Kotlin Basics

KotlinDrew Stephens
<drew@dinomite.net>
@dinomite
Forum For All
1

What do I know?
— JVM (Java1.4–8
, KotlinM14–1.1
, Groovy1.5
)
— Ruby1.9–2.4
& Rails3.0–5.0
— Perl5.5–5.12
— PHP5.2–5.3
— JavaScript1.5–ECMAScript 6/ES2016/ES6 Harmony/WTF
(when I have to)
— Android & iOS in a pinch
Drew Stephens • <drew@dinomite.net> • github.com/dinomite/talks 2

What is it?
— A language for the JVM
— As easy to integrate as Groovy
— Syntax like Java, but cleaner
— Written by JetBrains, who wrote your IDE1
1
If you use the correct IDE

———————————
———————————
Statics———————————
———————————Drew Stephens • <drew@dinomite.net> • github.com/dinomite/talks 4

Hate Java
Don't hate types

Static Dynamic
Java Ruby
C# Python
ObjectiveC JavaScript
Swift Perl
Kotlin PHP
Groovy Groovy !

Statics, generally
int foo = 7;
foo = "bar"; // compile error
if (foo == "bar") { // compile error
...
}
if (fooo == 7) { // compile error
...
}

Statics, generally
int foo = 7;
...
}
...
}

Statics, generally
— Prevents misspelled variables
— Catch NoMethodError, NameError, undeﬁned early
— Add context to the code

Equality

Productivity
— Unit tests can cover functionality, not borders
— Easy refactoring, thanks to powerful tools
— Hard to break things accidentally
— Much less reading the source APIs you're using
— No tracking down parameters

Updating a user's address
POST /address
{
"user_id": 44,
"address": {
"number": 2446,
"street": "Belmont Rd NW",
"zip": 20008
}
}

Param checking in Ruby
def update_address
check_params
...
end
def check_address_params
required_params = params.key?(:user_id) && params.key?(:address)
fail ActionController::ParameterMissing unless has_required_params
address = params[:address]
fail ActionController::ParameterMissing
unless address.key?(:number) && address.key?(:street) && address.key?(:zip)
# Doesn't verify types of anything
end

Param checking in Java
boolean updateAddress(AddressRequest newAddress) {
// Guaranteed to be well-formed
...
}
class AddressRequest {
int userId;
Address address;
}
class Address {
int number;
String street;
int zip;
}

———————————
———————————
Features———————————

Hello, world! (Java)
package demo;
public static void main(Array<String> args) {
try {
System.out.printf("Hello, world!");
} catch (IllegalFormatException e) {
e.printStackTrace();
}
}

Hello, World!
package demo
fun main(args: Array<String>) {
println("Hello, world!")
}

Variables
— vals are immutable (read-only; like final in Java)
— vars are mutable
— Type inference
val foo = "Foo" // is a String
foo = "Bar" // nope, can't change a val
var num = 7
num = 3 // ok
// Java
// final String foo = "Foo";

Null safety
var neverNull: String = "foobar"
var maybeNull: String? = "baz"
neverNull = null // Compiler error
neverNull.length // Guaranteed OK
maybeNull.length // Compiler error
maybeNull?.length // Possible null instead of length
maybeNull!!.length // Possible NPE
var valueOrSadness = maybeNull ?: "it was null !"

Null safety

Null tracking
if (maybeNull != null && maybeNull.length > 5) {
// maybeNull guaranteed to not be
print("It is ${maybeNull.length} characters")
} else {
print("The variable is null !")
}

Type inference
fun returnsAListOfStrings(): List<String> {
return listOf("foo", "bar")
}
// Java
// final List<String> listOfStrings = returnsAListOfStrings();
// final String firstString = listOfStrings[0];
val listOfStrings = returnsAListOfStrings()
val firstString = listOfStrings[0]

Type inference
}
// Java

POJO
public class Pagechat {
private final int id;
private final String url;
private final boolean fresh;
public Pagechat(int id, String url, boolean fresh) {
this.id = id;
this.url = url;
this.fresh = fresh;
}
public int getId() { return id; }
public String getUrl() { return url; }
public boolean getFresh() { return fresh; }
public boolean equals(Object obj) { … }
public int hashCode() { … }
public String toString() { … }
}

Data class (POJO equivalent)
data class Pagechat(val id: Int,
val url: String,
val fresh: Boolean)
Free stuff:
- Accessors for ﬁelds; setters for vars
- equals()/hashCode()
- toString()
- copy()

Param checking in Kotlin
fun updateAddress(newAddress: UpdateAddress): Boolean {
// Guaranteed to be well-formed
...
}
data class UpdateAddress(val userId: Int, val address: Address )
data class Address(val number: Int, val street: String, val zip: Int)

Using the copy constructor
data class Pagechat(val id: Int, val url: String, val fresh: Boolean)
...
fun createPagechat(url: String, fresh: Boolean): Pagechat {
val pagechat = Pagechat(0, "http://foobar.com", true)
// { id: 0, url: "http://foobar.com", fresh: true }
val newId = dao.insert(pagechat)
return pagechat.copy(id = newId)
// { id: $newId, url: "http://foobar.com", fresh: true }
}

Be!er switch statement (when)
when (view) {
is TextView -> toast(view.text)
is RecyclerView -> toast("Item count = ${view.adapter.itemCount}")
is SearchView -> toast("Current query: ${view.query}")
else -> toast("View type not supported")
}

Named arguments & defaults
data class User(name: String, email: String, admin: Boolean = false)
...
val drew = User("Drew", "drew@dinomite.net")
// drew.admin is false
val plague = User("The Plague", "babbage@ellingson.com", true)
// plague.admin is true
val named = User(email = "foo@bar.com", name = "Foo Bar")

Tail recursion4
tailrec fun findFixPoint(x: Double = 1.0): Double {
val cosine = Math.cos(x)
if (x == cosine) {
return x
} else {
return findFixPoint(cosine)
}
}
// Shorter
tailrec fun findFixPoint(x: Double = 1.0): Double
= if (x == Math.cos(x)) x else findFixPoint(Math.cos(x))
4
https://kotlinlang.org/docs/reference/functions.html#tail-recursive-functions

Be!er enums
enum class Matcher(val cssQuery: String,
val attr: String) {
OG_TITLE("meta[property=og:title]", "content"),
META_TITLE("meta[name=title]", "content"),
TITLE("title", null)
}

Be!er streams syntax
val cars = mapOf(
2 to "Vandoorne", 3 to "Ricciardo",
5 to "Vettel", 7 to "Räikkönen",
…
)
val qualified = listOf(44, 5, 77, 7, 33, …)
...
val grid = qualified.map { cars[it] }
// Java: qualified.stream().map(e -> cars.get(e))
// [Hamilton, Vettel, Bottas, Räikkönen, …]

———————————
———————————
How?———————————

How do I use it?

build.gradle
buildscript {
repositories {
mavenCentral()
}
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.1.2"
}
}
apply plugin: 'kotlin'

Tell me more
- Kotlin docs—https://kotlinlang.org/docs/reference/
- Try Kotlin in your browser—http://try.kotlinlang.org/
- My Kotlin repos—https://github.com/dinomite
- Dynamic vs Static—https://youtu.be/Uxl_X3zXVAM
Drew Stephens
<drew@dinomite.net>
@dinomite
https://forumforall.net

———————————
———————————
———————————
———————————
———————————

JSON3
{
"id": 7,
"email": "drew@dinomite.net"
}
data class User(id: Int, email: String)
3
https://medium.com/square-corner-blog/kotlins-a-great-language-for-json-fcd6ef99256b

Simpler lambdas
takesLambdaArg(e -> e.getValue())
takesLambdaArg({ it.getValue() })

String interpolation
val foo = "foobar"
println("The variable is $foo")
// The variable is foobar

Embracing null2
fun findUser(id: Int): User?
...
findUser(7)?.let {
// Only executes if user exists
it.transmogrify(42)
}
2
http://beust.com/weblog/2016/01/14/a-close-look-at-kotlins-let/

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