Introduction to Kotlin - Kotlin 을 아직 사용해보지 않은 Java 개발자들을 대상으로 Kotlin 언어에 대하여 설명합니다.

1. Svetlana Isakova
Why Kotlin?

2. - modern
- pragmatic
Kotlin Programming
Language

3. Kotlin developers
0
750
1500
2250
3000
2016 2017 2018
156K
700K
2.2M

4. Timeline
… …
2017 Official on Android
2010 Project started
2016 Kotlin 1.0
2018 Kotlin 1.3

5. Programming language for:

6. Pragmatic
- good tooling
- good Java
interoperability

7. From

8. has good tooling

9. - completion
- navigation
- refactorings
- inspections
…

10. can be easily mixed with
Java code

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

12. You can gradually add Kotlin to
your existing app

13. Modern
- concise
- safe
- expressive

14. concise & readable

15. 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;
}
}

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

17. 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, /*...*/; }

18. 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
}
// ...
}

19. 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)
}
// ...
}

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

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

22. val (description, colour) = when {
degrees < 5 -> Pair("cold", BLUE)
degrees < 23 -> Pair("mild", ORANGE)
else -> Pair("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;
}

23. safe

24. Billion Dollar Mistake

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

26. 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 ✗

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

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

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

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

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

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

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

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

35. Using annotated Java types from Kotlin
Type
behaves like
regular Java type
@ParametersAreNonnullByDefault
@Nullable
@NotNull Type
Type?
Type
Type
@MyNonnullApi
Type/
Type?

36. expressive

37. more code reuse!
expressive

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

39. Extension Functions

40. Extension Functions
val c: Char = "abc".lastChar()
fun String.lastChar() = get(length - 1)

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

42. 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");

43. 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?

44. Lambdas

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

46. { 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
)

47. 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()

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

49. Extension Function & Lambda
Lambda with receiver

50. 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()

51. 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 receiverwith 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

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

53. Under the Hood
No performance overhead
Lambdas can be inlined

54. with (window) {
width = 300
height = 200
isVisible = true
}
The with function
class Window(
var width: Int,
var height: Int,
var isVisible: Boolean
)

55. window.width = 300
window.height = 200
window.isVisible = true
with (window) {
width = 300
height = 200
isVisible = true
}
Inline functions
is declared as
inline function
generated bytecode is similar to:
No performance overhead for creating a lambda!

56. The apply function
val mainWindow =
windowById["main"]?.apply {
width = 300
height = 200
isVisible = true
} ?: return
Applies the given actions
and returns receiver as a result:

57. DSLs

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

59. plugins {
application
kotlin("jvm") version "1.3.50"
}
application {
mainClassName = "samples.HelloWorldKt"
}
dependencies {
compile(kotlin("stdlib"))
}
repositories {
jcenter()
}
Gradle Build Script in Kotlin

60. router {
accept(TEXT_HTML).nest {
GET("/") { ok().render("index") }
GET("/sse") { ok().render("sse") }
GET("/users", userHandler::findAllView)
}
"/api".nest {
accept(APPLICATION_JSON).nest {
GET("/users", userHandler::findAll)
}
accept(TEXT_EVENT_STREAM).nest {
GET("/users", userHandler::stream)
}
}
resources("/**", ClassPathResource("static/"))
}
Kofu* (Kotlin + functional) DSL for Spring Boot
*part of Spring Fu
(an incubator for
new Spring features)

61. Multi-platform Projects

62. Multi-platform projects
Browser
Kotlin/JS
Android
Kotlin/JVM
iOS
Kotlin/Native
Server
Kotlin/JVM
common
code

63. Sharing common code
• Sharing business logic
• Keeping UI platform-dependent
• The shared part might vary

64. Common code
• you define expect declarations in the common
code and use them
• you provide different actual implementations
for different platforms

65. Time measurement example
expect fun measureTime(action: () -> Unit): Duration
Expected platform-specific API:
Expected API can be used in the common code:
measureTime {
// operation
}

66. Platform-specific Implementations
expect fun measureTime(action: () -> Unit): Duration
actual fun measureTime(action: () -> Unit): Duration {
// implementation using System.nanoTime()
}
actual fun measureTime(action: () -> Unit): Duration {
// implementation using window.performance.now()
}
actual fun measureTime(action: () -> Unit): Duration {
// implementation using std::chrono::high_resolution_clock
}
Kotlin/JVM
Kotlin/JS
Kotlin/Native

67. Common code
• can use the standard library
• can define expect declarations and use them
• can use other multi-platform libraries

68. Multi-platform libraries
• Standard library
• Ktor HTTP client
• kotlinx.serialization
• kotlinx.coroutines
• … and more

69. Many apps already in production
Going Native: How I used Kotlin Native to Port 6 years
of Android Game Code to iOS in 6 months
Shipping a Mobile Multiplatform Project on iOS &
Android
Your Multiplatform Kaptain has Arrived: iOS release is
powered by Kotlin Multiplatform

70. play.kotlinlang.org

72. Kotlin course at Coursera

73. Have a nice Kotlin!