ADG Poznań - Kotlin for Android developers

Kotlin
for Android developers
Bartosz Kosarzycki - Apr 07, 2016

Kotlin for Android developers
❏ What is Kotlin?
❏ Why Kotlin?
❏ JAVA 8 JACK compiler
❏ Advantages
❏ Nullpointer safety
❏ Advantages
❏ Data objects
❏ Traits**
❏ Delegation pattern
❏ Properties
❏ Utility classes
❏ Project structure in Android
❏ Cost of Kotlin in Android
❏ Higher-order functions
❏ Lambda & getters and setters
❏ Kotterknife
❏ Dagger 2
❏ Kotlin Anko
❏ Sample Activity in Kotlin
❏ SWIFT & KOTLIN comparison
❏ Command line compiler
AGENDA
❏ Language name origins
❏ Live templates
❏ Enum translation
❏ Calling extension functions from Kotlin/Java
❏ Constructors with backing fields
❏ Warnings
❏ Annotation processing - KAPT
❏ SAM conversions
❏ Type equality
❏ Lambda vs Closure
❏ Reified generics
❏ Fluent interfaces
❏ Infix notation
❏ Static extension methods in Kotlin
❏ Generic types
❏ Dokka - documentation in Kotlin
❏ J2K converter
❏ Real-world example

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

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
** JACK compiler is in beta (Android N)

JAVA 8 JACK
● Default and static
interface methods
● Lambda expressions
● Repeatable annotations
** JACK compiler is in beta (Android N)
@Schedule(dayOfMonth= "last")
@Schedule(dayOfWeek= "Fri", hour="23")
public void doPeriodicCleanup () { ... }
Repeatable annotations
printPersons(
roster,
(Person p) ->
p.getGender() == Person.Sex.MALE
&& p.getAge() >= 18
&& p.getAge() <= 25
);
Lambda expressions
public interface Java8DefaultInterface {
default void closeSession() {
Log.i("TAG", "Session closed");
}
}
Default interface methods

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");
}
}
**JAVA 8 (JACK - Android)

Delegation
pattern
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; //helper object
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();
In Delagation Pattern an object instead of performing one of its stated
tasks, delegates that task to an associated helper object. There is an
Inversion of Responsibility.

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

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

Language
name origins
RUSSIAN WARSHIP
ISLAND
source:
https://en.wikipedia.org/wiki/Kotlin
CITY IN POLAND
KETCHUP BRANDPROGRAMMING
LANGUAGE

Live templates
exfun
fun Any.f(): Unit {
}
closure
{ x -> x }
iter
for (i in iterable) {
}
● let’s start with something
simple - templates

Live templates
exvar
var Any.v: Any
get() {
}
set(value) {
}
main
fun main(args: Array<String>) {
}
inn
if (i != null) {
}

Enum translation
Kotlin
enum class SliderActivityType
private constructor(val title: Int) {
PORTFOLIO(R.string.portfolio),
TEAM(R.string.team)
}
Java
public enum SliderActivityType {
PORTFOLIO(R.string.portfolio),
TEAM(R.string.team);
private final int title;
SliderActivityType(int title) {
this.title = title;
}
public int getTitle() {
return title;
}
}
● easier to read
● more concise
● help to avoid typos and boilerplate code
In Kotlin translated enums are:

Calling extension
functions
Get app-version ext. function:
@file:JvmName("ActivityUtil") //@file:JvmMultifileClass
package com.stxnext.stxinsider.util
fun Activity.getAppVersion(activity: Activity): String {
try {
val manager = activity.packageManager
val info = manager.getPackageInfo(activity.packageName, 0).versionName
} catch (e: PackageManager.NameNotFoundException) { /* ignore */ }
return "0.0.0"
}
Kotlin call:
versionTextView.setText(getAppVersion(this))
Java call:
versionTextView.setText(ActivityUtil.getAppVersion(MainActivity.this,
MainActivity.this));

Constructors with
backing fields
● using constructors with backing fields in
a proper way saves a lot of boiler-plate
code
“Java-style”
kotlin code:
class TeamCategoryFragment : Fragment() {
internal val TAG = TeamCategoryFragment::class.simpleName
lateinit var teamCategoryHeader: TeamCategoryHeader
lateinit var teamListRecyclerView: RecyclerView
fun teamCategoryHeader (teamCategoryHeader: TeamCategoryHeader): TeamCategoryFragment {
this.teamCategoryHeader = teamCategoryHeader
return this
}
override fun onCreateView(): View? {
val view = inflater!!.inflate(R.layout.fragment_layout, container, false)
teamListRecyclerView = view.findViewById(R.id.fragment_list) as RecyclerView
return view;
}
}
class TeamCategoryFragment (var teamCategoryHeader: TeamCategoryHeader) : Fragment() {
internal val TAG = TeamCategoryFragment::class.simpleName
lateinit var teamListRecyclerView: RecyclerView
override fun onCreateView(): View? {
val view = inflater!!.inflate(R.layout.fragment_layout, container, false)
teamListRecyclerView = view.findViewById(R.id.fragment_list) as RecyclerView
return view;
}
}
Contructor
with backing
field

Warnings
Checks:
Full list of supression contants:
https://github.com/JetBrains/kotlin/blob/master/compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java
@Suppress("UNCHECKED_CAST")
@Suppress("CANNOT_CHECK_FOR_ERASED")
@Suppress("SENSELESS_COMPARISON")
@Suppress("GENERIC_THROWABLE_SUBCLASS")
etc.

Annotation
processing
Implementation state:
● old implementation: kapt worked by intercepting
communication between annotation processors and javac,
and added already-compiled Kotlin classes on top of the
Java classes
● new implementation of KAPT: generates stubs of Kotlin classes
before running javac and llows the usage of APT based
libraries we already have at our current java stack
KAPT
kapt {
generateStubs = true
}
dependencies {
kapt 'com.google.dagger:dagger-compiler:2.0.2'
}
Example config:
● JSR 269 Annotation
Processing available in Kotlin
since M12
● Dagger 2 works :)
● DBFlow works

SAM
conversions
SAM call:
● function literals can be converted into implementations
of Java interfaces with a single non-default method
● the parameter types of the interface method must match
the parameter types of the Kotlin function
Lambda call:
mInsiderApiService.getTeamsAsync({ list ->
list.forEach { item -> print(item.description) } },
{ /* do nothing on error */ } )
versionTextView.setOnClickListener(
View.OnClickListener { print("Message content") } )
executor.execute(Runnable {
println("This runs in a thread pool") })
mInsiderApiService.getTeamsAsync(
object : Callback<List<SliderItem>> {
override fun onResponse(
p0: Call<List<SliderItem>>?,
response: Response<List<SliderItem>>?) {
/* something */ }
override fun onFailure(
p0: Call<List<SliderItem>>?,
p1: Throwable?) { /* something */ }
})
Java interface call:

Type equality
Checking type equality
BaseClass
TallItemView
if (this instanceof TallItemView) { .... }
// instanceof makes it very easy to be asymmetric
if (this.getClass()
.equals(TallItemView.class) ) { .... }
Java
if (this.javaClass
.isAssignableFrom(TallItemView::class.java) )
Kotlin
Referential equality
referential equality is checked by the ===
operation
Structural equality
structural equality is checked by the ==
operation
== is translated to: a?.equals(b) ?: (b === null)
a == null is translated to: a === null
if a is not null, it calls the equals(Any?)
function, otherwise b === null

Lambda vs
Closure
LAMBDA CLOSURE
● language construct
● a syntax for
anonymous function
● can be assigned to a
variable
● “closes over” the
environment in which
it was defined
● lambda which
references fields
external to its body
● function which is
evaluated in its own
environment

Reified generics
● in Java generic type parameters are not
reified: they are not available at runtime
● for inline functions
(inserting the function code at the address of each function call)
● safe casting of generic types
● problem comes from type-erasure
class MyClass<T> {
private final T o;
public MyClass() {
this.o = new T(); //type parameter ‘T’ cannot be instantiated directly
}
}
Example (Java)
public class MyClass2<T> {
@SuppressWarnings("unchecked")
public T doSomething() {
return (T) new MyClass(); //unchecked cast
}
}
class MyClass2 {
inline fun <reified T> doSomething() : T {
return MyClass() as T;
}
}
class MyClass
Kotlin

Fluent interfaces
Kotlin:
https://plugins.jetbrains.com/plugin/7903
Fluent setter generator plugin for Android Studio:
(JAVA)
public SampleActivity firstVariable(int firstVariable) {
this.firstVariable = firstVariable;
return this;
}
public SampleActivity secondVariable(int secondVariable) {
this.secondVariable = secondVariable;
return this;
}
● almost like an internal DSL
● ideal for filtering, creating,
customizing etc.
● used for model classes
Java:
Snakbar snack = Snackbar
.make(mainView , "Sample snackbar" , Snackbar.LENGTH_LONG)
.setAction( "Undo", undoClickListener) ;
Fluent interface example:
class Person(val name: String) {
val parents : List<String> = arrayListOf()
constructor(name: String, parent: String)
: this(name) {
parents.plus(parent)
}
fun parent(parent: String): Person {
parents.plus(parent); return this
}
}

Fluent interfaces /**
* Fluent sort
*/
fun <T : kotlin.Comparable<T>> kotlin.collections.MutableList<T>.
sortList(): MutableList<T> {
sort()
return this
}
/**
* For-each fluent interface
*/
fun <T : kotlin.Comparable<T>> kotlin.collections.MutableList<T>.
forEachList(action: (T) -> kotlin.Unit): MutableList<T> {
for (elem in this)
action.invoke(elem)
return this
}
Additional functions:
Fluent lists example:
val list = listOf(1, 2, 3, 4, 5, 6, 7, 8)
list .forEach { println(it) }
list forEachLoop { println(it) }
/**
* In-place forEach loop (discouraged in 1.0 release)
*/
infix fun <T> kotlin.collections.Iterable<T>
.forEachLoop(action: (T) -> kotlin.Unit): kotlin.Unit {
this.forEach { action }
}
val outList = list
.filter { it < 100 }
.filterNot { it == 1 }
.toMutableList()
.sortList()
.forEachList { it + 1 }
.filter { it % 2 == 0 }
.first()
//result: 2

Infix notation
infix fun Int.minus(x: Int): Int {
return this.minus(x)
}
infix extension function:
val result = 1 minus 2
println(3 minus 4)
type isPortfolio { println("Do something") }
type isTeam { println("Do something else") }
infix fun SliderActivityType.isPortfolio( execClosure : () -> Unit ) {
if (this.equals(SliderActivityType.PORTFOLIO))
execClosure.invoke()
}
infix fun SliderActivityType.isTeam( execClosure : () -> Unit ) {
if (this.equals(SliderActivityType.TEAM))
execClosure.invoke()
}
this displayToast "This is a message"
infix fun Activity.displayToast(txt : String) {
Toast.makeText(this, txt, Toast.LENGTH_SHORT).show()
}
● only one method
argument
● function literal can be
passed outside the
parentheses

Infix notation
if (this isGranted Manifest.permission.READ_CONTACTS) {
//do something here
}
infix fun Activity.isGranted(permissionStr : String) :
Boolean {
if (ContextCompat.checkSelfPermission(this,
permissionStr) !=
PackageManager.PERMISSION_GRANTED)
return false
return true
}
Handle Android
permissions check:
this loge "I really don't like errors"
infix fun Activity.loge(txt : String) {
Log.e(this.javaClass.simpleName, txt)
}
Log errors:

Generic types
Create generic object instance:
● in JVM generic types are lost due to type erasure
class MyClass<T> {
private final T o;
public MyClass() {
this.o = new T(); //type parameter ‘T’ cannot be instantiated directly
}
}

Generic types
val bindFunc = { baseView: FrameLayout , item: ListItem , position: Int , clickListener: View.OnClickListener ->
val nameTextView = baseView.findViewById(R.id. item_simple_list_main_header) as TextView
nameTextView. text = item.title
baseView.setOnClickListener(clickListener)
}
val adapter = SimpleItemListAdapter<ListItem , ListItemView<ListItem>>(onClickFunc ,
{ ListItemView <ListItem> (R.layout. item_simple_list, bindFunc , baseContext , null /* attrs */ ) } );
Create generic object instance:
● in JVM generic types are lost due to type erasure
override fun onCreateItemView(parent: ViewGroup, viewType: Int): TView {
val view = factory()
return view as TView
}
//not really convenient
override fun onCreateItemView(parent: ViewGroup, viewType: Int, classParam : Class<T>): T {
val v: T = classParam.constructors[0].newInstance(mContext, null) as T
return v as T
}

Click
annotations
KClick:
init { R.id.sendButton bind KClick(this, { v: View -> onSendButtonClick(v) }) }
fun onSendButtonClick(v: View) {
[...]
}
● custom function written in
Kotlin
● uses infix notation and
extension functions
Bind button click using DSL:
/** init {
* R.id.loginButton bind KClick(this, { v: View -> onLoginButtonClick(v) })
* R.id.logoutButton bind KClick(this, { v: View -> onLogoutButtonClick(v) })
* }
*/
infix fun Int.bind(kClick: KClick): kotlin.Unit {
var kViewEntries = Util.kViewsMap[(kClick.activity.javaClass.name)]
if (kViewEntries == null)
kViewEntries = mutableListOf()
kViewEntries.add(KViewEntry(this, { v: View -> kClick.action.invoke(v) }) )
Util.kViewsMap.put(kClick.activity.javaClass.name, kViewEntries)
}
data class KClick(val activity: Activity, val action: (View) -> kotlin.Unit)
data class KViewEntry(val id: Int, val action: (View) -> kotlin.Unit)
fun Activity.bindKViews() {
for (mutEntry in Util.kViewsMap)
if (mutEntry.key == this.javaClass.name) {
val kViewEntries = mutEntry.value;
for (kViewEntry in kViewEntries)
findViewById(kViewEntry.id).setOnClickListener { v: View -> kViewEntry.action.invoke(v) }
Util.kViewsMap.remove(mutEntry.key)
}
}
bindKViews()
[...]
}

Dokka
What is dokka:
KDoc documentation:
● similar do Javadoc
● supports stale Javadoc out of the box
● markdown support included
/**
* # Beacon SDK initialization
*
* This method registers 3 beacons with IDs taken from Estimote Cloud.
* Invoke this method in [onCreate].
*
* ## Showcase demo
*
* Steps:
* * Grant application bluetooth, GPS and WiFi permissions
* * Wait for about 1 minute (beacons broadcast in 0.1 ~ 2.0 sec intervals)
* * Snackbar should appear on the app's main activity
*
*/
private fun initializeNearables() { ….. }
● generates documentation in
html/markdown/javadoc
● maintained by Jetbrains
● generated from
gradle/maven/ant
● standalone executable jar
available
● [ref] instead of @see ref
● https://github.com/Kotlin/dokka

Dokka
Standalone jar:
● java -jar dokka-fatjar.jar ./src/
buildscript {
dependencies {
classpath "org.jetbrains.dokka:dokka-gradle-plugin:0.9.7"
}
}
sourceSets {
main.java.srcDirs += 'src/main/kotlin'
}
dokka {
outputFormat = 'html' //'markdown', 'javadoc'
outputDirectory = "$buildDir/kotlindocs"
}
Dokka gradle plugin:
HTML output with css styles:

J2K converter
Simple Activity
conversion:
Call Java 2 Kotlin converter:
● There is no Kotlin 2 Java as for now
public class SampleActivity extends Activity {
@Override
protected void onCreate(Bundle
savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}
class SampleActivity : Activity() {
setContentView(R.layout.activity_main)
}
}

Real-world
example
Method count:
Library Method count
cardview-v7/23.1.0 872
play-services-maps 1369
play-services-base 700
okhttp 1463
dagger-compiler 1582
kotlin-stdlib 2411
kotlin-runtime 705
support-vector-drawable 635
butterknife 635
okio 712
(617 after proguarding)
LoC: 1857 kotlin + 503 java lines
Compilation time (debug): 45.584 sec (6 sec incremental)
Compilation time (release): 44.142 sec
(proguard + zipaligning)
Intel® Core™ i5-3470 CPU @ 3.20GHz × 4
8 GB RAM 1333 MHz, Ubuntu 15.10, Linux kernel 4.4.2-040402-generic

Real-world
example
Method count:
Library Method count
joda-time 1707
converter-gson 236
com.google.android.gms 1912
kotterknife 456
com.google.dagger 290
retrofit-2.0.0-beta2 593
com.android.support/design 1017
com.android.support/recyclerview-v7 1579
LoC: 1857 kotlin + 503 java lines
Compilation time (debug): 47.135 sec (6 sec incremental)
Compilation time (release): 53.173 sec
(proguard + zipaligning)
Mac Mini late 2014, SSD 256 GB drive USB-3.0
2.6 GHz Intel Core i5
8 GB 1600 MHz DDR3, OsX El Capitan 10.11.2 (15C50)
Try it yourself - STXInsider example project:
https://github.com/kosiara/stx-insider

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/

Resources
RESOURCES:
● https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)
● https://kotlinlang.org/docs/reference/generics.html
● http://www.cs.cornell.edu/~ross/publications/mixedsite/mixedsite-tate-fool13.pdf
● http://blog.jetbrains.com/kotlin/2015/06/better-annotation-processing-supporting-stubs-in-kapt/
● https://yanniss.github.io/varj-ecoop12.pdf
● https://schneide.wordpress.com/2015/05/11/declaration-site-and-use-site-variance-explained/
● http://stackoverflow.com/questions/4231305/how-does-javas-use-site-variance-compare-to-cs-declaration-site-variance
● http://www.cs.cornell.edu/~ross/publications/tamewild/
● http://stackoverflow.com/questions/26507099/lambda-expressions-in-kotlin
● http://gafter.blogspot.com/2006/11/reified-generics-for-java.html
● http://stackoverflow.com/questions/1927789/why-should-i-care-that-java-doesnt-have-reified-generics
● https://github.com/KeepSafe/dexcount-gradle-plugin
● http://blog.jetbrains.com/kotlin/2015/09/kotlin-m13-is-out/
● http://blog.jetbrains.com/kotlin/2011/10/dsls-in-kotlin-part-1-whats-in-the-toolbox-builders/
● http://stackoverflow.com/questions/28210188/static-extension-methods-in-kotlin
● http://antonioleiva.com/collection-operations-kotlin/

Thankyou!
Bartosz Kosarzycki
bartosz.kosarzycki@stxnext.pl
Twitter: @bkosarzycki
Online compiler:
http://try.kotlinlang.org/
STXInsider example project in Kotlin:
https://github.com/kosiara/stx-insider

ADG Poznań - Kotlin for Android developers

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