Introduction to Android development in Kotlin based on real concrete examples. Also side to side comparisons with Java.

1. Delightul hacking
with Kotlin
Klemen Kresnik

2. developer @

3. Kotlin?

4. • Designed and developed by JetBrains
• Statically-typed programming language
• Compiles to JVM bytecode, runs on JVM
• v1.0 released on February 16, 2016
• Latest version 1.0.2 (Android Lint
Checks, method count reduced ~1500)

5. Main features

6. • Null safety
• Functions
• Sequences
• Extensions
• Data classes

7. Other sugars

8. • String templates
• ‘when' expression
• Smart casts
• Kotlin from Java - @JvmOverloads

9. Kotlin & Android

10. • Install Kotlin IDEA plugin
• Enable Kotlin Android plugin
• Enable Kotlin Android Extensions
plugin
• Add stdlib dependency

11. Setup with AS
Install IDEA Kotlin plugin

12. Add Kotlin plugin to build.gradle
buildscript {
ext.kotlin_version = ‘<kotlin_version>'
repositories {
mavenCentral()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
}
}
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
android {
...
sourceSets {
main.java.srcDirs += 'src/main/kotlin'
}
}
dependencies {
...
compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
}

13. Kotlin Android Extensions
• Part of Kotlin IDEA plugin
• View binder library similar to
Butterknife
• Plugin for the Kotlin compiler
• Based on extensions properties

14. Add Kotlin Android Extensions plugin to
build.gradle
apply plugin: 'kotlin-android-extensions'
Usage
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"/>
Without extensions
findViewById(R.id.container)?.visibility = View.INVISIBLE
With extensions
import kotlinx.android.synthetic.main.activity_delightful.*
container.visibility = View.INVISIBLE

15. Null safety

16. • Aims to eliminate NPE danger in code
• NPE can be thrown explicitly or by
usage of !! operator (for NPE lovers
:))
• By default references cannot be null
• Allow null with ? operator

17. Do not allow null values
data class Clip(val title: String)
data class FeedPost(var clip: Clip)
var data = FeedPost(Clip(null)) //Won't compile
Allow null values
data class Clip(val title: String?)
data class FeedPost(var clip: Clip?)
var data = FeedPost(Clip(null)) //Will compile
Results to
var data:FeedPost? = fetchFromBackend()
//Some code that maybe sets data reference
textView.text = (data?.clip?.title)
Java
if(data != null && data.clip != null && data.clip.title != null) {
//Code that uses data
}
What about this?
fun setText(title:String) {
}
setText(data?.clip?.title!!)

18. Functions

19. • Top level functions support
• Default Arguments
• Named Arguments
• Higher-Order Functions and Lambdas
• Inline functions
• Extension Functions

20. Default arguments
Java
private void setData(List<Clip> clipList, Set<String> ids, boolean reset) {
...
}
Kotlin
private fun setData(clipList: List<Clip>, ids: Set<String> = emptySet<String>(),
reset: Boolean = false) {
...
}
setData(loadedDataFromBackend, emptySet(), true)
setData(loadedDataFromBackend)

21. Named arguments
Let’s add a few more parameters to
setData function
Java
private void setData(List<Clip> clipList, Set<String> ids, boolean reset, boolean orderDescending,
boolean skipViewed) {
}
setData(loadedDataFromBackend, new HashSet<String>(), false, true, false);
Kotlin
private fun setData(clipList: List<Clip>, ids: Set<String> = emptySet(), reset: Boolean = false,
orderDescending: Boolean = false, skipViewed: Boolean = false) {
}
setData(clipList = emptyList(), reset = true, skipViewed = false, orderDescending = true)

22. Higher-Order Functions
and Lambdas

23. • A higher-order function takes
functions as parameter, or returns a
function
• A lambda expression or an anonymous
function is a function that is not
declared, but passed immediately as
an expression

24. Lambda syntax for MenuItemClickListener
var menuItemClickListenerLambdaFunction: (MenuItem) -> Boolean = { menuItem -> true }
Concise version
var menuItemClickListenerLambdaFunction = { menuItem: MenuItem ->
if (menuItem.itemId == R.id.menu_settings) {
true
}
false
}
MenuItemClickListener in Java
OnMenuItemClickListener onMenuItemClickListener = new Toolbar.OnMenuItemClickListener() {
@Override
public boolean onMenuItemClick(MenuItem item) {
return false;
}
};

25. Compared to lambdas in Java
• Retrolambda (replaces lambdas with
anonymous class through bytecode
manipulation)
• Compile with Android N (lambda is
compiled into class with constructor
and method)
• Increasing method count and creating
pressure on GC
• Kotlin generates bytecode from
lambdas using inline functions

26. Sequences

27. • Inside kotlin.sequences
• Similar to Java 8 Streams API
• Implemented as extension functions
• zip, map, reduce, filter etc…

28. Suppose we have a collection of video clips, where
each clip has an id. We would like to store ids of
clips where the title length is less than 10
characters in a separate collection
Java
for(Clip item : items) {
if(item.getTitle().length() < 10) {
ids.add(item.getId());
}
}
Kotlin
items.map { it -> it.id }.filter { it -> it.length < 10 }.forEach { it -> ids.add(it) }
items.map { it.id }.filter { it.length < 10 }.forEach { ids.add(it) }

29. In Java?
• Streams API in Java 8 with Android N
• RxJava

30. Extensions

31. • Add functionality to a class without
extending it
• Support through extension functions
and extension properties
• Get rid of StringUtils, ViewUtils
classes

32. Extension properties
val <Context> Context.context: android.content.Context?
get() = getApplicationContext()
val <Activity> Activity.view_pager: android.support.v4.view.ViewPager?
get() = findViewById(R.id.view_pager) as android.support.v4.view.ViewPager?

33. • Prefix function name with class name
that we are extending
• All class methods and fields are
available inside the extension
function
• No actual classes are modified.
Functions are callable with the dot-
notation on instances of this class
Extension Functions

34. Example 1
Java
public static float convertDpToPixel(Context context, float dp){
Resources resources = context.getResources();
DisplayMetrics metrics = resources.getDisplayMetrics();
return dp * ((float)metrics.densityDpi / DisplayMetrics.DENSITY_DEFAULT);
}
Kotlin
fun Context.dpToPx(dp: Float): Int {
val resources = this.resources
val metrics = resources.displayMetrics
return (dp * (metrics.densityDpi / DisplayMetrics.DENSITY_DEFAULT)).toInt()
}
//Inside Activity
val dp = dpToPx(6f)

35. Example 2
Java
Context context = getApplicationContext();
Drawable tintedDrawable = DrawableCompat.wrap(ContextCompat.getDrawable(context, R.drawable.ic_arrow));
DrawableCompat.setTint(tintedDrawable, ContextCompat.getColor(context, R.color.colorAccentDark));
DrawableCompat.unwrap(tintedDrawable);
Kotlin
fun Drawable.tint(context: Context,
@ColorRes color: Int): Drawable {
val tintedDrawable = DrawableCompat.wrap(this)
DrawableCompat.setTint(tintedDrawable, ContextCompat.getColor(context, color))
DrawableCompat.unwrap<Drawable>(this)
return tintedDrawable
}
ContextCompat.getDrawable(applicationContext, R.drawable.ic_arrow).tint(applicationContext,
R.color.colorAccentDark)

36. More complex example
fun ImageView.loadUrl(path: String?,
transformation: RequestCreator.() -> RequestCreator = { this }) =
Picasso.with(context).load(path).transformation().into(this)
thumbnailView.loadUrl(thumbnailUrl) {
transform(RoundedCornersTransformation(
thumbnailView.context.dpToPx(4f), 0
)).fit()
}

37. • loadUrl function takes functions as a
second parameter
• when calling loadUrl function we pass
a function that will be invoked

39. Data classes

40. • Intented to hold data
• Auto generated equals(), hashCode(),
toString() and copy()
• At least one parameter in
constructor
• Can’t be abstract
• Can implement, can’t extend…for now

41. Example
data class Clip(val id: String, val title: String)
@PaperParcel
data class Clip(val id: String, val title: String) : PaperParcelable {
companion object {
@JvmField val CREATOR = PaperParcelable.Creator(Clip::class.java)
}
}
with Parcelable

42. Other sugars

43. String templates
private fun setData(clipList: List<Clip>, ids: Set<String> = emptySet(), reset: Boolean = false,
orderDescending: Boolean = false, skipViewed: Boolean = false) {
Timber.i("Log parameters number of clip:${clipList.size} orderDescending:$orderDescending ")
}

44. When expression
Create view holder
Java
@Override
public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
switch (viewType) {
case VIEW_TYPE_ONE:
return new ViewHolderOne(LayoutInflater
.from(parent.getContext())
.inflate(R.layout.item_layout_one, parent, false));
case VIEW_TYPE_TWO:
return new ViewHolderTwo(LayoutInflater
.from(parent.getContext())
.inflate(R.layout.item_layout_two, parent, false));
case VIEW_TYPE_THREE:
return new ViewHolderThree(LayoutInflater
.from(parent.getContext())
.inflate(R.layout.item_layout_three, parent, false));
default:
throw new RuntimeException();
}
}

45. Kotlin
override fun onCreateViewHolder(parent: ViewGroup, viewType: Int) = when (viewType) {
VIEW_TYPE_ONE -> ViewHolderOne(parent.inflate<View>(R.layout.item_layout_one))
VIEW_TYPE_TWO -> ViewHolderTwo(parent.inflate<View>(R.layout.item_layout_two))
VIEW_TYPE_THREE -> ViewHolderThree(parent.inflate<View>(R.layout.item_layout_three))
else -> throw RuntimeException("Invalid view type.")
}

46. Smart casts
• Cast by using is or !is operator
• Compiler tracks is-checks for
immutable values and inserts casts
automatically

47. Java
@Override
public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
if (holder instanceof ViewHolderOne) {
ViewHolderOne holderOne = (ViewHolderOne) holder;
} else if (holder instanceof ViewHolderTwo) {
ViewHolderTwo holderTwo = (ViewHolderTwo) holder;
} else if (holder instanceof ViewHolderThree) {
ViewHolderThree holderThree = (ViewHolderThree) holder;
}
}
Example with binding view holder

48. Kotlin
override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
if (holder is ViewHolderOne) {
val holderOne:ViewHolderOne = holder
} else if (holder is ViewHolderTwo) {
val holder:TwoViewHolderTwo = holder
} else if (holder is ViewHolderThree) {
val holder:ThreeViewHolderThree = holder
}
}
override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) = when(holder){
is ClipAdapter.ViewHolderOne -> ...
is ClipAdapter.ViewHolderTwo -> ...
is ClipAdapter.ViewHolderThree -> ...
}
or even better

49. @JvmOverloads
• Call Kotlin from Java
• Provide all versions of constructor

50. Example by extending a view
class ActivityView(context: Context,
attrs: AttributeSet? = null,
defStyleAttr: Int = 0) : RelativeLayout(context, attrs, defStyleAttr){
constructor(context: Context):this(context, null, -1) {
}
constructor(context: Context, attrs: AttributeSet? ):this(context, attrs, -1) {
}
}
class ActivityView : RelativeLayout {
@JvmOverloads constructor(context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0)
: super(context, attrs, defStyleAttr)
}

51. Future is bright
• Gradle scripting in Kotlin
• Incremental compilation
• Data class inheritance support
• Java 8/9 support
• Jack & Jill toolchain integration

52. Any apps out there?
• Expedia Hotels, Flights & Cars
• BlueCrew
• Level Money
• LifeHack Chisinau
• Changelogs

53. I want more
• http://antonioleiva.com/kotlin/
• https://kotlinlang.org/docs/
reference/
• http://kotlinlang.slack.com/

54. Questions?

55. Thanks
Code Android with joy. Try Kotlin.