Are you stuck in the Java world? I’ll share my story about convincing my team and the client of the benefits of Kotlin. Furthermore I’ll delve into how we migrated an existing Java Android app, with 300k active users, to Kotlin. Even if you have never seen Kotlin before, come and see how you will create better apps with this modern and elegant language. At the end of this talk you’ll be able to convince your team / client why it’s a great to use Kotlin. The power of Kotlin can be leveraged everywhere you use Java, since it compiles to JVM bytecode. So even if you’re not an Android developer, check out this session to get acquainted with Kotlin! No excuses: switch to Kotlin 

1. NO EXCUSES,
SWITCH TO KOTLIN

2. NO EXCUSES,
SWITCH TO KOTLIN
t
Thijs Suijten
Mobile developer @ Q42
@tsuijten

9. GETTING EVERYBODY ONBOARD
TEAM Q42 CLIENT

12. val s1: String = "Hello" // Immutable variable
val s2 = "how are you" // Type is inferred
Variables
var s3 = "John" // Mutable variable
s3 = "Thijs" // Assign a different value
// w00t! String Interpolation
println("$s1, $s2 $s3?")
//> Hello, how are you Thijs?

13. fun reformat(str: String,
normalize: Boolean = true,
upperCase: Boolean = true): String {
...
}
Functions
reformat("Hello world")
reformat("Hello world", false, false)
reformat("Hello world", upperCase = false)
// Single-Expression functions
fun twiceTheFun(x: Int) = x * 2

14. class Person(val firstName: String,
val age: Int,
val locale: Locale = ENGLISH) {
fun sayHello() = println("Hi $firstName!")
}
Classes
val thijs = Person("Thijs", 34)
thijs.sayHello() // Hi Thijs!
class Cat(name: String) {
val name: String = name
}

15. val x: String? = getSomeOptionalString()
x.length // Does not compile.
Null safety / optionals
val y: String = null // Does not compile.
if (x != null) {
x.length // Compiles!
}
// Safe call
val optLength = foo?.bar?.length // Returns Int?
// Elvis operator.
val length = optLength ?: -1

16. Data classes
class User(val name: String, val age: Int)
val thijs = User("Thijs", 35)
println(thijs) // User(name=Thijs, age=35)
val bday = thijs.copy(age = 36) // Named params!
println(bday) // User(name=Thijs, age=36)
data

17. Data classes
public class User {
private String name;
private int age;
public User(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
User user = (User) o;
if (age != user.age) return false;
return name != null ? name.equals(user.name) : user.name == null;
}
@Override public int hashCode() {
int result = name != null ? name.hashCode() : 0;
result = 31 * result + age;
return result;
}
@Override public String toString() {
return "User{" +
"name='" + name + ''' +
", age=" + age +
'}';
}
}
// Remember this?
data class User(val name: String, val age: Int)
public class User {
private String name;
private int age;
public User(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
User user = (User) o;
if (age != user.age) return false;
return name != null ? name.equals(user.name) : user.name == null;
}
@Override public int hashCode() {
int result = name != null ? name.hashCode() : 0;
result = 31 * result + age;
return result;
}
@Override public String toString() {
return "User{" +
"name='" + name + ''' +
", age=" + age +
'}';
}
}

18. Functional / lambas
val sum = listOf(1, 2, 3, 4, 5) // 1,2,3,4,5
.filter { number -> number < 5 } // 1,2,3,4
.map { it * 2 } // 2,4,6,8
.sum() // 20
listOf("Zack", "Bob", "Thijs", "Julie", "Rob")
.filter { it.length > 3 } // Zack, Thijs, Julie
.sorted() // Julie, Thijs, Zack

19. Collections
// Interface List<String>
val fruits = listOf("Apple", "Pear", "Orange")
fruits.add("Strawberry") // Compiler error
// Interface MutableList<String>
val veggies = mutableListOf("Kale", "Spinach", "Lettuce")
veggies.add("Tomato")
veggies += "Cucumber" // Operator overloading

20. When statement
// Java
if (firstName.equals("Dan")) {
person.setTeam(programmers);
} else if (lastName.equals("Jamie")) {
person.setTeam(designers);
} else {
person.setTeam(others);
}
// Kotlin
when {
firstName == “Dan" -> person.team = programmers
lastName == "Jamie" -> person.team = designers
else -> person.team = others
}

21. When statement
// Java
switch (firstName) {
case "Dan":
case "Jay":
person.setTeam(programmers);
break;
case "Jamie":
person.setTeam(designers);
break;
default:
person.setTeam(others);
}
// Kotlin
when (firstName) {
"Dan", "Jay" -> person.team = programmers
"Jamie" -> person.team = designers
else -> person.team = others
}

22. When statement
when (x) {
1 -> print("x = 1")
is Int -> print(x + 1)
is String -> print(x.length + 1)
is IntArray -> print(x.sum())
in 10..20 -> print("Between 10 and 20")
}
val outcome = when(x) {
1 -> "x = 1"
"Hello" -> "is it me you're looking for?"
in 10..20 -> "Between 10 and 20"
}

23. Extensions
// Java
StringUtils.capitalize(AddressUtils.extractStreet(address))
// Kotlin
address.street.capitalize()
// Extension function
fun String.capitalize() = StringUtils.capitalize(this)
// Extension property
val String.street: String get() = AddressUtils.extractStreet(this)

24. Kotlin Android extensions
<TextView android:id="@+id/greeting"
android:layout_width="match_parent"
android:layout_height="wrap_content"/>
<Button android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Click me!"/>
override fun onCreate(savedInstanceState: Bundle?) {
//…
greeting.text = "Hello"
button.onClick { toast("Button clicked!") }
}

25. Interop Demo

26. Who is using Kotlin

29. NO EXCUSES,
SWITCH TO KOTLIN

30. ..
thijs@q42.nl / @tsuijten
try.kotlinlang.org