Introduction to Kotlin

Showing the power of Kotlin including basic syntax and functional approach. Kotlin User Group Bosnia first meetup.

Transcript

1. otlin Bajic Dusko Introduction to @bajicdusko - dusko.bajic@codecentric.de
2. What? • JVM based language • Statically-typed (type inference) • Usage of functional programming concepts • Null safe(r) • Expressive • Awesome IDE support - IntelliJ
3. Why? • Why not? • Android developers are in pain • Java 8, 9? Nope, Java7. • Port libraries • JVM - bytecode - Kotlin (Groovy, Scala, Clojure) • Highly interoperable with Java
4. Basics name val public String getName() { return name; } = "John" final String name = "John"; private private String name = "John" public String getName() { return name; } public void setName(String name){ this.name = name; } name var = "John" user.name = "Jane" val tempName = user.name
5. var name: String get() = field.toUpperCase() set(value){ field = "Name: $value" } val isAndroid: Boolean get() os == "Android" = //class Device if(device.isAndroid){ //do something } Basics
6. class User { } (username: String, displayName: String) init { log.debug("Yay $username is created") } constructor(profile: Profile) : this(profile.username, profile.displayName){ } construcutor open class Admin : User("admin", "ADMIN") { } val admin = Admin() Basics
7. data class User(val firstName: String, val lastName: String, val isAdmin: Boolean = false) class User{ private String firstName; private String lastName; private boolean isAdmin; public User(String firstName, String lastName, boolean isAdmin){ this.firstName = firstName; this.lastName = lastName; this.isAdmin = isAdmin; } public User(String firstName, String lastName){ this(firstName, lastName, false); } //getters and setters for all fields } Basics
8. interface Tappable { fun tap() } class View : Tappable fun click() = println("I'm clicked") override fun tap(){
 if(touchEvent.duration in 1..100){
 //do something
 } } } else {
 click()
 } val area: Area override val area: Area = Area(10, 40) { Basics
9. Flow control: "if" •If is an expression •There is no ternary operator var res = if (x > y) x else y var res = if(x > y){ println("Result is $x") x } else { println("Result is $y") y }
10. Flow control: "when" •Replaces switch statement •Matching an argument sequentially against all branches •Is also an expression. Matching branch is returned. •Else block is required (in most cases)
11. Flow control: "when" when(x) { 2 -> println("Correct result is 2") 4 -> println("Correct result is 4") else -> { println("Wrong result is $x") } } when(x) { in 1..10 -> println("Result is in safe range") 11, 12 -> println("Result is 11 or 12") else -> { println("Result $x is out of range") } }
12. Flow control: "for(each)" val numbers = listOf(1, 2, 3, 4) for(n in numbers){ println(n) } for(n in 1..100){ println(n) } for(index in ){
 println(index) } 0 until users.size for(index in 100 downTo 1 ){
 println(index) } step 2
13. Functions fun stringSize(value: String): Int { return value.length } fun printStringSize(value: String) println("Size ${value.length}") =
14. Functions var list = emptyList<Int>() var list = listOf(1, 2, 3) println(list.last()) // 3 println(list.max()) // 3 println(list.sortedDescending()) // [3, 2, 1] println(joinToString(list, prefix = "# ", suffix = ";")) //# 1, 2, 3; var users = mutableListOf(User(18), User(22)) println(users.sortedBy({ it.age })) //User(age = 18), User(age = 22) println(users.sortedBy({ it.age }).map({ it.age }) //18, 22
15. Extension Functions • Util hell • Adding methods to other people's classes • We're good as long as it is compiled to Java class fun String.lastChar(): Char = this.get(this.length - 1) Receiver type Function name Receiver object fun String.lastChar(): Char = get(length - 1) import some_package_name.lastChar println("John".lastChar()) val String.lastChar: Char = get() = get(length - 1) println("John".lastChar)
16. Extension Functions public class ViewUtil{ public static void fadeIn(Button button){ button.setAlpha(0F) button.setVisibility(View.VISIBLE) button.animate().alpha(1F).duration(250).start() } } ViewUtil.fadeIn(button)
17. Extension Functions fun View.fadeIn(){ alpha = 0F visibility = View.VISIBLE animate().alpha(1F).duration(250).start() } button.fadeIn()
18. Nullability • "Unfortunately, the application X has stopped" - java.lang.NullPointerException • Goal: Converting runtime errors into compile-time errors • Introduced nullable types var name: String = null var name: String? = null fun stringSize(value: String ) = value .length ? ? ?: 0 Int
19. Nullability var car: Car? = Tesla("Model 3") fun energySource(car: Car) = if (car.battery.kwhEnergy >= 60) "Electricity" else "Unknown" val energySource = energySource(car)
20. Nullability var car: Car? = Tesla("Model 3") fun energySource(car: Car) = if (car.battery.kwhEnergy >= 60) "Electricity" else "Unknown" val energySource = energySource(car) if(car != null){ }
21. if (car.battery.kwhEnergy >= 60) "Electricity" else "Unknown" Nullability var car: Car = Tesla("Model 3") fun energySource(car: Car?) = val energySource = energySource(car)
22. Nullability var car: Car = Tesla("Model 3") fun energySource(car: Car?) = val energySource = energySource(car) if (car?.battery?.kwhEnergy ?: 0 >= 60) "Electricity" else "Unknown"
23. Nullability var car: Car = Tesla("Model 3") fun energySource(car: Car?) = val energySource = energySource(car) if (car battery kwhEnergy >= 60) "Electricity" else "Unknown" !! !!
24. High-Ordered Functions • Function that takes another function as an argument or returns one • Function types val sum: (Int, Int) -> Int = { x, y -> x + y } Parameter Types Return Type val sum = { x: Int, y: Int -> x + y } Int
25. High-Ordered Functions val sum = { x: Int, y: Int -> x + y } val div = { x: Int, y: Int -> x / y } val mul = { x: Int, y: Int -> x * y } fun <T : Number> execute(opOne: T, opTwo: T, opFn: (T, T) -> T) = opFn(opOne, opTwo) val sumRes = execute(2, 3, sum) val multiplicationResult = execute(2, 3, mul) val divideResult = execute(2, 3, div)
26. High-Ordered Functions lateinit var pref: Pref val editor = pref.edit() editor.putString("name", "John") editor.apply() fun Editor.applyChange(execute: (Editor) -> Unit){
 execute(this)
 apply()
 } typealias SharedPreferences = Pref
 typealias SharedPreferences.Editor = Editor
27. High-Ordered Functions lateinit var pref: Pref val editor = pref.edit() editor.putString("name", "John") editor.apply() fun Editor.applyChange(execute: (Editor) -> Unit){
 execute(this)
 apply()
 } typealias SharedPreferences = Pref
 typealias SharedPreferences.Editor = Editor pref.edit().applyChange { .putString("name", "John") } editor -> editor
28. High-Ordered Functions lateinit var pref: Pref fun Editor.applyChange(execute: (Editor) -> Unit){
 execute(this)
 apply()
 } pref.edit().applyChange { .putString("name", "John")} it
29. High-Ordered Functions lateinit var pref: Pref val editor = edit()
 execute(editor)
 editor.apply()
 } -> Unit){ fun Pref.applyChange(execute: (Editor).() pref.applyChange { putString("name", "John")}
30. High-Ordered Functions lateinit var pref: Pref val editor = edit()
 execute(editor)
 editor.apply()
 } -> Unit){ fun Pref.applyChange(execute: (Editor).() fun Pref.save( ){ when(value){ is String -> putString(key, value) else -> throw NotMappedTypeException(value) } } applyChange { } val (value, key) = valueKeyFn() valueKeyFn: () -> Pair<Any, String> pref.save { "John" to "name" }
31. High-Ordered Functions SharedPreferences.Editor editor = sharedPreferences.edit() editor.putString("name", "John") editor.apply() SharedPreferences.Editor editor = sharedPreferences.edit() editor.putString("age", 29) editor.apply() SharedPreferences.Editor editor = sharedPreferences.edit() editor.putBoolean("adminStatus", false) editor.apply() cache.save { "John" to "name" } cache.save { 29 to "age" } cache.save { false to "adminStatus" }
32. High-Ordered Functions
33. Thank you and Have a nice Kotlin " " Andrey Breslav - JetBrains