Kotlin is a new language for the JVM that aims to be a "better Java". Made in-house by Jetbrains, the company behind IntelliJ IDEA and also Android Studio, it's been in development for more than 5 years. Just a few weeks ago the final version of Kotlin 1.0 saw the light of day.
This talk will start with a brief introduction into Kotlin and its core language features. After this brief foray into concepts like Kotlin's immutable variables, null behaviour and other smarts like the syntactic sugar it provides for dealing with types and properties you'll discover what's in store for Android developers.
Particularly noteworthy are the decrease in annoying Java boilerplate code, fewer of the irritating "Process has stopped unexpectedly" messages and the easily available Kotlin Android Extensions. You will also explore the toolchain and compiler tools Kotlin provides for Android development, the differences to using Kotlin for Java development as well as the integration into Android Studio.
4. WHAT IS KOTLIN?
SOME FUNDAMENTALS
▸ Statically typed programming
language for the JVM and Android
▸ Started as internal language “Project
Kotlin” at Jetbrains in 2010
▸ Now: Open-Source, Apache License
▸ Kotlin SDK plus tool support for IntelliJ,
Android Studio, Eclipse
▸ Named after an island in the Gulf of
Finland
5. WHAT IS KOTLIN?
MOTIVATION FOR KOTLIN
▸ The Java platform is awesome, but it has its issues:
▸ sometimes tied to backwards/legacy compatibility
▸ can be a very verbose language and produce bloated code
▸ type system has various flaws
▸ Kotlin aims to fix a lot of those issues
▸ compiles to Java 6 byte code therefore caters well for Android runtimes
6. WHAT IS KOTLIN?
HOW DOES A SIMPLE CONVERSION LOOK LIKE?
public String listConvert(Collection<Integer>
collection) {
StringBuilder sb = new StringBuilder();
sb.append("{");
Iterator<Integer> iterator =
collection.iterator();
while (iterator.hasNext()) {
Integer element = iterator.next();
sb.append(element);
if (iterator.hasNext()) {
sb.append(", ");
}
}
sb.append("}");
return sb.toString();
}
fun listConvert(collection: Collection<Int>): String {
val sb = StringBuilder()
sb.append("{")
val iterator = collection.iterator()
while (iterator.hasNext()) {
val element = iterator.next()
sb.append(element)
if (iterator.hasNext()) {
sb.append(", ")
}
}
sb.append("}")
return sb.toString()
}
fun listConvertKt(collection: Collection<Int>): String {
return collection.joinToString(prefix = "{",postfix = "}")
}
8. LANGUAGE IDIOMS & CONCEPTS
OVERVIEW
▸ Immutability
▸ String templates & Enum classes
▸ Null safety
▸ Properties and Fields
▸ Type inference and casts
▸ Data classes
▸ Syntactic sugar (loops, ranges etc)
▸ Extension functions
▸ Lambdas
▸ Collection API
▸ Type-safe builders
▸ Java-Kotlin-Interop
9. LANGUAGE IDIOMS & CONCEPTS
IMMUTABILITY
▸ Built-in support for mutable and
immutable variables, properties and
fields
▸ Keywords var and val
▸ val - immutable (recommended)
▸ var - mutable
▸ Similar concept applies for class
properties, val creates getters, var
creates getters and setters (more later)
val a: Int = 1
val b = 1
val c: Int
c = 1
var x = 23
x += 1
10. LANGUAGE IDIOMS & CONCEPTS
STRING TEMPLATES & ENUM CLASSES (I)
▸ Kotlin Strings can contain template
expressions
▸ String templates start with a $ character
and
▸ can contain simple references such
as $s, as well as
▸ complex expressions in curly braces:
${s.length}
val s = "abc"
val str = "$s.length is ${s.length}"
Output:
abc.length is 3
11. LANGUAGE IDIOMS & CONCEPTS
STRING TEMPLATES & ENUM CLASSES (II)
▸ Kotlin has a dedicated and expressive
enum class
▸ Can be used very nicely in conjunction
with String templates
enum class Locale(val hello: String) {
DE_DE("Hallo"),
EN_NZ("Hello"),
MI_NZ("Kia Ora")
}
class Customer(val firstName:String,
val lastName:String,
val locale: Locale = Locale.DE_DE) {
fun sayHello() = println(
"${locale.hello}, $firstName $lastName")
}
fun main(args : Array<String>) {
val myCustomer = Customer("Sandra",
"Musterfrau",
Locale.MI_NZ)
myCustomer.sayHello()
}
12. LANGUAGE IDIOMS & CONCEPTS
NULL SAFETY
▸ Motivation: A better way to deal with NPEs
▸ Kotlin differentiates nullable types from non-
nullable types by adding a ? to the type:
▸ String: no nullable
▸ String?: nullable
▸ Handle manually
▸ Use Safe Call operator ?.
▸ Use the !! operator to allow/trigger a NPE
// Won't compile
var lastName: String = null
// Will compile
var lastNameNullable: String? = null
// Will also not compile
println(lastNameNullable.length)
// Option 1 (-1)
println(if (lastNameNullable != null)
lastNameNullable.length else -1)
// Option 2 (null)
println(lastNameNullable?.length)
// Option 3 (NPE)
println(lastNameNullable!!.length)
13. LANGUAGE IDIOMS & CONCEPTS
PROPERTIES AND FIELDS
▸ Kotlin classes have mutable or
immutable properties
▸ Default getter/setters for properties,
can be customised
▸ An automated backing field can be
provided by the compiler (if required)
▸ Alternative: use an explicit backing
property
var counter = 0
set(value) {
if (value >= 0)
field = value
}
public class Address {
public var name: String = ...
public var street: String = ...
public var city: String = ...
public var state: String? = ...
public var zip: String = ...
}
14. LANGUAGE IDIOMS & CONCEPTS
TYPE INFERENCE AND CASTS (I)
▸ When possible, Kotlin will infer the type
of variables
▸ Explicit conversions, type widening and
inference
▸ Smaller types are not subtypes of
bigger types, no implicit conversion
▸ Types are often inferred from the
context
val b: Byte = 1
// This won't work
val i: Int = b
// This will
val i: Int = b.toInt()
val l: Long = 1L + 3
15. LANGUAGE IDIOMS & CONCEPTS
TYPE INFERENCE AND CASTS (II)
▸ is or !is checks if an object adheres to a
certain type
▸ Smart cast: Compiler tracks is-expressions
for immutable values
▸ works for val local variables and private,
internal or in-module performed casts
▸ works for var local variables if the
variable hasn’t been modified between
check and usage, never for var
properties
fun whatIs(x: Any) {
when (x) {
is Int -> println(x + 42)
is String -> println(x.length)
is IntArray -> println(x.sum())
}
}
whatIs(4) // 46
whatIs("4") // 1
whatIs(intArrayOf(1,2,3,4,5)) // 15
16. LANGUAGE IDIOMS & CONCEPTS
DATA CLASSES
▸ The POJOs/Beans of other languages
▸ Data classes implicitly create:
▸ getters/setters (non-data classes
have those too) - recommend to use
val as often as possible.
▸ useful implementations for equals(),
hashCode(), toString(), copy()
▸ copy() has default parameters and
can be used to alter a copy
data class ChromeEncryptedPayload(
val encryptedPayload: String,
val encryptionHeader: String,
val cryptoKeyHeader: String)
val pl = ChromeEncryptedPayload(
encryptedPayload = "...",
encryptionHeader = "...",
cryptoKeyHeader = "...")
val anotherPl = pl.copy(
encryptedPayload = "...")
17. LANGUAGE IDIOMS & CONCEPTS
EXTENSION FUNCTIONS
▸ Allow adding new functionality to a
class without inheritance or Decorators
▸ Kotlin has extension functions as well
as extension properties
▸ Resolved statically, don’t actually
modify the class (excellent example
why this has to be the case at http://
goo.gl/EN6bTs
fun Int.sum(otherInt: Int): Int =
this + otherInt
3.sum(7)
fun Activity.toast(message: CharSequence,
duration: Int =
TOAST.LENGTH_SHORT) {
Toast.makeText(this, message,
duration).show()
}
// In onCreate of an Activity
override fun onCreate(...) {
...
toast("Hi there")
...
}
18. LANGUAGE IDIOMS & CONCEPTS
LAMBDAS
▸ Anonymous function or “function
literal”, closely related to the ideas of
Higher-Order-Functions
val sum = { x: Int, y: Int -> x + y }
val sum: (Int, Int) -> Int = { x, y ->
x + y }
private fun convertPetListToDomain(list: List<Pet>): List<PetDomain> {
return list.map { convertPetItemToDomain(it) }
}
private fun convertPetItemToDomain(pet: Pet): PetDomain {
return PetDomain(pet.age,pet.size,pet.id,pet.name,pet.sex,pet.breed)
}
19. LANGUAGE IDIOMS & CONCEPTS
TYPE-SAFE BUILDERS (I)
▸ Builders are a very popular approach in
Groovy to define data in a declarative
way
▸ Often used for:
▸ generating XML or JSON
▸ UI layout (Swing components) etc
▸ In Kotlin, builders even can be type-
checked
JsonBuilder builder = new JsonBuilder()
builder.records {
car {
name 'HSV Maloo'
make 'Holden'
year 2006
country 'Australia'
}
}
String json = JsonOutput.prettyPrint
(builder.toString())
20. LANGUAGE IDIOMS & CONCEPTS
TYPE-SAFE BUILDERS (II)
▸ html() is a function with a lambda as an
argument (init)
▸ init’s type is a function type with
receiver, this allows you to:
▸ pass receiver (of type HTML) to
function
▸ call members of instance inside the
function
fun result(args: Array<String>) =
html {
head {
title {”HTML in Kotlin"}
}
body {
...
}
}
fun html(init: HTML.() -> Unit): HTML {
val html = HTML()
html.init()
return html
}
21. LANGUAGE IDIOMS & CONCEPTS
TYPE-SAFE BUILDERS (III)
▸ HTML class has functions to build the
head and the body elements of the
DSL.
▸ Not shown: classes further down in the
hierarchy:
▸ Head, Body etc.
▸ Complete HTML builder example at:
http://goo.gl/TndcC9
class HTML {
...
fun head(headBuilder: Head.() -> Unit) {
head = Head()
head?.headBuilder()
}
fun body(bodyBuilder: Body.() -> Unit) {
body = Body()
body?.bodyBuilder()
}
}
22. LANGUAGE IDIOMS & CONCEPTS
JAVA-KOTLIN-INTEROP
▸ Java and Kotlin are fully interoperable from an integration point of view
▸ Your Java code can call and use Kotlin code
▸ Your Kotlin code can call and use Java code
▸ The latter is in particular useful because it means you can continue to use pretty
much any existing Android/Java library
▸ Check out Hendrik Kokocinski’s sample Kotlin app that uses all kinds of well
known Android/Java libs: https://goo.gl/xdYqf5
23. LANGUAGE IDIOMS & CONCEPTS
OVERVIEW
▸ Immutability
▸ String templates & Enum classes
▸ Null safety
▸ Properties and Fields
▸ Type inference and casts
▸ Data classes
▸ Syntactic sugar (loops, ranges etc)
▸ Extension functions
▸ Lambdas
▸ Collection API
▸ Type-safe builders
▸ Java-Kotlin-Interop
26. KOTLIN AND ANDROID
PROJECT SETUP
▸ Use Android Studio 2.x or IntelliJ 2016
▸ Install Kotlin plugin (comes within “Jetbrains plugins”)
▸ Gradle dependencies project-level:
▸ classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.0.4"
▸ Gradle dependencies module-level:
▸ compile 'org.jetbrains.kotlin:kotlin-stdlib:1.0.4'
▸ apply plugin: 'kotlin-android'
▸ main.java.srcDirs += 'src/main/kotlin'
27. KOTLIN AND ANDROID
KOTLIN EXTENSIONS FOR ANDROID
▸ Provides of a set of synthetic properties that bind views to those properties
▸ Alternative to Butter Knife-style bindings, no need for additional runtime
library (Kotlin Extensions for Android are a Kotlin compiler plugin)
▸ import kotlinx.android.synthetic.main.<layout>.*
▸ import kotlinx.android.synthetic.main.<layout>
▸ usage: <componentid>.doSomething()
30. ANKO DSL
WHAT IS A DSL?
▸ Domain-Specific Language
▸ Limited expressiveness:
▸ DSLs are usually not general-purpose languages
▸ strongly focussed on a particular domain
▸ examples: SQL, Ant XML, XSLT, Gradle etc.
31. ANKO DSL
A DSL FOR LAYOUTS
▸ The most important element of Anko is the Layout DSL
▸ Idea: replace XML layout definitions by Kotlin code - without having to build
the layout in a fully programmatic sense
▸ Anko DSL is modular and extensible
33. ANKO DSL
PROGRAMMATIC LAYOUT IN KOTLIN
val act = this
val layout = LinearLayout(act)
layout.orientation = LinearLayout.VERTICAL
val name = EditText(act)
val button = Button(act)
button.text = "Say Hello"
button.setOnClickListener {
Toast.makeText(act, "Hello, ${name.text}!", Toast.LENGTH_SHORT).show()
}
layout.addView(name)
layout.addView(button)
35. ANKO DSL
ANKO DSL INTERNALS
▸ Anko DSL example from previous slide looks very similar to the earlier HTML
builder example
▸ Anko uses extension functions arranged into type-safe builders and lambdas
▸ You don’t have to write all those extensions by hand though - Anko generates
them based on the Android SDK’s android.jar
36. ANKO DSL
GETTING STARTED WITH ANKO DSL (I)
▸ Depending on minSdkVersion of project, import:
compile "org.jetbrains.anko:anko-sdk{15|19|21|23}:0.9"
▸ If the project uses an Android Support library, import matching Anko library:
compile "org.jetbrains.anko:anko-recyclerview-v7:0.9"
37. ANKO DSL
GETTING STARTED WITH ANKO DSL (II)
▸ General approach:
▸ Anko DSL automatically becomes
available in onCreate() in an Activity
▸ no need for setContentView(), Anko
DSL also automatically sets the
content view for activities
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
}
}
}
38. ANKO DSL
ANKO DSL IN FRAGMENTS
▸ In fragments, use
onCreateView(…)
▸ Returns UI {…}.view:
▸ mandatory before Anko 0.8
▸ works well for Fragments
▸ createTodo() is a fragment-private
method
override fun onCreateView(inflater: LayoutInflater?,
container: ViewGroup?,
savedInstanceState: Bundle?): View? {
return UI {
verticalLayout {
padding = dip(24)
var title = editText {
id = R.id.title
hintResource = R.string.hint
}
...
button {
id = R.id.add
textResource = R.string.add
onClick { view -> createTodo(title, desc) }
}
}
}.view
}
39. ANKO DSL
ANKO COMPONENTS
class EditFragmentUI : AnkoComponent<EditFragment> {
override fun createView(ui: AnkoContext<EditFragment>) = with(ui) {
verticalLayout {
padding = dip(24)
var title = editText {
id = R.id.title
hintResource = R.string.hint
}
...
button {
id = R.id.add
textResource = R.string.add
onClick { view -> ui.owner.createTodoFrom(title, desc) }
}
}
}
}
override fun onCreateView(...)= EditFragmentUI().createView(AnkoContext.create(ctx,this))
40. ANKO DSL
EXTENDING ANKOS DSL FUNCTIONALITY
▸ Multiple ways to do this:
▸ insert an existing XML layout into an Anko DSL declaration:
▸ adding new DSL elements to the language
include<View>(R.layout.mylayout) { ... }
inline fun ViewManager.customView(theme: Int = 0) = customView(theme) {}
inline fun ViewManager.customView(theme: Int = 0, init: CustomView.() -> Unit) =
ankoView({ CustomView(it) }, theme, init)
inline fun Activity.customView(theme: Int = 0) = customView(theme) {}
inline fun Activity.customView(theme: Int = 0, init: CustomView.() -> Unit) =
ankoView({ CustomView(it) }, theme, init)
41. ANKO DSL
THERE’S MORE
▸ Intent wrappers for various purposes: e.g. sendSMS(number, [text])
▸ Intent builders
▸ Service shortcuts
▸ Configuration qualifiers: configuration(screenSize = ScreenSize.LARGE,
orientation = Orientation.LANDSCAPE) { … }
▸ Asynchronous tasks
▸ SQLLite
44. LIBRARIES AND TOOLS
KOTTER KNIFE (AND BUTTER KNIFE)
▸ Kotter Knife provides view binding in a similar way to Butter Knife for Android/
Java
▸ Why *Knife over KAE?
▸ Porting code from Java/Butter Knife to Kotlin
▸ Features like listener binding and resources binding that don’t exist in KAE.
▸ Sample code for Butter Knife/Kotlin in the official Jetbrains Kotlin Examples repo
(https://goo.gl/rH7vBu)
45. LIBRARIES AND TOOLS
KOTTER KNIFE VS BUTTER KNIFE VS KAE (I)
findViewById(R.id.send).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Log.d("MainActivity", "onClick: send")
}
});
@OnClick(R.id.send)
void clickSend(View v) {
Log.d("MainActivity", "onClick: send")
}
46. LIBRARIES AND TOOLS
KOTTER KNIFE VS BUTTER KNIFE VS KAE (II)
findViewById(R.id.send).setOnClickListener { view -> Log.d("MainActivity",
"onClick: send") };
val btnSend: Button by bindView(R.id.send)
btnSend.setOnClickListener({ view -> Log.d("MainActivity", "onClick: send") })
import kotlinx.android.synthetic.activity_main.*
btn_send.setOnClickListener({ view -> Log.d("MainActivity", "onClick: send") })
47. LIBRARIES AND TOOLS
KANDROID
▸ View binding (again)
▸ TextWatcher
▸ SeekBar Extension
▸ SearchView Extension
▸ Shortcuts to system services
▸ Logging
▸ Dealing with Intents
▸ SDK Version API (from/to)
▸ Thread management
48. LIBRARIES AND TOOLS
KOVENANT
▸ Core: foundations of a Promise
framework
▸ Tasks & Callbacks
▸ Chaining (Then, ThenApply)
▸ Lazy Promises
▸ Cancelling and Voiding
▸ Combine: combines 2-20 promises
▸ Functional: adds map, bind and
apply to support more advanced
HOF constructs in Kovenant
▸ JVM: Executors and Throttles (thread
pools)
▸ Android: UI callbacks and interacting
with UI Thread
51. FINAL THOUGHTS
PERFORMANCE
▸ Runtime is pretty much on-par with Java
▸ Pre Kotlin 1.0.2: Build process was slower than a comparable app in Java -
mainly due to how the Kotlin compiler works (no partial builds/compilation)
▸ Kotlin libraries do add to the size of the application as well as to the method
count
▸ Kotlin runtime + stdlib are similar in method count to support-v4 or play-
services-base and add significantly less than Scala or Groovy
52. FINAL THOUGHTS
LANGUAGE AND MATURITY
▸ Kotlin 1.0 was a big step for the language
▸ Surprisingly mature for a 1.0 release (but 5+ years in the making), full of great
concepts and idioms, getting better with each release
▸ Anko is very stable given it’s still a 0.x release
▸ Anko makes UI declaration actually pleasurable but offers way more than only
a layout DSL
53. FINAL THOUGHTS
WHAT DID WE LEARN?
▸ What is Kotlin?
▸ Language idioms & concepts
▸ Kotlin and Android
▸ Anko DSL
▸ Libraries and Tools
▸ Final thoughts