Kotlin Statically typed, modern language for industry Not swift of Android Nevin Chen @Mozilla 

Agenda • Overview • Kotlin Basics • Work with Android • Adoption • Resources • Testing 

Kotlin • Build and used by JetBrain • Since 2010, now v1.0.4 • IDE support • Very low learning curve (<100hr) • Expressive, Safe, Interoperable 

Expressive public class Person{
 
 private String id;
 private String name;
 private String height;
 private String weight;
 
 public Person(String id, String name, String height, String weight) {
 this.id = id;
 this.name = name;
 this.height = height;
 this.weight = weight;
 }
 
 public String getId() {
 return id;
 }
 
 public void setId(String id) {
 this.id = id;
 }
 
 public String getName() {
 return name;
 }
 
 public void setName(String name) {
 this.name = name;
 }
 
 public String getHeight() {
 return height;
 }
 
 public void setHeight(String height) {
 this.height = height;
 }
 
 public String getWeight() {
 return weight;
 }
 
 public void setWeight(String weight) {
 this.weight = weight;
 }
 } class Person(
 var id: String,
 var name: String,
 var weight: String,
 var height: String
 ) getter / setter for free 

Expressive public class Person{
 
 private String id;
 private String name;
 private String height;
 private String weight;
 
 public Person(String id, String name, String height, String weight) {
 this.id = id;
 this.name = name;
 this.height = height;
 this.weight = weight;
 }
 
 public String getId() {
 return id;
 }
 
 public void setId(String id) {
 this.id = id;
 }
 
 public String getName() {
 return name;
 }
 
 public void setName(String name) {
 this.name = name;
 }
 
 public String getHeight() {
 return height;
 }
 
 public void setHeight(String height) {
 this.height = height;
 }
 
 public String getWeight() {
 return weight;
 }
 
 public void setWeight(String weight) {
 this.weight = weight;
 }
 } data class Person(
 var id: String,
 var name: String,
 var weight: String,
 var height: String
 ) var peter = Person("0","Peter","60","170")
 var john = Person("1","John","70","180")
 
 peter.toString()
 peter.hashCode()
 peter.equals(john)
 peter.copy() For Free! Add “data” getter / setter for free 

Safe • Null Safety • Smart Cast 

Null safety var a: String = "abc"
 a = null //compilation error
 
 var b: String? = null // OK!
 b = bob?.department?.name // safe chained calls val l = b!!.length() // YOYO (NPE-lovers) // default value if var is null val l = b?.length ?: -1 

Smart Cast // Safe cast. cast fail will return null val x: String? = y as? String // smart cast
 fun demo(x: Any) { if (x is String) {
 print(x.length) }
 } 

Extension functions package com.nevin
 // scope : within the same package
 fun Int.beforeKitkat()= this < Build.VERSION_CODES.KITKAT
 // usage in Kotlin if (Build.VERSION.SDK_INT.beforeKitkat()){
 // do something…
 } extend function name caller return type is inferred Usage in Kotlin 

MyHelper.kt Interoperable package com.nevin
 // scope : within the same package
 fun Int.beforeKitkat()= this < Build.VERSION_CODES.KITKAT
 Just like helper method in Java // usage in Java if (com.nevin.MyHelperKt.beforeKitkat(Build.VERSION.SDK_INT)){
 // do something…
 } 

variable 

No content

string template & conditional expressions 

Everything in Kotlin is an expression ${….} is calculated first 

function 

return type inferred “void” in Java 

First class citizen package boo.bar
 //normal class A {
 fun foo(){ //…………
 }
 } //package level
 fun foo(){ //……… } A package level class is auto created using “<file name>Kt” 

default value & named param fun cal(x: Int = 1, y: Int = 2){ print("x+y=${x + y}”) } 
 fun main(args: Array) {
 val a = A()
 cal(100) // 102 
 cal(x = 200) // 202 cal(y = 300) // 301 
 a.cal(y = 400, x = 500) // 900
 }
 
 

range 

No content

When when (x) {
 0, 1 -> print("x == 0 or x == 1”) 2 -> print("x == 2") true -> print("true") is String -> x.startsWith("prefix") is File -> x.getFreeSpace() else -> { // Note the block
 print("x is funny")
 }
 } No need to cast again 

No content

Exception • Exception is an expression val a: Int? = try { Int.parseInt("300") } catch (e: NumberFormatException) { null }
 • There’s no checked Exception [1] [2] Can receive value 

Class • Final by default. With Android in mind • Main constructor must be called • Parent’s constructor must be called class MyView : View {
 constructor(ctx: Context) : super(ctx) {
 }
 constructor(ctx: Context, attrs: AttributeSet) : super(ctx, attrs) {
 }
 } default constructor must called 

Function expression // normal function fun sum(x: Int, y: Int): Int {
 return x + y
 } // function expression val sum = fun(x: Int, y: Int): Int {
 return x + y
 } // one liner val sum = fun(x: Int, y: Int): Int = x + y 

Lambda • function literal • Surrounded by curly braces, {} • parameters (if any) are declared before -> • The body goes after -> 

Function expression // normal function fun sum(x: Int, y: Int): Int {
 return x + y
 } // function expression val sum = fun(x: Int, y: Int): Int {
 return x + y
 } // one liner val sum = fun(x: Int, y: Int): Int = x + y 

Lambda val sum = { : Int, : Int -> } x y x + y // one liner val sum = fun(x: Int, y: Int): Int = x + y 

Lambda val sum = { : Int, : Int -> } x y x + y 

Lambda val sum : (Int,Int) -> Int = { , -> } x y x + y (Int, Int) -> Int 

Higher-order function // function that take functions as parameter, // or return a function fun calculate(x: Int, y: Int, formula: {
 } (Int, Int) -> Int println("the result is ${formula(x, y)}") // usage
 fun main(args: Array) {
 calculate(1,5, )
 calculate(1,5, ) 
 } sum { x, y -> x + y } 

Extension Function // Extension Function fun Int.sum(other: Int): Int = this + other
 // Extension Function Expressions
 val sum = fun Int.(other: Int): Int = this + other // usage 1.sum(2) 

Sample // scope : within the same package fun Int.beforeKitkat(): Boolean{
 return this < Build.VERSION_CODES.KITKAT
 } if (Build.VERSION.SDK_INT.beforeKitkat()){
 // do something…
 } 

Sample fun String.bd() = BigDecimal(this) fun main(args: Array) { //0.06
 println("0.05".bd().add("0.01".bd()) ) //0.060000000000000005
 println(0.05+0.01)
 } 

Function Expression val validator: (String) -> Boolean = { it.length > 2 }
 
 
 fun EditText.validateWith(check: (String) -> Boolean){
 val content = this.text.toString()
 if (check(content)) {
 context.toast("Welcome $content!")
 } else {
 context.toast("Name not valid")
 }
 }
 

observable delegated property 

No content

Anko • Why Anko ? (sample) • type safe builder for view • null-safe (findViewById(….) return null) • avoid XML parsing (save CPU and battery) 

Anko val layout = LinearLayout(context)
 layout.orientation = LinearLayout.VERTICAL
 val name = EditText(act)
 val button = Button(act)
 button.text = "Say Hello"
 button.setOnClickListener {
 Toast.makeText(act, .....).show()
 } layout.addView(name) layout.addView(button) 

Anko verticalLayout {
 val name = editText()
 button("Say Hello") {
 onClick { toast(“HI!,${name.text}!") }
 }
 } 

Anko • Anko DSL preview plugin 

Anko longToast("Halo"))
 info(“info”) warn(“warn”) debug(“debug”) async {
 var result = readDataFromServer();
 uiThread {
 warn(result + "")
 halo.text = result
 }
 } 

How to…. 

calling Java from Kotlin 

No content

calling Kotlin from Java 

No content

Adoption 

Steps to convert • Start small (ex. convert POJO into data class) • When you got NPE , Cast Exception • Check all !!s , make them ? or non-null • Check the difference between Java and Kotlin 

Other difference • Invariant array • No raw types • No private fields • Smaller type is not a subtype of bigger types • Calling Java code from Kotlin • Calling Kotlin from Java 

Other tips • std-lib : let() , apply() , with() ,run(), use() • Not a completely new language, very handy • A great place to learn functional programming • Droidcon talks may not up-to-date. Some issues are fixed. 

Other tips • How to return from lambda • Don’t use inheritance with data class • Pass in function instead of object • Almost everything in Kotlin is an expression ( can return value) • Just “convert Java code to Kotlin” and “Config Kotlin in Project” 

Resources • Official materials , try.kotlinlang.org , and Slack(#android) • Koans web and github • Jake Wharton : Using Project Kotlin for Android and Advancing Development with the Kotlin • twitter #kotlin @cbeust @madisp [JRebel] • Links : antonio leva , Jcconf 2015 

Testing 

Testing • Sample Project( Android Testing) • Testing with Kotlin • Instrumentation Testing Robots 

No content

No content

No content

No content

No content

No content

Usage • Swing: PokemonGoBot • Backend Frontend • Testing • Web , backend , and other list of Kotlin Projects • Hot on Github • Gradle (sample) (talk) (Groovy) 

email : [email protected] email : [email protected] twitter : ChenNevin Thanks!