Kotlin is a JVM language developed by Jetbrains. Its version 1.0 (production ready) was released at the beginning of the year and made some buzz within the android community. This session proposes to discover this language, which takes up some aspects of groovy or scala, and that is very close to swift in syntax and concepts. We will see how Kotlin boosts the productivity of Java & Android application development and how well it accompanies reactive development.

1. Smarter development for the JVM
#GeekBreakFast
Kotlin

2. @arnogiu
Work @ ekito
App & Gear Maker
Mobile & Cloud DevArnaud GIULIANI

3. 3
« production ready »
15.02.2016

4. Yet another JVM
Language ?

5. 5
The new « Swift »
for Android ?
https://goo.gl/W5g6Do - mid 2014

6. Limits of java
- Verbose
- Type inference
- No properties / Lazy / Delegate
- Checked exception
- NullPointerException
- Extensibility
- End of lines with ;
https://goo.gl/HIrmC9 @sdeleuze

7. But Java is great …
- Fast
- Optimized bytecode
- Static typing
- Simple to learn
- Amazing ecosystem

8. Today’s IT
Challenge
Responsive
Resilient
Scalable
Productive

10. (not only) Functional
programming
First class functions, immutability, no side effects,
conciseness …

11. 11
Ready for Reactive
Programming

12. How K can help
you ?
- Conciseness
- Null Safety & Immutability protection
- Static Typing & Smart Casts
- Open programming styles (lambdas, high order functions …)
- Java interop

13. What’s
Kotlin ?
- Fully open source (built by Jetbrains)
- Run on Java 6+
- Static typing & type inference
- Modern language features
- 1st grade tooling

14. What’s inside
Kotlin ?
- String templates
- Properties
- Lambdas
- Data class
- Smart cast
- Null safety
- Lazy property
- Default values for function
parameters
- Extension Functions
- No more ;
- Single-expression
functions
- When expression
- let, apply, use, with
- Collections
- Android Extensions Plugin

15. Compare with
Same conciseness and expressive code, but
Kotlin static typing and null-safety make a big
difference.
"Some people say Kotlin has 80% the power of
Scala, with 20% of the features" *
"Kotlin is a software engineering language in
contrast to Scala which is a computing science
language." *
Swift and Kotlin are VERY similar. Swift is LLVM
based and has C interop while Kotlin is JVM based
and has Java interop.
* Quotes from Kotlin:TheYing andYang of Programming Languages

16. Use Cases
Source : poll on Kotlin Slack (early 2016)https://goo.gl/HIrmC9 @sdeleuze

17. 17
Want some nougat ?

18. 18
Slowing Deployment
https://goo.gl/2sOGBd

19. 19

20. 20
is Kotlin Android friendly ?
https://github.com/SidneyXu/AndroidDemoIn4Languages
Method counts by Language

21. Where to use Kotlin ?*
*everywhere you use Java

22. 22
let’s go write some Kotlin !

23. Ready to use
Easy to run

24. 24
Getting started with
gradle
buildscript {
ext.kotlin_version = '<version to use>'
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
}
}
apply plugin: "kotlin"
dependencies {
compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
}
1.0.5-2
also available with maven

25. 25
IntelliJ

26. 26

27. Gradle tips for Kotlin
# Gradle
org.gradle.jvmargs=-Xmx2048m -XX:MaxPermSize=512m -XX:
+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8
org.gradle.parallel=true
org.gradle.deamon=true
# Kotlin
kotlin.incremental=true
# Android Studio 2.2+
android.enableBuildCache=true
In your gradle.properties
https://medium.com/keepsafe-engineering/kotlin-vs-java-compilation-speed-e6c174b39b5d#.k44v7axsk

28. Smarter
development
with K

29. 29
Typing & Inference
val a: Int = 1
val b = 1 // `Int` type is inferred
var x = 5 // `Int` type is inferred
x += 1
Infered values
Mutable values
val : constant value - IMMUTABLE
var : variable value - MUTABLE
USE VAL AS MUCH AS POSSIBLE !

30. 30
Safety with Optionals
var stringA: String = "foo"
stringA = null //Compilation error - stringA is a String (non optional)
var stringB: String? = "bar" // stringB is an Optional String
stringB = null //ok
Optional value

31. 31
Late initialization, Lazy, Delegates …
// set length default value manually
val length = if (stringB != null) stringB.length else -1
//or with the Elvis operator
val length = stringB?.length ?: -1
DefaultValue & Elvis Operator
val length = stringB.length // Compilation error - stringB can be null !
// use ? the safe call operator
val length = stringB?.length //Value or null - length is type Int?
val length = stringB!!.length //Value or explicit throw NPE - length is type Int
Safe call with ?. or Explicit call with !!.

32. 32
Example

33. 33
Class
class User (
var userName: String,
var firstName: String,
var lastName: String,
var location: Point? = null
)
POJO +
- Getter
- Setter
- Constructors

34. 34
Data Class
data class User (
var userName: String,
var firstName: String,
var lastName: String,
var location: Point? = null
)
POJO +
- Getter
- Setter
- Constructors
- toString
- hashcode
- equals
- copy

35. 35
POJO ?

36. 36
Properties
// readonly property
val isEmpty: Boolean
get() = this.size == 0
Properties can be declared in constructor or in class
You can also handle getter & setter
// customer getter & setter
var stringRepresentation: String
get() = this.toString()
set(value) {
setDataFromString(value) // parses the string and assigns values to other properties
}
class ApiKey(var weatherKey: String, var geocodeKey: String){
var debug : Boolean = false
}

37. 37
Object Component
// singleton
object Resource {
val name = "Name"
}
println(" my resource is : ${Resource.name}")
class StringCalculator{
// class helper
companion object{
val operators = arrayOf("+","-","x","/")
}
}
println(" my operators are : ${StringCalculator.operators}")
Singleton Class
Companion Object

38. 38
Example
data class User(val name: String = "", val age: Int = 0)
val jack = User(name = "Jack", age = 1)
val anotherJack = jack.copy(age = 2)
json class
data class usage

39. 39
When
when (s) {
1 -> print("x == 1")
2 -> print("x == 2")
else -> { // Note the block
print("x is neither 1 nor 2")
}
}
Flow Control (replace your old if blocks)
when (x) {
in 1..10 -> print("x is in the range")
in validNumbers -> print("x is valid")
!in 10..20 -> print("x is outside the range")
else -> print("none of the above")
}
Pattern Matching
in <range> ->
is <type> ->
expression ->
when (s) {
is String -> print("s is a string")
else -> print("s is not a string")
}

40. 40
Example

41. 41
Functions
fun reformat(str: String,
normalizeCase: Boolean = true,
upperCaseFirstLetter: Boolean = true,
wordSeparator: Char = ' '): String {
}
Named Parameters & default values
reformat(str, true, true, '_') // old way to call
reformat(str, wordSeparator = '_') // using default values & named params
fun String.hello(): String = "Hello " + this
val hi = "Arnaud !".hello()
println("$hi") // Hello Arnaud !
Extension

42. 42
Lambdas
val fab = findViewById(R.id.fab) as FloatingActionButton
fab.setOnClickListener { view -> popLocationDialog(view) }
A lambda expression or an anonymous function is a “function literal”, i.e. a
function that is not declared, but passed immediately as an expression
- A lambda expression is always surrounded by curly braces,
- Its parameters (if any) are declared before -> (parameter types may be omitted),
- The body goes after -> (when present).
numberString.split("n").flatMap { it.split(separator) }
.map(Integer::parseInt)
.map(::checkPositiveNumber)
.filter { it <= 1000 }
.sum()

43. 43
Destructuring Data
fun extractDelimiter(input: String): Pair<String, String> = …
val (separator, numberString) = extractDelimiter(input)
data class User(var name : String,var age : Int)
val toto = User("toto",42)
val (name,age) = toto
Returning two values with Pair
Destructured values with data classes

44. 44
Collections
// immutable list
val list = listOf("a", "b", "c","aa")
list.filter { it.startsWith("a") }
// immutable map
val map = mapOf("a" to 1, "b" to 2, "c" to 3)
for ((k, v) in map) {
println("$k -> $v")
}
// mutable map
val map2 = HashMap<String,String>()
// direct map access
map2["a"] = "my value"
println(map2["a"])
// range
for (i in 1..100) { //... }

45. 45
InterOp
object UserSingleton{
fun stringify(user : User) : String{
val (name,age) = user
return "[name=$name][age=$age]"
}
}
fun WhatIsyourAge(user : User){
println("Your age is ${user.age}")
}
data class User (val name: String, val age : Int){
companion object{
fun sayHello(user : User){
println("Hello ${user.name}")
}
}
}
User u = new User("toto",42);
User.Companion.sayHello(u);
UserUtilsKt.WhatIsyourAge(u);
System.out.println("stringify : "+UserSingleton.INSTANCE.stringify(u));
Kotlin code
Calling Kotlin from Java

46. Sweet stuffs for

47. 47
Kotlin’s Android
Extensions
android {
...
sourceSets {
main.java.srcDirs += 'src/main/kotlin'
}
}
apply plugin: 'com.android.application'
apply plugin:‘kotlin-android’
apply plugin:‘kotlin-android-extensions’ // if use extensions
In your build.gradle

48. Kotlin
Java

49. Anko - Android DSL
Async task in few lines
doAsync {
// Long background task
uiThread {
result.text = "Done"
}
}
Layout DSL
verticalLayout {
val name = editText()
button("Say Hello") {
onClick { toast("Hello, ${name.text}!") }
}
}
https://github.com/Kotlin/anko

51. One of our longest journey …

52. Reactive
Programming

53. Lamba expressions
Functional Programming
Reactive Streams
http://reactivex.io/documentation/operators.html
http://www.reactive-streams.org/
https://spring.io/blog/2016/06/07/notes-on-reactive-
programming-part-i-the-reactive-landscape

54. 54
Example - Rx/Retrofit
Kotlin
Java

55. 55
Example - Rx/Realm

56. 56
Just another hype
JVM stuff ?

57. 57
Ready to go ?

58. 58
Production
ready ?

59. 59
Always hard to start …

60. 60
Our feedback
with K
- Easy to start small & grow (no big bang)
- Tool support is very good
- Great feedback about the language itself
- Stable in production !
- Magical conversion … but take care
- Hard to come back to java …

61. 61
IF YOU DON'T LOOK AT
JAVA AND THINK, "THIS
COULD BE BETTER", DON'T
SWITCH.
@RunChristinaRun

62. 62
Learning
Kotlin

63. 63
Try Kotlin online
http://try.kotlinlang.org/#/Examples/
Kotlin Reference
https://kotlinlang.org/docs/reference/
Kotlin Cheat Sheet (by ekito)
http://www.slideshare.net/arnaudgiuliani/kotlin-
cheat-sheet-by-ekito
Kotlin Community
https://kotlinlang.org/community.html

64. 64
Kotlin 1.1

65. Thank you :)