % 2 != 0) destination.add(item) }G val odds = destination filter it
58. 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
59. 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
60. 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( )
61. 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
62. 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
63. 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
64. 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
65. 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>()
66. 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>() ALOAD 7 # item INSTANCEOF java/lang/Integer IFEQ L8 ALOAD 5 # destination ALOAD 7 # item INVOKEINTERFACE java/util/Collection.add (Ljava/lang/Object;)Z
67. class User {D
 val name = "Jake"
 }A
68. class User(name: String) {D
 val name = name
 }A 
 "Jake"
69. class User(val name: String) {
 }A
70. class User(val name: String)
71. class User(val name: String) val jake = User("Jake") println("Hello, $jake!")
72. class User(val name: String) val jake = User("Jake") println("Hello, $jake!") Hello, User@3a71f4dd!
73. data class User(val name: String) val jake = User("Jake") println("Hello, $jake!") Hello, User@3a71f4dd!
74. @3a71f4dd data class User(val name: String) val jake = User("Jake") println("Hello, $jake!") Hello, User(name=Jake)!
75. data class User(val name: String) val jake = User("Jake") println("Hello, $jake!") Hello, User(name=Jake)!
76. class UserPersisence(db: SqlDatabase) { private val deleteByName = db.createStatement("DELETE FROM user WHERE name = ?") fun delete(name: String) { deleteByName.bind(1, name) deleteByName.execute() } }
77. class UserPersisence(db: SqlDatabase) { private val deleteByName = db.createStatement("DELETE FROM user WHERE name = ?") fun delete(name: String) { deleteByName.bind(1, name) deleteByName.execute() }B }A
78. 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
79. val deleteByName by lazy { db.createStatement("DELETE FROM user WHERE name = ?") }C
80. 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") }
81. 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") } var address by Delegates.notNull<String>()
82. 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") } var address by Delegates.notNull<String>() val nameView by bindView<TextView>(R.id.name)
83. 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") } var address by Delegates.notNull<String>() val nameView by bindView<TextView>(R.id.name)
84. fun main(vararg args: String) = runBlocking<Unit> { val jobs = List(100_000) { launch(CommonPool) { delay(1000L) print(".") } } jobs.forEach { it.join() } }
85. Plugin Author Buildscript Author App Developer
86. Plugin Author Buildscript Author App Developer
87. apply plugin: 'org.jetbrains.kotlin.jvm'
88. apply plugin: 'org.jetbrains.kotlin.jvm' src/ main/ java/ Fizz.java Buzz.java
89. apply plugin: 'org.jetbrains.kotlin.jvm' src/ main/ java/ Fizz.java Buzz.kt
90. *.java *.kt
91. *.java *.kt kotlinc
92. *.java *.kt kotlinc
93. *.java *.kt kotlinc javac
94. *.java *.kt kotlinc javac
95. *.java *.kt kotlinc javac
96. kotlin-stdlib *.java *.kt kotlinc javac
97. kotlin-stdlib *.java *.kt kotlinc javac
98. kotlin-stdlib *.java *.kt kotlinc javac project(':some-library')
99. apply plugin: 'org.jetbrains.kotlin.jvm'
100. apply plugin: 'org.jetbrains.kotlin.jvm' apply plugin: 'org.jetbrains.kotlin.android'
101. apply plugin: 'org.jetbrains.kotlin.jvm' apply plugin: 'org.jetbrains.kotlin.android' apply plugin: 'kotlin2js'
102. apply plugin: 'org.jetbrains.kotlin.jvm' apply plugin: 'org.jetbrains.kotlin.android' apply plugin: 'kotlin2js' apply plugin: 'konan' # Different classpath dependency
103. apply plugin: 'org.jetbrains.kotlin.jvm' apply plugin: 'org.jetbrains.kotlin.android' apply plugin: 'kotlin2js' apply plugin: 'konan' # Different classpath dependency or apply plugin: 'org.jetbrains.kotlin.platform.common' apply plugin: 'org.jetbrains.kotlin.platform.jvm' apply plugin: 'org.jetbrains.kotlin.platform.js' apply plugin: 'konan' # Different classpath dependency
104. Plugin Author Buildscript Author App Developer
105. Plugin Author Buildscript Author App Developer
106. 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 }
107. 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 }
108. 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 }
109. 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 }
110. 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 }
111. 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 }
112. 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 }
113. 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 }
114. android { // ... testOptions { unitTests.all { systemProperty('robolectric.dependency.repo.id', 'example-nexus') systemProperty('robolectric.dependency.repo.url', 'https://nexus.example.com/content/groups/public') } } }
115. android { // ... testOptions { unitTests.all { systemProperty('robolectric.dependency.repo.id', 'example-nexus') systemProperty('robolectric.dependency.repo.url', 'https://nexus.example.com/content/groups/public') } } }
116. android { // ... testOptions { unitTests.all { systemProperty('robolectric.dependency.repo.id', 'example-nexus') systemProperty('robolectric.dependency.repo.url', 'https://nexus.example.com/content/groups/public') }
 
 }
117. 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 }A java build.gradle
118. 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
119. 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
120. 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
121. 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
122. 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
123. 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
126. Plugin Author Buildscript Author App Developer
127. Plugin Author Buildscript Author App Developer
128. class MyPlugin : Plugin<Project> { override fun apply(project: Project) { // ... }B }A
129. import org.gradle.kotlin.dsl.* class MyPlugin : Plugin<Project> { override fun apply(project: Project) { // ... }B }A
130. kotlin-stdlib *.java *.kt kotlinc javac
131. kotlin-stdlib *.java *.kt kotlinc javac apply plugin: 'your-gradle-plugin'
134. +
135. jakewharton jakewharton jakewharton twitter.com/ github.com/ .com A Builder's Intro to Kotlin