val kotlin: Introduction

Chris Dail - @chrisdail

Senior Director, Software Engineering

Slides - github.com/chrisdail/talk-kotlin-intro

Kotlin

Fastest Growing Programming Language

PYPL - pypl.github.io

StackOverflow

But Why?

Google IO 2017

Why Kotlin?

From The Authors:

"We want to become more productive by switching to a more expressive language. At the same time, we cannot accept compromises in terms of either Java interoperability or compilation speed." - JetBrains Blog

Kotlin Is

General purpose programming language
Open Source (Apache 2)
Statically Typed
Concise, safe, pragmatic
Interoperable
Object Oriented, Procedural, Functional

Kotlin Myths

Misconceptions about the language

Kotlin is Google's answer to Apple's Swift

--

Kotlin started in 2011 by JetBrains with 1.0 in 2016

Android support was only announced in 2017

Kotlin is a Swift Clone

--

Swift started in 2010 but Apple only announced it in 2014

Kotlin is only for Java Developers

--

Kotlin is a modern, pragmatic language with excellent interop

Initially meant JVM but now extends to JavaScript and Native

Kotlin offers little over Java 8/9/10

--

Java has made some improvements recently but is still far from modern

Don't take my word for it...

Basics

Creating Variables

name : type
Variables require initialization

var message: String = "Hello World!"

--

Inferred Types

var message = "Hello World!"

--

Immutable is preferable

val message = "Hello World Forever!"

Strings

Interpolation

var planet = "Earth"
println("Hello $planet")
println("HELLO ${planet.toUpperCase()}")

Nulls

Any - All classes extend from Any

--

Any? - Nullable Type

var neverNull: String = "hi"
//neverNull = null   // Compilation Error
var message: String? = "hi"
message = null

--

?. - Null-safe calls

//message.length   // Compilation Error
message?.length
// Equivalent Java: (message == null) ? null : message.length

Scoping Function: `let`

val person = lastName.let {
    Person(it)
}

Takes the argument as a parameter (default: it)
Result is the last value (functional like a transform/map) --
More concise Null-checking

val username = usernameOrNull()
if (username != null) {

}

usernameOrNull()?.let { username ->

}

Elvis Operator

`?:` - Elvis Operator

Returns left if not null, otherwise right

val length = message?.length ?: -1

Can be combined with throw or return

val map = mapOf("name" to "Ivy")
val name = map["name"] ?: throw IllegalArgumentException()

Control Flow

If / Else

Traditional

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

As Expression

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

When

Switch-like

when (x) {
    -1 -> println("INVALID")
    0, 1 -> println("0 or 1")
    in 2..100 -> println("x == $x")
    else -> println("otherwise")
}

if-else chain

when {
    x.isOdd() -> print("x is odd")
    x.isEven() -> print("x is even")
    else -> print("x is funny")
}

Exceptions

No checked exceptions
- Debated topic in the Java community (Spring)
- Leads to empty try/catch blocks
try/catch is an expression (can return a value)

val a: Int = try {
    value.toInt()
} catch (e: NumberFormatException) {
    -1
}

--

val b = value.toIntOrNull() ?: -1

For

Loop over list

for (value in ints) {
    println(value)
}

With Ranges

for (i in 1..3) {
    println(i)
}

Putting it all together

Fizz Buzz

for (n in 1..100) {
    println(when {
        n % 15 == 0 -> "FizzBuzz"
        n % 3 == 0 -> "Fizz"
        n % 5 == 0 -> "Buzz"
        else -> n
    })
}

Functions

No class required
Simple function

fun reverse(message: String): String {
    return message.reversed()
}
println(reverse("Hello World"))

// dlroW olleH

One Liner

fun add(a: Int, b: Int) = a + b
println(add(1, 1))

// 2

Arguments

Default/Optional Arguments

fun addUser(name: String, city: String = "Moncton", province:
        String = "NB", country: String = "Canada") {
    // ...
}

addUser("Charlie")

Named Arguments

addUser("Charlie", province = "NS", city = "Halifax")

Lambdas / Closures

Last line is the returned value

val sum = { x: Int, y: Int ->
        x + y
}

Higher Order Functions

val diff = { x: Int, y: Int -> x - y }
fun eval(x: Int, y: Int, binaryOp: (Int,Int) -> Int): Int {
        return binaryOp(x, y)
}
println(eval(2, 1, sum))
// 3
println(eval(2, 1, diff))
// 1

Classes

Class

Simple Constructor with init block

class Person(name: String) {
        init {
            println("Created person with $name")
        }
}

Class with properties

class User(var name: String, var age: Int)
val user = User("Charlie", 20)
println("Created user: ${user.name} age: ${user.age}")

Inheritance

interface Processor {
    fun process()
}

open class Base(val p: Int) : Processor {
    private val privateField = "cannot see me"

    private fun privateFunction() = println(privateField)
    override fun process() = println("Processed")
    open fun allowsOverride() = println("Can Override")
}

class Derived(p: Int) : Base(p) {
    override fun allowsOverride() = println(p)
}

Properties

Basic declaration

class Address {
        var name: String = ""
        var street: String = ""
        var city: String = ""
        var state: String? = null
        var zip: String = ""
}

Custom getter/setter

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

Data Classes

Easy way to create DTO / POJO
Most data classes can be one liners

data class User(var name: String, var age: Int)

Adds some default functions based on the constructor:
- equals() and hashCode() pair
- toString() of the form User(name=Ivy, age=42)
- copy() function - Creates a copy of this class
Destructuring

val (name, age) = User("Ivy", 1)

Objects

Singleton as object declaration

object UserService {
        fun createUser(name: String) {
            // ...
        }
}
UserService.createUser("charlie")

Companion Object

Kotlin has no static keyword
companion object is an associated object
Fields and functions on the class

class SomeClass() {
    val id: Int

    init {
        id = nextId++
    }

    companion object {
        private var nextId = 1
    }
}

Extension

Adding to existing classes
Extension Functions

fun <T> HttpResponse<T>.successful() = when (statusCode()) {
        in 200..299 -> true
        else -> false
}

No 'magic' here. Imported like any other function

Collections

Lists and Sets

val empty = emptyList<String>()
val list = listOf(1, 1, 2, 3)
val mutableList = mutableListOf<String>()
mutableList.add("hello")

println(list[0])
// 1

println(list + listOf(5, 8))
// [1, 1, 2, 3, 5, 8]

println(setOf(1, 1, 2, 3))
// [1, 2, 3]

Maps

val emptyMap = emptyMap<String,String>()
val map = mapOf("a" to 1, "b" to 2, "c" to 3)
println(map["b"])
// 2
val mutableMap = mutableMapOf<String,String>()
mutableMap["hello"] = "world"

Collection Manipulation

Great standard library for manipulating
filter, map, find, fold, forEach, groupBy, min/max, reduce, sort
Example:
- Find the minimum ttl value between both sources and destinations

data class DnsResult(val target: String, val ttl: Long)
val sources = listOf<DnsResult>(/* ... */)
val destinations = listOf<DnsResult>(/* ... */)

--

Solution:

val minTtl = (sources + destinations).map { it.ttl }.min()

And Lots More ...

Advanced Topics

Coroutines
Inline functions
Generics, Covariance, Reified
Type Safe Builders
Lazy initialization
Operator overloading
Java Interop: Platform Types
Scoping Functions: let, run, apply, also
Kotlin.js

Java to Kotlin - By Example

Example using Open Source project: Floodlight

Conversion tooling (IntelliJ)

Collection<IFloodlightModule> mods = serviceMap.get(s);
if (mods == null) {
    mods = new ArrayList<IFloodlightModule>();
    serviceMap.put(s, mods);
}
mods.add(m);
int dupInConf = 0;
for (IFloodlightModule cMod : mods) {
    if (mList.contains(cMod.getClass().getCanonicalName()))
        dupInConf += 1;
}
if (dupInConf > 1) {
    StringBuilder sb = new StringBuilder();
    for (IFloodlightModule mod : mods) {
        sb.append(mod.getClass().getCanonicalName());
        sb.append(", ");
    }
    String duplicateMods = sb.toString();
    String mess = "ERROR! The configuration file " +
            "specifies more than one module that " +
            "provides the service " +
            s.getCanonicalName() +
            ". Please specify only ONE of the " +
            "following modules in the config file: " +
            duplicateMods;
    throw new FloodlightModuleException(mess);
}

Java

Collection<IFloodlightModule> mods = serviceMap.get(s);
if (mods == null) {
    mods = new ArrayList<IFloodlightModule>();
    serviceMap.put(s, mods);
}
mods.add(m);

Kotlin

val mods = serviceMap.getOrPut(s) { mutableListOf() }
mods.add(m)

Java

int dupInConf = 0;
for (IFloodlightModule cMod : mods) {
    if (mList.contains(cMod.getClass().getCanonicalName()))
        dupInConf += 1;
}
if (dupInConf > 1) {
    //...
}

Kotlin

val intersection = mList.intersect(mods.map {
    it.javaClass.canonicalName
})
if (intersection.isNotEmpty()) {
    //...
}

Java

StringBuilder sb = new StringBuilder();
for (IFloodlightModule mod : mods) {
    sb.append(mod.getClass().getCanonicalName());
    sb.append(", ");
}
String duplicateMods = sb.toString();
String mess = "ERROR! The configuration file " +
        "specifies more than one module that " +
        "provides the service " +
        s.getCanonicalName() +
        ". Please specify only ONE of the " +
        "following modules in the config file: " +
        duplicateMods;
throw new FloodlightModuleException(mess);

Kotlin

throw FloodlightModuleException("""ERROR! The configuration file
    |specifies more than one module that provides the service
    |${s.canonicalName}. Please specify only ONE of the following
    |modules in the config file: ${mods.joinToString(", ")}"""
    .trimMargin())

Kotlin Result

val mods = serviceMap.getOrPut(s) { mutableListOf() }
mods.add(m)

val intersection = mList.intersect(mods.map {
    it.javaClass.canonicalName
})
if (intersection.isNotEmpty()) {
    throw FloodlightModuleException("""ERROR! The configuration file
        |specifies more than one module that provides the service
        |${s.canonicalName}. Please specify only ONE of the following
        |modules in the config file: ${mods.joinToString(", ")}"""
        .trimMargin())
}

Files

slides.md

Latest commit

History

slides.md

File metadata and controls

val kotlin: Introduction

Kotlin

Fastest Growing Programming Language

PYPL - pypl.github.io

StackOverflow

But Why?

Google IO 2017

Why Kotlin?

Kotlin Is

Kotlin Myths

Kotlin is Google's answer to Apple's Swift

Kotlin is a Swift Clone

Kotlin is only for Java Developers

Kotlin offers little over Java 8/9/10

Basics

Creating Variables

Strings

Nulls

Scoping Function: let

Elvis Operator

?: - Elvis Operator

Control Flow

If / Else

When

Exceptions

For

Putting it all together

Functions

Functions

Arguments

Lambdas / Closures

Classes

Class

Inheritance

Properties

Data Classes

Objects

Companion Object

Extension

Collections

Lists and Sets

Maps

Collection Manipulation

And Lots More ...

Advanced Topics

Java to Kotlin - By Example

Kotlin Result

Scoping Function: `let`

`?:` - Elvis Operator