You can do better with Kotlin. Svetlana Isakova

Transcription

1 You can do better with Kotlin Svetlana Isakova

2 Kotlin Programming Language - modern - pragmatic - Android-friendly

3 Official on Android

4 Not only Android

5 Pragmatic - tooling - Java interop

6 From

7 has good tooling

8 - completion - navigation - refactorings - inspections

9 can be easily mixed with Java code

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

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

12 You can gradually add Kotlin to your existing app

13 Android-friendly

14 Android Studio is based on IntelliJ IDEA

15 just another library for your app rxjava kotlin-stdlib

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

17 Modern - concise - safe - expressive

18 concise

19 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; } }

20 data class Person(val name: String, val age: Int) - equals - hashcode - tostring

21 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()

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

23 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

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

25 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 } //... }

26 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) } //... }

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

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

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

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

31 safe

32 Billion Dollar Mistake

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

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

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

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

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

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

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

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

41 Nullable Types Under annotations No performance overhead

42 Annotate your Java types in Kotlin Type behaves like regular Java Type Type

43 Nullable types Optional class Optional<T>(val value: T) { fun ispresent() = value!= null } fun get() = value?: throw NoSuchElementException("No value present")

44 expressive

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

46 Extension Functions

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

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

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

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

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

52 Extensions for Android val intent = Intent(this, NewActivity::class.java) intent.putextra("answer", 42) startactivity(intent) this.startactivity<newactivity>( ANSWER" to 42)

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

54 Lambdas

55 Lambdas button.addactionlistener { println("hi") }

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

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

58 Kotlin library: extensions on collections filter count flatmap map reduce find any groupby

59 Under the Hood Lambdas can be inlined No performance overhead

60 Extension Function & Lambda Lambda with receiver

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

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

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

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

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

66 Avoiding Duplication 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"))

67 Inline functions is declared as inline function db.intransaction { } delete("users", "first_name =?", arrayof("jake")) generated bytecode is similar to db.begintransaction() try { db.delete("users", "first_name =?", arrayof("jake")) db.settransactionsuccessful() } finally { db.endtransaction() }

68 ANKO-DSL DSL for dynamic layouts

69 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

70 Custom layouts customview { verticallayout { val = edittext { hint = " " Password } Log In val password = edittext { hint = "Password" transformationmethod = PasswordTransformationMethod.getInstance() } positivebutton("log In") { } login( .text, password.text) } }

71 kotlinlang.org

72 Gradle & Kotlin Writing Gradle build scripts and plugins in Kotlin

73 try.kotlinlang.org

74 Kotlin Koans

75

76 Have a nice Kotlin!