You can do better with Kotlin

Introduction to Kotlin talk.

Transcript

1. Svetlana Isakova You can do better with Kotlin
2. - modern - pragmatic - Android-friendly Kotlin Programming Language
3. Official on Android
4. Not only Android
5. Multiplatform projects
6. Pragmatic - tooling - Java interop
7. From
8. has good tooling
9. - completion - navigation - refactorings - inspections …
10. can be easily mixed with Java code
11. *.java *.class *.dex compiled to Java bytecode *.kt
12. Kotlin code Java code You can have Java & Kotlin code in one project
13. You can gradually add Kotlin to your existing app
14. Android-friendly
15. Android Studio is based on IntelliJ IDEA
16. just another library for your app rxjava-2.1.2 kotlin-stdlib-1.1.4 6315 10212
17. No Kotlin SDK …just JDK + extensions small runtime jar easy Java interop
18. Modern - concise - safe - expressive
19. concise
20. 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;
 }
 }
21. - equals - hashCode - toString 
 data 
 class Person(val name: String, val age: Int)
22. 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()
23. 
 class Person( val name: String, val age: Int ) person.getName()
24. 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
25. 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, /*...*/; }
26. 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
 }
 // ...
 }
27. 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)
 } // ...
 }
28. fun updateWeather(degrees: Int) {
 val (description, colour) =
 if (degrees < 5) {
 Pair("cold", BLUE)
 } else if (degrees < 23) {
 Pair("mild", ORANGE)
 } else {
 Pair("hot", RED)
 } // ...
 }
29. fun updateWeather(degrees: Int) {
 val (description, colour) = when {
 degrees < 5 -> Pair("cold", BLUE)
 degrees < 23 -> Pair("mild", ORANGE)
 else -> Pair("hot", RED)
 } // ...
 }
30. fun updateWeather(degrees: Int) {
 val (description, colour) = when {
 degrees < 5 -> "cold" to BLUE
 degrees < 23 -> "mild" to ORANGE
 else -> "hot" to RED
 }
 }
31. 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;
 }
32. safe
33. Billion Dollar Mistake
34. Modern approach: to make NPE compile-time error, not run-time error
35. 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 ✗
36. val s: String? if (s != null) {
 s.length
 } Dealing with Nullable Types
37. s?.length val s: String? Dealing with Nullable Types
38. val length = if (s != null) s.length else null val s: String? Nullability operators val length = s?.length
39. val length = if (s != null) s.length else null val s: String? Nullability operators val length: Int? = s?.length
40. val length = if (s != null) s.length else 0 val s: String? Nullability operators val length: Int = s?.length ?: 0
41. val s: String? if (s == null) fail()
 s.length Control-flow analysis
42. val s: String? Making NPE explicit s!! throws NPE if s is null s!!.length
43. Nullable Types Under the Hood No performance overhead @Nullable, @NotNull annotations
44. Annotate your Java types in Kotlin Type behaves like regular Java type @Nullable @NotNull Type Type? Type Type @ParametersAreNonnullByDefault @MyNonnullApi Type/Type?
45. class Optional<T>(val value: T) { 
 fun isPresent() = value != null
 
 fun get() = value ?:
 throw NoSuchElementException("No value present") } Nullable types ≠ Optional
46. expressive
47. you can avoid any repetition you can make the code look nicer you can create API looking like DSL expressive
48. Extension Functions
49. fun String.lastChar() = get(length - 1) this can be omitted Extension Functions fun String.lastChar() = this.get(this.length - 1)
50. import com.example.util.lastChar import com.example.util.* Extension Functions val c: Char = "abc".lastChar() fun String.lastChar() = get(length - 1)
51. 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");
52. 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?
53. Extensions for Android Toast.makeText(this, "Thank you!”, Toast.LENGTH_SHORT).show() Activity extension function on Activity toast("Thank you!") this.toast("Thank you!")
54. this.startActivity<NewActivity>(“ANSWER" to 42) val intent = Intent(this, NewActivity::class.java)
 intent.putExtra("ANSWER", 42)
 startActivity(intent) Extensions for Android
55. infix fun <A, B> A.to(that: B) = Pair(this, that) "ANSWER".to(42) "hot" to RED mapOf(0 to "zero", 1 to "one") The to extension function
56. Lambdas
57. Lambdas button.addActionListener { println("Hi") }
58. { employee: Employee -> employee.city == City.PRAGUE } What’s an average age of employees working in Prague? Working with collections with Lambdas val employees: List<Employee> employees.filter { it.city == City.PRAGUE }.map { it.age }.average() data class Employee( val city: City, val age: Int )
59. What’s an average age of employees working in Prague? Working with collections with Lambdas val employees: List<Employee> data class Employee( val city: City, val age: Int ) extension functions employees.filter { it.city == City.PRAGUE }.map { it.age }.average()
60. Kotlin library: extensions on collections • filter • map • reduce • count • find • any • flatMap • groupBy • …
61. Under the Hood No performance overhead Lambdas can be inlined
62. Extension Function & Lambda Lambda with receiver
63. 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()

64. 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
65. with (sb) {
 appendln("Alphabet: ")
 ...
 } inline fun <T, R> with( receiver: T, block: T.() -> R ): R = receiver.block() The with function declaration
66. Lambda with receiver val sb = StringBuilder()
 with (sb) {
 appendln("Alphabet: ")
 for (c in 'a'..'z') {
 this.append(c)
 }
 } lambda with implicit this
67. html {
 table {
 for (product in products) {
 tr {
 td { text(product.description) }
 td { text(product.price) }
 td { text(product.popularity) }
 }
 }
 }
 } HTML Builders lambdas with receiver
68. 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
69. 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
70. ANKO-DSL DSL for dynamic layouts
71. 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
72. 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
73. kotlinlang.org
74. Gradle & Kotlin Writing Gradle build scripts and plugins in Kotlin
75. try.kotlinlang.org
76. Kotlin Koans
78. Have a nice Kotlin!