Kotlin Development - Speaker Deck

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Kotlin Statically typed, modern language for industry Not swift of Android [email protected]

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

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Kotlin • Build and used by JetBrain • Since 2010, • 1.0 -Beta 2 • Tooling support • Very low learning curve

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Statically typed programming language • for Android , JVM , Browser • Concise , Safe , Interoperable • work with Java echo system ( libraries, JRebel )

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function

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First class citizen package boo.bar  class A {  fun foo(){  }  }   fun foo(){}

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Function fun cal(x: Int = 1, y: Int = 2){ print("x+y=${x + y}”) // string template    fun main(args: Array) {  val a = A()  cal(1) //default value  cal(x = 3)   a.cal(x = 4, y = 5) // named param  }   

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Function fun sum(x: Int, y: Int): Int {  return x + y  } fun sum(x: Int, y: Int): Int = x + y fun sum(x: Int, y: Int) = x + y

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variable

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string template & conditional expressions

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Null safety var a: String = "abc"  a = null //compilation error    var b: String? = null // OK!  b = bob?.department?.name // safe call using ?. val l = b!!.length() // YOYO (NPE-lovers)

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Elvis operator val l: Int = if (b != null) b.length else -1    val l = b?.length ?: -1

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range

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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")  }  }

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Smart cast / Auto cast // Once type is checked, no need to cast again when (x) {  is String -> x.length()  is File -> x.getFreeSpace() else -> { // Note the block  print("x is neither String nor File")  } } // safe cast, x must be nullable val x: String? = y as? String

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type safety recap - null safety

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Equity val a: Int = 10000  print(a == a) // Prints 'true'  print(a === a) // Prints ‘true'   val boxedA: Int? = a  val anotherBoxedA: Int? = a  print(boxedA == anotherBoxedA) // 'true'  print(boxedA === anotherBoxedA)// 'false'

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Equity • Smaller types are NOT implicitly converted to bigger types val a: Int? = 1 // A boxed Int  val b: Long? = a print(a == b) // Won’t compile

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

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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) {  }  }

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data class

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Data class // Declaration in Java public class Person(){ private String name; private int weight; // getter… setter… } 

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Data class // Declaration in Kotlin data class Person(val name: String,var weight: Int)  val nevin = Person("Nevin",82)  nevin.name = "Nevin Chen” // compile error   nevin.weight = 7 // ok

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Data class val p2 = Person("Nevin", 82)  val p3 = Person("Mark", 75)    println("nevin == p2: ${nevin == p2}”) // true  println("nevin == p3: ${nevin == p3}”) // false println(user.copy("Max", 0.33))

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

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

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

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Function expression // function expression val sum = fun(x: Int, y: Int): Int {  return x + y  } // anonymous function only val sum = fun(x: Int, y: Int): Int = x + y

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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,sum)  calculate(1,5, { x, y -> x + y }) calculate(1,5){ x, y -> x + y }  calculate(1,5,fun(x: Int, y: Int): Int = x + y )  }

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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)

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

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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)  }

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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")  }  } 

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delegated property

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observable delegated property

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

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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)

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

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Anko • Anko DSL preview plugin • Design Support Library compile ‘org.jetbrains.anko:anko-design:0.7.2’ • Work great if you create view programatically • Other good stuff

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

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Android Extension // import import kotlinx.android.synthetic.layout.* // code  txt.text= "WELCOME !!”// use widget id directly

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Take Away • Less code , Less bug • Easy to read and code • Lambda • Extension function

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How to….

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calling Java from Kotlin

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calling Kotlin from Java

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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

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Static method • There’s no static in Kotlin. Use companion object // Declaration class SomeClass{  companion object {  fun method(){  }  }  } // Usage SomeClass.Companion.method()

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Mutable non-nullable lateinit var username: String 

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Immutable non-nullable val url: String by lazy { // compute the string  if (isProduction){  return@lazy "http://prod.url.com"  }else{  return@lazy "http://stg.url.com"  }  }

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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

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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.

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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”

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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

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Testing

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

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

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Thanks!