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Svetlana Isakova You can do better with Kotlin

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- modern - pragmatic - Android-friendly Kotlin Programming Language

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Official on Android

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Not only Android

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Multiplatform projects

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Pragmatic - tooling - Java interop

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From

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has good tooling

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- completion - navigation - refactorings - inspections …

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can be easily mixed with Java code

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*.java *.class *.dex compiled to Java bytecode *.kt

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Kotlin code Java code You can have Java & Kotlin code in one project

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You can gradually add Kotlin to your existing app

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Android-friendly

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Android Studio is based on IntelliJ IDEA

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just another library for your app rxjava-2.1.2 kotlin-stdlib-1.1.4 6315 10212

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No Kotlin SDK …just JDK + extensions small runtime jar easy Java interop

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Modern - concise - safe - expressive

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concise

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public class Person {
 private final String name;
 private final int age;
 
 public Person(String name, int age) {
 this.name = name;
 this.age = age;
 }
 
 public String getName() {
 return name;
 }
 
 public int getAge() {
 return age;
 }
 }

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- equals - hashCode - toString 
 data 
 class Person(val name: String, val age: Int)

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public class Person {
 private final String name;
 private final int age;
 
 public Person(String name, int age) {
 this.name = name;
 this.age = age;
 }
 
 public String getName() {
 return name;
 }
 
 public int getAge() {
 return age;
 }
 } 
 class Person( val name: String, val age: Int ) person.name person.getName()

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 class Person( val name: String, val age: Int ) person.getName()

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public class Person {
 private final String name;
 private final int age;
 
 public Person(String name, int age) {
 this.name = name;
 this.age = age;
 }
 
 public String getName() {
 return name;
 }
 
 public int getAge() {
 return age;
 }
 } person.name

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public void updateWeather(int degrees) {
 String description;
 Colour colour;
 if (degrees < 5) {
 description = "cold";
 colour = BLUE;
 } else if (degrees < 23) {
 description = "mild";
 colour = ORANGE;
 } else {
 description = "hot";
 colour = RED;
 }
 // ...
 } enum Colour { BLUE, ORANGE, RED, /*...*/; }

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fun updateWeather(degrees: Int) {
 val description: String
 val colour: Colour
 if (degrees < 5) {
 description = "cold"
 colour = BLUE
 } else if (degrees < 23) {
 description = "mild"
 colour = ORANGE
 } else {
 description = "hot"
 colour = RED
 }
 // ...
 }

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fun updateWeather(degrees: Int) {
 val (description: String, colour: Colour) =
 if (degrees < 5) {
 Pair("cold", BLUE)
 } else if (degrees < 23) {
 Pair("mild", ORANGE)
 } else {
 Pair("hot", RED)
 } // ...
 }

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fun updateWeather(degrees: Int) {
 val (description, colour) =
 if (degrees < 5) {
 Pair("cold", BLUE)
 } else if (degrees < 23) {
 Pair("mild", ORANGE)
 } else {
 Pair("hot", RED)
 } // ...
 }

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fun updateWeather(degrees: Int) {
 val (description, colour) = when {
 degrees < 5 -> Pair("cold", BLUE)
 degrees < 23 -> Pair("mild", ORANGE)
 else -> Pair("hot", RED)
 } // ...
 }

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fun updateWeather(degrees: Int) {
 val (description, colour) = when {
 degrees < 5 -> "cold" to BLUE
 degrees < 23 -> "mild" to ORANGE
 else -> "hot" to RED
 }
 }

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val (description, colour) = when {
 degrees < 5 -> "cold" to BLUE
 degrees < 23 -> "mild" to ORANGE
 else -> "hot" to RED
 } String description;
 Colour colour; if (degrees < 5) {
 description = "cold";
 colour = BLUE;
 } else if (degrees < 23) {
 description = "mild";
 colour = ORANGE;
 } else {
 description = "hot";
 colour = RED;
 }

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safe

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Billion Dollar Mistake

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Modern approach: to make NPE compile-time error, not run-time error

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Nullable types in Kotlin val s1: String = "always not null" 
 val s2: String? = null s1.length ✓ ✗ s2.length "can be null or non-null" null ✗

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val s: String? if (s != null) {
 s.length
 } Dealing with Nullable Types

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s?.length val s: String? Dealing with Nullable Types

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val length = if (s != null) s.length else null val s: String? Nullability operators val length = s?.length

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val length = if (s != null) s.length else null val s: String? Nullability operators val length: Int? = s?.length

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val length = if (s != null) s.length else 0 val s: String? Nullability operators val length: Int = s?.length ?: 0

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val s: String? if (s == null) fail()
 s.length Control-flow analysis

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val s: String? Making NPE explicit s!! throws NPE if s is null s!!.length

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Nullable Types Under the Hood No performance overhead @Nullable, @NotNull annotations

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Annotate your Java types in Kotlin Type behaves like regular Java type @Nullable @NotNull Type Type? Type Type @ParametersAreNonnullByDefault @MyNonnullApi Type/Type?

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class Optional(val value: T) { 
 fun isPresent() = value != null
 
 fun get() = value ?:
 throw NoSuchElementException("No value present") } Nullable types ≠ Optional

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expressive

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you can avoid any repetition you can make the code look nicer you can create API looking like DSL expressive

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Extension Functions

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fun String.lastChar() = get(length - 1) this can be omitted Extension Functions fun String.lastChar() = this.get(this.length - 1)

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import com.example.util.lastChar import com.example.util.* Extension Functions val c: Char = "abc".lastChar() fun String.lastChar() = get(length - 1)

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fun String.lastChar() = get(length - 1) Calling Extension Functions from Java code StringExtensions.kt char c = StringExtensionsKt.lastChar("abc"); JavaClass.java import static StringExtensionsKt.lastChar; char c = lastChar("abc");

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No. Because it’s a regular static method under the hood. fun String.lastChar() = get(length - 1) Extension Functions Is it possible to call a private member of String here?

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Extensions for Android Toast.makeText(this, "Thank you!”, Toast.LENGTH_SHORT).show() Activity extension function on Activity toast("Thank you!") this.toast("Thank you!")

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this.startActivity(“ANSWER" to 42) val intent = Intent(this, NewActivity::class.java)
 intent.putExtra("ANSWER", 42)
 startActivity(intent) Extensions for Android

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infix fun A.to(that: B) = Pair(this, that) "ANSWER".to(42) "hot" to RED mapOf(0 to "zero", 1 to "one") The to extension function

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Lambdas

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Lambdas button.addActionListener { println("Hi") }

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{ employee: Employee -> employee.city == City.PRAGUE } What’s an average age of employees working in Prague? Working with collections with Lambdas val employees: List employees.filter { it.city == City.PRAGUE }.map { it.age }.average() data class Employee( val city: City, val age: Int )

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What’s an average age of employees working in Prague? Working with collections with Lambdas val employees: List data class Employee( val city: City, val age: Int ) extension functions employees.filter { it.city == City.PRAGUE }.map { it.age }.average()

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Kotlin library: extensions on collections • filter • map • reduce • count • find • any • flatMap • groupBy • …

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Under the Hood No performance overhead Lambdas can be inlined

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Extension Function & Lambda Lambda with receiver

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val sb = StringBuilder()
 with (sb) {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 append(c)
 } toString()
 } The with function with is a function val sb = StringBuilder()
 sb.appendln("Alphabet: ")
 for (c in 'a'..'z') {
 sb.append(c)
 }
 sb.toString()


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lambda is its second argument val sb = StringBuilder()
 with (sb) {
 this.appendln(“Alphabet: ")
 for (c in 'a'..'z') {
 this.append(c)
 } this.toString()
 } val sb = StringBuilder()
 with (sb, { ->
 this.appendln(“Alphabet: ")
 for (c in 'a'..'z') {
 this.append(c)
 } this.toString()
 }) lambda is its second argument Lambda with receiver with is a function this is an implicit receiver in the lambda val sb = StringBuilder()
 with (sb) {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 append(c)
 } toString()
 } this can be omitted

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with (sb) {
 appendln("Alphabet: ")
 ...
 } inline fun with( receiver: T, block: T.() -> R ): R = receiver.block() The with function declaration

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Lambda with receiver val sb = StringBuilder()
 with (sb) {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 this.append(c)
 }
 } lambda with implicit this

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html {
 table {
 for (product in products) {
 tr {
 td { text(product.description) }
 td { text(product.price) }
 td { text(product.popularity) }
 }
 }
 }
 } HTML Builders lambdas with receiver

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val db: SQLiteDatabase = … db.beginTransaction()
 try {
 db.delete("users", "first_name = ?", arrayOf("Jake"))
 db.setTransactionSuccessful()
 } finally {
 db.endTransaction()
 } db.inTransaction {
 delete("users", "first_name = ?", arrayOf("Jake"))
 } Avoiding Duplication

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db.beginTransaction()
 try {
 db.delete("users", "first_name = ?", arrayOf("Jake"))
 db.setTransactionSuccessful()
 } finally {
 db.endTransaction()
 } db.inTransaction {
 delete("users", "first_name = ?", arrayOf("Jake"))
 } Inline functions is declared as inline function generated bytecode is similar to

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ANKO-DSL DSL for dynamic layouts

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Alerts fun Activity.showAreYouSureAlert(process: () -> Unit) {
 alert(title = "Are you sure?",
 message = "Are you really sure?") {
 positiveButton("Yes") { process() }
 negativeButton("No") { }
 }.show()
 } Are you sure? Are you really sure? No Yes

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customView {
 verticalLayout {
 val email = editText {
 hint = "Email"
 }
 
 val password = editText {
 hint = "Password"
 transformationMethod = PasswordTransformationMethod.getInstance()
 }
 
 positiveButton("Log In") {
 logIn(email.text, password.text)
 }
 }
 } Password Log In Email Custom layouts

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kotlinlang.org

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Gradle & Kotlin Writing Gradle build scripts and plugins in Kotlin

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try.kotlinlang.org

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Kotlin Koans

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No content

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Have a nice Kotlin!