Kotlin Developer Starter in Android projects

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
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

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:

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)

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();

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:
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

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
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);

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

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
@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()
}
}

Kotlin Anko
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
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
SDKMAN
or homebrew
Linux
FreeBSD
Cygwin
SDKMAN

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);
}

What is
KOTLIN :)?
RUSSIAN WARSHIP
ISLAND
source:
https://en.wikipedia.org/wiki/Kotlin
CITY IN POLAND
KETCHUP BRANDPROGRAMMING
LANGUAGE

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

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/

Thankyou!
Bartosz Kosarzycki
bartosz.kosarzycki@stxnext.pl
Twitter: @bkosarzycki
Online compiler:
http://try.kotlinlang.org/

Kotlin Developer Starter in Android projects

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