Introducing Kotlin — JVM-based language made by JetBrains. Kotlin takes aim at the age and perceived uncoolness of the Java with fresh and popular language features. It can do anything the Java language can do and more, but often with a more concise and pleasant syntax.

1. Exploring Kotlin on
Android
by Deepanshu & Malcolm

2. What is Kotlin?
• Statically typed programming language
• JVM-based language developed by JetBrains
• From industry not academia
• 100% inter-operable with the Java language
Open Source - https://github.com/JetBrains/kotlin

3. Why Kotlin
Concise - Drastically reduce the amount of boilerplate code you need to
write.
Safe - Avoid entire classes of errors such as null pointer exceptions.
Versatile - Build server-side applications, Android apps or front-end code
running in the browser.

4. Kotlin in Expedia Android
Kotlin code is at 23% and rising.
Refactoring started mid 2015.
Hotels, Packages, Flights, Services and MockWebServer & tests
classes are 90% in Kotlin
Cars/LX/Legacy tablet code still in java.
Other teams in Expedia are experimenting with Kotlin in the web.

5. Features
• Null type and safety
• Lambdas
• Optional params
• Data classes
• Extension functions
• Delegates
• Smart Casting

6. Not so good about Kotlin!
• Tools support
• Compile time

7. Kotlin
Roadmap for Android
• Incremental compilation
• support for Jack and Jill toolchain
• Instant run works for cold swaps right now not
for hot swaps
• Lint checks in Kotlin 1.0.2

8. Kotlin syntax 101

9. Defining Functions
fun sum(a: Int, b: Int): Int {
return a + b
}

10. Defining Functions
fun sum(a: Int, b: Int): Int {
return a + b
}
fun sum(a: Int, b: Int) = a + b

11. fun printSum(a: Int, b: Int): Unit {
print(a + b)
}
Defining Functions

12. fun printSum(a: Int, b: Int): Unit {
print(a + b)
}
fun printSum(a: Int, b: Int) {
print(a + b)
}
Defining Functions

13. fun doSomething(): Int {
val a: Int = 1
val b = 1
val c: Int
c = a + b
return c
}
Defining local variables

14. fun doSomething(): Int {
val a: Int = 1
var b = 1
val c: Int
b += 5
c = a + b
return c
}
Defining local variables

15. Using conditional
expressions
fun doSomething(a: Int, b: Int): Int {
if (a > b)
return a
else
return b
}

16. Using conditional
expressions
fun doSomething(a: Int, b: Int): Int {
if (a > b)
return a
else
return b
}
fun doSomething(a: Int, b: Int) = if (a > b) a else b

17. Using a for loop
fun doSomething(args: Array<String>) {
for (arg in args)
print(arg)
}

18. Using a for loop
fun doSomething(args: Array<String>) {
for (arg in args)
print(arg)
}
fun doSomething(args: Array<String>) {
for (i in args.indices)
print(args[i])
}

19. Using when expression
fun doSomething(obj: Any) {
when (obj) {
1 -> print("One")
"Hello" -> print("Kotlin")
is Long -> print("Long")
!is String -> print(“!string")
else -> print("Unknown")
}
}

20. Using ranges
fun doSomething(x: Int, y: Int) {
if (x in 1..y - 1)
print("In")
for (x in 1..5)
print(“N")
if (x !in 1..y - 1)
print(“In")
}

21. Using ranges
fun doSomething(x: Int, y: Int) {
if (x in 1..y - 1)
print("In")
for (x in 1..5)
print("N")
if (x !in 1..y - 1)
print(“In")
}

22. Using ranges
fun doSomething(x: Int, y: Int) {
if (x in 1..y - 1)
print("In")
for (x in 1..5)
print("N")
if (x !in 1..y - 1)
print(“In")
}

23. Class in Java
public class SomeClass {
private String variable;
private final String defaultVar;
SomeClass(String variable) {
this.variable = variable;
this.defaultVar = "Java";
}
SomeClass(String variable, String defaultVar) {
this.variable = variable;
this.defaultVar = defaultVar;
}
}
// Use
new SomeClass("Kotlin", "Java");
new SomeClass("Kotlin");

24. Class in Kotlin
class SomeClass(var variable: String, val defaultValue: String = "Java") {
}
// Use
SomeClass("Kotlin", "Java")
SomeClass("Kotlin")

25. Lambdas
fun doSomething() {
val items = ArrayList<String>()
items.sortBy { item ->
item.length
}
}

26. Lambdas
fun doSomething() {
val items = ArrayList<String>()
items.sortBy { it.length }
}

27. Lambdas
//In Kotlin
fun doSomething() {
val items = ArrayList<String>()
items.sortBy { it.length }
}
//In Java
ArrayList<String> items = new ArrayList();
Collections.sort(items, new Comparator<String>(){
@Override
public int compare(String s1, String s2) {
return s1.length() - s2.length();
}
});

28. button.setOnClickListener(new View.OnClickListener(){
public void onClick(View v) {
Toast.makeText(this.MainActivity, "Button",
Toast.LENGTH_LONG).show();
}
});
In Java

29. button.setOnClickListener(object: View.OnClickListener {
override fun onClick(view: View): Unit {
Toast.makeText(context, "Button",
Toast.LENGTH_LONG).show()
}
})
In Kotlin

30. button.setOnClickListener { view ->
Toast.makeText(this, "Button",
Toast.LENGTH_LONG).show()
}
In Kotlin

31. button.setOnClickListener {
Toast.makeText(this, "Button",
Toast.LENGTH_LONG).show()
}
In Kotlin

32. //In Kotlin
button.setOnClickListener {
Toast.makeText(this, "Button",
Toast.LENGTH_LONG).show()
}
//In Java
button.setOnClickListener(new View.OnClickListener(){
public void onClick(View v) {
Toast.makeText(this.MainActivity, "Button",
Toast.LENGTH_LONG).show();
}
});

33. class Vehicle(var value: String) {
}
fun doSomething() {
val type = Vehicle("Car")
println("${type.value}")
}

34. class Vehicle(var value: String) {
}
fun doSomething() {
val type = Vehicle("Car")
println("${type.value}")
}

35. class Vehicle(var value: String) {
}
fun doSomething() {
val type = Vehicle("Car")
println("${type.value}")
}

36. class Vehicle(var value: String) {
fun print() = println("$value")
}
fun doSomething() {
val type = Vehicle("Car")
type.print()
}

37. class Vehicle(var value: String) {
fun print() = println("$value")
}
fun doSomething() {
val type = Vehicle("Car")
type.print()
}

38. class Vehicle(var value: String) {
fun print() = println("$value")
}
fun doSomething() {
val types = listOf(
Vehicle("Car"),
Vehicle("Truck"),
Vehicle("Bike"))
}

39. class Vehicle(var value: String) {
fun print() = println("$value")
}
fun doSomething() {
val types = listOf(
Vehicle("Car"),
Vehicle("Truck"),
Vehicle("Bike"))
for (type in types) {
type.print()
}
}

40. class Vehicle(var value: String) {
fun print() = println("$value")
}
fun doSomething() {
val types = listOf(
Vehicle("Car"),
Vehicle("Truck"),
Vehicle("Bike"))
types.forEach { type ->
type.print()
}
}

41. class Vehicle(var value: String) {
fun print() = println("$value")
}
fun doSomething() {
val types = listOf(
Vehicle("Car"),
Vehicle("Truck"),
Vehicle("Bike"))
types.forEach { it.print() }
}

42. class Vehicle(var value: String) {
fun print() = println("$value")
}
fun doSomething() {
val types = listOf(
Vehicle("Car"),
Vehicle("Truck"),
Vehicle("Bike"))
types.forEach { it.print() }
}

43. class Vehicle(var value: String) {
fun print() = println("$value")
}
fun doSomething() {
listOf(
Vehicle("Car"),
Vehicle("Truck"),
Vehicle("Bike")
).forEach { it.print() }
}

44. open class Vehicle(var value: String) {
fun print() = println("$value")
}
class Car(var type: String) : Vehicle(type){
}
fun doSomething() {
listOf(
Car("Car"),
Vehicle("Truck"),
Vehicle("Bike")
).forEach{it.print()}
}

45. open class Vehicle(var value: String) {
fun print() = println("$value")
}
class Car(var type: String) : Vehicle(type){
}
fun doSomething() {
listOf(
Car("Car"),
Vehicle("Truck"),
Vehicle("Bike")
).forEach{it.print()}
}

46. open class Vehicle(var value: String) {
open fun print() = println("$value")
}
class Car(var type: String) : Vehicle(type){
override fun print() = println("Yo, Its a $value")
}
fun doSomething() {
listOf(
Car("Car"),
Vehicle("Truck"),
Vehicle("Bike")
).forEach{it.print()}
}

47. open class Vehicle(var value: String) {
open fun print() = println("$value")
}
class Car(var type: String) : Vehicle(type){
override fun print() = println("Yo, Its a $value")
}
fun doSomething() {
listOf(
Car("Car"),
Vehicle("Truck"),
Vehicle("Bike")
).forEach{it.print()}
}

48. Nullability
I call it my billion-dollar mistake. It was the invention of the null reference …. has led to
innumerable errors, vulnerabilities, and system crashes, which have probably caused a
billion dollars of pain and damage in the last forty years.
-Sir Charles Antony Richard Hoare

49. Null and type safety
fun doSomething() {
var a: String = "abc"
a = null // compilation error
}

50. fun doSomething() {
var a: String = "abc"
a = null // compilation error
var b: String? = "abc"
b = null // ok
}
Null and type safety

51. fun doSomething() {
var a: String = "abc"
var b: String? = "abc"
b = null // ok
val l = a.length
}
Null and type safety

52.
fun doSomething() {
var a: String = "abc"
var b: String? = "abc"
b = null // ok
val l1 = a.length
val l2 = b.length // error:variable 'b' can be null
}
Null and type safety

53. Checking for null in conditions
fun doSomething() {
var a: String = "abc"
var b: String? = "abc"
b = null // ok
val l1 = a.length
val l = if (b != null) b.length else -1
}

54. Safe Calls
fun doSomething() {
var a: String = "abc"
var b: String? = "abc"
b = null // ok
val l1 = a.length
val l2 = b?.length
}

55. Elvis Operator
fun doSomething() {
var a: String = "abc"
var b: String? = "abc"
b = null // ok
val l1 = a.length
val l = if (b != null) b.length else -1
}

56. fun doSomething() {
var a: String = "abc"
var b: String? = "abc"
b = null // ok
val l1 = a.length
val l2 = b?.length ?: -1
}

57. if (originLocation != null) {
if (originLocation.hierarchyInfo != null) {
if (originLocation.hierarchyInfo.airport != null) {
if (originLocation.hierarchyInfo.airport.airportCode != null) {
departureAirportCode =
originLocation.hierarchyInfo.airport.airportCode;
}
}
}
}
else {
departureAirportCode = "";
}

58. val departureAirportCode = originLocation?.hierarchyInfo?.airport?.airportCode ?: ""
if (originLocation != null) {
if (originLocation.hierarchyInfo != null) {
if (originLocation.hierarchyInfo.airport != null) {
if (originLocation.hierarchyInfo.airport.airportCode != null) {
departureAirportCode =
originLocation.hierarchyInfo.airport.airportCode;
}
}
}
}
else {
departureAirportCode = "";
}

59. The !! Operator
fun doSomething() {
var a: String = "abc"
var b: String? = "abc"
b = null // ok
val l1 = a.length
val l2 = b!!.length
}

60. Optional-params
//In Java
void foo(int p1, boolean p2) {
}
void foo(int p1) {
foo(1, false);
}
void foo() {
foo(1, false);
}

61. Optional-params
//In Kotlin
fun foo(p1: Int = 1, p2: Boolean = false) {
}
foo()
foo(3)

62. Data class
data class Traveler(val name: String?, val age: Int)
Automatically implements:
equals()/hashCode()
toString() of the form "User(name=John, age=42)",
copy() function

63. val testTravlerJohn = Traveler(“John”, 42)
val testTravlerJane = Traveler(“Jane”, 41)
assertEquals(testTravlerJohn, testTravlerJane)
Data class in Testing

64. public class Traveler(String name, Int age)
@Override
public boolean equals(Object o) {
if (this == o) {
return true;
}
if (!(o instanceof Traveler)) {
return false;
}
Traveler traveler = (Traveler) o;
if (name != name) {
return false;
}
if (age != traveler.age) {
return false;
}
return true;
}

65. Extension Functions
//In Java
public class StrUtils {
public static String encodeString(String str) {
return str.replaceAll(" ", "_")
}
}
String text = "Hello World";
System.out.println(StrUtils.encodeString(text));

66. Extension Functions
//In Kotlin
fun String.encodeSpaces(): String {
return this.replaceAll(" ", "_")
}
val text = "hello world"
println(text.encodeSpaces())

67. val list = listOf("E", "H", "Hello", "Hi", "I")
val map = mapOf(Pair("A", 1), Pair("B", 2), Pair("C", 3))
list.forEach { s ->
s.length
}
"E, H, Hello, Hi, I"
Kotlin Collections Extensions

68. val list = listOf("E", "H", "Hello", "Hi", "I")
val map = mapOf(Pair("A", 1), Pair("B", 2), Pair("C", 3))
var count = 0
list.forEachIndexed { i, s ->
count += i
s.length
}
"E, H, Hello, Hi, I"

69. val list = listOf("E", "H", "Hello", "Hi", "I")
val map = mapOf(Pair("A", 1), Pair("B", 2), Pair("C", 3))
list.filter { s ->
s.contains("H")
}
"H, Hello, Hi"

70. val list = listOf("E", "H", "Hello", "Hi", "I")
val map = mapOf(Pair("A", 1), Pair("B", 2), Pair("C", 3))
list.first { s ->
s.startsWith("He")
}
"Hello"

71. val list = listOf("E", "H", "Hello", "Hi", "I")
val map = mapOf(Pair("A", 1), Pair("B", 2), Pair("C", 3))
map.forEach { entry ->
entry.value
}
"A, B, C"

72. //In Java
final int[] sponsoredIndexes = { 0, 50, 51 };
ArrayList<Property> sponsored = new ArrayList<Property>();
ListIterator<Property> it = properties.listIterator();
while (it.hasNext()) {
Property prop = it.next();
if (prop.isSponsored()) {
it.remove();
sponsored.add(prop);
}
}
for (int i = 0; i < sponsored.size() && i < sponsoredIndexes.length; i++) {
if (sponsoredIndexes[i] <= properties.size()) {
properties.add(sponsoredIndexes[i], sponsored.get(i));
}
}

73. //In Kotlin
fun putSponsoredItemsInCorrectPlaces(hotelList: List<Hotel>): List<Hotel> {
val (sponsored, nonSponsored) = hotelList.partition { it.isSponsoredListing }
val firstChunk = sponsored.take(1)
val secondChunk = nonSponsored.take(49)
val thirdChunk = sponsored.drop(1)
val rest = nonSponsored.drop(49)
return firstChunk + secondChunk + thirdChunk + rest
}

74. //In Java
Observable.just("Hello World")
.map(new Func1<String, Object>() {
@Override
public Object call(String s) {
return s + "!";
}
})
.subscribe(new Subscriber<String>() {
@Override
public void onCompleted() {
//Completion
}
@Override
public void onError(final Throwable e) {
//TODO : Handle error here
}
@Override
public void onNext(final String s) {
Log.e("Output",s);
}
});
Reative Java

75. Kotlin & Reactive
Observable.just("Hello World")
.map { it.plus("!") }
.subscribe { println("Output:" + it); }

76. Flow sensitive typing
//In Java
@Override
public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
if (holder.getItemViewType() != LOADING_VIEW) {
((ViewHolder) holder).updateHotel();
}
else if (holder.getItemViewType() != HOTEL_VIEW) {
((LoadingViewHolder) holder).loadView();
}
}

77. Flow sensitive typing
//In Kotlin
override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
when (holder) {
is HotelCellViewHolder-> holder.updateHotel()
is LoadingViewHolder -> holder.loadView()
}
}

78. Delegates
var p: String by Delegate()
Properties using the delegate expression will have its
get() and set() methods delegated to the expression.
* lazy
* observable
* notNull

79. val view: Presenter by lazy {
var view = stub.inflate() as Presenter
view
}
Delegates.lazy

80. Delegates.notNull
class Foo {
var bar : Bar by Delegates.notNull() // type is now Bar
init {
val s = bar // Using before setting throws an IllegalStateException!
bar = Bar()
}
}

81. Delegates.observable
val text: String by Delegates.observable("") {
prop, old, new ->
println("$old -> $new")
}

82. Custom delegate(KotterKnife)
fun <T : View> ViewGroup.bindView(id: Int): ReadOnlyProperty<Any, T> =
ViewBinding(this, id)
private class ViewBinding<T : View>(val source: Any, val id: Int) :
ReadOnlyProperty<Any, T> {
private val lazy = Lazy<T>()
override fun getValue(thisRef: Any, property: KProperty<*>): T = lazy.get {
findView<T>(source, id)
?: throw IllegalStateException("View ID $id for '${property.name}'
not found.")
}
}
val mapView: MapView by bindView(R.id.map_view)

83. Questions?