Privet Kotlin (Windy City DevFest)

Hello Kotlin
Cody Engel
Senior Software Engineer @ Yello
Free T-Shirts & Clean Code
Nick Cruz
Android Engineer @ Yello
Mr. Plantastic & Crispy Animations

2. Getting Started
3. Null Safety
4. Our Favorite Things About Kotlin
5. Kotlin At Work
1. Fun Facts

Developed in 2011 by JetBrains
(IntelliJ IDEA, RubyMine, PhpStorm, Android Studio)
Released 2012
Statically typed programming language
for the JVM

Developed in 2011 by JetBrains
(IntelliJ IDEA, RubyMine, PhpStorm)
Released 2012
Statically typed programming language
for the JVM

Runs on JVM = 100% Interoperability with Java

Privet Kotlin (Windy City DevFest)

1. Fun Facts
3. Null Safety
5. Kotlin At Work
2. Getting Started

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

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

fun sum(a: Int, b: Int) = a + b
Functions

Functions
fun printSum(a: Int, b: Int): Unit {
println("sum of $a and $b is ${a + b}")
}

fun printSum(a: Int, b: Int) {
println("sum of $a and $b is ${a + b}")
}
Functions

val immutable: Int = 1
Variables

val immutable: Int
immutable = 3
Variables

var mutable = 0
mutable++
mutable++
Variables

class Address {
var name: String = ...
var street: String = ...
var city: String = ...
var state: String? = ...
var zip: String = ...
}
Properties

fun copyAddress(address: Address): Address {
val result = Address()
result.name = address.name
result.street = address.street
return result
}
Properties

val isEmpty: Boolean
get() = this.size == 0
Properties

var stringRepresentation: String
get() = this.toString()
set(value) {
field = value.toString()
}
Properties

val = "Java"
println(
String.format("The way is terrible!", )
)
java
java%s
String Templates

String Templates
val = "Java"
println(
String.format("The way is terrible!", )
)
java
java%s

val = "Kotlin"
println("The way is awesome!")
kotlin
$kotlin
String Templates

String Templates
val = "Kotlin"
println("The way is awesome!")
kotlin
$kotlin

var max = a
if (a < b) max = b
Control Flow - If Expression

var max: Int
if (a > b) {
max = a
} else {
max = b
}

val max = if (a > b) a else b

val max = if (a > b) {
print("Returns a")
a
} else {
print("Returns b")
b
}

when (x) {
1 -> print("x == 1")
2 -> print("x == 2")
else -> {
print(“x is neither 1 nor 2")
}
}
When Expression - Replaces Switch

when (x) {
1, 2 -> print("x == 1 or a == 2")
else -> print(“x is neither 1 nor 2”)
}

when (x) {
in 1..10 -> print("x is in the range")
in validNumbers -> print("x is valid")
!in 10..20 -> print("x is outside the range")
else -> print("none of the above")
}

when {
x.isOdd() -> print("x is odd")
x.isEven() -> print("x is even")
else -> print("x is funny")
}
When Expression - Replaces If/Else If

for (item in collection) {
print(item)
}
For Loops

for (i in array.indices) {
print(array[i])
}
For Loops

for ((index, value) in array.withIndex()) {
println("the element at $index is $value")
}
For Loops

1. Fun Facts
2. Getting Started
5. Kotlin At Work
3. Null Safety

Null Safety
(and Immutability)

Caused by: java.lang.NullPointerException: Attempt
to invoke virtual method 'java.lang.String
java.lang.String.trim()' on a null object
reference

Nullable String s; val s: String?
NonNull String s; val s: String

val length = s!!.length
NonNull Assert

NonNull Assert
kotlin.KotlinNullPointerException
at part2.delegation.interceptor.LoggerList.add(LoggerList.kt:39)
(don’t do this)

val length = s?.length // Int?
NonNull Assert
Safe call, returns optional
kotlin.KotlinNullPointerException
at part2.delegation.interceptor.LoggerList.add(LoggerList.kt:39)
(don’t do this)

val length = s?.length ?: 0 // Int
Safe Call and Elvis Operator

val length = s?.length ?: 0 // Int
Safe Call and Elvis Operator
Other helpful operators
with, apply, let, run
s?.let {
val length = it.length // Int
// Do stuff with length
}

What about variables that become null?
What about variables that become non-null?

Mutable
Nullable
String s; var s: String?
Mutable
NonNull
String s; var s: String
Immutable
Nullable
final String s; val s: String?
Immutable
NonNull
final String s; val s: String

1. Fun Facts
2. Getting Started
3. Null Safety
5. Kotlin At Work

Data Classes
We frequently create classes whose main purpose is to
hold data. In such a class some standard functionality
and utility functions are often mechanically derivable
from the data. In Kotlin, this is called a data class.

Data Classes
data class User(val name: String, val age: Int)

data class User(
val name: String = "",
val age: Int = 0
)
Data Classes

val user = User(“Jane Smith", 27)
user.name
user.age
user.toString()
user.hashCode()
user.equals()
Data Classes

val jack = User(name = "Jack", age = 1)
val olderJack = jack.copy(age = 2)
Data Classes

val jane = User("Jane", 35)
val (name, age) = jane
println("$name, $age years of age")
Data Classes

Sealed Classes
Sealed classes are used for representing restricted
class hierarchies, when a value can have one of the
types from a limited set, but cannot have any other type.
They are, in a sense, an extension of enum classes: the
set of values for an enum type is also restricted, but
each enum constant exists only as a single instance,
whereas a subclass of a sealed class can have multiple
instances which can contain state.

sealed class Expr {
data class Const(val number: Double) : Expr()
data class Sum(val e1: Expr, val e2: Expr) : Expr()
object NotANumber : Expr()
}
Sealed Classes

Sealed Classes
fun eval(expr: Expr): Double = when(expr) {
is Const -> expr.number
is Sum -> eval(expr.e1) + eval(expr.e2)
NotANumber -> Double.NaN
}

Lambdas
& Higher-Order Functions

A higher-order function is a function
that takes functions as parameters,
or returns a function.
fun transform1Through5(transform: (Int) -> Int) {
for (int in 1..5) {
println(transform(int))
}
}

for (int in 1..5) {
}
}
Method Reference
class NumberDoubler {
fun run(int: Int) = int * 2
}
fun doubleNumbersMethodReference() {
val numberDoubler = NumberDoubler()
transform1Through5(numberDoubler::run)
}

Lambda
for (int in 1..5) {
}
}
fun doubleNumbersLambda() {
transform1Through5 { num -> num * 2 }
}

transform1Through5 { num -> num * 2 }

transform1Through5 { it * 30 }

transform1Through5 { it * it }

listOf(1, 2, 3, 4, 5)
.filter { it % 2 == 1 } // Filter evens: [1, 3, 5]
.map { it * 2 } // Double the results: [2, 6, 10]
.forEach { println(it) } // Print them
Kotlin Collections

listOf(1, 2, 3, 4, 5)
.filter { it % 2 == 1 } // Filter evens: [1, 3, 5]
.map { it * 2 } // Double the results: [2, 6, 10]
.forEach { println(it) } // Print them
Kotlin Collections
Observable.just(1, 2, 3, 4, 5)
.filter { it % 2 == 1 }
.map { it * 2 }
.forEach { println(it) }
RxJava Observable (in Kotlin)

1. Fun Facts
2. Getting Started
3. Null Safety
5. Kotlin At Work

Introduce Kotlin
• Discuss the beneﬁts of Kotlin.
• Give snippets in Pull Request comments.
• Get others on our team interested.
@

Learn Together
• Work through Kotlin Koans together.
• Watch Kotlin videos together.
• Make this a team eﬀort.
@

Kotlin In Production
• Determine code style rules as a team.
• Find the path of least resistance.
• Java-style Kotlin in the beginning.
• Focus on continuous improvement.
@

twitter.com/POTUS404
medium.com/@CodyEngel
github.com/CodyEngel github.com/NickCruz
medium.com/@NickCruz26
twitter.com/ReactiveNick
medium.com/yello-offline
instagram.com/yello_offline
twitter.com/yello_offline

Resources
Kotlin Koans
https://goo.gl/MTLEYg
KotlinLang
https://kotlinlang.org
Life is Great and Everything Will Be Ok, Kotlin Is Here
https://goo.gl/yhp2Gn
Kotlin Uncovered
https://goo.gl/fbAFW5
Introduction To Kotlin
https://goo.gl/iCwYHd

Privet Kotlin (Windy City DevFest)

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