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Kotlin lang - basics
(Android projects)
Bartosz Kosarzycki - StxNext Lightning Talks - Feb 12, 2016
talented developers | ...
KOTLINhttp://kotlinlang.org/
Required knowledge:
● basic Android development skills
● functional programming
● familiarity...
What is it?
KOTLIN is:
● safe
● versatile
● interoparable
● IDE support
● fast
JAVA SCALA
KOTLIN
+ fast compilation
+ simp...
Why KOTLIN?
● no javax.time from JDK8
● no try-with resources
● no lambdas!
● no new java stream api
● no way to add metho...
Advantages
fun finish(obj: Any) {
if (obj is Activity)
obj.finish()
}
Auto-casting:
Named args in func calls:
fun circle(x...
Nullpointer
safety
var output : String?
output = null
println(output!!.length)
Exception in thread "main" kotlin.KotlinNul...
Advantages
val arr = arrayOf(D("1A", "1B"),
D( "2A", "2B"), D("3A", "3B"));
for ((first, second) in arr )
println("a: $fir...
Data objects
data class Point(val x: Double = 0.0, val y: Double = 0.0, var descr: String?)
val point1 = Point( x = 1.0, y...
Traits
TRAITS - java-like interfaces
with default implementation
class ExampleActivity :
AppCompatActivity(), ActivitySess...
Class
delegation
Built-in delegate pattern:
class Derived(b: Base) : Base by b
class BaseImpl(val x: Int) : Base {
overrid...
Properties
Properties & read-only properties:
public class Address(addr : String) {
public var name: String = ""
public va...
Utility
classes
UTILS:
StringUtil
ActivityUtil
ListUtil
JAVA utility class
public class StringUtils {
public static String...
+ KOTLIN
Project
structure
Android kotlin project structure: Kotlin & Java intertwined:
Gradle dependencies:
build.gradle:
buildscr...
Cost
Reference application:
one blank Activity app generated with Android Studio
Higher-order
functions
Introduction
//extension function
fun Date.isTuesday() = day == 2
//function expression as contant
...
Android
Compactness
mDescriptionTextView.setOnClickListener({ activityPrivateMethod() })
mDescriptionTextView.setOnClickLi...
Helpers ● Lambda expressions in EditText listeners
● usually handled by kotlin android libs
class TextWatcherFunctions : T...
Kotterknife
val mHeaderTextView: TextView by bindView(R.id.activity_team_details_team_header)
val mDescriptionTextView: Te...
Dagger 2
& KOTLIN
@Module
class AndroidModule(private val application: Application) {
@Provides
@Singleton
fun provideLoca...
Kotlin Anko
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
verticalLayout {
paddi...
Sample
Activity
class TeamDetailsActivity : AppCompatActivity() {
val mHeaderTextView: TextView by bindView(R.id.activity_...
SWIFT
var myVariable = 42 //Variable
val explicitDouble: Double = 70.0 //Explicit Type Constant
for (i in 1..5) { print(i)...
Command line
compiler
$ curl -s get.sdkman.io | bash
$ source "$HOME/.sdkman/bin/sdkman-init.sh"
$ sdk install kotlin
Do y...
Command line
compiler
Kotlin:
class Capturing {
fun run2(func: Runnable) {
func.run()
}
}
fun main(args: Array<String>) {
...
What is
KOTLIN :)?
RUSSIAN WARSHIP
ISLAND
source:
https://en.wikipedia.org/wiki/Kotlin
CITY IN POLAND
KETCHUP BRANDPROGRAM...
Comparison
KOTLIN
WORSE THAN SCALA: BETTER THAN SCALA:
● no static members (BY DESIGN) - if you need something that
is not...
Resources
RESOURCES:
● http://kotlinlang.org/
● Jake Wharton - Using Project Kotlin for Android - https://t.co/9t8qsBGPlo
...
Thankyou!
Bartosz Kosarzycki
bartosz.kosarzycki@stxnext.pl
Twitter: @bkosarzycki
Online compiler:
http://try.kotlinlang.or...
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Kotlin Developer Starter in Android projects

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StxNext Lightning Talks - Feb 12, 2016

Kotlin - one of the popular programming languages built on top of Java that runs on JVM. Thanks to JetBrains support and excellent IDE integration, it’s an ideal choice when it comes to Android development. 100% Java compatibility, interoperability and no runtime overhead is just the beginning of a long list of strengths. Kotlin is supposed to be a subset of SCALA, on one hand covering major advantages for developers and on the other - keeping short compile times.

This presentation is a Developer Starter - a set of hand-picked information allowing a person with no knowledge of Kotlin to start writing basic Android activities and set up an Android-kotlin project. It starts with language background, reasons for its creation and advantages. Then presents basic use cases, syntax, structures and patterns. Later on Kotlin is presented in Android context. Simple project structure, imports and Kotlin usage with Android SDK is explained. In the end cost of Kotlin usage is presented and the language is compared to SCALA and SWIFT.

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Kotlin Developer Starter in Android projects

  1. 1. Kotlin lang - basics (Android projects) Bartosz Kosarzycki - StxNext Lightning Talks - Feb 12, 2016 talented developers | flexible teams | agile experts
  2. 2. KOTLINhttp://kotlinlang.org/ Required knowledge: ● basic Android development skills ● functional programming ● familiarity with JDK 6,7,8 ● Scala is a plus
  3. 3. What is it? KOTLIN is: ● safe ● versatile ● interoparable ● IDE support ● fast JAVA SCALA KOTLIN + fast compilation + simplicity + swift’s syntax is similar Online compiler: http://try.kotlinlang.org/ SWIFT
  4. 4. Why KOTLIN? ● no javax.time from JDK8 ● no try-with resources ● no lambdas! ● no new java stream api ● no way to add methods to platform data types (e.g. View) List<string> names = students.stream() .map(Student::getName) .filter(name->name.startsWith("B")) .collect(Collectors.toList()); ZoneId zone = ZoneId.systemDefault(); Clock clock = Clock.system(zone); LocalDate date = LocalDate.now(); LocalTime time = LocalTime.now(); time = time.plus(Period.ofDays(12)); javax.time Java Stream API static String readFirstLineFromFile(String path) throws IOException { try (BufferedReader br = new BufferedReader(new FileReader(path))) { return br.readLine(); } } try-with resources
  5. 5. Advantages fun finish(obj: Any) { if (obj is Activity) obj.finish() } Auto-casting: Named args in func calls: fun circle(x: Int, y: Int, rad: Int, stroke: Int) {…} circle(15, 40, rad = 20, stroke = 1); Built-in lambdas: val numbers: IntArray = intArrayOf(11, 20, 31, 40, 51) val predicate: (Int) -> Boolean = { it -> it % 2 == 1 } val list1 = numbers.filter { it % 2 == 1 } val list2 = numbers.filter(predicate) println("Lists identical: " + list1.equals(list2)); > Lists identical: true Compactness: * no new statement: val a = B(); * optional brackets, return statement and one-line function declarations: class A { var field1: String = "No "; fun printNews () = field1 + " news for you" ; } println(A().printNews()) ● all of these are much-needed in Android development
  6. 6. Nullpointer safety var output : String? output = null println(output!!.length) Exception in thread "main" kotlin.KotlinNullPointerException at Simplest_versionKt.main(Simplest version.kt:11) Java-like !! Operator: (for NPE lovers) - Optional.get() equivalent var output : String? output = null println(output?.length) val len = output?.length ?: -1 //elvis operator println(len) > null > -1 ?. Safe calls: (for if not null -> call function; return null otherwise) Kotlin type aliases - planned in roadmap (not yet released - as of Feb 2016) Java JDK 10 will push Optional onto the default stack ~ 2018 Optional<> pattern no longer needed! kotlin.Unit If a function does not return any useful value, its return type is Unit
  7. 7. Advantages val arr = arrayOf(D("1A", "1B"), D( "2A", "2B"), D("3A", "3B")); for ((first, second) in arr ) println("a: $first, b: $second") class D { public var nameA: String = "" public var nameB: String = "" constructor (nameA: String , nameB: String) { this.nameA = nameA this.nameB = nameB } operator fun component1() : String { return nameA } operator fun component2() : String { return nameB } } (a, b) Destructuring Declaration Singleton: object SampleSingleton { var baseUrl: String = "https://aaa.bbb" fun printUrl() = println(baseUrl) } SampleSingleton.printUrl() SampleSingleton.baseUrl = "https://ccc.ddd" SampleSingleton.printUrl() > https://aaa.bbb > https://ccc.ddd ● Singletons and destructuring declarations are built-in:
  8. 8. Data objects data class Point(val x: Double = 0.0, val y: Double = 0.0, var descr: String?) val point1 = Point( x = 1.0, y = 2.0, descr = "no description"); val point2 = Point( descr = "no description", y = 2.0, x = 1.0); println(point1.equals(point2)) println(point1.hashCode().equals(point2.hashCode()) ) println(point1.toString().equals(point2.toString()) ) println(point1.toString()) Data object: hashCode() toString() equals() + properties Automatically generated:● removes most of the boilerplate code
  9. 9. Traits TRAITS - java-like interfaces with default implementation class ExampleActivity : AppCompatActivity(), ActivitySessionHandling { override fun onDestroy() { super.onDestroy() closeSession() } } open interface ActivitySessionHandling { fun closeSession() = println("Session closed") } JAVA JDK 8 - extension methods - default interface implementation public class ItemListActivity extends AppCompatActivity implements Java8DefaultInterface { @Override protected void onDestroy() { super.onDestroy(); closeSession(); } } public interface Java8DefaultInterface { default void closeSession() { Log.i("TAG", "Session closed"); } } (not available in Android)
  10. 10. Class delegation Built-in delegate pattern: class Derived(b: Base) : Base by b class BaseImpl(val x: Int) : Base { override fun print() { println(x) } } interface Base { fun print() } val b = BaseImpl(10) Derived(b).print() > 10 Java equivalent: class Derived { Base base; public Derived(Base b) { this.base = b; } void print(){ base.print(); } } class BaseImpl implements Base { int val; public BaseImpl(int v) { this.val = v; } public void print() { System.out.println(val); } } interface Base { void print(); } BaseImpl base = new BaseImpl(10); new Derived(base).print();
  11. 11. Properties Properties & read-only properties: public class Address(addr : String) { public var name: String = "" public val address: String = addr //read-only } val address = Address(addr = "Low street 123") address.name = "Mickey mouse" println(address.address) println(address.name) > Low street 123 > Mickey mouse address.address = "Another street 123" //Error: val cannot be reassigned Getters & setters: public class Address() { var address: String get() = "Lorem ipsum" set(value) { println(value) } } public class Address() { var address: String = "" get //default getter private set //default private setter } Companion objects: class CarAssemblyFactory { companion object Factory { fun createCar(): String = String().plus("This is a car") } } println(CarAssemblyFactory.createCar())
  12. 12. Utility classes UTILS: StringUtil ActivityUtil ListUtil JAVA utility class public class StringUtils { public static String encodeString(String str) { return str.replaceAll(" ", "_"); } } KOTLIN utility class fun String.encodeSpaces():String = this.replace(" ", "_") println("Neque porro quisquam".encodeSpaces()) Separate packages: package main.kotlin.utils fun String.encodeSpaces(): String = this.replace(" ", "_") import main.kotlin.utils.encodeSpaces println("Neque porro quisquam".encodeSpaces()) ● no utils hell ● extend final classes ● classes in Kotlin are final by default @JvmName("DateUtil") fun Date.isTuesday() = day == 2 //in fact it's: //Date.getDay() == 2
  13. 13. + KOTLIN
  14. 14. Project structure Android kotlin project structure: Kotlin & Java intertwined: Gradle dependencies: build.gradle: buildscript { dependencies { classpath 'com.android.tools.build:gradle:2.0.0-beta3' classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.0.0-rc-1036' } } app.gradle: apply plugin: 'kotlin-android'
  15. 15. Cost Reference application: one blank Activity app generated with Android Studio
  16. 16. Higher-order functions Introduction //extension function fun Date.isTuesday() = day == 2 //function expression as contant val addition = { x: Int, y:Int -> x + y } //higher order function fun higherOrder(x : Int, y: Int, func : (x : Int, y : Int) -> Int ) : Int { return func.invoke(x, y); } Higher-order extension function fun Int.addCustomFunc(arg: Int, func : (x : Int, y : Int) -> Int ) : Int { return func.invoke(this, arg); } val addition = { x: Int, y:Int -> x + y } val result = 1.addCustomFunc(5, addition);
  17. 17. Android Compactness mDescriptionTextView.setOnClickListener({ activityPrivateMethod() }) mDescriptionTextView.setOnClickListener({ mDescriptionTextView.text = "Current time: " + DateTime.now() }) Lambda expression to the rescue private fun bind(item: Team) { mHeaderTextView.text = item.header mDescriptionTextView.text = item.description } mSampleEditText.addTextChangedListener{ onTextChanged { text:CharSequence, a:Int, b:Int, c:Int -> Toast.makeText(applicationContext, text, LENGTH_SHORT).show() } } ● simplify development ● remove boilerplate ● improve multi-threading Auto getters&setters
  18. 18. Helpers ● Lambda expressions in EditText listeners ● usually handled by kotlin android libs class TextWatcherFunctions : TextWatcher { private var _beforeTextChanged : ((CharSequence , Int, Int, Int) -> Unit)? = null private var _onTextChanged : ((CharSequence , Int, Int, Int) -> Unit)? = null private var _afterTextChanged : ((Editable) -> Unit)? = null override fun beforeTextChanged (s: CharSequence , start: Int , count: Int , after: Int) : Unit = _beforeTextChanged ?.invoke(s , start, count, after) ?: Unit ; override fun onTextChanged (s: CharSequence , start: Int , before: Int , count: Int) : Unit = _onTextChanged ?.invoke(s , start, before, count) ?: Unit override fun afterTextChanged (s: Editable) : Unit = _afterTextChanged ?.invoke(s) ?: Unit fun beforeTextChanged (function: (CharSequence , Int, Int, Int) -> Unit) { _beforeTextChanged = function } fun onTextChanged (function: (CharSequence , Int, Int, Int) -> Unit) { _onTextChanged = function } fun afterTextChanged (function: (Editable) -> Unit) { _afterTextChanged = function } } fun EditText.addTextChangedListener (init: TextWatcherFunctions.() -> Unit): TextWatcher { val watcher = TextWatcherFunctions() watcher.init() addTextChangedListener(watcher) return watcher }
  19. 19. Kotterknife val mHeaderTextView: TextView by bindView(R.id.activity_team_details_team_header) val mDescriptionTextView: TextView by bindView(R.id.activity_team_details_team_description) val textViews: List<TextView> by bindViews(R.id.activity_team_details_team_header, R.id.activity_team_details_team_description) // List binding with optional items being omitted. val nameViews: List<TextView> by bindOptionalViews(R.id.first_name, R.id.middle_name, R.id.last_name) ● bindView() instead of @Bind annotation ● developed by Jake Wharton ● still not pushed to maven central
  20. 20. Dagger 2 & KOTLIN @Module class AndroidModule(private val application: Application) { @Provides @Singleton fun provideLocationManager(): LocationManager { return application .getSystemService(Context.LOCATION_SERVICE) as LocationManager } @Provides @Singleton @Named("something") fun provideSomething(): String { return "something" } } class MainActivity : AppCompatActivity() { @Inject lateinit var locationManager: LocationManager @field:[Inject Named("something")] lateinit var something: String override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) } } ● compatible with KOTLIN since M13 ● introduction of lateinit property Late-initialized property: e.g. for unit tests public class MyTest { lateinit var subject: TestSubject @SetUp fun setup() { subject = TestSubject() } @Test fun test() { subject.method() } }
  21. 21. Kotlin Anko override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) verticalLayout { padding = dip(30) editText { hint = "Name" textSize = 24f } editText { hint = "Password" textSize = 24f } button("Login") { textSize = 26f } } } ● from Jetbrains ● create layouts from code dependencies { compile 'org.jetbrains.anko:anko-sdk15:0.8' compile 'org.jetbrains.anko:anko-support-v4:0.8' compile 'org.jetbrains.anko:anko-appcompat-v7:0.8' } verticalLayout { val name = editText() button("Say Hello") { onClick { toast("Hello, ${name.text}!") } } }
  22. 22. Sample Activity class TeamDetailsActivity : AppCompatActivity() { val mHeaderTextView: TextView by bindView(R.id.activity_team_details_team_header) var mTeam: Team? = null override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_team_details) supportActionBar!!.title = "Team description" supportActionBar!!.setDisplayHomeAsUpEnabled(true) mTeam = Gson().fromJson<Team>(intent.getStringExtra("item"), Team::class.java) bind(mTeam!!) } private fun bind(item: Team) { mHeaderTextView.text = item.header } override fun onOptionsItemSelected(item: MenuItem): Boolean { if (item.itemId == android.R.id.home) finish() return super.onOptionsItemSelected(item) } } ● 100% Java compatibility ● kotterknife ● Android SDK usage in Kotlin is simple
  23. 23. SWIFT var myVariable = 42 //Variable val explicitDouble: Double = 70.0 //Explicit Type Constant for (i in 1..5) { print(i) } //Inclusive Range Operator val a = "A"; val b = "B"; val str = "I have ${a + b} "; //String interpolation var shoppingList = arrayOf("catfish" , "water") //Array Creation var hashMap = hashMapOf("Malcolm" to "Captain" ); //Maps val emptyArray = arrayOf<String>() //Empty Typed Array interface Nameable { fun name(): String } //Interface val movie = obj as Movie //Downcasting fun Double.km() : Double = this * 1000; //Extension function KOTLIN SWIFT var myVariable = 42 //Variable let explicitDouble: Double = 70 //Explicit Type Constant for i in 1...5 { println( i) } //Inclusive Range Operator let a = "A"; let b = "B"; let str = "I have (a + b) " //String interpolation var shoppingList = [ "catfish" , "water"] //Array Creation var occupations = [ "Malcolm" : "Captain" ] //Maps let emptyArray = String[]() //Empty Typed Array protocol Nameable { func name() -> String } //Protocol (Interface) let movie = object as Movie //Downcasting extension Double { var km: Double { return self * 1_000.0 } } //Extension function ● Kotlin’s syntax is similar Swift
  24. 24. Command line compiler $ curl -s get.sdkman.io | bash $ source "$HOME/.sdkman/bin/sdkman-init.sh" $ sdk install kotlin Do you want kotlin 1.0.0-rc-1036 to be set as default? (Y/n): Y Setting kotlin 1.0.0-rc-1036 as default. Installation: ● Let us compare the resulting bytecode of kotlin and java compilation OsX SDKMAN or homebrew Linux FreeBSD Cygwin SDKMAN
  25. 25. Command line compiler Kotlin: class Capturing { fun run2(func: Runnable) { func.run() } } fun main(args: Array<String>) { Capturing().run2(Runnable { println("Hey! $args") }) } $ kotlinc Capturing.kt $ javap -p Compiled from "Capturing.kt" public final class Capturing { public final void run2(java.lang.Runnable); public Capturing(); } public final class CapturingKt { public static final void main(java.lang.String[]); } Java: ● Let us compare the resulting bytecode of kotlin and java compilation import java.util.Arrays; class Capturing { public static void main(final String... args) { run(new Runnable() { @Override public void run() { System.out.println("Hey! " + Arrays.toString(args)); } }); } private static void run(Runnable run) { run.run(); } } $ javac Capturing.java $ javap -p Capturing Compiled from "Capturing.java" class Capturing { Capturing(); public static void main(java.lang.String...); private static void run(java.lang.Runnable); }
  26. 26. What is KOTLIN :)? RUSSIAN WARSHIP ISLAND source: https://en.wikipedia.org/wiki/Kotlin CITY IN POLAND KETCHUP BRANDPROGRAMMING LANGUAGE
  27. 27. Comparison KOTLIN WORSE THAN SCALA: BETTER THAN SCALA: ● no static members (BY DESIGN) - if you need something that is not attached to an instance of any class, you define it in a package ● no checked exceptions (BY DESIGN) no functions like: void copy(CharSequence csq) throws IOException ● primitive types that are not classes ● non-private fields (i.e. java’s public int field; ) - by design - in kotlin one should use properties instead of public fields IN JAVA NOT IN KOTLIN: ● Overridable type members ● Path-dependent types ● Macros ● Existential types ● Complicated logic for initialization of traits ● Custom symbolic operations ● Built-in XML ● Parallel collections ● Structural types ● Value types ● Yield operator ● Actors ● smart-casts ● first-class delegation (built-in delegate pattern) ● no overhead in extension functions (compared to implicit class in Scala [link]) ● no overhead in null-safety
  28. 28. Resources RESOURCES: ● http://kotlinlang.org/ ● Jake Wharton - Using Project Kotlin for Android - https://t.co/9t8qsBGPlo ● Mike Gouline - Kotlin the Swift of Android - - https://blog.gouline.net/2014/08/31/kotlin-the-swift-of-android/ ● Paweł Gajda - Kotlin Android - http://slides.com/pawegio/kotlin-android, https://github.com/pawegio/KAndroid ● Jetbrains Kotlin Anko - https://github.com/Kotlin/anko ● Amit Phaltankar Java 8 streams - https://dzone.com/articles/understanding-java-8-streams-1 ● Salomon Brys - https://github.com/SalomonBrys ● Gautier Mechling - http://nilhcem.github.io/swift-is-like-kotlin/
  29. 29. Thankyou! Bartosz Kosarzycki bartosz.kosarzycki@stxnext.pl Twitter: @bkosarzycki Online compiler: http://try.kotlinlang.org/

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