The document provides a comprehensive overview of Kotlin, a statically-typed programming language designed and developed by JetBrains, including its main features such as null safety, extensions, and data classes. It offers detailed guidance on setting up Kotlin in Android, utilizing its unique functionalities like higher-order functions, smart casts, and extension functions. Additionally, it highlights Kotlin's compatibility with Java and the advantages of using Kotlin in modern application development.

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.