Develop your next app with Kotlin

@arnogiu Work @ ekito Core & Gear Maker Mobile & Cloud Arnaud GIULIANI

« production ready » 15.02.2016

1.1.1 @ 15.03.2017

Yet another JVM Language ?

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

Limits of java - Verbose - Type inference - No properties / Lazy / Delegate - Checked exception - NullPointerException - Extensibility - End of lines with ; @ sdeleuze

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

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- Conciseness & Smarter API - Null Safety & Immutability protection - Type Inference, Static Typing & Smart Casts - Open programming styles - Java Interop Why Kotlin ?

What’s Kotlin ? - Fully open source (built by Jetbrains) - Based on Java & run on Java 6+ - Modern language features - 1st grade tooling

More About 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

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: The Ying and Yang of Programming Languages @ sdeleuze

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

Java 8 on Android ? https://android-developers.googleblog.com/2017/03/future-of-java-8-language-feature.html

KOTLIN is not just about writing your app with lesser lines. It’s all about writing SAFER & BETTER APPS !

Kotlin clearly considered by the Java community ! https://spring.io/blog/2017/01/04/introducing-kotlin-support-in- spring-framework-5-0 http://www.javamagazine.mozaicreader.com/ #&pageSet=5&page=0&contentItem=0 (March/April 2017) https://www.thoughtworks.com/radar/languages-and-frameworks/kotlin

Ready to use Easy to run

19 IntelliJ

20

Getting started with gradle buildscript { ext.kotlin_version = '' dependencies { classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version" } } apply plugin: "kotlin" dependencies { compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version" }

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

Let’s go !

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

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 with ?

// set length default value manually
 val length = if (stringB != null) stringB.length else -1
 //or with the Elvis operator
 val length = stringB?.length ?: -1 Default Value & 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 !!.

Example Working with optional values

Example Null check safety & Smart cast Securing with default values

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

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

POJO ? Kotlin Java

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 lateinit var weatherWS : WeatherWS
 
 @Test
 fun testMyWeatherWS() {
 weatherWS = Inject.get()
 // ... 
 } class ApiKey(var weatherKey: String, var geocodeKey: String){
 var debug : Boolean = false
 } Late & Lazy initialization

Example data class User(val name: String = "", val age: Int = 0) val jack = User(name = "Jack", age = 1) //no new keyword
 val anotherJack = jack.copy(age = 2) Data Class usage A simple GSon Class

Object Class // singleton
 object Resource {
 val name = "Name"
 } class StringCalculator{ // class helper
 companion object{
 val operators = arrayOf("+","-","x","/")
 }
 } Singleton Class Companion Object

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 switch and 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

Example Page Adapter Menu Select

Collections // immutable map
 val map = mapOf("a" to 1, "b" to 2, "c" to 3)
 for ((k, v) in map) {
 println("$k -> $v")
 } 
 map["a"] = "my value" // direct map access with array style // range
 for (i in 1..100) { //... } // immutable list
 val list = listOf("a", "b", "c","aa")
 list.filter { it.startsWith("a") } * collections operators : map, filter, take, flatMap, forEach, firstOrNull, last …

Example Kotlin Collection Java 7 Collection

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). myNumber.split("\n").flatMap { it.split(separator) }
 .map(Integer::parseInt)
 .map(::checkPositiveNumber)
 .filter { it <= 1000 }
 .sum() * Method references are not surrounded by curly braces !

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

Extensions Functions Extension declaration Usage

Destructuring Declarations In lambdas, Since Kotlin 1.1 map.mapValues { (key, value) -> "$value!" } Destructured variables declaration val person = Person("john",31) //data class Person(val name:String, val age : Int)
 val (name,age) = person
 println("I'm $name, my age is $age") * Works with Map, Data Class, Pair, Triple … => you can return 2 values in function !

InterOp Kotlin/Java Java Kotlin

InterOp Java/Kotlin object UserSingleton{
 fun stringify(user : User) : String{
 // …
 }
 } 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){ //…
 }
 }
 } User u = new User("toto",42);
 User.Companion.sayHello(u); UserUtilsKt.WhatIsyourAge(u); UserSingleton.INSTANCE.stringify(u)

Sweet stuffs for

Kotlin’s Android Extensions apply plugin: 'com.android.application' apply plugin: ‘kotlin-android’ apply plugin: ‘kotlin-android-extensions’ // the kotlin-android extension ! In your build.gradle

Kotlin Java

Reactive Programming

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

Example - Rx/Retrofit Kotlin Java 7

Example - Rx/Realm Load first element & update UI

52 Ready to Go ?

53 Just another hype JVM stuff ?

54 Production ready ?

It’s always hard to start !

Our feedback - Easy to start on existing project - Great learning curve - Good doc & tools support - Don’t use anymore Butterknife, Dagger … - T_T hard to come back to Java https://www.ekito.fr/people/kotlin-in-production-one-year-later/

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

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Try Kotlin online http://try.kotlinlang.org/#/Examples/ Kotlin Reference https://kotlinlang.org/docs/reference/ Kotlin Cheat Sheet (by ekito) https://speakerdeck.com/agiuliani/kotlin-cheat- sheet

70% Less ! Develop a weather app with K https://github.com/Ekito/2017-handson-kotlinAndroid

Thank you :)