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kotlin: NewHope In a Java 6 Wasteland

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100% interoperable with Java™ Statically typed programming language targeting the JVM and JavaScript

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concise /kənˈsīs/ adjective giving a lot of information clearly and in a few words; brief but comprehensive.

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expressive /ikˈspresiv/ adjective effectively conveying thought or feeling.

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safe /sāf/ adjective protected from or not exposed to danger or risk; not likely to be harmed or lost.

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versatile /ˈvərsədl/ adjective able to adapt or be adapted to many different functions or activities.

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interoperable /ˌin(t)ərˈäp(ə)rəb(ə)l/ adjective (of computer systems or software) able to exchange and make use of information.

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Why not wait for Java 8?

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Java 6 2006

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Java 6 2006 Android 1.0 2008

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Java 6 2006 Java 7 2011 Android 1.0 2008

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Java 6 2006 Java 7 2011 Java 7 Support 2013 Android 1.0 2008

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Java 6 2006 Java 7 2011 Java 8 2014 Java 7 Support 2013 Android 1.0 2008

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Java 6 2006 Java 7 2011 Java 8 2014 Java 7 Support 2013 Java 8 Support ???? Android 1.0 2008

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KitKat Jelly Bean Lollipop Froyo Gingerbread Ice Cream Sandwich

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KitKat Jelly Bean Lollipop Froyo Gingerbread Ice Cream Sandwich

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Problems with Java

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Null references “I call it my billion-dollar mistake… [which] has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.” — Tony Hoare

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Raw types List numbers = getNumberList(); int sum = 0; for (Object num : numbers) { sum += (Integer) num; // Unchecked cast }

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Covariant arrays String[] strings = { "hello" }; Object[] objects = strings; objects[0] = 1; // java.lang.ArrayStoreException

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SAM types interface Func1 { R call(T1 t1); } interface Func2 { R call(T1 t1, T2 t2); } interface Func3 { R call(T1 t1, T2 t2, T3 t3); } // ...

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Wildcards “I am completely and totally humbled. Laid low. I realize now that I am simply not smart at all. I made the mistake of thinking that I could understand generics. I simply cannot. I just can't. This is really depressing. It is the first time that I've ever not been able to understand something related to computers, in any domain, anywhere, period.” "We simply cannot afford another wildcards” — Joshua Bloch

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Checked exceptions “… requiring exception specifications could both enhance developer productivity and enhance code quality, but experience with large software projects suggests a different result – decreased productivity and little or no increase in code quality.” — Bruce Eckel

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Kotlin to the rescue!

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What Kotlin removes • Checked exceptions • Non-class primitive types • Static members • Non-private fields • Wildcard types

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What Kotlin adds • Lambdas • Data classes • Function literals & inline functions • Extension functions • Null-safety • Smart casts • String templates • Properties • Primary constructors • Class delegation • Type inference • Singletons • Declaration-site variance • Range expressions

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Dalvik ART Android Java source javac Kotlin source kotlinc bytecode JVM Backend JVM Desktop

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Hello, Kotlin!

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fun main(args: Array): Unit { println("Hello, World!") } > Hello, World!

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fun main(args: Array): Unit { println("Hello, World!") } Function keyword

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fun main(args: Array): Unit { println("Hello, World!") } Function name

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fun main(args: Array): Unit { println("Hello, World!") } Argument name

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fun main(args: Array): Unit { println("Hello, World!") } Argument type

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fun main(args: Array): Unit { println("Hello, World!") } Return type

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fun main(args: Array): Unit { println("Hello, World!") }

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fun main(args: Array): Unit {. println("Hello, World!") }. Unit inferred

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fun main(args: Array) {. println("Hello, World!") }.

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fun main(args: Array) {. println("Hello, World!") }.

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fun main(args: Array) {. var name = "World" println("Hello, $name!") }.

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fun main(args: Array) {. var name = "World" println("Hello, $name!") }. Variable declaration

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fun main(args: Array) {. var name = "World" println("Hello, $name!") }. String interpolation

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fun main(args: Array) {. var name = "World" if (args.isNotEmpty()) { name = args[0] } println("Hello, $name!") }.

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fun main(args: Array) {. var name = "World" if (args.isNotEmpty()) { name = args[0] } println("Hello, $name!") }.

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fun main(args: Array) {. val name = "World" if (args.isNotEmpty()) { name = args[0] } println("Hello, $name!") }.

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fun main(args: Array) {. val name = "World" if (args.isNotEmpty()) { name = args[0] } println("Hello, $name!") }. Constant declaration

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fun main(args: Array) {. val name = "World" if (args.isNotEmpty()) { name = args[0] }. println("Hello, $name!") }. Val cannot be reassigned

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fun main(args: Array) {. val name = "World" if (args.isNotEmpty()) { name = args[0] }. println("Hello, $name!") }.

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fun main(args: Array) {. val name = if (args.isNotEmpty()) { args[0] } else { "World" }. println("Hello, $name!") }.

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fun main(args: Array) {. val name = if (args.isNotEmpty()) { args[0] } else { "World" }. println("Hello, $name!") }.

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fun main(args: Array) {. val name = if (args.isNotEmpty()) { args[0] } else { "World" }. println("Hello, $name!") }. Conditional assignment block

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fun main(args: Array) { val name = if (args.isNotEmpty()) { args[0] } else { "World" }. println("Hello, $name!") }.

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fun main(args: Array) { val name = if (args.isNotEmpty()) args[0] else "World" println("Hello, $name!") }.

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class Person(var name: String) fun main(args: Array) { val name = if (args.isNotEmpty()) args[0] else "World" println("Hello, $name!") }.

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class Person(var name: String) fun main(args: Array) { val name = if (args.isNotEmpty()) args[0] else "World" println("Hello, $name!") } Class keyword

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class Person(var name: String) fun main(args: Array) { val name = if (args.isNotEmpty()) args[0] else "World" println("Hello, $name!") } Class name

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class Person(var name: String) fun main(args: Array) { val name = if (args.isNotEmpty()) args[0] else "World" println("Hello, $name!") } Primary constructor

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class Person(var name: String) fun main(args: Array) { val name = if (args.isNotEmpty()) args[0] else "World" println("Hello, $name!") } Non-final class member

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class Person(var name: String) fun main(args: Array) { val name = if (args.isNotEmpty()) args[0] else "World" println("Hello, $name!") }

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class Person(var name: String) fun main(args: Array) { println("Hello, $name!") }

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class Person(var name: String) fun main(args: Array) { val person = Person("Michael") println("Hello, $name!") }

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class Person(var name: String) fun main(args: Array) { val person = Person("Michael") println("Hello, $name!") }. Instance declaration

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class Person(var name: String) fun main(args: Array) { val person = Person("Michael") println("Hello, $name!") }.

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class Person(var name: String) fun main(args: Array) { val person = Person("Michael") println("Hello, ${person.name}!") }. > Hello, Michael!

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") } class Person(var name: String) fun main(args: Array) { val person = Person("Michael") println("Hello, ${person.name}!") }.

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }.. class Person(var name: String). fun main(args: Array) { val person = Person("Michael") println("Hello, ${person.name}!") }.

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }.. class Person(var name: String, var lang: Language). fun main(args: Array) { val person = Person("Michael") println("Hello, ${person.name}!") }.

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }.. class Person(var name: String, var lang: Language = Language.EN). fun main(args: Array) { val person = Person("Michael") println("Hello, ${person.name}!") }. Default value

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... class Person(var name: String, var lang: Language = Language.EN) fun main(args: Array) { val person = Person("Michael") println("Hello, ${person.name}!") }..

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") }. fun main(args: Array) { val person = Person("Michael") println("Hello, ${person.name}!") }..

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") } class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") } fun main(args: Array) { val person = Person("Michael") println("Hello, ${person.name}!") }

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") } class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") } fun main(args: Array) { val person = Person("Michael") }

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") } class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") } fun main(args: Array) { val person = Person("Michael") person.greet() } > Hello, Michael!

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") }.. fun main(args: Array) { }.

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") }.. fun main(args: Array) { val people = listOf( Person("Michael"), Person("Miguel", Language.SP), Person("Michelle", Language.FR) ). }.

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") }.. fun main(args: Array) { val people = listOf( Person("Michael"), Person("Miguel", Language.SP), Person("Michelle", Language.FR) ). for (person in people) {. person.greet() }. }.

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") }.. fun main(args: Array) { val people = listOf( Person("Michael"), Person("Miguel", Language.SP), Person("Michelle", Language.FR) ). people.forEach {.person -> person.greet() }. }.

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") }.. fun main(args: Array) { val people = listOf( Person("Michael"), Person("Miguel", Language.SP), Person("Michelle", Language.FR) ). people.forEach { it.greet() }. }.

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") }.. fun main(args: Array) { listOf( Person("Michael"), Person("Miguel", Language.SP), Person("Michelle", Language.FR) ).forEach { it.greet() }. }. > Hello, Michael! > Hola, Miguel! > Bonjour, Michelle!

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... open class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") }.. fun main(args: Array) { listOf( Person("Michael"), Person("Miguel", Language.SP), Person("Michelle", Language.FR) ).forEach { it.greet() }. }. Non-final

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... open class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") }.. class Hispanophone(name: String) : Person(name, Language.ES) class Francophone(name: String) : Person(name, Language.FR) fun main(args: Array) { listOf( Person("Michael"), Person("Miguel", Language.SP), Person("Michelle", Language.FR) ).forEach { it.greet() } }.

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... open class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") }.. class Hispanophone(name: String) : Person(name, Language.ES) class Francophone(name: String) : Person(name, Language.FR) fun main(args: Array) { listOf( Person("Michael"), Hispanophone("Miguel"), Francophone("Michelle") ).forEach { it.greet() } }.

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enum class Language(val greeting: String) { EN("Hello"), ES("Hola"), FR("Bonjour") }... open class Person(var name: String, var lang: Language = Language.EN) { fun greet() = println("${lang.greeting}, $name!") }.. class Hispanophone(name: String) : Person(name, Language.ES) class Francophone(name: String) : Person(name, Language.FR) fun main(args: Array) { listOf( Person("Michael"), Hispanophone("Miguel"), Francophone("Michelle") ).forEach { it.greet() } }.

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What Kotlin Adds to Java

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Type inference

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Type inference String string = "";

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Type inference var string: String = ""

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Type inference var string = ""

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Type inference var string = "" var char = ' '

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Type inference var string = "" var char = ' ‘ var int = 0

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Type inference var string = "" var char = ' ‘ var int = 0 var long = 0L

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Type inference var string = "" var char = ' ‘ var int = 0 var long = 0L var float = 0F

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Type inference var string = "" var char = ' ‘ var int = 0 var long = 0L var float = 0F var double = 0.0

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Type inference var string = "" var char = ' ' var int = 0 var long = 0L var float = 0F var double = 0.0 var boolean = true

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Type inference var string = "" var char = ' ' var int = 0 var long = 0L var float = 0F var double = 0.0 var boolean = true var foo = MyFooType()

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Null-safety

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Null-safety String a = null;

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Null-safety String a = null; System.out.println(a.length());

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Null-safety String a = null; System.out.println(a.length()); NullPointerException

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Null-safety val a: String = null

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Null-safety val a: String = null Non-null type

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Null-safety val a: String? = null

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Null-safety val a: String? = null

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Null-safety val a: String? = null println(a.length())

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Null-safety val a: String? = null println(a.length()) Unsafe call

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Null-safety val a: String? = null println(a?.length())

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Null-safety val a: String? = null println(a?.length()) > null

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Null-safety int length = a != null ? a.length() : -1

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Null-safety int length = a != null ? a.length() : -1 Null check

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Null-safety int length = a != null ? a.length() : -1 Assignment selector

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Null-safety var length = if (a != null) a.length() else -1

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Null-safety var length = if (a != null) a.length() else -1 Null check

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Null-safety var length = if (a != null) a.length() else -1 Assignment selector

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Null-safety var length = a?.length() ?: -1

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Null-safety var length = a?.length() ?: -1 Null check

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Null-safety var length = a?.length() ?: -1 Assignment selector

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Smart casts

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Smart casts if (x is String) { print(x.length()) }

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Smart casts if (x is String) { print(x.length()) } Type check

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Smart casts if (x is String) { print(x.length()) } Smart cast

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Smart casts if (x !is String) { return } print(x.size())

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Smart casts if (x !is String) { return } print(x.size()) Type check

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Smart casts if (x !is String) { return } print(x.size())

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Smart casts if (x !is String) { return } print(x.size()) Smart cast

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Smart casts if (x !is String || x.size() == 0) { return }

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Smart casts if (x !is String || x.size() == 0) { return } Type check

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Smart casts if (x !is String || x.size() == 0) { return }

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Smart casts if (x !is String || x.size() == 0) { return } Smart cast

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Smart casts if (x is String && x.size() > 0) { print(x.size()) }

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Smart casts if (x is String && x.size() > 0) { print(x.size()) } Type check

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Smart casts if (x is String && x.size() > 0) { print(x.size()) }

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Smart casts if (x is String && x.size() > 0) { print(x.size()) } Smart cast

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Smart casts if (x is String && x.size() > 0) { print(x.size()) } Smart cast

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Smart casts when (x) { is Int -> print(x + 1) is String -> print(x.size() + 1) is Array -> print(x.sum()) }

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Smart casts when (x) { is Int -> print(x + 1) is String -> print(x.size() + 1) is Array -> print(x.sum()) } Type check

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Smart casts when (x) { is Int -> print(x + 1) is String -> print(x.size() + 1) is Array -> print(x.sum()) } Smart cast

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Smart casts when (x) { is Int -> print(x + 1) is String -> print(x.size() + 1) is Array -> print(x.sum()) } Type check

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Smart casts when (x) { is Int -> print(x + 1) is String -> print(x.size() + 1) is Array -> print(x.sum()) } Smart cast

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Smart casts when (x) { is Int -> print(x + 1) is String -> print(x.size() + 1) is Array -> print(x.sum()) } Type check

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Smart casts when (x) { is Int -> print(x + 1) is String -> print(x.size() + 1) is Array -> print(x.sum()) } Smart cast

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String templates

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String templates val apples = 4 println("I have " + apples + " apples.") > I have 4 apples.

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String templates val apples = 4 println("I have $apples apples.") > I have 4 apples. > I have 4 apples.

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String templates val apples = 4 val bananas = 3 println("I have $apples apples and " + (apples + bananas) + " fruits.") > I have 4 apples. > I have 4 apples. > I have 4 apples and 7 fruits.

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String templates val apples = 4 val bananas = 3 println("I have $apples apples and ${apples+bananas} fruits.") > I have 4 apples. > I have 4 apples and 7 fruits. > I have 4 apples and 7 fruits.

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Range expressions

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Range expressions if (1 <= i && i <= 10) { println(i) }

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Range expressions if (1 <= i && i <= 10) { println(i) }. if (IntRange(1, 10).contains(i)) { println(i) }.

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Range expressions if (1 <= i && i <= 10) { println(i) }. if (1.rangeTo(10).contains(i)) { println(i) }.

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Range expressions if (1 <= i && i <= 10) { println(i) }. if (i in 1..10) { println(i) }. Range operator

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Range expressions for (i in 1..4) { print(i) }. > 1234

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Range expressions for (i in 1..4 step 2) { print(i) }. > 1234 > 13

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Range expressions for (i in 4 downTo 1 step 2) { print(i) }. > 1234 > 13 > 42

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Range expressions for (i in 1.0..2.0) { print("$i ") }. > 13 > 42 > 1.0 2.0

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Range expressions for (i in 1.0..2.0 step 0.3) { print("$i ") }. > 42 > 1.0 2.0 > 1.0 1.3 1.6 1.9

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Higher-order functions & lambdas

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Higher-order functions & lambdas public interface Function { R call(T t); }

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Higher-order functions & lambdas public interface Function { R call(T t); } public static List filter(Collection items, Function f) { final List filtered = new ArrayList(); for (T item : items) if (f.call(item)) filtered.add(item); return filtered; }

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Higher-order functions & lambdas public interface Function { R call(T t); } public static List filter(Collection items, Function f) { final List filtered = new ArrayList(); for (T item : items) if (f.call(item)) filtered.add(item); return filtered; } filter(numbers, new Function() { @Override public Boolean call(Integer value) { return value % 2 == 0; } });

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Higher-order functions & lambdas public interface Function { R call(T t); } public static List filter(Collection items, Function f) { final List filtered = new ArrayList(); for (T item : items) if (f.call(item)) filtered.add(item); return filtered; } filter(numbers, new Function() { @Override public Boolean call(Integer value) { return value % 2 == 0; } });

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Higher-order functions & lambdas public interface Function { R call(T t); } public static List filter(Collection items, Function f) { final List filtered = new ArrayList(); for (T item : items) if (f.call(item)) filtered.add(item); return filtered; } filter(numbers, new Function() { @Override public Boolean call(Integer value) { return value % 2 == 0; } }); Functional interface

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Higher-order functions & lambdas public interface Function { R call(T t); } public static List filter(Collection items, Function f) { final List filtered = new ArrayList(); for (T item : items) if (f.call(item)) filtered.add(item); return filtered; } filter(numbers, new Function() { @Override public Boolean call(Integer value) { return value % 2 == 0; } }); Interface argument

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Higher-order functions & lambdas public interface Function { R call(T t); } public static List filter(Collection items, Function f) { final List filtered = new ArrayList(); for (T item : items) if (f.call(item)) filtered.add(item); return filtered; } filter(numbers, new Function() { @Override public Boolean call(Integer value) { return value % 2 == 0; } }); Interface function call

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Higher-order functions & lambdas public interface Function { R call(T t); } public static List filter(Collection items, Function f) { final List filtered = new ArrayList(); for (T item : items) if (f.call(item)) filtered.add(item); return filtered; } filter(numbers, new Function() { @Override public Boolean call(Integer value) { return value % 2 == 0; } }); Anonymous implementation

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Higher-order functions & lambdas fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered }

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Higher-order functions & lambdas fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered } filter(numbers, { value -> value % 2 == 0 })

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Higher-order functions & lambdas fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered } filter(numbers, { value -> value % 2 == 0 })

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Higher-order functions & lambdas fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered } filter(numbers, { value -> value % 2 == 0 }) Function type

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Higher-order functions & lambdas fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered } filter(numbers, { value -> value % 2 == 0 }) Function call

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Higher-order functions & lambdas fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered } filter(numbers, { value -> value % 2 == 0 }) Anonymous function

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Higher-order functions & lambdas fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered }. filter(numbers, { value -> value % 2 == 0 } . )

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Higher-order functions & lambdas fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered }. filter(numbers) { value -> value % 2 == 0 } .

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Higher-order functions & lambdas fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered }. filter(numbers) { it % 2 == 0 } .

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Higher-order functions & lambdas fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered }. filter(numbers) { it % 2 == 0 } .

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Inline functions

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Inline functions fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered }. filter(numbers) { it % 2 == 0 }

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Inline functions inline fun filter(items: Collection, f: (T) -> Boolean): List { val filtered = arrayListOf() for (item in items) if (f(item)) filtered.add(item) return filtered }. filter(numbers) { it % 2 == 0 }

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Inline functions val filtered = arrayListOf() for (item in items) if (it % 2 == 0) filtered.add(item)

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Inline functions fun call(f: () -> Unit) { f() } call { return }

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Inline functions fun call(f: () -> Unit) { f() } call { return } Return not allowed

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Inline functions fun call(f: () -> Unit) { f() }. call { return }.

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Inline functions inline fun call(f: () -> Unit) { f() }. call { return }.

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Inline functions inline fun call(f: () -> Unit) { f() }. call { return }. Return allowed

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Inline functions inline fun View.findViewParent(): T? { var parent = getParent() while (parent != null && parent !is T) { parent = parent.getParent() } return parent as T }

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Inline functions inline fun View.findViewParent(): T? { var parent = getParent() while (parent != null && parent !is T) { parent = parent.getParent() } return parent as T } Type erasure

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Inline functions inline fun View.findViewParent(): T? { var parent = getParent() while (parent != null && parent !is T) { parent = parent.getParent() } return parent as T } Unchecked cast

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Inline functions inline fun View.findViewParent(): T? { var parent = getParent() while (parent != null && parent !is T) { parent = parent.getParent() }. return parent as T }.

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Inline functions inline fun View.findViewParent(): T? { var parent = getParent() while (parent != null && parent !is T) { parent = parent.getParent() }. return parent as T }. Reified type

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Inline functions inline fun View.findViewParent(): T? { var parent = getParent() while (parent != null && parent !is T) { parent = parent.getParent() } return parent as T } Type check allowed

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Inline functions inline fun View.findViewParent(): T? { var parent = getParent() while (parent != null && parent !is T) { parent = parent.getParent() } return parent as T } Type cast allowed

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Extension functions

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Extension functions public fun isLollipopOrGreater(code: Int): Boolean { return code >= Build.VERSION_CODES.LOLLIPOP }.

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Extension functions public fun isLollipopOrGreater(code: Int): Boolean { return code >= Build.VERSION_CODES.LOLLIPOP }.

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Extension functions public fun Int.isLollipopOrGreater(code: Int): Boolean { return code >= Build.VERSION_CODES.LOLLIPOP }.

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Extension functions public fun Int.isLollipopOrGreater(): Boolean { return code >= Build.VERSION_CODES.LOLLIPOP }.

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Extension functions public fun Int.isLollipopOrGreater(): Boolean {. return this >= Build.VERSION_CODES.LOLLIPOP }..

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Extension functions public fun Int.isLollipopOrGreater(): Boolean {. return this >= Build.VERSION_CODES.LOLLIPOP }.. if (Build.VERSION.SDK_INT.isLollipopOrGreater) { // ... }.

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Extension functions public fun Int.isLollipopOrGreater(): Boolean {. return this >= Build.VERSION_CODES.LOLLIPOP }. if (16.isLollipopOrGreater) { // ... }.

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Extension functions final Function customerMapper = // ... final Function orderFilter = // ... final Function orderSorter = // ... final List vipOrders = sortBy(filter(map(customers, customerMapper), orderFilter), orderSorter);

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Extension functions final Function customerMapper = // ... final Function orderFilter = // ... final Function orderSorter = // ... final List vipOrders = sortBy(filter(map(customers, customerMapper), orderFilter), orderSorter);

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Extension functions final Function customerMapper = // ... final Function orderFilter = // ... final Function orderSorter = // ... final List vipOrders = sortBy(filter(map(customers, customerMapper), orderFilter), orderSorter);

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Extension functions final Function customerMapper = // ... final Function orderFilter = // ... final Function orderSorter = // ... final List vipOrders = sortBy(filter(map(customers, customerMapper), orderFilter), orderSorter);

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Extension functions final Function customerMapper = // ... final Function orderFilter = // ... final Function orderSorter = // ... final List vipOrders = sortBy(filter(map(customers, customerMapper), orderFilter), orderSorter);

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Extension functions val vipOrders = customers .map { it.lastOrder } .filter { it.total >= 500F } .sortBy { it.total }

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Extension functions val vipOrders = customers .map { it.lastOrder } .filter { it.total >= 500F } .sortBy { it.total }

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Extension functions val vipOrders = customers .map { it.lastOrder } .filter { it.total >= 500F } .sortBy { it.total }

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Extension functions val vipOrders = customers .map { it.lastOrder } .filter { it.total >= 500F } .sortBy { it.total }

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Extension functions val vipOrders = customers .map { it.lastOrder } .filter { it.total >= 500F } .sortBy { it.total }

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Extension functions val vipOrders = customers .map { it.lastOrder } .filter { it.total >= 500F } .sortBy { it.total }

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Properties

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Properties class Customer { private String firstName; private String lastName; private String email; public String getFirstName() { return firstName; } public String getLastName() { return lastName; } public String getEmail() { return email; } public void setFirstName(String firstName) { this.firstName = firstName } public void setLastName(String lastName) { this.lastName = lastName } public void setEmail(String email) { this.email = email } }

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Properties class Customer { private String firstName; private String lastName; private String email; public String getFirstName() { return firstName; } public String getLastName() { return lastName; } public String getEmail() { return email; } public void setFirstName(String firstName) { this.firstName = firstName } public void setLastName(String lastName) { this.lastName = lastName } public void setEmail(String email) { this.email = email } }

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Properties class Customer { private String firstName; private String lastName; private String email; public String getFirstName() { return firstName; } public String getLastName() { return lastName; } public String getEmail() { return email; } public void setFirstName(String firstName) { this.firstName = firstName } public void setLastName(String lastName) { this.lastName = lastName } public void setEmail(String email) { this.email = email } }

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Properties class Customer { private String firstName; private String lastName; private String email; public String getFirstName() { return firstName; } public String getLastName() { return lastName; } public String getEmail() { return email; } public void setFirstName(String firstName) { this.firstName = firstName } public void setLastName(String lastName) { this.lastName = lastName } public void setEmail(String email) { this.email = email } }

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Properties class Customer { var firstName: String = // ... var lastName: String = // ... var email: String = // ... }

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Properties class Customer { var firstName: String = // ... var lastName: String = // ... var email: String = // ... } val customer = Customer() customer.firstName = "Michael" customer.lastName = "Pardo" customer.email = "michael@michaelpardo.com"

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Primary constructors

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Primary constructors class Customer {. var firstName: String = // ... var lastName: String = // ... var email: String = // ... }.

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Primary constructors class Customer(firstName: String, lastName: String, email: String) {. var firstName: String = // ... var lastName: String = // ... var email: String = // ... }.

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Primary constructors class Customer(firstName: String, lastName: String, email: String) {. var firstName: String = firstName var lastName: String = lastName var email: String = email }.

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Primary constructors class Customer(firstName: String, lastName: String, email: String) {. var firstName: String var lastName: String var email: String init { this.firstName = firstName this.lastName = lastName this.email = email } }.

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Primary constructors class Customer(firstName: String, lastName: String, email: String)

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Primary constructors class Customer(var firstName: String, var lastName: String, var email: String)

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Primary constructors class Customer( var firstName: String, var lastName: String, var email: String)

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Primary constructors class Customer( val firstName: String, val lastName: String, val email: String)

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Singletons

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Singletons public class Singleton { private static volatile Singleton instance; private Singleton() { } public static Singleton getInstance() { if (instance == null ) { synchronized (Singleton.class) { if (instance == null) { instance = new Singleton(); } } } return instance; } }

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Singletons public class Singleton { private static volatile Singleton instance = null; private Singleton() { } public static synchronized Singleton getInstance() { if (instance == null) { instance = new Singleton(); } return instance; } }

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Singletons public class Singleton { private static final Singleton INSTANCE = new Singleton(); private Singleton() {} public static Singleton getInstance() { return INSTANCE; } }

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Singletons public class Singleton { private static final Singleton instance; static { try { instance = new Singleton(); } catch (Exception e) { throw new RuntimeException("Darn, an error occurred!", e); } } public static Singleton getInstance() { return instance; } private Singleton() { } }

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Singletons public class Singleton { private Singleton() { } private static class SingletonHolder { private static final Singleton INSTANCE = new Singleton(); } public static Singleton getInstance() { return SingletonHolder.INSTANCE; } }

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Singletons public enum Singleton { INSTANCE; public void execute (String arg) { } }

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Singletons object Singleton

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Singletons object Logger { val tag = "TAG" fun d(message: String) { Log.d(tag, message) } }

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Companion objects

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Companion objects class LaunchActivity extends AppCompatActivity { public static final String TAG = LaunchActivity.class.getName(); public static void start(Context context) { context.startActivity(new Intent(context, LaunchActivity.class)); } }

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Companion objects class LaunchActivity extends AppCompatActivity { public static final String TAG = LaunchActivity.class.getName(); public static void start(Context context) { context.startActivity(new Intent(context, LaunchActivity.class)); } }

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Companion objects class LaunchActivity extends AppCompatActivity { public static final String TAG = LaunchActivity.class.getName(); public static void start(Context context) { context.startActivity(new Intent(context, LaunchActivity.class)); } }

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Companion objects class LaunchActivity extends AppCompatActivity { public static final String TAG = LaunchActivity.class.getName(); public static void start(Context context) { context.startActivity(new Intent(context, LaunchActivity.class)); } } Timber.v("Starting activity %s", LaunchActivity.TAG);

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Companion objects class LaunchActivity extends AppCompatActivity { public static final String TAG = LaunchActivity.class.getName(); public static void start(Context context) { context.startActivity(new Intent(context, LaunchActivity.class)); } } Timber.v("Starting activity %s", LaunchActivity.TAG); LaunchActivity.start(context);

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Companion objects class LaunchActivity { companion object { val TAG: String = LaunchActivity::class.simpleName fun start(context: Context) { context.startActivity(Intent(context, LaunchActivity::class)) } } }

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Companion objects class LaunchActivity { companion object { val TAG: String = LaunchActivity::class.simpleName fun start(context: Context) { context.startActivity(Intent(context, LaunchActivity::class)) } } }

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Companion objects class LaunchActivity { companion object { val TAG: String = LaunchActivity::class.simpleName fun start(context: Context) { context.startActivity(Intent(context, LaunchActivity::class)) } } }

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Companion objects class LaunchActivity { companion object { val TAG: String = LaunchActivity::class.simpleName fun start(context: Context) { context.startActivity(Intent(context, LaunchActivity::class)) } } }

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Companion objects class LaunchActivity { companion object { val TAG: String = LaunchActivity::class.simpleName fun start(context: Context) { context.startActivity(Intent(context, LaunchActivity::class)) } } } Timber.v("Starting activity ${LaunchActivity.TAG}")

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Companion objects class LaunchActivity { companion object { val TAG: String = LaunchActivity::class.simpleName fun start(context: Context) { context.startActivity(Intent(context, LaunchActivity::class)) } } } Timber.v("Starting activity ${LaunchActivity.TAG}") LaunchActivity.start(context)

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Class delegation

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Class delegation public class MyList implements List { private List delegate; public MyList(delegate: List) { this.delegate = delegate; } // ... public E get(int location) { return delegate.get(location) } // ... }

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Class delegation public class MyList implements List { private List delegate; public MyList(delegate: List) { this.delegate = delegate; } // ... public E get(int location) { return delegate.get(location) } // ... }

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Class delegation public class MyList implements List { private List delegate; public MyList(delegate: List) { this.delegate = delegate; } // ... public E get(int location) { return delegate.get(location) } // ... }

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Class delegation public class MyList implements List { private List delegate; public MyList(delegate: List) { this.delegate = delegate; } // ... public E get(int location) { return delegate.get(location) } // ... }

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Class delegation public class MyList implements List { private List delegate; public MyList(delegate: List) { this.delegate = delegate; } // ... public E get(int location) { return delegate.get(location) } // ... }

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Class delegation class MyList(list: List) : List by list

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Class delegation class MyList(list: List) : List by list

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Class delegation class MyList(list: List) : List by list

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Class delegation class MyList(list: List) : List by list

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Declaration-site variance

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Declaration-site variance String[] strings = { "hello", "world" }; Object[] objects = strings;

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Declaration-site variance List strings = Arrays.asList("hello", "world"); List objects = strings;

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Declaration-site variance List strings = Arrays.asList("hello", "world"); List objects = strings; Error: incompatible types

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Declaration-site variance List strings = Arrays.asList("hello", "world"); List objects = strings;

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Declaration-site variance List strings = Arrays.asList("hello", "world"); List extends Object> objects = strings;

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Declaration-site variance public interface List extends Collection { public boolean addAll(Collection extends E> collection); public E get(int location); }

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Declaration-site variance public interface List : Collection { public fun get(index: Int): E } public interface MutableList : List, MutableCollection { override fun addAll(c: Collection): Boolean }

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Declaration-site variance public interface List : Collection { public fun get(index: Int): E } public interface MutableList : List, MutableCollection { override fun addAll(c: Collection): Boolean }

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Declaration-site variance public interface List : Collection { public fun get(index: Int): E } public interface MutableList : List, MutableCollection { override fun addAll(c: Collection): Boolean }

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Declaration-site variance val strings: List = listOf("hello", "world") val objects: List = strings

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Declaration-site variance val strings: List = arrayListOf("hello", "world") val objects: List = strings

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Declaration-site variance val strings: MutableList = arrayListOf("hello", "world") val objects: List = strings

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Declaration-site variance val strings: MutableList = arrayListOf("hello", "world") val objects: MutableList = strings

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Declaration-site variance val strings: MutableList = arrayListOf("hello", "world") val objects: MutableList = strings Type mismatch

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Operator overloading

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Operator overloading enum class Coin(val cents: Int) { PENNY(1), NICKEL(5), DIME(10), QUARTER(25), }

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Operator overloading enum class Coin(val cents: Int) { PENNY(1), NICKEL(5), DIME(10), QUARTER(25), }. class Purse(var amount: Float)

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Operator overloading enum class Coin(val cents: Int) { PENNY(1), NICKEL(5), DIME(10), QUARTER(25), }. class Purse(var amount: Float) { fun plusAssign(coin: Coin): Unit { amount += (coin.cents / 100f) } } Reserved function name

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Operator overloading enum class Coin(val cents: Int) { PENNY(1), NICKEL(5), DIME(10), QUARTER(25), } class Purse(var amount: Float) { fun plusAssign(coin: Coin): Unit { amount += (coin.cents / 100f) } } var purse = Purse(1.50f)

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Operator overloading enum class Coin(val cents: Int) { PENNY(1), NICKEL(5), DIME(10), QUARTER(25), } class Purse(var amount: Float) { fun plusAssign(coin: Coin): Unit { amount += (coin.cents / 100f) } } var purse = Purse(1.50f) purse += Coin.QUARTER // 1.75

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Operator overloading enum class Coin(val cents: Int) { PENNY(1), NICKEL(5), DIME(10), QUARTER(25), } class Purse(var amount: Float) { fun plusAssign(coin: Coin): Unit { amount += (coin.cents / 100f) } } var purse = Purse(1.50f) purse += Coin.QUARTER // 1.75 purse += Coin.DIME // 1.85

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Operator overloading enum class Coin(val cents: Int) { PENNY(1), NICKEL(5), DIME(10), QUARTER(25), } class Purse(var amount: Float) { fun plusAssign(coin: Coin): Unit { amount += (coin.cents / 100f) } } var purse = Purse(1.50f) purse += Coin.QUARTER // 1.75 purse += Coin.DIME // 1.85 purse += Coin.PENNY // 1.86

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Operator overloading a + b a - b a * b a / b a % b a == b a != b a +- b a -= b a *= b a /= b a %= b a > b a < b a >= b a <= b a++ a-- a[] a() a..b a in b a !in b

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Adding Kotlin to your project

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app └── src └── main ├── java └── kotlin

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app └── src └── main ├── java └── kotlin

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buildscript { repositories { mavenCentral() } dependencies { classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:' } } apply plugin "kotlin-android" sourceSets { main.kotlin.srcDirs += 'src/main/myKotlin' } repositories { mavenCentral() } dependencies { compile 'org.jetbrains.kotlin:kotlin-stdlib:' }

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buildscript { repositories { mavenCentral() } dependencies { classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:' } } apply plugin "kotlin-android" sourceSets { main.kotlin.srcDirs += 'src/main/myKotlin' } repositories { mavenCentral() } dependencies { compile 'org.jetbrains.kotlin:kotlin-stdlib:' }

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buildscript { repositories { mavenCentral() } dependencies { classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:' } } apply plugin "kotlin-android" sourceSets { main.kotlin.srcDirs += 'src/main/myKotlin' } repositories { mavenCentral() } dependencies { compile 'org.jetbrains.kotlin:kotlin-stdlib:' }

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buildscript { repositories { mavenCentral() } dependencies { classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:' } } apply plugin "kotlin-android" sourceSets { main.kotlin.srcDirs += 'src/main/myKotlin' } repositories { mavenCentral() } dependencies { compile 'org.jetbrains.kotlin:kotlin-stdlib:' }

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No content

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Resources • http://kotlinlang.org/docs/reference/ • http://kotlinlang.org/docs/tutorials/koans.html • http://kotlinlang.org/docs/reference/comparison-to-java.html

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Questions? @pardom