Introduction to Kotlin

Introduction to Kotlin

Transcript

1. Introduction to Kotlin 2020
2. What is Kotlin?
3. Kotlin is a cross-platform, statically typed, general-purpose programming language with type inference.
4. Open Source - Apache 2.0 JetBrains - 2011
5. • Android • JVM • iOS, macOS, watchOS, and tvOS • Windows • Linux • JavaScript (KotlinJs) • WebAssembly • LLVM (Kotlin/Native)
6. History
7. • 2011 - Born • 2015 - v1.0 released • 2017 - Google announced first-class support for Kotlin on Android. • 2017 - Sharing code between JVM and JavaScript platforms • 2018 - Coroutines support for asynchronous programming. • 2019 - Google announced that the Kotlin programming language is now its preferred language for Android app developers. • 2020 - Support for Apple's platforms.
8. 100% Interop with Java
9. Null-safe
10. Less code
11. Easy to use the latest version
12. Share code between platforms
13. Fully supported since Spring 5.0
14. Gradle DSL
15. Syntax
16. val immutable = "Can't change" immutable = "New value" // Compile error var mutable = "Can change" mutable = "New value"
17. val str: String = "Hello"
18. val str = "Hello"
19. var canBeNull: String? = "abc" canBeNull = null // ok var notNull: String = "abc" notNull = null // Compile error
20. var name: String? = "Name" name.toUpperCase() // Compile error name?.toUpperCase() // ok
21. // Elvis print( name?.toUpperCase() ?: "UNKNOWN" )
22. print( name!!.toUpperCase() // NullPointerException )
23. if (some != null && some.length() > 0){ // } if (!some.isNullOrEmpty()) { // }
24. // older way if (user != null) { user.save(); } // kotlin way user?.save()
25. // Optional chaining bob?.department?.head?.name
26. // Early return myVar ?: return println(myVar)
27. // simple name in template: var a = 1 val s1 = "a is $a" a = 2 // arbitrary expression in template: val s2 = "${s1.replace("is", "was")}, but now is $a"
28. // Smart cast if (obj is String) { // obj is automatically cast to String in this branch return obj.length }
29. val items = listOf("apple", "banana", "kiwi") val array = arrayOf(1, 2, 3) val numbers = mutableListOf(1, 2) val nums = arrayListOf(1) val map = mapOf( "one" to 1, "two" to 2 )
30. fun describe(obj: Any): String = when (obj) { 1 -> "One" "Hello" -> "Greeting" is Long -> "Long" !is String -> "Not a string" else -> "Unknown" }
31. fun convert(x: String): Int { return when { x == "unknown" -> 0 x.length < 11 -> 1 else -> 2 } }
32. Functions
33. fun foo(input: String): Int { ... }
34. // expression functions fun maxOf(a: Int, b: Int) = if (a > b) a else b
35. fun generate(): Int { return 5 } fun generate(): Int = 5 fun generate() = 5
36. // Default function values fun foo(a: Int = 0, b: String = "") { ... }
37. fun reformat(str: String, normalizeCase: Boolean = true, wordSeparator: Char = ' ') { ... } reformat(str) reformat(str, false, '_') reformat(str, wordSeparator = '_')
38. infix fun Int.shl(x: Int): Int { ... } // call the function using the infix notation 1 shl 2 // is the same as 1.shl(2)
39. fun myfun(graph: Graph) { fun myInnerFun() { .... } myInnerFun() }
40. // Higher order functions fun <T> ArrayList<T>.myFilter(condition: (T) -> Boolean): ArrayList<T>{ val result = arrayListOf<T>() for (item in this){ if (condition(item)){ result.add(item) } } return result }
41. inline fun onlyIf(check: Boolean, operation: () -> Unit) { if (check) { operation() } } onlyIf(shouldPrint) { println(“Hello, Kotlin”) } // which will be inlined to this if (shouldPrint) { // no need to create lambda println(“Hello, Kotlin”) }
42. // Older way val logger = Logger.getLogger(SomeLogger::class.java) inline fun <reified T> loggerOf() = Logger.getLogger(T::class.java) // Kotlin way val logger = loggerOf<SomeLogger>
43. @Test fun testSeeAnEmptyListUserSelected(){ ... } @Test fun `Test see an empty list user selected`(){ ... }
44. // Extensions fun String.isValid() = length > 6 "hello".isValid() // returns false
45. fun Int?.orZero() = this ?: 0 val myInt:Int? = null println("${myInt.orZero()}") // will print 0
46. Functions are final by default Arguments are always final Functions can be defined in a file outside of a class
47. // Getter & setter var email:String = "" set(value) { if (value.isValidEmail()){ field = value } }
48. // lambdas val fruits = listOf( "banana", "avocado", "apple", "kiwifruit" ) fruits .filter { it.startsWith("a") } .sortedBy { it } .map { it.toUpperCase() } .forEach { println(it) }
49. class MyClass() { // non-null, but not initialized lateinit var myView: View override fun onCreate() { // initialized here myView = findViewById(R.id.view) } }
50. Delegates
51. // Initialized at first call val preference: String by lazy { sharedPreferences.getString(PREFERENCE_KEY) }
52. class Example { var username: String by Delegate("key") fun main(){ println(username) // Reads from database username = "newName" // Writes to databse } } class Delegate(private val key: String) { operator fun getValue(thisRef: Example, prop: KProperty<*>): String { return thisRef.readFromDb(key) } operator fun setValue(thisRef: Example, prop: KProperty<*>, value: String) { thisRef.writeToDb(key, value) } }
53. Classes
54. // All classes and methods are final by default class MyClass { ... } open class Base { fun thisIsFinal() { ... } open fun notFinal() { ... } } class NewClass: Base { override fun notFinal() { ... } }
55. sealed class NetworkResult{ data class Success(val result: String): NetworkResult() data class Failure(val error: Error): NetworkResult() } // one observer for success and failure fun parse(data: NetworkResult){ when(data) { is Success -> showResult(data.result) // smart cast to Success is Failure -> showError(data.error) // smart cast to Failure } }
56. data class User( val name: String = "", val age: Int = 0 ) val user = User("name", 20) val newUser = user.copy(age = 33) // User("name", 33)
57. // Destructing val user = User("Jane", 35) val (name, age) = user println("$name, $age years of age") // prints "Jane, 35 years of age"
58. // No need to handle concurrency object Singleton { var token = "" }
59. apply, also, run, let
60. // T -> operation -> T User user = new User(); user.name = "Name; user.save(); val user = User().apply { // `this` represents User name = "Name" save() }
61. // Builder class MyBuilder { var name = "" ... fun setName(newName: String) = builder.apply(name = newName) ... }
62. // T -> operation -> Y UserVm wrap(User user){ if (user != null) { return new UserVm(user); } return null; } fun wrap(user: User?) = user?.let { UserVm(it) }
63. typealias MyMap = Map<String, Lis<User>> fun filter(userMap: MyMap){ ... }
64. DSL
65. fun result() = html { head { title {+"XML encoding with Kotlin"} } body { h1 {+"XML encoding with Kotlin"} p {+"this format can be used as an alternative markup to XML"} a(href = "http://kotlinlang.org") {+"Kotlin"} p {+"some text"} p { for (arg in args) +arg } } }
66. fun html(init: HTML.() -> Unit): HTML { val html = HTML() html.init() return html } fun head(init: Head.() -> Unit) : Head { val head = Head() head.init() children.add(head) return head } html { head { ... } body { ... } }
67. Coroutines
68. • Asynchronous programming • Light-weight • Non-blocking • Structured concurrency • Flow
69. fun main() = runBlocking { // this: CoroutineScope launch { delay(200L) println("Task from runBlocking") } coroutineScope { // Creates a coroutine scope launch { delay(500L) println("Task from nested launch") } delay(100L) println("Task from coroutine scope") // This line will be printed before the nested launch } println("Coroutine scope is over") // This line is not printed until the nested launch completes }
70. • 2018 - Github Fastest growing language https://octoverse.github.com/2018/projects#lan guages • 4th language most loved by developers https://insights.stackoverflow.com/survey/2019# most-popular-technologies • 70% of the top 1000 Android apps use Kotlin https://developer.android.com/kotlin/stories
72. • https://kotlinlang.org • https://github.com/JetBrains/kotlin • https://try.kotlinlang.org/ • https://en.wikipedia.org/wiki/Kotlin_(programming_language) • https://www.thoughtworks.com/radar/languages-and-frameworks/ko tlin • https://spring.io/blog/2017/01/04/introducing-kotlin-support-in-sprin g-framework-5-0 • https://blog.duolingo.com/migrating-duolingos-android-app-to-100-ko tlin/
73. Thanks