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Learning Kotlin An Android Developer’s Journey Rebecca Franks @riggaroo

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What is Kotlin?

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What is Kotlin?

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Why Learn Kotlin?

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My Journey

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I saw general hype about Kotlin...

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I watched a video...

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I wrote an article...

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Then I forgot about it..

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Google I/O happened

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Google I/O 2017 - Kotlin Support Announcement

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In my mind

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I tried Kotlin Koans...

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try.kotlinlang.org

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I tried an Online Course

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Udemy

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I bought a Kotlin book

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I bought another Kotlin Book…

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I decided to CMD +ALT+ SHIFT + K a few files

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Eventually I decided to just try write Kotlin code

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You don’t need to know all the features to write Kotlin code. Learn as you go.

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Kotlin Things

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Semicolons - there are none *well unless you want them

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var / val

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var = variable value

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val = constant value

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var temperature : Int = 1 // has type Int val numberMembers = 5 //automatically inferred type Int val percentage = 80 / 100 percentage = 30 //does not compile var percentage = 80 / 100 percentage = 30 //compiles

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var temperature : Int = 1 // has type Int val numberMembers = 5 //automatically inferred type Int val percentage = 80 / 100 percentage = 30 //does not compile var percentage = 80 / 100 percentage = 30 //compiles

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var temperature : Int = 1 // has type Int val numberMembers = 5 //automatically inferred type Int val percentage = 80 / 100 percentage = 30 //does not compile var percentage = 80 / 100 percentage = 30 //compiles

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var temperature : Int = 1 // has type Int val numberMembers = 5 //automatically inferred type Int val percentage = 80 / 100 percentage = 30 //does not compile var percentage = 80 / 100 percentage = 30 //compiles

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Nullability

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The Kotlin Type system distinguishes between references that can hold null and those that can not

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Avoid NullPointerException *well unless you want them

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//No Nulls Allowed var average : Double = 1.0 average = 2.0 average = null

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//No Nulls Allowed var average : Double = 1.0 average = 2.0 average = null //Allows nulls var average : Double? = 1.0 average = 2.0 average = null

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// Double !! -> NullPointerException val l = name!!.length // Safe Call ?. with chains val name = bob?.department?.head?.name // Elvis Operator val length = b?.length ?: -1

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// Double Bang !! -> NullPointerException val l = b!!.length // Safe Call ?. with chains val name = bob?.department?.head?.name // Elvis Operator val length = b?.length ?: -1

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// Double Bang !! -> NullPointerException val l = b!!.length // Safe Call ?. with chains val name = bob?.department?.head?.name // Elvis Operator ?: val length = b?.length ?: -1

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val name : String? = "hello" name?.let { //Now name is not null at this point textView.text = name } // There are whole bunch more - .with{} .run{} - with slight differences // Can also just do a normal null check!

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lateinit

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class EventsActivity : AppCompatActivity() { private lateinit var recyclerViewAdapter: SimpleAdapter override fun onCreate(savedInstanceState: Bundle?) { // Other initialization stuff recyclerViewAdapter = SimpleAdapter(/*..*/) } }

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lazy

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val lazyValue: String by lazy { println("computed!") "Hello" } fun main(args: Array) { println(lazyValue) println(lazyValue) } computed! Hello Hello

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when

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when (s) { 1 -> print("x == 1") 2 -> print("x == 2") else -> { // Note the block print("x is neither 1 nor 2") } }

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enum class Currency { GBP, ZAR; } when (currency) { Currency.GBP -> doSomething() Currency.ZAR -> doAnotherThing() }

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enum class Currency { GBP, USD, ZAR; } when (currency) { Currency.GBP -> doSomething() Currency.ZAR -> doAnotherThing() }

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enum class Currency { GBP, USD, ZAR; } when (currency) { Currency.GBP -> doSomething() Currency.ZAR -> doAnotherThing() else -> doNothing() }

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Functions

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Functions are first-class citizens

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fun calculateSum(a: Int, b: Int) : Int { return a + b } fun calculateSum(a: Int, b: Int) = a + b

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fun calculateSum(a: Int, b: Int) : Int { return a + b } fun calculateSum(a: Int, b: Int) = a + b

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Extension Functions

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fun String.hello(): String = "Hello " + this // use the new hello() method val hi = "Kotlin !".hello() Hello Kotlin ! fun TextView.getFloatValue() : Float { return 0f } val float = textViewHello.getFloatValue()

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fun String.hello(): String = "Hello " + this // use the new hello() method val hi = "Kotlin !".hello() Hello Kotlin ! fun TextView.getFloatValue() : Float { return 0f } val num = textViewHello.getFloatValue()

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Default Values

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fun calculateSum(a: Int = 2, b: Int) : Int { return a + b } val sum = calculateSum(b = 3) 5 //Can be done on Constructors as well as functions

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Higher Order Functions & Lambdas

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val exampleArray = arrayListOf(3,324,5,6,3,2,6,8,1,2,4,3,2) exampleArray.filter { it % 2 == 0 } [324, 6, 2, 6, 8, 2, 4, 2] val numberList = arrayListOf(1,2,3,4,5,6,7,8,9) numberList.map { it * 2 } .filter { it % 3 == 0} [6, 12, 18]

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val exampleArray = arrayListOf(3,324,5,6,3,2,6,8,1,2,4,3,2) exampleArray.filter { it % 2 == 0 } [324, 6, 2, 6, 8, 2, 4, 2] val numberList = arrayListOf(1,2,3,4,5,6,7,8,9) numberList.map { it * 2 } .filter { it % 3 == 0} [6, 12, 18]

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fun calcPercentage(value: Int, onComplete: (output : Double) -> Unit) { val calculation = value / 100.0 onComplete(calculation) } //called in onCreate or wherever calculatePercentage(2) { Log.d("MainActivity", "Calculation Complete: $it") } Calculation Complete: 0.02

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fun calcPercentage(value: Int, onComplete: (input : Double) -> Unit) { val calculation = value / 100.0 onComplete(calculation) } //called in onCreate or wherever calcPercentage(2) { Log.d("MainActivity", "Calculation Complete: $it") } Calculation Complete: 0.02

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fun executeWork(job: () -> T) : T { Log.d(TAG, "Work started") val result = job() Log.d(TAG, "Work Fin") return result } val number = executeWork { 100 * 5 } Log.d(TAG, "Result: $number") val stringCalc = executeWork { "Rebecca".plus(" Hello") } Log.d(TAG, "String: $stringCalc") Work started Work Fin Result: 500 Work started Work Fin String: Rebecca Hello

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fun executeWork(job: () -> T) : T { Log.d(TAG, "Work started") val result = job() Log.d(TAG, "Work Fin") return result } val number = executeWork { 100 * 5 } Log.d(TAG, "Result: $number") val stringCalc = executeWork { "Rebecca".plus(" Hello") } Log.d(TAG, "String: $stringCalc") Work started Work Fin Result: 500 Work started Work Fin String: Rebecca Hello

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fun executeWork(job: () -> T) : T { Log.d(TAG, "Work started") val result = job() Log.d(TAG, "Work Fin") return result } val number = executeWork { 100 * 5 } Log.d(TAG, "Result: $number") val stringCalc = executeWork { "Rebecca".plus(" Hello") } Log.d(TAG, "String: $stringCalc") Work started Work Fin Result: 500 Work started Work Fin String: Rebecca Hello

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Classes

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class Admin : User { val name: String val age: Int constructor(name : String, age : Int){ this.name =name this.age = age } fun isAdult(): Boolean { return age >= 18 } }

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class Admin(val name: String, val age: Int) : User { fun isAdult(): Boolean { return age >= 18 } }

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class Admin(val name: String, val age: Int) : User { fun isAdult() = age >= 18 }

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Data Classes

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// data classes data class Admin(val name: String, val age: Int) // Automatically generates: - toString() - hashCode() / equals() - copy()

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Coroutines *Experimental

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Simplify Async Programming Lightweight thread *Spin up 1mil coroutines easily

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Coroutines are computations that can be suspended without blocking a thread. https://kotlinlang.org/docs/tutorials/coroutines-basic-jvm.html

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//onCreate job = launch(background) { val result = requestData() launch(UI) { textViewHello.text = result } } //suspend function suspend fun requestData(): String { delay(3000) return "DONE" } //onDestroy job.cancel()

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//onCreate job = launch(background) { val result = requestData() launch(UI) { textViewHello.text = result } } //suspend function suspend fun requestData(): String { delay(3000) return "DONE" } //onDestroy job.cancel()

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//onCreate job = launch(background) { val result = requestData() launch(UI) { textViewHello.text = result } } //suspend function suspend fun requestData(): String { delay(3000) return "DONE" } //onDestroy job.cancel()

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https://kotlinlang.org/docs/tutorials/c oroutines-basic-jvm.html

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Android KTX

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github.com/android/android-ktx

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A set of Kotlin extensions for Android

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//Before sharedPreferences.edit() .putBoolean("key", value) .apply() //After - with Android KTX sharedPreferences.edit { putBoolean("key", value) }

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//Before view.viewTreeObserver.addOnPreDrawListener( object : ViewTreeObserver.OnPreDrawListener { override fun onPreDraw(): Boolean { viewTreeObserver.removeOnPreDrawListener(this) actionToBeTriggered() return true } }) //After - with Android KTX view.doOnPreDraw { actionToBeTriggered() }

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val (a,r,g,b) = Color.MAGENTA val drawable = customDrawable.toBitmap() canvas.withTranslation(x, y) { // block } val (year, month, day) = LocalDate.now()

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The difficult bits *for me...

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Mocking - All classes in Kotlin are final unless you tell it otherwise (ie mark it as open ) - Workarounds but seem unnecessary https://antonioleiva.com/mockito-2-kotlin/ http://hadihariri.com/2016/10/04/Mocking-Kotlin-With-Mockito/

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Tooling IntelliJ is great… but it doesn’t write all the code. Battled with Lambda syntax, it sometimes doesn’t fill in all the blanks nicely Solution - Write it in Java, then copy paste the java into your Kotlin file and it’ll auto convert

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Nothing, Unit & Any Nothing - never returns - ie infinite loop or exception throwing Unit - default return type for a function Any - Default Super Type (similar to Java’s Object type) http://natpryce.com/articles/000818.html

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Concerns ● Packaging another library in your app (size 9kbs when proguarded - not much) ● Another barrier to entry for new developers ● Java 8 works with Android.. (not all features ie Stream API only available in 24+) ● Certain Java plugins not supported (yet) - Sonarqube https://discuss.kotlinlang.org/t/sonarqube-support/3657

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Resources

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Online Courses O’Reilly Introduction to Kotlin YouTube Search “Kotlin on Android” https://www.youtube.com/results?search_query=Kotlin+on+Android

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Practise Kotlin Koans https://try.kotlinlang.org Kotlin REPL Kotlin Android Codelab

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try.kotlinlang.org

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Books ● Kotlin in Action - Svetlana Isakova - Dmitry Jemerov ● Kotlin for Android Developers - Some common patterns etc

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Online Community kotlinweekly.net talkingkotlin.com reddit.com/r/kotlin

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Just start writing Kotlin code

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Thank you! Questions? Rebecca Franks @riggaroo