#DevoxxFR 13 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
#DevoxxFR 15 - Fully open source (built by Jetbrains) - Run on Java 6+ - Static typing & type inference - Modern language features - 1st grade tooling What’s Kotlin ?
#DevoxxFR 17 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
#DevoxxFR 21 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 !
#DevoxxFR 23 Late initialization, Lazy, Delegates … // 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 !!.
#DevoxxFR 26 Class class User ( val userName: String, val firstName: String, val lastName: String, var location: Point? = null ) POJO + - Getter - Setter - Constructors public / closed by default optional Body
#DevoxxFR 27 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
#DevoxxFR 29 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 // customer getter & setter var stringRepresentation: String set(value) { … } class ApiKey(var weatherKey: String, var geocodeKey: String){ var debug : Boolean = false } * Lateinit, Lazy, Delegates …
#DevoxxFR 31 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
#DevoxxFR 32 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 old 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 * can match with operators : in, as, is, range …
#DevoxxFR 34 Collections // immutable map val map = mapOf("a" to 1, "b" to 2, "c" to 3) for ((k, v) in map) { println("$k -> $v") }
map2["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 …
#DevoxxFR 37 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)
} Named Parameters & default values reformat(str, true, true, '_') // old way to call reformat(str, wordSeparator = '_') // using default values & named params
#DevoxxFR 53 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
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