Kotlin lang - basics(Android projects)

Bartosz Kosarzycki - StxNext Lightning Talks - Feb 12, 2016

talented developers | flexible teams | agile experts

KOTLINhttp://kotlinlang.org/

Required knowledge:● basic Android development skills● functional programming● familiarity with JDK 6,7,8● Scala is a plus

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

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

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

Nullpointer

safetyvar output : String? output = nullprintln(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 = nullprintln(output?.length)

val len = output?.length ?: -1 //elvis operatorprintln(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.UnitIf a function does not return any useful value, its return type is Unit

Advantagesval 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:

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

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)

Classdelegation

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();

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())

Utility classes

UTILS:StringUtilActivityUtilListUtil

JAVA utility classpublic class StringUtils { public static String encodeString(String str) { return str.replaceAll(" ", "_"); }}

KOTLIN utility classfun 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

+ KOTLIN

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'

Cost

Reference application:

one blank Activity app generated with Android Studio

Higher-order

functionsIntroduction//extension functionfun Date.isTuesday() = day == 2

//function expression as contantval addition = { x: Int, y:Int -> x + y }

//higher order functionfun higherOrder(x : Int, y: Int, func : (x : Int, y : Int) -> Int ) : Int { return func.invoke(x, y); }

Higher-order extension functionfun 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);

Android

CompactnessmDescriptionTextView.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() } }

Lambda expression to the rescue

● simplify development● remove boilerplate● improve multi-threading

Auto getters&setters

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}

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

Dagger 2

& KOTLIN

@Moduleclass 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 KOTLINsince 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() }}

Kotlin Ankooverride 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}!") } }}

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

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

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

SDKMANor homebrew

LinuxFreeBSDCygwin

SDKMAN

Command line

compilerKotlin:

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);}

What is KOTLIN :)?

RUSSIAN WARSHIP

ISLAND

source:https://en.wikipedia.org/wiki/Kotlin

CITY IN POLAND

KETCHUP BRANDPROGRAMMING LANGUAGE

Comparison KOTLINWORSE 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

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/

Thank you!

Bartosz Kosarzyckibartosz.kosarzycki@stxnext.plTwitter: @bkosarzycki

Online compiler:

http://try.kotlinlang.org/