JetBrains Open Day, Москва, 19 апреля 2018 года.

1. Kotlin.
Что это и зачем?
—
Роман Белов,
PMM at Kotlin
JetBrains Open Day
Moscow

2. WHO R U?

3. В разделе личные качества
стоит только одно слово:
Прагматичен

4. “We’ve built tools to
support so many nice
languages, and we’re still
using Java”
JetBrains, 2010

5. Hello Kotlin!

6. • Kotlin is a modern language for JVM, JS and Native
• Statically-typed
• Object-oriented and functional paradigms
• Open source under Apache 2.0
• Ver 1.0 in 2016
• Compatibility commitment
• Now at version 1.2

7. Kotlin on Android

9. Gradle Script Kotlin

10. …

11. Server
Android
iOS Browser
Kotlin/JVM
Kotlin/JVM
Kotlin/Native Kotlin/JS
Multiplatform projects

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

13. fun main(args: Array<String>): Unit {
println("Hello, World!")
}
Function keyword

14. fun main(args: Array<String>): Unit {
println("Hello, World!")
}
Function name

15. fun main(args: Array<String>): Unit {
println("Hello, World!")
}
Argument name

16. fun main(args: Array<String>): Unit {
println("Hello, World!")
}
Argument type

17. fun main(args: Array<String>): Unit {
println("Hello, World!")
}
Return type

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

45. class Person(var name: String)
fun main(args: Array<String>) {
val person = Person(“Roman”)
println("Hello, $name!")
}.
Instance declaration

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

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

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

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

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

51. 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<String>) {
val person = Person("Roman")
println("Hello, ${person.name}!")
}.
Default value

52. 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<String>) {
val person = Person("Roman")
println("Hello, ${person.name}!")
}..

53. 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<String>) {
val person = Person("Roman")
println("Hello, ${person.name}!")
}..

54. 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<String>) {
val person = Person("Roman")
println("Hello, ${person.name}!")
}

55. 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<String>) {
val person = Person("Roman")
}

56. 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<String>) {
val person = Person("Roman")
person.greet()
}
> Hello, Roman!

57. 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<String>) {
}.

58. 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<String>) {
val people = listOf(
Person("Roman"),
Person("Romario", Language.SP),
Person("Roman de Belov", Language.FR)
).
}.

59. 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<String>) {
val people = listOf(
Person("Roman"),
Person("Romario", Language.SP),
Person("Roman de Belov", Language.FR)
).
for (person in people) {.
person.greet()
}.
}.

60. 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<String>) {
val people = listOf(
Person("Roman"),
Person("Romario", Language.SP),
Person("Roman de Belov", Language.FR)
).
people.forEach {.person ->
person.greet()
}.
}.

61. 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<String>) {
val people = listOf(
Person("Roman"),
Person("Romario", Language.SP),
Person("Roman de Belov", Language.FR)
).
people.forEach { it.greet() }.
}.

62. 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<String>) {
listOf(
Person("Roman"),
Person("Romario", Language.SP),
Person("Roman de Belov", Language.FR)
).forEach { it.greet() }.
}.
> Hello, Roman!
> Hola, Romario!
> Bonjour, Roman de Belov!

63. Null-safety

65. “Billion dollar mistake”
— Sir C. A. R. Hoare

66. Java String s = null;
s.length();
Errors At Runtime
Kotlin val s: String
s.length()
val s: String? = null
s.length()
Errors At Compile Time
= null
Nullable type

67. Check and use val s: String? = …
if (s != null) {
s.length()
}
Check and exit if (s == null) return
s.length()
Rock’n’Roll s?.length()
s!!.length()
(s ?: “…”).length()

68. public class JavaClass {
public String foo(List<String> l) {…}
}

69. public class JavaClass {
public String foo(List<String> l) {…}
}
String
String?
List<String>
List<String?>
List<String>?
List<String?>?
Safest!
public class JavaClass {
public String foo(List<String> l) {…}
}

70. Java Interop: All Nullable
javaValue.toString().length() + 1
javaValue?.toString()?.length()!! + 1
val l: List<String> = javaValue?.getList()!!
ArrayList<String?>

71. Your safest option doesn’t work!

72. Annotations
public class JavaClass {
public String foo(List<String> l) {…}
}
String
String?
List<String>
List<String?>
List<String>?
List<String?>?
public class JavaClass {
@NotNull
public String foo(@NotNull List<String> l) {…}
}

73. Annotations
are cumbersome
and don’t
really help!

74. • Null-safety
• Convenience
• Java Interop
Pick Two
Platform Types!
Thanks to
Dr. Ross Tate of

75. String! List<String!>!
public class JavaClass {
public String foo(List<String> l) {…}
}
Java: Platform Types
Platform Type

76. Dereferencing Platform Values
s: String s: String? s: String!
s.length() s.length() s.length()
s?.length() s?.length() s?.length()
s!!.length() s!!.length() s!!.length()
NPE

77. Assignability
String String?
String!
Platform Type
Not Transitive!
NPE

78. Some Notes
• Platform Types are Not Denotable!
– String! is notation, not syntax
• Pure Kotlin is Null-Safe
• Kotlin+Java is as safe as Java
• Annotations Still Applicable
– @NotNull String in Java becomes String in Kotlin
• Dynamic Type for JavaScript Interop
– JS is even unsafer than Java

79. Interoperability
• Use all Java libraries
from Kotlin
• Use Kotlin code from Java
• Keep Java and Kotlin code
side-by-side in project
• Search/refactorings work
cross-language

80. • Modern
• Concise
• Safe
• Extensible
• Interoperable
• Fun to work with!
Kotlin

81. Pragmatism

82. Resources
• kotlinlang.org
• discuss.kotlinlang.org
• try.kotlinlang.org
• slack.kotlinlang.org
• kotlinconf.com
(3-5 Oct. 2018, Amsterdam)

83. Спасибо
за внимание
—