Table

of Contents
Introduction 1.1
Preface 1.2

Getting started 2.1

Kotlin features 2.1.1

Hello, world 2.1.2

Hello, world (version 2) 2.1.3

The REPL 2.1.4

Two types of variables 2.1.5

The type is optional 2.1.6

Built-in types 2.1.7
Two notes about Strings 2.1.8

I/O 2.1.9
Control structures 3.1
if/then/else 3.1.1

for loops 3.1.2

while, do/while 3.1.3
when (like switch) 3.1.4

try/catch/finally 3.1.5
Classes and Objects 4.1
Constructors and members 4.1.1

Imports and packages 4.1.2

Class members 4.1.3
Getters and setters 4.1.4

Constructor default values 4.1.5

Secondary constructors 4.1.6

Open and final classes 4.1.7

Abstract classes 4.1.8

Interfaces 4.1.9
== and === 4.1.10

Enumerations 4.1.11

A complete class 4.1.12

Data classes 4.1.13

Objects 4.1.14

Companion objects 4.1.15

Visibility modifiers 4.1.16

Functions 5.1

Extension functions 5.1.1

2
Infix functions 5.1.2
Anonymous functions 5.1.3
Passing functions around 5.1.4

vararg parameters 5.1.5

Nullability 6.1
Nullable types 6.1.1

Safe-call operator 6.1.2

Elvis operator 6.1.3

let operator 6.1.4

!! operator 6.1.5

Nullability example 6.1.6

Nullability summary 6.1.7

Collections 7.1
Array 7.1.1

List 7.1.2
Map 7.1.3
Set 7.1.4

Sequence methods 7.1.5

Map methods 7.1.6
Miscellaneous 8.1

A Swing example 8.1.1

Build tools 8.1.2
Idioms 8.1.3

An OOP example 8.1.4

An FP example 8.1.5
Command line 8.1.6

Android 9.1

Contributors 10.1
License 10.2

About 10.3

3
Introduction

Kotlin Quick Reference

Introduction
Kotlin Quick Reference is intended to provide a quick reference to the Kotlin programming language. Each chapter in
the book demonstrates Kotlin syntax and provides examples to explain the chapter’s topic. This is done in a brisk
manner, with as few words as necessary.

Audience
Because this book provides only a quick reference to the Kotlin language, it’s intended for developers who have
experience in other programming languages and just need a fast reference to Kotlin features. If you need a more
complete book, Kotlin in Action is an excellent resource.

4
Introduction

5
Preface

Preface

Release information
This is Release 0.1 of Kotlin Quick Reference.

Loving Scala ... and Kotlin

Since 2011 I’ve been using the Scala programming language. I fell in love with as soon as I saw it, so much so that I
wrote three books about it:

Scala Cookbook
Functional Programming, Simplified
Hello, Scala

The only problem I have with Scala is that I enjoy developing Android applications, and with the exception of the Scala
on Android project — which hasn’t had a new release since February 24, 2017 — there isn’t a simple way to create
Android applications with Scala. So when I looked into Kotlin and saw that it was very similar to Scala, well, I was
pretty happy.

What I love about Kotlin

What I love about both Kotlin (and Scala) can be summed up this way:

The syntax is as elegant and concise as Ruby

Everything feels dynamic, but it’s statically typed
Source code compiles to class files that run on the JVM
You can use all of the thousands of Java libraries in existence
Just as I’ve seen with Scala, the Kotlin creators state that as a rough estimate, Kotlin requires about 40% fewer
lines of code than Java
Developing Android applications with Kotlin is just as easy (easier!) as creating Android applications with Java

Goal of this book

While there are now many good Kotlin books available, and the documentation on the Kotlin website is excellent, I’ve
found that what I want is a quick reference to the Kotlin language. Having used multiple programming languages
before — including Scala — I don’t need a lot of discussion about a new language, I mostly just need to see the
language’s syntax and some good examples.

Therefore, my goal in this book is to provide a quick reference to the Kotlin language — light on words, and heavy on
demonstrating syntax and examples.

The history of this book

This book originally started as a light introduction to the Kotlin language, similar to my own “Hello, Scala” book, whose
goal is to help programmers learn Scala fast. However, as I was working on it I realized that what I wanted was a
quick reference, similar in style to the O’Reilly “Nutshell” books, or the book, Scala for the Impatient.

6
Preface

Therefore, I started converting my original “Hello, Kotlin” book into this Kotlin Quick Reference. Because of that, and
because this is also a very early release of the book, some of the writing style is inconsistent. Some lessons —
especially the early lessons — are written in a “Hello, Kotlin” tutorial style, while others are written in a “Nutshell” style.
My goal is that in the long term all lessons will be written in the Nutshell style, with lots of source code examples and
few words.

Alvin Alexander
alvinalexander.com

7
Getting started

Getting Started With Kotlin

In this book I assume that you’re familiar with at least one other programming language like Java or Scala, so I don’t
spend much time on programming basics. That is, I assume that you’ve seen things like for-loops, classes, and
methods before, so I don’t try to explain object-oriented or functional programming, I generally only write, “This is how
you create a class in Kotlin,” that sort of thing.

That being said, if you’re new to Kotlin, there are a few good things to know before you jump in.

Download Kotlin
To run the examples in this book you’ll need to download the Kotlin compiler and runtime environment. Visit
kotlinlang.org for information about how to use Kotlin from the command line, or in the IntelliJ IDEA, Android Studio,
and Eclipse IDEs.

In this book I assume that you have the Java SDK and Kotlin command line tools installed.

Comments
Comments in Kotlin are just like comments in Java (and many other languages):

// a single line comment

/*
* a multiline comment
*/

/**
* also a multiline comment
*/

Naming conventions
Kotlin naming conventions follow the same “camel case” style as Java and Scala:

Class names: Person , StoreEmployee

Variable names: name , firstName
Method names: convertToInt , toUpper

Kotlin source files

Kotlin source code files end with the .kt filename extension.

Coding style
A few notes about the coding style in this book:

I indent source code lines with four spaces, but most people seem to use two spaces. I find that four spaces
makes code easier to read.

8
Getting started

In this book I use val variables in all places unless the feature I’m showing specifically requires the use of var .
It’s a best practice to always use val unless there’s a good reason not to.

9
Kotlin features

The Kotlin Programming Language

Kotlin is a modern programming language created by JetBrains, the makers of the IntelliJ IDEA IDE (and other
products). Kotlin first appeared in 2011, was open-sourced in 2012, and is now used by companies throughout the
world, including Square, Pinterest, Basecamp, Evernote, and many more.

Here are a few nuggets about Kotlin:

Per the Kotlin Language Documentation, it’s design is influenced by Java, C#, JavaScript, Scala and Groovy.
It’s a high-level language.
It’s statically typed.
It has a sophisticated type inference system.
It’s syntax is concise but still readable. From the Kotlin Language Documentation: “Rough estimates indicate
approximately a 40% cut in the number of lines of code (compared to Java).”
It’s a pure object-oriented programming (OOP) language. Every variable is an object, and every “operator” is a
method.
It can also be used as a functional programming (FP) language, so functions are also variables, and you can
pass them into other functions. You can write your code using OOP, FP, or combine them in a hybrid style.
Kotlin source code compiles to “.class” files that run on the JVM. Kotlin lets you choose between generating Java
6 and Java 8 compatible bytecode.
Kotlin works extremely well with the thousands of Java libraries that have been developed over the years.
Kotlin doesn’t have its own collections classes, it just provides extensions to the Java collections classes.
Kotlin can be used to create server-side applications, GUI applications with Swing and JavaFX, and Android
applications. It can also be compiled to JavaScript to create web browser client-side applications.
There’s also a Kotlin/Native project “to allow compilation for platforms where virtual machines are not desirable or
possible (such as iOS or embedded targets).”
A great thing about Kotlin is that you can be productive with it on Day 1, but it’s also a deep language, so as you
go along you’ll keep learning and finding newer, better ways to write code.
Of all of Kotlin’s benefits, what I like best is that it lets you write concise, readable code. The time a programmer
spends reading code compared to the time spent writing code is said to be at least a 10:1 ratio, so writing code
that’s concise and readable is a big deal. Because Kotlin has these attributes, programmers say that it’s
expressive.

As a historical tidbit, the name comes from Kotlin Island, which is near St. Petersburg, Russia, where the JetBrains
team has an office. Since Java was named after the Indonesian island of Java, the team thought it would be
appropriate to name their new language after an island.

10
Hello, world

Hello, World
Since the release of C Programming Language, most programming books have begun with a simple “Hello, world”
example, and in keeping with tradition, here’s the source code for a Kotlin “Hello, world” example:

fun main(args: Array<String>) {

println("Hello, world")
}

Using a text editor, save that source code in a file named Hello.kt.

This code is similar to Java (and Scala), but notice a few things about it:

A Kotlin function is declared with the fun keyword

Compared to Java, you only have to write println as opposed to System.out.println
In Kotlin, functions don’t have to be inside classes, so it’s perfectly legal to put a function like this in a file by itself

A fun thing about Kotlin is that you can easily create a jar file with the kotlinc compiler, so run this command at your
command line prompt to compile that source code and create a jar file named Hello.jar:

$ kotlinc Hello.kt -include-runtime -d Hello.jar

Then execute the jar file with this java command:

$ java -jar Hello.jar

Hello, world

Welcome to the Kotlin world!

11
Hello, world (version 2)

Hello, World (Part 2)

To understand how Kotlin works, let’s take a look at that Hello.kt file again:

fun main(args: Array<String>) {

println("Hello, world")
}

This time, rather than creating an executable jar file, just compile the code like this:

$ kotlinc Hello.kt

That command creates a file named HelloKt.class. Since this is a normal JVM class file, use the javap command to
disassemble it and see what’s inside:

$ javap HelloKt.class

Compiled from "Hello.kt"

public final class HelloKt {
public static final void main(java.lang.String[]);
}

As shown, Kotlin creates a class named HelloKt , and it contains a normal Java public static void main method.
Kotlin creates the class with the name HelloKt because we didn’t supply a class name. The class file name comes
from the filename — Hello.kt becomes HelloKt .

A little more fun

Before we move on, let’s have a little more fun. Save this source code to a file named HelloYou.kt:

fun main(args: Array<String>) {

if (args.size == 0)
println("Hello, you")
else
println("Hello, ${args[0]}")
}

I hope you can see how it works:

if/else statements work just like Java.

Kotlin supports String Templates, which are similar to String Interpolation in languages like Scala, Groovy, and
Ruby, so ${args[0]} prints the first command line argument.

Now compile that file and create a new executable jar file:

$ kotlinc HelloYou.kt -include-runtime -d HelloYou.jar

Then run it with and without a command line argument:

$ java -jar HelloYou.jar

Hello, you

$ java -jar HelloYou.jar Alvin

12
Hello, world (version 2)

Hello, Alvin

Extra credit
To have a little more fun with this example, run this Java jar command on HelloYou.jar:

$ jar tvf HelloYou.jar

If you know Java and how the JVM works, you can guess that the initial output of that command looks like this:

79 Wed Aug 01 13:54:12 MDT 2018 META-INF/MANIFEST.MF

1249 Wed Aug 01 13:54:12 MDT 2018 HelloYouKt.class

The rest of the output is too long to include here — 654 lines total, to be precise — but I encourage you to run that
command to get an idea of what’s needed to create an executable jar file.

13
The REPL

The Kotlin REPL

The Kotlin REPL (“Read-Evaluate-Print-Loop”) is a command-line interpreter that you use as a “playground” area to
test your Kotlin code. To start a REPL session, just type kotlinc at your operating system command line, and you’ll
see this:

> kotlinc
Welcome to Kotlin version 1.2.51 (JRE 1.8.0_181-b13)
Type :help for help, :quit for quit
>>> _

The prompt for the Kotlin REPL is >>> , but because I don’t like that I only show one > character for the
prompt in this book.

Because the REPL is a command-line interpreter, it just sits there waiting for you to type something. Once you’re in
the REPL, you can type Kotlin expressions to see how they work:

> 1 + 1
2

> 4 / 2
2

> 4 % 2
0

As those examples show, when you type simple expressions, the Kotlin REPL shows the result of the expression on
the line after the prompt. For other expressions you may have to type the variable name you created to see its value:

> val x = 1 + 1

> x
2

You can also try Kotlin online at try.kotlinlang.org/

Take the REPL for a spin

You’re going to use the Kotlin REPL a lot in this book, so go ahead and start it (with kotlinc ) and experiment with it.
Here are a few expressions you can try to see how it all works:

1 + 1
4 / 2
5 / 2
4 % 2
5 % 2
5 / 2.0
1 + 2 * 3
(1 + 2) * 3
val name = "John Doe"
"hello"[0]
"hello"[1]
"hello, world".take(5)
println("hi")
if (2 > 1) println("greater") else println("lesser")
for (i in 1..3) println(i)

14
The REPL

listOf(1,2,3).forEach { println(it) }

As a little more advanced exercise, here’s how to define a class named Person , create an instance of it, and then
show the value of the name field:

class Person(var name: String)

val p = Person("Kim")
p.name

Notice that you don’t need the new keyword when creating an instance of a class.

Use javaClass to see an object’s type

To see an object’s type in the REPL, call .javaClass on the instance:

> "foo".javaClass
class java.lang.String

> 1.javaClass
int

> 1.0.javaClass
double

Here’s a map:

// paste this into the repl

val map = mapOf(
1 to "one",
2 to "two"
)

> map.javaClass
class java.util.LinkedHashMap

Here’s a list:

> listOf(1,2,3).javaClass
class java.util.Arrays$ArrayList

The following examples on the result of intArrayOf show other calls you can make after javaClass :

> val x = intArrayOf(1,2,3)

> println(x.javaClass.name)
[I

> println(x.javaClass.kotlin)
class kotlin.IntArray

> println(x.javaClass.kotlin.qualifiedName)
kotlin.IntArray

15
The REPL

16
Two types of variables

Two Types of Variables

Kotlin has two types of variable declarations:

val creates an immutable variable (like final in Java)

var creates a mutable variable

Examples:

val s = "hello" // immutable

var i = 42 // mutable

val p = Person("Hala")

Notes:

Kotlin can usually infer the variable’s data type from the code on the right side of the = sign.
This is considered an implicit form.
You can also explicitly declare the variable type if you prefer:

val s: String = "hello"

var i: Int = 42

The implicit form is generally preferred

I use the explicit form when I don’t think the type is obvious; code is easier to maintain that way

The difference between val and var

The REPL shows what happens when you try to reassign a val field:

> val a = 'a'

> a = 'b'
error: val cannot be reassigned
a = 'b'
^

That fails with a “val cannot be reassigned” error, as expected. Conversely, you can reassign a var :

> var a = 'a'

> a = 'b'

> a
b

The general rule is that you should always use a val field unless there’s a good reason not to. This simple rule has
several benefits:

It makes your intention obvious: you don’t want this field to be reassigned
It makes your code more like algebra
If you ever want to go there, it helps get you started down the path to functional programming, where all fields are
immutable

17
Two types of variables

“Hello, world” with a val field

Here’s a “Hello, world” app with a val field:

fun main(args: Array<String>) {

val s = "Hello, world"
println(s)
}

As before:

Save that code in a file named HelloVal.kt

Compile it with kotlinc HelloVal.kt -include-runtime -d HelloVal.jar
Run it with java -jar HelloVal.jar

Deferred initialization of val

Per Kotlin in Action, “a val variable must be initialized exactly once during the execution of the block where it’s
defined ... but you can initialize it with different values depending on some condition.” Examples:

val name: String

val num: Int

val r = (1..10).shuffled().first()

// assign `name` and `num`

name = if (r % 2 == 0) "Alvin" else "Alexander"
num = r

println("name = $name, num = $num")

A note about val fields in the REPL

You can’t reassign a val field in the REPL:

> val age = 18

> age = 19
error: val cannot be reassigned
age = 19
^

However, you can do this in the REPL:

> val age = 18

> val age = 19

I thought I’d mention that because I didn’t want you to see it one day and think, “Hey, Al said val fields couldn’t be
reassigned.” They can be reassigned like that, but only in the REPL.

18
Two types of variables

19
The type is optional

Declaring the Type is Optional

As I showed in the previous lesson, when you create a new variable in Kotlin you can explicitly declare its type, like
this:

val count: Int = 1

val name: String = "Alvin"

However, you can generally leave the type off and Kotlin can infer it for you:

val count = 1
val name = "Alvin"

In most cases your code is easier to read when you leave the type off, so this implicit form is preferred.

The explicit form feels verbose

One thing you’ll find is that the explicit form feels verbose. For instance, in this example it’s obvious that the data type
is Person , so there’s no need to declare the type on the left side of the expression:

val p = Person("Candy")

By contrast, when you put the type next to the variable name, the code feels unnecessarily verbose:

val p: Person = Person("Leo")

Summary:

val p = Person("Candy") // preferred

val p: Person = Person("Candy") // unnecessarily verbose

Use the explicit form when you need to be clear

One place where you’ll want to show the data type is when you want to be clear about what you’re creating. That is, if
you don’t explicitly declare the data type, the compiler may make a wrong assumption about what you want to create.
Some examples of this are when you want to create numbers with specific data types. I show this in the next lesson.

20
Built-in types

Built-In Data Types

Kotlin comes with the standard numeric data types you’d expect, and in Kotlin all of these data types are full-blown
objects — not primitive data types.

How to declare variables of the basic numeric types:

val b: Byte = 1
val i: Int = 1
val l: Long = 1
val s: Short = 1
val d: Double = 2.0
val f: Float = 3.0f

In the first four examples, if you don’t explicitly specify a type, the number 1 will default to an Int , so if you want
one of the other data types — Byte , Long , or Short — you need to explicitly declare those types, as shown.
Numbers with a decimal (like 2.0) will default to a Double , so if you want a Float you need to declare a Float , as
shown in the last example. You can also declare Long and Float types like this:

val l = 1L
val f = 3.0f

Because Int and Double are the default numeric types, you typically create them without explicitly declaring the
data type:

val i = 123 // defaults to Int

val x = 1.0 // defaults to Double

All of those data types have the same data ranges as their Java equivalents:

Type Bit width

Byte 8

Short 16

Int 32

Long 64

Float 32

Double 64

(For more information on those, see my article, JVM bit sizes and ranges.)

BigInteger and BigDecimal

In Kotlin you can use the java.math.BigInteger class:

> import java.math.BigInteger

> val x = BigInteger("1")

Kotlin also has convenient extension functions to help you convert other data types to BigInteger :

21
Built-in types

> val y = 42.toBigInteger()

> val y = 42L.toBigInteger()

Kotlin lets you use the Java BigDecimal class in similar ways:

> import java.math.BigDecimal

> val x = BigDecimal("1.0")
> 1.0.toBigDecimal()

See these links for more information:

Kotlin BigInteger extension functions

Kotlin BigDecimal extension functions

String, Char, and Boolean

Kotlin also has String , Char , and Boolean data types, which I always declare with the implicit form:

val name = "Bill"

val c = 'c'
val b = true

22
Two notes about Strings

Strings
This lesson shows:

String templates
Multiline strings
How to convert a list or array to a String

String templates
Kotlin has a nice, Ruby- and Scala-like way to merge multiple strings known as String Templates. Given these three
variables:

val firstName = "John"

val mi = 'C'
val lastName = "Doe"

you can append them together like this:

val name = "$firstName $mi $lastName"

This creates a very readable way to print multiple strings:

println("Name: $firstName $mi $lastName")

You can also include complete expressions inside strings:

> val x = 2
> val y = 3
> println("$x times $y is ${x * y}.")
2 times 3 is 6.

Multiline strings
You can create multiline strings by including the string inside three parentheses:

val speech = """Four score and

seven years ago
our fathers ..."""

One drawback of this basic approach is that lines after the first line are indented, as you can see in the REPL:

> speech
Four score and
seven years ago
our fathers ...

A simple way to fix this problem is to put a | symbol in front of all lines after the first line, and call the trimMargin
function after the string:

23
Two notes about Strings

val speech = """Four score and

|seven years ago
|our fathers ...""".trimMargin()

The REPL shows that when you do this, all of the lines are left-justified:

> speech
Four score and
seven years ago
our fathers ...

How to convert a list to a String

How to convert a list or array to a String :

val nums = listOf(1,2,3,4,5)

> nums.joinToString()
1, 2, 3, 4, 5

> nums.joinToString(
separator = ", ",
prefix = "[",
postfix = "]",
limit = 3,
truncated = "there’s more ..."
)
[1, 2, 3, there’s more ...]

Another example:

val words = arrayOf("Al", "was", "here")

> words.joinToString()
Al, was, here

> words.joinToString(separator = " ")

Al was here

24
I/O

I/O With Kotlin

To get ready to show for loops, if expressions, and other Kotlin constructs, let’s take a look at how to handle I/O
with Kotlin.

Writing to STDOUT and STDERR

Write to standard out (STDOUT) using println :

println("Hello, world") // includes newline

print("Hello without newline") // no newline character

Because println is so commonly used, there’s no need to import it.

Writing to STDERR:

System.err.println("yikes, an error happened")

Reading command-line input

A simple way to read command-line (console) input is with the readLine() function:

print("Enter your name: ")

val name = readLine()

readLine() provides a simple way to read input. For more complicated needs you can also use the java.util.Scanner

class, as shown in this example:

import java.util.Scanner
val scanner = Scanner(System.`in`)
print("Enter an int: ")
val i: Int = scanner.nextInt()
println("i = $i")

Just be careful with the Scanner class. If you’re looking for an Int and the user enters something else, you’ll end up
with a InputMismatchException :

>>> val i: Int = scanner.nextInt()

1.1
java.util.InputMismatchException
at java.util.Scanner.throwFor(Scanner.java:864)
at java.util.Scanner.next(Scanner.java:1485)
at java.util.Scanner.nextInt(Scanner.java:2117)
at java.util.Scanner.nextInt(Scanner.java:2076)

I write more about the Scanner class in my article, How to prompt users for input in Scala shell scripts.

Reading text files

25
I/O

There are a number of ways to read text files in Kotlin. While this approach isn’t recommended for large text files, it’s a
simple way to read small-ish text files into a List<String> :

import java.io.File
fun readFile(filename: String): List<String> = File(filename).readLines()

Here’s how you use that function to read the /etc/passwd file:

val lines = readFile("/etc/passwd")

And here are two ways to print all of those lines to STDOUT:

lines.forEach{ println(it) }
lines.forEach{ line -> println(line) }

Other ways to read text files

Here are a few other ways to read text files in Kotlin:

fun readFile(filename: String): List<String> = File(filename).readLines()

fun readFile(filename: String): List<String> = File(filename).bufferedReader().readLines()

fun readFile(filename: String): List<String> = File(filename).useLines { it.toList() }

fun readFile(filename: String): String = File(filename).inputStream().readBytes().toString(Charsets.UTF_8)

val text = File("/etc/passwd").bufferedReader().use { it.readText() }

The file-reading function signatures look like this:

// Do not use this function for huge files.

fun File.readLines(
charset: Charset = Charsets.UTF_8
): List<String>

fun File.bufferedReader(
charset: Charset = Charsets.UTF_8,
bufferSize: Int = DEFAULT_BUFFER_SIZE
): BufferedReader

fun File.reader(
charset: Charset = Charsets.UTF_8
): InputStreamReader

fun <T> File.useLines(

charset: Charset = Charsets.UTF_8,
block: (Sequence<String>) -> T
): T

// This method is not recommended on huge files. It has an internal limitation of 2 GB byte array size.
fun File.readBytes(): ByteArray

// This method is not recommended on huge files. It has an internal limitation of 2 GB file size.
fun File.readText(charset: Charset = Charsets.UTF_8): String

See the Kotlin java.io.File documentation for more information and caveats about the methods shown ( readLines ,
useLines , etc.).

26
I/O

Writing text files

There are several ways to write text files in Kotlin. Here’s a simple approach:

File(filename).writeText(string)

That approach default to the UTF-8 character set. You can also specify the Charset when using writeText :

File(filename).writeText(string, Charset.forName("UTF-16"))

Other ways to write files

There are other ways to write to files in Kotlin. Here are some examples:

File(filename).writeBytes(byteArray)
File(filename).printWriter().use { out -> out.println(string) }
File(filename).bufferedWriter().use { out -> out.write(string) }

The file-writing function signatures look like this:

fun File.writeText(
text: String,
charset: Charset = Charsets.UTF_8
)

fun File.writeBytes(array: ByteArray)

fun File.printWriter(
charset: Charset = Charsets.UTF_8
): PrintWriter

fun File.bufferedWriter(
charset: Charset = Charsets.UTF_8,
bufferSize: Int = DEFAULT_BUFFER_SIZE
): BufferedWriter

See the Kotlin java.io.File documentation for more information about the methods shown.

27
Control structures

Kotlin Control Structures

Kotlin has the basic control structures you’d expect to find in a programming language, including:

if/then/else-if/else
for loops

try/catch/finally
while and do..while loops

It also has unique constructs, including:

when expressions, which are like improved Java switch statements

I’ll demonstrate those in the following lessons.

28
if/then/else

if/then/else
if statements in Kotlin are just like Java, with one exception: they are an expression, so they always return a result.

Basic syntax
A basic Kotlin if statement:

if (a == b) doSomething()

You can also write that statement like this:

if (a == b) {
doSomething()
}

if/else
The if / else construct:

if (a == b) {
doSomething()
} else {
doSomethingElse()
}

if/else-if/else
The complete Kotlin if/else-if/else expression looks like this:

if (test1) {
doX()
} else if (test2) {
doY()
} else {
doZ()
}

if expressions always return a result

The if construct always returns a result, meaning that you can use it as an expression. This means that you can
assign its result to a variable:

val minValue = if (a < b) a else b

This means that Kotlin doesn’t require a special ternary operator.

29
if/then/else

30
for loops

Kotlin for Loops

for loops are similar to Java, but the syntax is different

Unlike in Scala, Kotlin for loops are not expressions

Basic syntax
Kotlin for loops have this basic syntax:

for (e in elements) { // do something with e ...}

For example, given a list:

val nums = listOf(1,2,3)

here’s a single-line algorithm:

for (n in nums) println(n)

The REPL shows how it works:

>>> for (n in nums) println(n)

1
2
3

You can also use multiple lines of code in your algorithm:

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

Here are a couple of other examples:

// directly on listOf
for (n in listOf(1,2,3)) println(n)

// 1..3 creates a range

for (n in 1..3) println(n)

Using for with Maps

You can also use a for loop with a Kotlin Map (which is similar to a Java HashMap ). This is how you create a Map
of movie names and ratings:

val ratings = mapOf(

"Lady in the Water" to 3.0,
"Snakes on a Plane" to 4.0,
"You, Me and Dupree" to 3.5
)

31
for loops

Given that map, you can print the movie names and ratings with this for loop:

for ((name,rating) in ratings) println("Movie: $name, Rating: $rating")

Here’s what that looks like in the REPL:

>>> for ((name,rating) in ratings) println("Movie: $name, Rating: $rating")

Movie: Lady in the Water, Rating: 3.0
Movie: Snakes on a Plane, Rating: 4.0
Movie: You, Me and Dupree, Rating: 3.5

In this example, name corresponds to each key in the map, and rating is the name I assign for each value in the
map.

for is not an expression

If you’re coming to Kotlin from Scala, it’s important to note that for is NOT an expression. That means that code like
this does not work:

// error
val x = for (n in 1..3) {
return n * 2
}

32
while, do/while

while and do..while

Kotlin’s while and do..while constructs are just like Java.

while
Here’s while :

var i = 1

while (i < 5) {
println(i++)
}

Here’s its output in the REPL:

>>> while (i < 5) {

... println(i++)
... }
1
2
3
4

do..while
Here’s do..while:

var i = 1

do {
println(i++)
} while (i < 0)

and its REPL output:

>>> do {
... println(i++)
... } while (i < 0)
1

Notice that do..while always runs at least once because it executes its body before running the first test.

33
when (like switch)

when Expressions
Kotlin when expressions are like Java switch statements, but you’ll see in this lesson, they’re much more powerful.
If you’re experienced with Scala, when is similar to Scala’s match expression.

Replacement for switch

In its most basic use, when can be used as a replacement for a Java switch statement:

val dayAsInt = 1

when (dayAsInt) {
1 -> println("Sunday")
2 -> println("Monday")
3 -> println("Tuesday")
4 -> println("Wednesday")
5 -> println("Thursday")
6 -> println("Friday")
7 -> println("Saturday")
else -> {
// notice you can use a block
println("invalid day")
}
}

when is an expression

when can also be used as an expression, meaning that it returns a value:

val dayAsInt = 1

val dayAsString = when (dayAsInt) {

1 -> "Sunday"
2 -> "Monday"
3 -> "Tuesday"
4 -> "Wednesday"
5 -> "Thursday"
6 -> "Friday"
7 -> "Saturday"
else -> "invalid day"
}

println(dayAsString)

Matches must be exhaustive

When when is used as an expression you generally must include an else clause. If you don’t, you risk the chance of
getting an error, as in this example:

val i = 0

val result = when (i) {

1 -> "a little odd"
2 -> "a little even"
}

34
when (like switch)

That code produces this error message:

error: 'when' expression must be exhaustive, add necessary 'else' branch

val result = when (i) {
^

Multiple branch conditions

When you have multiple branch conditions with constants that should be handled the same way, they can be
combined in one statement with a comma:

when (i) {
1,2,3 -> println("got a 1, 2, or 3")
4,5,6 -> println("got a 4, 5, or 6")
else -> println("something else")
}

Testing against ranges

You can also check a value for being in a range ( in ) or not in a range ( !in ):

val i = 1

when (i) {
in 1..3 -> println("1, 2, or 3")
!in 4..5 -> println("not a 4 or 5")
else -> println("something else")
}

// result: 1, 2, or 3

The order of the expressions is important. In this example I reverse the first two possible matches, and get a different
result:

val i = 1

when (i) {
!in 4..5 -> println("not a 4 or 5")
in 1..3 -> println("1, 2, or 3")
else -> println("something else")
}

// result: not a 4 or 5

These examples also show that the when expression stops on the first statement that matches the given value.

in with listOf
Similar to ranges, you can also use in with listOf as the predicate condition:

val i = 1
when (i) {
in listOf(1,3,5) -> println("a little odd")
in listOf(2,4,6) -> println("a little even")
else -> println("something else")
}

35
when (like switch)

Expressions as branch conditions

when expressions aren’t limited to using only constants, you can also use any sort of predicate as a branch condition.

Here are some tests with Kotlin’s is operator:

val x: Any = 11.0

when (x) {
is Boolean -> println("$x is a Boolean")
is Double -> println("$x is a Double")
is String -> println("$x is a String")
!is String -> println("$x is not a String")
else -> println("$x is something else")
}

When you run that code as shown the result is:

11.0 is a Double

Here’s an example that demonstrates how to use a function ( toUpperCase() ) in a branch condition:

fun isUpperCase(s: String): Boolean {

return when (s) {
s.toUpperCase() -> true
else -> false
}
}

The REPL shows how this works:

>>> isUpperCase("FOO")
true

>>> isUpperCase("foo")
false

>>> isUpperCase("Foo")
false

when without arguments

If you don’t give when an argument, it can be used as a replacement for an if/else expression:

val i = 1

when {
i < 0 -> println("less than zero")
i == 0 -> println("zero")
else -> println("greater than zero")
}

Key point: Notice that this example uses when , and not when(i) .

This syntax is useful for using when as the body of a function:

36
when (like switch)

fun intToString(i: Int): String = when {

i < 0 -> "a negative number!"
i == 0 -> "0"
else -> "a positive number!"
}

Notice in this example that when is used as the body of the intToString function, and returns a String . If you
haven’t seen that approach before, it can help to see it broken down into steps:

fun intToString(i: Int): String {

// convert the Int to a String
val string = when {
i < 0 -> "a negative number!"
i == 0 -> "0"
else -> "a positive number!"
}
// return the String
return string
}

Smart casts with when

When you use an is expression as a branch condition you can check the type of the variable passed into when ,
and call methods on it on the right-hand side of the expression. Here’s an example:

fun getInfo (x: Any): String {

return when (x) {
is Int -> "The Int plus one is ${x + 1}"
is String -> "Got a String, length is " + x.length
else -> "Got something else"
}
}

Here’s what the function looks like if you call it several times with different types:

println(getInfo(1)) //The Int plus one is 2

println(getInfo("foo")) //Got a String, length is 3
println(getInfo(1L)) //Got something else

Even more!
While that covers most of when ’s functionality, there are even more things you can do with it.

37
try/catch/finally

Kotlin try/catch/finally Expressions

Like Java, Kotlin has a try/catch/finally construct to let you catch and manage exceptions. If you’ve used Java, there’s
only one new feature here: a try expression is truly an expression, meaning you can assign its result to a variable.
(See the “Try is an expression” section below for those details.)

Basic syntax
Kotlin’s try/catch/finally syntax looks like this:

try {
// some exception-throwing code
}
catch (e: SomeException) {
// code
}
catch (e: SomeOtherException) {
// code
}
finally {
// optional
}

As shown in the comment, the finally clause is optional. Everything works just like Java.

Example
Because there are easier ways to read files in Kotlin, this example is a little contrived, but it shows an example of how
to use try with multiple catch clauses and a finally clause:

import java.io.*

fun main(args: Array<String>) {

var linesFromFile = listOf<String>()

var br: BufferedReader? = null

try {
br = File("/etc/passwd2").bufferedReader()
linesFromFile = br.readLines()
}
catch (e: IOException) {
e.printStackTrace()
}
catch (e: FileNotFoundException) {
e.printStackTrace()
}
finally {
br?.close()
}

linesFromFile.forEach { println("> " + it) }

38
try/catch/finally

Don’t worry about the ? symbols in the code for now, I just wanted to show a complete try/catch/finally example. Just
know that the question marks are needed for working with null values in Kotlin.

Or, if you want to learn about them, jump ahead to the “Nullability” lessons later in this book.

Try is an expression

Try in Kotlin is different than Java because Kotlin’s try is an expression, meaning that it returns a value:

val s = "1"

// using try as an expression

val i = try {
s.toInt()
}
catch (e: NumberFormatException) {
0
}

That expression can be read as, “If s converts to an Int , return that integer value; otherwise, if it throws a
NumberFormatException , return 0 .”

You can write that try expression more concisely:

val i = try { s.toInt() } catch (e: NumberFormatException) { 0 }

Try that expression with different values like these to confirm for yourself that it works as expected:

val s = "1"
val s = "foo"

39
Classes and Objects

Classes and Objects

This section of the book covers everything related to classes and objects, including:

Classes
Data classes
Objects
The object keyword
Companion objects
More ...

40
Constructors and members

Constructors and Members

In support of object-oriented programming (OOP), Kotlin provides a class construct. The syntax is much more concise
than languages like Java and C#, but it’s also still easy to use and read.

Overview
Classes are created with the class keyword
Primary constructor parameters go in the class header
var parameters are read-write, and val parameters are read-only

Class instances are created without the new keyword

Think of it like you’re calling a function
Classes can have initializer blocks
Classes can have properties and methods (functions)
Classes can have nested classes and inner classes

Creating a class
Classes are created with class keyword:

class Foo
class Person constructor(var firstName: String, var lastName: String)

The primary constructor is part of the class header. If the primary constructor does not have any annotations or
visibility modifiers, the constructor keyword can be omitted:

class Person(var firstName: String, var lastName: String)

I show annotations and visibility modifiers at the end of this lesson.

How to create an instance of a class

New instances of classes are created without the new keyword. You can think of them as being like function calls:

val f = Foo()
val p = Person("Bill", "Panner")

Comparison to Java
With the exception of the names of the getter and setter functions, if you’re coming to Kotlin from Java, this Kotlin
code:

class Person(var firstName: String, var lastName: String)

is the equivalent of this Java code:

41
Constructors and members

public class Person {

private String firstName;

private String lastName;

public Person(String firstName, String lastName) {

this.firstName = firstName;
this.lastName = lastName;
}

public String getFirstName() {

return this.firstName;
}

public void setFirstName(String firstName) {

this.firstName = firstName;
}

public String getLastName() {

return this.lastName;
}

public void setLastName(String lastName) {

this.lastName = lastName;
}

Constructor parameter visibility

Class constructor parameters are defined in the class header:

class Person(var firstName: String, var lastName: String)

--- ---

class Person(val firstName: String, val lastName: String)

--- ---

var means fields can be read-from and written-to (they are mutable); they have both a getter and a setter. val

means the fields are read-only (they are immutable).

Accessing constructor parameters

Given this definition with var fields:

class Person(var firstName: String, var lastName: String)

--- ---

you can create a new Person instance like this:

val p = Person("Bill", "Panner")

You access the firstName and lastName fields like this:

println("${p.firstName} ${p.lastName}")
Bill Panner

42
Constructors and members

The REPL shows that the fields can be updated:

> class Person(var firstName: String, var lastName: String)

> val p = Person("Bill", "Panner")

> p.firstName = "William"

> p.lastName = "Bernheim"

> println("${p.firstName} ${p.lastName}")

William Bernheim

val fields are read-only

Had the fields been defined as val :

class Person(val firstName: String, val lastName: String)

--- ---

you can still access them (via a getter method), but you can’t change their values (their is no setter method):

> class Person(val firstName: String, val lastName: String)

> val p = Person("Bill", "Panner")

// can access the fields

> println(p.firstName)
Bill
> println(p.lastName)
Panner

// can’t mutate the fields

> p.firstName = "William"
error: val cannot be reassigned
p.firstName = "William"
^
> p.lastName = "Bernheim"
error: val cannot be reassigned
p.lastName = "Bernheim"
^

Summary:

val fields are read-only

var fields are read-write

Pro tip: If you use Kotlin to write OOP code, create your fields as var fields so you can easily mutate them. If
you prefer an FP style of programming you may prefer Kotlin’s data classes. (More on this later.)

A note about annotations and visibility modifiers

Per the Kotlin documentation, “If the constructor has annotations or visibility modifiers, the constructor keyword is
required, and the modifiers go before it”:

class Customer public @Inject constructor(name: String) { ... }

-----------

Visibility modifiers

43
Constructors and members

Visibility modifiers are:

private
protected
internal
public

Per this kotlinlang.org link, “classes, objects, interfaces, constructors, functions, properties and their setters can have
visibility modifiers.”

Here’s an example of internal :

// `internal` is visible everywhere in the same “module”

// see https://kotlinlang.org/docs/reference/visibility-modifiers.html
class TabAdapter internal constructor(fm: FragmentManager) : FragmentStatePagerAdapter(fm) {...
--------

44
Imports and packages

Imports and Packages

Import and package statements are very similar to Java, with just a few additions/improvements.

Key points
Packages:

Package statements are just like Java, but they don’t have to match the directory the file is in

Import statements:

There is no import static syntax

You can rename a class when you import it
The import statement is not restricted to only importing class

You can also import:

Top-level functions and properties

Functions and properties declared in object declarations
Enum constants

Finally, a collection of classes and functions are imported for you by default, such as kotlin.*

Package statements
Put package statements at the top of a file:

package foo.bar.baz

// the rest of your code here ...

The only real difference with Java is that the package name doesn’t have to match the name of the directory that the
file is in.

Files don’t have to contain class declarations

Because a Kotlin file doesn’t have to contain a class, it’s perfectly legal to put one or more functions in a file:

package foo.bar

fun plus1(i: Int) = i + 1

fun double(i: Int) = i * 2

Because of the package name, you can now refer to the function plus1 in the rest of your code as foo.bar.plus1 .
Or, as you’ll see in the import examples that follow, you can import the function into the scope of your other code like
this:

import foo.bar.plus1

45
Imports and packages

Import statements
Import statements work just like Java, with only a few differences:

There is no import static syntax

You can rename a class when you import it
The import statement is not restricted to only importing class

No “import static”
Kotlin doesn’t have a separate import static syntax. Just use a regular import declaration to import static methods
and fields:

>>> import java.lang.Math.PI

>>> PI
3.141592653589793

>>> import java.lang.Math.pow

>>> pow(2.0, 2.0)
4.0

Renaming a class when you import it

To avoid namespace collisions you can rename a class when you import it:

import java.util.HashMap as JavaHashMap

The REPL shows how it works:

>>> import java.util.HashMap as JavaHashMap

>>> val map = JavaHashMap<String, String>();

>>> map.put("first_name", "Alvin")

null

>>> map
{first_name=Alvin}

Here’s a combination of the two techniques just shown, (a) importing a static field and (b) renaming it during the
import process:

import java.lang.Integer.MAX_VALUE as MAX_INT

import java.lang.Long.MAX_VALUE as MAX_LONG

Here are the values in the REPL:

>>> MAX_INT
2147483647

>>> MAX_LONG
9223372036854775807

Things you can import

46
Imports and packages

Per the official Kotlin documentation, in addition to importing classes, you can also import:

Top-level functions and properties

Functions and properties declared in object declarations
Enum constants

Default imports
When you first start working with Kotlin it can be a surprise that you can use the println function without having to
write System.out.println every time. This is partly due to the fact that a collection of packages are imported into
every Kotlin file by default:

kotlin.*
kotlin.annotation.*
kotlin.collections.*
kotlin.comparisons.* (since 1.1)
kotlin.io.*
kotlin.ranges.*
kotlin.sequences.*
kotlin.text.*

The println function is declared in the kotlin.io package, and it’s automatically made available to you.

When working with the Java Virtual Machine (JVM), these packages are also automatically imported:

java.lang.*
kotlin.jvm.*

When you’re compiling Kotlin code to JavaScript this package is imported by default:

kotlin.js.*

47
Class members

Class Members
Classes can contain:

Constructors
Constructor initializer blocks
Functions
Properties
Nested and Inner Classes
Object Declarations

Constructors
Basic constructor syntax:

class Person

class Person (name: String, age: Int) {

// class code here ...
}

The constructor keyword is only needed if the class has annotations or visibility modifiers:

class Person private constructor (name: String) { ... }

class Person public @Inject constructor (name: String) { ... }
class Person @JvmOverloads constructor (name: String, age: Int) { ... }

Primary constructor initialization code goes in an init block:

class Person (name: String) {

init {
println("Person instance created")
}
}

Initializer blocks
Classes can have initializer blocks that are called when the class is constructed, and those blocks can access the
constructor parameters:

// a network socket
class Socket(var timeout: Int, var linger: Int) {
init {
println("Entered 'init' ...")
println("timeout = ${timeout}, linger = ${linger}")
}
}

Here’s what it looks like when a Socket is created:

> val s = Socket(2000, 3000)

Entered 'init' ...

48
Class members

timeout = 2000, linger = 3000

You can have multiple initializer blocks, and the code in those blocks is effectively part of the primary constructor.

Methods (functions)
Just like Java and other languages, classes can have methods (which are referred to as functions):

class Person(val firstName: String, val lastName: String) {

fun fullName() = "$firstName $lastName"
}

Here’s a REPL example:

> val p = Person("Hala", "Cina")

> p.fullName()
Hala Cina

Properties (fields)
Just like Java and other languages, classes can have properties (fields):

class Pizza () {
val toppings = mutableListOf<String>()
fun addTopping(t: String) { toppings.add(t) }
fun showToppings() { println(toppings) }
// more code ...
}

Here’s an example in the REPL:

> val p = Pizza()

> p.addTopping("Cheese")
> p.addTopping("Pepperoni")
> p.showToppings()
[Cheese, Pepperoni]

Nested and inner classes

Classes can be nested in other classes. Note that a nested class can’t access a parameter in the outer class:

class Outer {
private val x = 1
class Nested {
//fun foo() = 2 * x //this won’t compile
fun foo() = 2
}
}

In the REPL:

> val foo = Outer.Nested().foo()

> foo

49
Class members

Inner classes
Mark a class as inner if you need to access members of the outer class:

class Outer {
private val x = 1
inner class Inner {
fun foo() = x * 2
}
}

In the REPL:

> val foo = Outer().Inner().foo()

> foo
2

Syntax
Notice the difference between the syntax when using a nested class versus an inner class:

val foo = Outer.Nested().foo()

val foo = Outer().Inner().foo()
--

If you try to create an inner class without first creating an instance of the outer class you’ll see this error:

> val foo = Outer.Inner().foo()

error: constructor of inner class Inner can be called only with receiver of containing class
val foo = Outer.Inner().foo()
^

50
Getters and setters

Custom Class Field Getters and Setters

Key points
Kotlin classes can have properties (what we call fields in Java)
Properties can be defined as val or var
val fields are read-only, var fields are read-write

Fields are accessed directly by their name, just like constructor parameters
TODO: Properties are public by default, but can also be private
You can write custom getter and setter (accessor and mutator) methods for your properties with a special syntax
The getter/setter methods can use a “backing field” named field (TODO: field is kind of a reserved word in
Kotlin but not really)

A field without custom getters or setters

If you just want a simple property in a class that you can read and modify, declare the field as a var :

class Person() {
var name = ""
}

// create an instance
>>> val p = Person()

// no initial value
>>> p.name

// set the value

>>> p.name = "Al"

// get (access) the value

>>> p.name
Al

That shows how to get and set values of a simple property.

Custom get/set methods

Conversely, if you want to create custom get/set methods, define custom get() and set() methods just below the
property. For example, imagine that you want to make a log entry every time someone accesses or updates the name
of a Person :

class Person {
var name: String = "<no name>"
get() {
println("OMG, someone accessed 'name'")
return field
}
set(s) {
println("OMG, someone updated 'name' to be '$s'")
field = s
}
}

51
Getters and setters

The REPL shows how this works:

>>> val p = Person()

>>> p.name
OMG, someone accessed 'name'
<no name>

>>> p.name = "Al"

OMG, someone updated 'name' to be 'Al'

>>> p.name
OMG, someone accessed 'name'
Al

Key points:

The getter/setter methods can use a “backing field” named field

field is a way of referencing the value you’re referencing, in this case the name field

field is made available to you, but you don’t have to use it

Per the official Kotlin documentation, “A backing field will be generated for a property if it uses the default
implementation of at least one of the accessors, or if a custom accessor references it through the field identifier.”

More examples
Here are a few more examples of Kotlin’s getter/setter approach.

First, a one-line getter with a multiline setter, both using the special backing field:

class Foo {
var prop: String = "initial value"
get() = field //no real need for this, just showing syntax
set(s) {
// do whatever you want with `s` ...
// then:
field = s
}
}

Second, multiline getters and setters:

class Bar {
var name = ""
get() {
val s = if (field == "") "hello" else "hello, ${field}"
return s
}
set(s) {
// do whatever you want with `s`
field = s
}
}

This is how you can use your own “backing field” instead of field :

class Baz {
private var _name = ""
var name = ""
fun get() = _name
fun set(s: String) {

52
Getters and setters

_name = s
}
}

Finally, here’s an example that shows get/set methods alongside other functions in a class:

class BlogPost {
var content = ""
get() {
logAccess(Date())
return field
}
set(s) {
logUpdate(Date(), s)
field = s
}
fun logAccess(d: Date) {
println("logAccess called ...")
//...
}
fun logUpdate(d: Date, s: String) {
println("logUpdate called ...")
//...
}
}

53
Constructor default values

Constructor Default Values and Named Arguments

Kotlin has two nice features that you’ll also find in Scala:

You can supply default values for constructor parameters

You can use named arguments when calling a constructor

Default values for constructor parameters

A convenient Kotlin feature is that you can supply default values for constructor parameters. For example, you could
define a Socket class like this:

class Socket(var timeout: Int, var linger: Int) {

override def toString = s"timeout: $timeout, linger: $linger"
}

That’s nice, but you can make this class even better by supplying default values for the timeout and linger
parameters:

class Socket(var timeout: Int = 2000, var linger: Int = 3000) {

override fun toString(): String = "timeout: ${timeout}, linger: ${linger}"
}

By supplying default values for the parameters, you can now create a new Socket in a variety of different ways:

Socket()
Socket(1000)
Socket(4000, 6000)

This is what those examples look like in the REPL:

>>> Socket()
timeout: 2000, linger: 3000

>>> Socket(1000)
timeout: 1000, linger: 3000

>>> Socket(4000, 6000)

timeout: 4000, linger: 6000

As those examples shows:

When all parameters have default values, you don’t have to provide any values when creating a new instance
If you supply one value, it’s used for the first named parameter
You can override the default values with your own values

An important implication of this is that default values have the effect of letting consumers consumers create instances
of your class in a variety of ways — in a sense they work just as though you had created multiple, different
constructors for your class.

When you don’t provide defaults for all parameters

54
Constructor default values

As a word of caution, it generally doesn’t make any sense to provide a default value for an early parameter without
providing a default for subsequent parameters.

// don't do this
class Socket(var timeout: Int = 5000, var linger: Int) {
override fun toString(): String = "timeout: ${timeout}, linger: ${linger}"
}

If you do that, you’ll get errors when trying to construct an instance with zero or one parameters:

>>> val s = Socket()

error: no value passed for parameter 'linger'
val s = Socket()
^

>>> val s = Socket(2)

error: no value passed for parameter 'linger'
val s = Socket(2)
^

If you’re not going to provide default values for all parameters, you should only provide default values for the last
parameters in the constructor:

// this is a little better

class Socket(var timeout: Int, var linger: Int = 5000) {
override fun toString(): String = "timeout: ${timeout}, linger: ${linger}"
}

With this approach the zero-argument constructor still fails, but you can use the one-argument constructor:

>>> val s = Socket()

error: no value passed for parameter 'timeout'
val s = Socket()
^

>>> val s = Socket(10)

>>> s
timeout: 10, linger: 5000

Named arguments
You can use named arguments when creating new class instances. For example, given this class:

class Socket(var timeout: Int, var linger: Int) {

override fun toString(): String = "timeout: ${timeout}, linger: ${linger}"
}

you can create a new Socket like this:

val s = Socket(timeout=2000, linger=3000)

I don’t use this feature too often, but it comes in handy in certain situations, such as when all of the class constructor
parameters have the same type (such as Int in this example). For example, some people find that this code:

val s = new Socket(timeout=2000, linger=3000)

55
Constructor default values

is more readable than this code:

val s = new Socket(2000, 3000)

56
Secondary constructors

Secondary Class Constructors

Key points
Here are a few rules to know about Kotlin secondary class constructors:

A class can have zero or more secondary class constructors

A secondary constructor must call the primary constructor; this can happen by directly calling the primary
constructor, or by calling another secondary constructor that calls the primary constructor
You call other constructors of the same class with the this keyword
The @JvmOverloads annotation lets Kotlin classes that have default parameter values be created in Java code

Secondary constructor examples

Here’s an example that shows a primary constructor and two different auxiliary constructors:

class Pizza constructor (

var crustSize: String,
var crustType: String,
val toppings: MutableList<String> = mutableListOf()
) {

// secondary constructor (no-args)

constructor() : this("SMALL", "THIN")

// secondary constructor (2-args)

constructor(crustSize: String, crustType: String) : this(crustSize, crustType, mutableListOf<String>())

override fun toString(): String = "size: ${crustSize}, type: ${crustType}, toppings: ${toppings}"

This function and its output shows how the constructors work:

fun main(args: Array<String>) {

val p1 = Pizza()
val p2 = Pizza("LARGE", "THICK")
val p3 = Pizza("MEDIUM", "REGULAR", mutableListOf("CHEESE", "PEPPERONI"))

println(p1)
println(p2)
println(p3)
}

// output
size: SMALL, type: THIN, toppings: []
size: LARGE, type: THICK, toppings: []
size: MEDIUM, type: REGULAR, toppings: [CHEESE, PEPPERONI]

Notes
Notice how default constructor values eliminate the need for secondary constructors like these. This code with default
constructor parameter values produces the same results:

57
Secondary constructors

class Pizza2 constructor (

var crustSize: String = "SMALL",
var crustType: String = "THIN",
val toppings: MutableList<String> = mutableListOf()
) {
override fun toString(): String = "size: ${crustSize}, type: ${crustType}, toppings: ${toppings}"
}

@JvmOverloads (Working with Java)

Per the Kotlin documentation, the @JvmOverloads annotation “Instructs the Kotlin compiler to generate overloads for
this function that substitute default parameter values.”

This doesn’t do too much for you when you’re working only in Kotlin, but it’s necessary if you want to generate multiple
constructors that will work with Java code. For example, this Kotlin code works just fine:

class Person constructor(

var firstName: String,
var lastName: String,
var age: Int = 0
) {
override fun toString(): String =
"$firstName $lastName is $age years old"
}

fun main(args: Array<String>) {

val p1 = Person("Alvin", "Alexander", 40)
val p2 = Person("Alvin", "Alexander")

println(p1)
println(p2)
}

But if you try to use the Person class constructors in Java you’ll find that the second line won’t compile:

Person p1 = new Person("John", "Doe", 30);

Person p2 = new Person("Jane", "Doe"); //won’t compile

The solution to work properly with Java is to add the @JvmOverloads annotation to the Kotlin class:

class Person @JvmOverloads constructor( //same as before...

-------------

With the rest of the class being the same, this annotation allows the Kotlin class to be used in Java code.

58
Open and final classes

Open and Final Classes

Key points
Because of a concern about the “fragile base class” problem, Kotlin classes and their functions are final by
default
To allow a class to be extended it must be marked open , meaning that it’s “open to be extended”
To allow class functions and fields to be overridden, they must also be marked open

Classes and functions are final by default

I’ll demonstrate how this works through a series of examples.

(1) Class and function are not open

In this first example, Child won’t compile because Parent and its function name are final (by default):

class Parent { //class is final by default

fun name() = "base" //function is final by default
}

// this won’t compile

class Child : Parent() {
override fun name() = "Child"
}

// error messages:
error: this type is final, so it cannot be inherited from
class Child : Parent() {
^
error: 'name' in 'Parent' is final and cannot be overridden
override fun name() = "Child"
^

(2) Class is open, function is not

In this example, Parent is now open, but because name isn’t open the code still won’t compile:

open class Parent {

fun name() = "base"
}

class Child : Parent() {

override fun name() = "Child" //intentional error
}

// error message:
error: 'name' in 'Parent' is final and cannot be overridden
override fun name() = "Child" //intentional error
^

(3) Success: Class and function are open

Finally, with Parent and name both being marked open , this code will compile:

59
Open and final classes

open class Parent {

open fun name() = "base"
}

class Child : Parent() {

override fun name() = "Child"
}

Also, note the need for the override modifier on Child::name .

Closing an open method

To begin looking at an interesting problem, notice that foo is defined to be open in class A , and then it is
overridden in both class B and then class C :

open class A {
open fun foo() = "foo in A"
}

open class B : A() {

override fun foo() = "foo in B"
}

open class C : B() {

override fun foo() = "foo in C"
}

This code is perfectly legal; so far, so good.

Now, if you’re writing class B and don’t want your subclasses to override foo , you can mark foo as final in your
class:

open class A {
open fun foo() = "foo in A"
}

open class B : A() {

// added `final` here
final override fun foo() = "foo in B"
}

open class C : B() {

// this won’t compile because `foo` is marked
// `final` in B
override fun foo() = "foo in C"
}

// error:
error: 'foo' in 'B' is final and cannot be overridden

Similarly, if you take the open off of class B , class C will no longer compile:

open class A {
open fun foo() = "foo in A"
}

// removed `open` here

class B : A() {
override fun foo() = "foo in B"
}

60
Open and final classes

// this code won’t compile because B is closed

open class C : B() {
override fun foo() = "foo in C"
}

// error message:
error: this type is final, so it cannot be inherited from
open class C : B() {
^

More information: Fragile base classes

To understand Kotlin’s approach with classes and functions, you first have to understand the “fragile base class”
problem. Wikipedia describes it like this:

“The fragile base class problem is a fundamental architectural problem of object-oriented programming systems
where base classes (superclasses) are considered ‘fragile’ because seemingly safe modifications to a base class,
when inherited by the derived classes, may cause the derived classes to malfunction. The programmer cannot
determine whether a base class change is safe simply by examining the methods of the base class in isolation.”

This is a problem in Java, where classes and methods are open by default. To fix the problem, as Joshua Block writes
in his classic book, Effective Java, to protect against this problem, developers should “Design and document for
inheritance or else prohibit it.”

Because of this experience in Java, the Kotlin designers decided to make all classes and methods final by default.
That way, you can “design for inheritance” by marking classes and functions as open only when you really want them
to be open to extension.

61
Abstract classes

Abstract Classes
Abstract classes in Kotlin are similar to abstract classes in Java and Scala, but Kotlin makes a clear distinction about
which methods are considered “open” to be overridden.

Key points
Abstract classes are similar to Java’s abstract classes
If you declare a class abstract it can’t be instantiated
Abstract classes can have:
Abstract and concrete methods
Abstract and concrete properties (fields)
Abstract classes and abstract members don’t have to be annotated with open ; the fact that they are abstract
implies that they are intended to be overridden
Abstract classes can have private and protected members

Functions in abstract classes

Functions can be:

Abstract: function signature only, no body

Concrete, but closed (i.e., final)
Concrete, and open to modification

Example:

abstract class Pet (name: String) {

abstract fun comeToMaster(): Unit //abstract, open by default
fun walk(): Unit = println("I’m walking") //concrete, closed (final)
open fun speak(): Unit = println("Yo") //concrete, open
}

Working code
Here’s a small, complete example that shows how abstract classes and functions work with inheritance:

abstract class Pet (name: String) {

abstract fun comeToMaster(): Unit //abstract, open by default
fun walk(): Unit = println("I’m walking") //concrete, closed (final)
open fun speak(): Unit = println("Yo") //concrete, open
}

class Dog(name: String) : Pet(name) {

override fun speak() = println("Woof")
override fun comeToMaster() = println("Here I come!")
}

class Cat(name: String) : Pet(name) {

override fun speak() = println("Meow")
override fun comeToMaster() = println("That’s not gonna happen")
}

fun main(args: Array<String>) {

62
Abstract classes

val d = Dog("Zeus")
d.walk()
d.speak()
d.comeToMaster()

val c = Cat("Rusty")
c.walk()
c.speak()
c.comeToMaster()

Other abstract class features

Abstract classes can also have private properties, abstract properties, and concrete properties, as shown in the last
three lines of this example:

abstract class Pet (name: String) {

abstract fun comeToMaster(): Unit //abstract method
fun walk(): Unit = println("I’m walking") //concrete, closed (final)
open fun speak(): Unit = println("Yo") //concrete, open

private var numberOfLegs = 4 //private

abstract var furColor: String //abstract (no initial value)
var actuallyLikesPeople = false //concrete, must have a value
}

Why use abstract classes (instead of interfaces)?

Interfaces can’t have constructor parameters, initialized properties, or private properties.

You can kind of simulate properties with default values, but interface properties can’t have backing fields:

interface Cat {
private var numLegs: Int
get() = 4
set(value) = TODO() //can’t use `field`
}

A note about designing base classes

This Reddit post has an interesting discussion about designing base classes. One interesting quote:

"Designing a base class, you should therefore avoid using open members in the constructors, property
initializers, and init blocks."

See that Reddit link and this Kotlin link for more details.

63
Interfaces

Kotlin Interfaces
If you’re familiar with Java, Kotlin interfaces are similar to Java 8 interfaces. If you know Scala, they also have
similarities to Scala traits.

Key points
What you’ll see in this lesson:

Interfaces are defined using the interface keyword

Interfaces can declare functions, which can be abstract or concrete
Interfaces can declare fields (properties):
They can be abstract
They can provide implementations for accessors
Interfaces can inherit/derive from other interfaces
A class or object can implement one or more interfaces
When you inherit from multiple interfaces that have methods with the same names and signatures, you must
manually resolve the conflict

As shown previously, Kotlin also has a concept of abstract classes. Use abstract classes instead of interfaces when:

You need constructor parameters (interfaces can’t have them)

You need to define concrete read/write fields
You need private members

Interfaces with abstract and concrete functions

This example shows common uses of interfaces, with both abstract and concrete functions in the interfaces, along
with overridden functions in the concrete Dog class:

package interfaces

interface Speaker {
//abstract function
fun speak(): String
}

interface TailWagger {
// concrete implementations
fun startTail() { println("tail is wagging") }
fun stopTail() { println("tail is stopped") }
}

class Dog : Speaker, TailWagger {

// override an abstract function

override fun speak(): String = "Woof!"

// override a concrete function

override fun stopTail() { println("can’t stop the tail!") }

fun main(args: Array<String>) {

val d = Dog()
println(d.speak()) //"Woof!"

64
Interfaces

d.startTail() //"tail is wagging"

d.stopTail() //"can’t stop the tail!"
}

Properties
Key points:

Interfaces can define properties (fields)

They can be abstract
You can define an accessor (getter) for a property

Examples:

interface PizzaInterface {
var numToppings: Int //abstract
var size: Int //abstract
val maxNumToppings: Int //concrete
get() = 10
}

class Pizza : PizzaInterface {

// simple override
override var numToppings = 0

// override with get/set

override var size: Int = 14 //14"
get() = field
set(value) { field = value}

// override on a `val`
override val maxNumToppings: Int
//get() = super.maxNumToppings
get() = 20
}

fun main(args: Array<String>) {

val p = Pizza()
println(p.numToppings) //0
println(p.size) //14
println(p.maxNumToppings) //20

p.numToppings = 5
p.size = 16
}

A note from the official Kotlin docs: “A property declared in an interface can either be abstract, or it can provide
implementations for accessors. Properties declared in interfaces can’t have backing fields, and therefore accessors
declared in interfaces can’t reference them.”

Inheritance
Interfaces can extend other interfaces, override their properties and functions, and declare new properties and
functions:

interface StarTrekCrewMember {
fun uniformColor(): String
}

65
Interfaces

interface Officer : StarTrekCrewMember {

override fun uniformColor(): String = "not red"
}

interface RedShirt : StarTrekCrewMember {

override fun uniformColor(): String = "red (sorry about your luck)"
fun diePainfulDeath(): String = "i’m dead"
}

// more code ...

Here’s a class that implements three interfaces:

interface Starship
interface WarpCore
interface WarpCoreEjector
class Enterprise : Starship, WarpCore, WarpCoreEjector

Resolving inheritance conflicts

When a class extends multiple interfaces and those interfaces have a common method, you must manually handle the
situation in the function in your class. This example shows how to handle the functions foo and bar in the class
C , where C extends both interfaces A and B :

interface A {
fun foo() { println("foo: A") }
fun bar(): Unit
}

interface B {
fun foo() { println("foo: B") }
fun bar() { println("bar: B") }
}

class C : A, B {
override fun foo() {
super<A>.foo()
super<B>.foo()
println("foo: C")
}
override fun bar() {
super<B>.bar()
println("bar: C")
}
}

fun main(args: Array<String>) {

val c = C()
c.foo()
println()
c.bar()
}

Here’s the output from main :

foo: A
foo: B
foo: C

bar: B
bar: C

66
Interfaces

This example is a simplified version of this kotlinlang.org example.

67
== and ===

Object Equality
Comparing objects — class instances — in Kotlin is a little different than Java, and very similar to Scala.

Key points
== calls equals under the hood (structural equality)

=== is used to test reference equality

Classes don’t have equals or hashCode methods by default, you need to implement them
Data classes have automatically generated equals and hashCode methods

== and ===

This code shows how object equality works when a class does not have an equals method ( Person ) and when a
class does have an equals method ( PersonWithEquals ):

class Person(var name: String)

class PersonWithEquals(var name: String) {

override fun equals(that: Any?): Boolean {
if (that == null) return false
if (that !is PersonWithEquals) return false
if (this.name == that.name) {
return true
} else {
return false
}
}
}

fun main(args: Array<String>) {

// Person (without `equals`)

val p1 = Person("Joe")
val p2 = Person("Joe")
println(p1 == p2) //false
println(p1 === p2) //false
println(p1 === p1) //true

// PersonWithEquals
val p1a = PersonWithEquals("Joe")
val p2a = PersonWithEquals("Joe")
println(p1a == p2a) //true
println(p1a === p2a) //false
println(p1a === p1a) //true

Notes:

You can write the equals function differently; I’m just trying to be obvious about the tests
In the real world you should implement hashCode
As you’ll see in the Data Classes lesson, a data class implements equals and hashCode for you
See the “nullability” lessons for the meaning of Any?

68
== and ===

69
Enumerations

Summary
Kotlin enumerations are similar to enumerations in other languages, such as Java
Enumerations are declared with enum class
Enumerations can have constructor parameters, properties, and functions
Enumerations can be used in if and when expressions and for loops

Basic enumerations
Simple enumerations are a useful tool for creating small groups of constants, things like the days of the week, months
in a year, suits in a deck of cards, etc., situations where you have a group of related, constant values. Here are the
days of the week:

enum class DayOfWeek {

SUNDAY, MONDAY, TUESDAY,
WEDNESDAY, THURSDAY, FRIDAY, SATURDAY
}

Here’s an enumeration for the suits in a deck of cards:

enum class Suit {

CLUBS, SPADES, DIAMONDS, HEARTS
}

The full power of enumerations

More than just simple constants, enumerations can have constructor parameters, properties, and functions, just like a
regular class. Here’s an example, adapted from this Oracle Java example:

enum class Planet(val mass: Double, val radius: Double) {

// just the first three planets
MERCURY (3.303e+23, 2.4397e6),
VENUS (4.869e+24, 6.0518e6),
EARTH (5.976e+24, 6.37814e6); //semi-colon is required

// universal gravitational constant (m3 kg-1 s-2)

val G = 6.67300E-11

fun surfaceGravity(): Double {

return G * mass / (radius * radius);
}

fun surfaceWeight(otherMass: Double): Double {

return otherMass * surfaceGravity();
}
}

fun main(args: Array<String>) {

val earthWeight = 200.0
val mass = earthWeight / Planet.EARTH.surfaceGravity()
for (p in Planet.values()) {
val s = String.format("Your weight on %s is %.2f", p, p.surfaceWeight(mass))
println(s)
}
}

70
Enumerations

Here’s the output from that code:

Your weight on MERCURY is 75.55

Your weight on VENUS is 181.00
Your weight on EARTH is 200.00

Enumerations with if , when , and for

Enumerations can be used with if and when expressions and for loops, as shown in the following examples.

if expressions:

enum class DayOfWeek {

SUNDAY, MONDAY, TUESDAY,
WEDNESDAY, THURSDAY, FRIDAY, SATURDAY
}

fun main(args: Array<String>) {

val today = DayOfWeek.MONDAY

if (today == DayOfWeek.MONDAY) {
println("MONDAY")
}
else if (today == DayOfWeek.TUESDAY) {
println("TUESDAY")
}
}

when expressions:

enum class Margin {

TOP, RIGHT, BOTTOM, LEFT
}

fun getMarginValue(margin: Margin) = when(margin) {

Margin.TOP -> "1em"
Margin.RIGHT -> "12px"
Margin.BOTTOM -> "1.5em"
Margin.LEFT -> "6px"
}

fun main(args: Array<String>) {

println(getMarginValue(Margin.TOP))
}

for loops:

for (m in Margin.values()) println(m)

71
A complete class

A Complete Kotlin Class

In this lesson you’ll create a Pizza class in Kotlin that might be used in a pizza-store point of sales system. The class
will be written in an object-oriented programming (OOP) style, and will use enumerations, constructor parameters, and
class properties and functions.

Pizza-related enumerations
I’ll start creating a pizza class by first creating some enumerations that will be helpful:

enum class Topping {

CHEESE, PEPPERONI, SAUSAGE, MUSHROOMS, ONIONS
}

enum class CrustSize {

SMALL, MEDIUM, LARGE
}

enum class CrustType {

REGULAR, THIN, THICK
}

Those enumerations provide a nice way to work with pizza toppings, crust sizes, and crust types.

A sample Pizza class

Given those enumerations I can define a Pizza class like this:

class Pizza (
var crustSize: CrustSize = CrustSize.MEDIUM,
var crustType: CrustType = CrustType.REGULAR
) {
val toppings = mutableListOf<Topping>()

fun addTopping(t: Topping): Boolean = toppings.add(t)

fun removeTopping(t: Topping): Boolean = toppings.remove(t)
fun removeAllToppings(): Unit = toppings.clear()
}

toppings can be a constructor parameter, but I wrote it this way to show a class property.

When you save the enumerations and that class in a file named Pizza.kt, you can compile it with kotlinc :

$ kotlinc Pizza.kt

That command creates the following class files:

CrustSize.class
CrustType.class
Pizza.class
Topping.class

There’s nothing to run yet because this class doesn’t have a main method, but ...

72
A complete class

A complete Pizza class with a main method

To see how this class works, replace the source code in Pizza.kt with the following code:

enum class Topping {

CHEESE, PEPPERONI, SAUSAGE, MUSHROOMS, ONIONS
}

enum class CrustSize {

SMALL, MEDIUM, LARGE
}

enum class CrustType {

REGULAR, THIN, THICK
}

class Pizza (
var crustSize: CrustSize = CrustSize.MEDIUM,
var crustType: CrustType = CrustType.REGULAR
) {

val toppings = mutableListOf<Topping>()

fun addTopping(t: Topping): Boolean = toppings.add(t)

fun removeTopping(t: Topping): Boolean = toppings.remove(t)
fun removeAllToppings(): Unit = toppings.clear()

override fun toString(): String {

return """
|Pizza:
| Crust Size: $crustSize
| Crust Type: $crustType
| Toppings: $toppings
""".trimMargin()
}
}

fun main(args: Array<String>) {

val p = Pizza()
p.addTopping(Topping.CHEESE)
p.addTopping(Topping.PEPPERONI)
p.addTopping(Topping.MUSHROOMS)
p.addTopping(Topping.ONIONS)
println(p)
p.removeTopping(Topping.MUSHROOMS)
p.removeTopping(Topping.ONIONS)
println(p)
p.removeAllToppings()
println(p)
}

Notice that I added a toString function to the Pizza class, and added a main function to test everything in the
Pizza class.

When you compile the code with this command:

$ kotlinc Pizza.kt -include-runtime -d Pizza.jar

then execute the jar file with this java command:

$ java -jar Pizza.jar

73
A complete class

you’ll see this output:

Pizza:
Crust Size: MEDIUM
Crust Type: REGULAR
Toppings: [CHEESE, PEPPERONI, MUSHROOMS, ONIONS]
Pizza:
Crust Size: MEDIUM
Crust Type: REGULAR
Toppings: [CHEESE, PEPPERONI]
Pizza:
Crust Size: MEDIUM
Crust Type: REGULAR
Toppings: []

This is a good start for creating an OOP-style class that uses enumerations, constructor parameters, and class
properties and functions.

74
Data classes

Data Classes
Sometimes you just need to create classes to hold data, like structs in the C programming language. Kotlin data
classes give you a way to create data structures that have automatically-generated functions that make them very
useful. If you’re familiar with Scala’s case classes, data classes are almost exactly the same.

Data classes are generally intended for use in a functional programming (FP) environment, but OOP
developers can use them as well.

Key points
In FP you create classes just to hold data, like structs in C
Kotlin data classes are useful for this purpose
The compiler automatically generates the following functions for data classes:
equals() and hashCode() methods

a useful toString() method

a copy() function that is useful in an update as you copy scenario
“componentN” functions are created to let you destructure an instance into its constructor fields
Data classes are like regular Kotlin classes, but because they’re intended to hold data, the primary constructor
must have at least one parameter

Functional programming
While data classes may be useful for OOP, they’re primarily intended for the FP paradigm:

FP developers create data classes with all val constructor parameters

All instances of data classes are also val

In FP, all variables and all collections are immutable, so the features of data classes are especially useful when
everything is immutable, the only way to update variables is to update them as you make a copy of them.

See my book, Functional Programming, Simplified for many more details on this.

Example
First, here are some operations on a regular Kotlin class:

class Person(
val name: String,
val age: Int
)

fun main(args: Array<String>) {

val p1 = Person("Jane Doe", 30)

val p2 = Person("Jane Doe", 30)

println(p1.name) //Jane Doe

println(p1.age) //30

println(p1 == p2) //false

println(p1) //Person@49476842

75
Data classes

Notice that p1 does not equal p2 and p1 does not print very well.

Next, just put the keyword data before the class to create a data class:

data class Person(

val name: String,
val age: Int
)

fun main(args: Array<String>) {

val p1 = Person("Jane Doe", 30)

val p2 = Person("Jane Doe", 30)

println(p1.name) //Jane Doe

println(p1.age) //30

println(p1 == p2) //true

println(p1) //Person(name=Jane Doe, age=30)

val p3 = p2.copy(name = "Jane Deer")

println(p3) //Person(name=Jane Deer, age=30)

Notice:

p1 equals p2 (because equals and hashCode are generated for you)

Person is readable when you print it out

Use the copy function to create a modified variable

Getter/setter functions for constructor parameters

For OOP developers, data class constructor parameters can also be a var :

data class Person (

var name: String,
var age: Int
)

fun main(args: Array<String>) {

val p = Person("Jane Doe", 30)
p.name = "Jane Deer" //setter function
p.age = 31 //setter function
println(p) //Person(name=Jane Deer, age=31)
}

Key points:

var constructor parameters will have both getter and setter methods

val constructor parameters will have only getter methods

All constructor parameters are included in the toString() output

Data class properties don’t print

76
Data classes

Data classes can also have properties — such as age in this next example — but notice that they don’t print out; only
constructor fields are included in toString() output:

data class Person (var name: String) {

var age: Int = 0
}

fun main(args: Array<String>) {

val p = Person("Jane Doe")
p.age = 30
println(p) //Person(name=Jane Doe) <-- no age here
}

equals() and hashCode()

As shown earlier, data classes have automatically-generated equals and hashCode methods, so instances can be
compared:

data class Person(

val name: String,
val age: Int
)

val p1 = Person("Jane Doe", 30)

val p2 = Person("Jane Doe", 30)

println(p1 == p2) //true

These methods also let you easily use data class instances in collections like sets and maps.

toString()

As shown, data classes have a good default toString() method implementation, which at the very least is helpful
when debugging code:

val p = Person("Jane Doe", 30)

p //Person(name=Jane Doe, age=30)

“componentN” functions
Functions are generated for data classes that enable them to be used in destructuring declarations:

// create a Person
val p = Person("Jane Doe", 30)

// destructure the person into its fields

val (name, age) = p
println("$name is $age") //"Jane Doe is 30"

// field names aren’t important

val (a, b) = p
println("$a is $b") //"Jane Doe is 30"

Only constructor fields are destructured this way.

77
Data classes

Copying
As shown earlier, a data class also has an automatically-generated copy method that’s extremely helpful when you
need to perform the process of a) cloning an object and b) updating one or more of the fields during the cloning
process:

> data class BaseballTeam(val name: String, val lastWorldSeriesWin: Int)

> val cubs1908 = BaseballTeam("Chicago Cubs", 1908)
> val cubs2016 = cubs1908.copy(lastWorldSeriesWin = 2016)

> cubs1908
BaseballTeam(name=Chicago Cubs, lastWorldSeriesWin=1908)

> cubs2016
BaseballTeam(name=Chicago Cubs, lastWorldSeriesWin=2016)

When you use the copy method just supply the names of the fields you want to modify during the cloning process.

Because you never mutate data structures in FP, this is how you create a new instance of a class from an existing
instance. I refer to this process as “update as you copy,” and I discuss this process in detail in my book, Functional
Programming, Simplified.

78
Objects

Create Singletons with Object

Key points
Use object instead of class to create a singleton (single instance of that class)
Useful for “utility” classes

Rules:

No constructor parameters
Can contain:
Properties
Functions
Initializer blocks
Objects can inherit from interfaces and classes
There is a difference between object expressions and declarations
Expressions are used on the right hand side of expressions
Declarations are when you use object to create a singleton or companion object

Singleton example
object MathUtils {
init {
println("MathUtils init was called")
}
val ONE = 1
fun plus1(i: Int) = i + 1
}

fun main(args: Array<String>) {

val x = MathUtils.plus1(MathUtils.ONE)
println(x)
}

Prints:

MathUtils init was called

2

Inheritance
This is from the Kotlin Language Reference (TODO: write my own example):

// objects can have supertypes:

object DefaultListener : MouseAdapter() {
override fun mouseClicked(e: MouseEvent) { ... }
override fun mouseEntered(e: MouseEvent) { ... }
}

79
Objects

80
Companion objects

Companion Objects (TODO)

Similar to Scala
Create a companion object inside a class
A way of adding static methods to a class
Factory pattern: private constructor in class + factory method in companion object
TODO: Document rules about relationship between a class and its companion object

Syntax
From Kotlin Language Reference (TODO: write my own example):

(1) Give the companion object a name:

class MyClass {
companion object Factory {
fun create(): MyClass = MyClass()
}
}

val instance = MyClass.create()

(2) No name on companion object:

class MyClass {
companion object { } // will be called "Companion"
}
fun MyClass.Companion.foo() { ... }

Notes
Style/Idiom: Use package-level functions instead of static methods in companion objects? (Noted in Kotlin
Language Reference)

TODO
@JvmStatic annotation

From Kotlin Language Reference, p. 83: “Note that, even though the members of companion objects look like
static members in other languages, at runtime those are still instance members of real objects, and can, for
example, implement interfaces:”

81
Visibility modifiers

public, private, etc.

i think this goes here, maybe not

TODO: need to discuss "module" (KIA, p.73)

Access modifiers in classes

From KIA, p.72

Modifier Meaning Comments

abstract must be overridden TODO

final can’t be overridden TODO

open can be overridden TODO

override override a member in a superclass or interface

Kotlin visibility modifiers

From KIA, p.73

Modifier Class Member

public visible everywhere

internal visible in a module

protected visible in subclasses

private visible in a class

82
Functions

A First Look at Kotlin Functions

In Kotlin, functions can be defined both inside and outside of classes.

Key points
Functions are defined with the fun keyword
The syntax:

// single-expression syntax
fun plus1(i: Int) = i + 1
fun plus1(i: Int): Int = i + 1

// multiline syntax
fun plus1(i: Int): Int {
return i + 1
}

// calling a function
val x = plus1(1)

Functions are called just like you call Java methods

Function parameters can have default values
Functions can be called with named parameters
Local functions can be created (functions defined within functions)
“Infix” functions can be created (and will be covered in the next lesson)
Extension functions let you extend existing types
TODO: functions don't have to be inside classes, they can be at a package level

One-line functions (single-expression)

One-line functions are called “single-expression” functions and they can be written like this:

// single-expression syntax
fun plus1(i: Int) = i + 1
fun plus1(i: Int): Int = i + 1

Notice that the return keyword is not used with the single-expression syntax.

Multiline functions with return values

A multiline function looks like this:

fun addThenDouble(a: Int, b: Int): Int {

val sum = a + b
val doubled = sum * 2
return doubled
}

The REPL shows how addThenDouble works:

83
Functions

> addThenDouble(1,1)
4

> addThenDouble(1,2)
6

Multiline functions without return values

If a multiline function doesn’t have a return value, you can either leave it off the function declaration:

fun addAndPrint(a: Int, b: Int) {

val sum = a + b
println(sum)
}

or declare the return type as Unit :

fun addAndPrint(a: Int, b: Int): Unit {

val sum = a + b
println(sum)
}

Kotlin’s Unit return type is similar to void/Void in other programming languages.

Function parameters can have default values

Just like constructors, you can specify default values for function parameters:

fun connect(timeout: Int = 5000, protocol: String = "http"): Unit {

println("timeout = ${timeout}, protocol = ${protocol}")
// more code here
}

Because of the default parameters, that function can be called in these ways:

connect()
connect(2500)
connect(10000, "https")

Here’s what those examples look like in the REPL:

> connect()
timeout = 5000, protocol = http

> connect(2500)
timeout = 2500, protocol = http

> connect(10000, "https")

timeout = 10000, protocol = https

Functions can use named arguments

Just as with constructors, you can use named arguments when calling a function:

84
Functions

fun printName(firstName: String, lastName: String) {

println("Your name is ${firstName} ${lastName}")
}

printName(firstName="Hala", lastName="Terra")

The REPL shows the result:

> printName(firstName="Hala", lastName="Terra")

Your name is Hala Terra

Local functions
When functions are only accessed by other functions in the local scope, you can declare those other functions to be
private , but in Kotlin you can make the other functions local to the outer function. In this example the functions add

and double are defined inside the outer addThenDouble function:

fun addThenDouble(a: Int, b: Int): Int {

fun add(a: Int, b: Int) = a + b //local function
fun double(a: Int) = a * 2 //local function
return double(add(a, b))
}

The REPL shows that this works as desired:

> addThenDouble(1,1)
4

> addThenDouble(1,2)
6

Benefits:

Minimize the scope of functions

Make it easier to understand the outer function because you don’t need to look around through other code to see
the source code for the other functions that it uses

85
Extension functions

Extension Functions
Extension functions let you add new methods to existing classes (like String , Int , etc.). If you’ve used implicit
classes in Scala, extension functions are just like those.

Example
In the years before Kotlin, I wanted an Android method that would let me write code like this:

if (x.between(1, 10)) ...

I couldn’t do that with Java, so I had to settle for less-readable code that looked like this:

if (between(x, 1, 10)) { ...

But these days with Kotlin I can get what I want with an extension function:

fun Int.between(a: Int, b: Int): Boolean {

return if (this >= a && this <= b) true else false
}

The REPL shows how to use that function:

>>> 5.between(1, 10)

true

>>> 22.between(1, 10)

false

How to declare an extension function

The key to writing an extension function is that you declare that you want to add a function to a specific data type:

fun Int.between(a: Int, b: Int): Boolean {

---

You also specify that you want the function to have this name:

fun Int.between(a: Int, b: Int): Boolean {

-------

these parameters:

fun Int.between(a: Int, b: Int): Boolean {

------ ------

and this return type:

fun Int.between(a: Int, b: Int): Boolean {

86
Extension functions

-------

In fact, an extension function is just like a normal function, except that it’s preceded by the name of the data type you
want the function to be added to, as shown in this example.

An extension function on Any

Here’s an extension function that I add to the Any type:

fun Any.printMe(): Unit = println("I am ${this}")

Because Any is the root of all Kotlin objects — like java.lang.Object in Java — it works on all Kotlin types:

1.printMe()
"Hello".printMe()
true.printMe()

Here’s what those examples look like in the REPL:

>>> 1.printMe()
I am 1

>>> "Hello".printMe()
I am Hello

>>> true.printMe()
I am true

87
Infix functions

Infix Functions
An infix function is a function that appears in between two variables. Kotlin makes it easy to create functions like this.

Key points
Infix functions are declared similarly to extension functions
They are declared by writing infix fun at the start of the function definition
When applied like 1 plus 2 , the function belongs to the variable on the left ( 1 ) and the variable on the right
( 2 ) is the function parameter
Can be used to create DSLs

Examples
Imagine that the + operator doesn’t exist and you want a function to add two integers, like this:

val x = 1 plus 1

Just write an infix function like this:

infix fun Int.plus(that: Int) = this + that

Here’s how it looks in the Kotlin REPL:

> 1 plus 1
2

Notes
Note the infix keyword:

infix fun Int.plus(that: Int) = this + that

-----

Notice that it’s an extension function for the Int type:

infix fun Int.plus(that: Int) = this + that

---

Understanding the “target” object

By changing the function:

infix fun Int.plus(that: Int): Int {

println("this: ${this}, that: ${that}")
return this + that
}

88
Infix functions

> 1 plus 2
this: 1, that: 2
3

You can see:

The variable on the left ( 1 ) is considered the target object

The variable on the right ( 2 ) is the function parameter

89
Anonymous functions

90
Passing functions around

91
vararg parameters

vararg Parameters
A Kotlin function parameter can be defined as a vararg parameter so it can be called with zero or more values.

Key points
Declare function parameters with the vararg keyword
When arrays are passed in, handle them with the * character
Note: “In Kotlin, a vararg parameter of type T is internally represented as an array of type T ( Array<T> ) inside
the function body.”

Basic example
Here’s a function that takes a vararg parameter:

fun printAll(vararg ints: Int) {

for (i in ints) println(i)
}

The REPL shows how it works:

> printAll(1)
1

> printAll(1,2,3)
1
2
3

> printAll()

Passing in an array
Passing in an array won’t work:

val arr = intArrayOf(1,2,3)

> printAll(arr)
error: type mismatch: inferred type is Array<Int> but Int was expected
printAll(arr)
^

The solution is that you need to pass the array into your vararg parameter with this syntax:

printAll(*arr)

The * character in this use is known as the “spread operator.” The spread operator lets you convert arrays — well,
most arrays — so they can be passed into a vararg parameter.

Not all arrays work (TODO)

92
vararg parameters

The spread operator only works with certain types of arrays with vararg parameters. For example, I just showed that
intArrayOf works:

> val intArray = intArrayOf(1,2,3)

> printAll(*intArray)
1
2
3

However, arrayOf does not work when it’s given a list of integers:

> val arrayOfInt = arrayOf(1,2,3)

> printAll(*arrayOfInt)
error: type mismatch: inferred type is Array<Int> but IntArray was expected
printAll(*arrayOfInt)
^

To understand this it helps to look at the data types in the Kotlin REPL. Here are some details about the result from
intArrayOf :

> val x = intArrayOf(1,2,3)

> println(x.javaClass.name)
[I

> println(x.javaClass.kotlin)
class kotlin.IntArray

> println(x.javaClass.kotlin.qualifiedName)
kotlin.IntArray

Similarly, here are some results for arrayOf :

> val x = arrayOf(1,2,3)

> println(x.javaClass.name)
[Ljava.lang.Integer;

> println(x.javaClass.kotlin)
class kotlin.Array

> println(x.javaClass.kotlin.qualifiedName)
kotlin.Array

As shown by those results, intArrayOf creates a type of kotlin.IntArray , which works with a vararg parameter of
type Int , but arrayOf creates a type of kotlin.Array , which does not work with the same vararg parameter. (This
is true as of Kotlin version 1.2.51.)

Bonus: Make it generic (TODO: NOT WORKING)

fun <T> printAll(vararg elems: T) {
for (e in elems) println(e)
}

fun Int.toString() = this printAll(arrayOf(1,2,3)) printAll(intArrayOf(1,2,3))

93
vararg parameters

val x = arrayOf(1,2,3) val x = intArrayOf(1,2,3)

94
Nullability

Nullability: Introduction
Nullability is a Kotlin feature that’s intended to help you eliminate NullPointerExceptions in your code. We all know that
null pointer exceptions are bad, and the features I introduce in the following lessons are Kotlin’s way of eliminating
them.

Two rules about using null values

Before you move into the following lessons, it’s worth mentioning that I like to think that there are two rules about
using null values in Kotlin:

1. Never use null values!

2. When you’re forced to use null values — such as by using an old Java library — use Kotlin’s nullable types and
the operators shown in the next few lessons.

With that point made, let’s jump into nullable types.

95
Nullable types

Nullable Types
Nullable types are an approach in Kotlin to help you eliminate null values and null pointer exceptions (the dreaded
NullPointerException ). Here are the main concepts.

Key points
These are the key points of this lesson:

A variable of type String can never be null

A variable of the nullable type String? can be null
The operations that can be performed on nullable types are very limited

I use String in those statements, but the same things can be said about every other type, including Int , Float ,
Person , etc.

Standard types can’t be null

Variables that are instances of standard Kotlin types can’t contain null values:

val> val s: String = null

error: null can not be a value of a non-null type String
val s: String = null
^

> val i: Int = null

error: null can not be a value of a non-null type Int
val i : Int = null
^

Similarly, variables that are instances of custom types can’t be null either:

> class Person (var name: String)

> val p: Person = null

error: null can not be a value of a non-null type Line_3.Person
val p: Person = null
^

Nullable types
A nullable type is a variation of a type that permits null values. You declare a type to be nullable by adding a question
mark after it:

var s: String? = null

_

This simple addition to your type allows your variable to contain null values. Notice that no exceptions are thrown in
the REPL when you declare a nullable type:

> var s: String? = null

96
Nullable types

> var i: Int? = null

> var p: Person? = null

Also notice that I intentionally declare those variables as var fields. I do this because they’re declared with null
values, but at some point later in their lifetime they’ll presumably contain more useful values.

Summary
A brief summary so far:

Default Kotlin types cannot contain null values.

A nullable type is a variation of an existing type, and can contain a null value. The ? operator says, “This
instance of this type is allowed to contain a null value.”

As mentioned in the previous chapter, you should write your code to never use null values. Allowing null values in
your code is considered a “bad smell.” (When I look at code that permits null values I think, “How quaint, this is like
code from 1996.”)

Places where you’ll use nullable types

You’ll use and encounter nullable types in several areas:

Variable assignment
Function parameters
Function return values
Converting a nullable type to its non-nullable equivalent

Here’s what those situations look like. You’ve already seen variable assignment:

var s: String? = null

This is a function parameter:

fun foo(s: String?) ...

This is a nullable type used as a function return value:

fun foo(): String? = ...

You may also need to handle the process of converting a nullable type to its non-nullable equivalent:

val x: String? = null

val y: String = x // error: this won’t compile

If you attempt to write the code as shown, you’ll see this error message on the second line:

//error: type mismatch: inferred type is String? but String was expected

I’ll show how to handle this situation in the following lessons.

Nullable types are limited

97
Nullable types

Because a nullable type can be null , in order for them to be safe to work with, the methods you can call on them are
intentionally very restricted. As you can imagine, if you try to call a function like length() on a null string, it will throw
a null pointer exception. Therefore, a String? instance doesn’t have a length property:

> var s: String? = "fred"

> s.length
error: only safe (?.) or non-null asserted (!!.) calls are allowed
on a nullable receiver of type String?
s.length
^

A String? instance also doesn’t have a toUpperCase() function:

> s.toUpperCase()
error: only safe (?.) or non-null asserted (!!.) calls are allowed
on a nullable receiver of type String?
s.toUpperCase()
^

That error message is Kotlin’s way of saying, “ String? doesn’t have this functionality.”

This is an important point: a String? isn’t the same as a String . For example, with Kotlin 1.5.2, a String instance
has well over 100 functions that can be called on it, while a String? has only 13. You can think of String? as being
a limited subset of a String .

Note 1: Nullability concepts are enforced by the compiler at compile-time.

Note 2: I’ll discuss the ?. and !!. operators you see referenced in those error messages in the lessons that
follow.

Coming soon: Operators to help you

While you can work with nullable types manually by checking for null values in if/then expressions:

val len = if (s != null) s.length else 0

that code is verbose, and it only gets worse when you have several nullable types. Therefore, this style of code is
considered a bad practice, and Kotlin has operators to help you work with nullable types. Those operators will be
demonstrated in the lessons that follow.

98
Safe-call operator

The Safe-Call Operator

A main operator you’ll use with nullable types is called the safe-call operator, and as you’ll see in this lesson, it’s
represented by the symbol ?. (a question mark followed by a period).

Background
If Kotlin had nullable types but didn’t have any other operators to work with them, you’d always have to use if/then
checks to work with them, like this:

fun toUpper(s: String?): String? = if (s != null) s.toUpperCase() else null

That function isn’t too helpful, but it does help keep you from throwing null pointer exceptions. The REPL shows how it
works:

>>> toUpper("al")
AL

>>> toUpper(null)
null

The safe-call operator

Fortunately Kotlin has a safe-call operator that does the same thing:

fun toUpper(s: String?): String? = s?.toUpperCase()

The REPL shows that this function works just like the previous one:

>>> toUpper("al")
AL

>>> toUpper(null)
null

In summary, these two pieces of code are equivalent:

Safe-Call Operator Manual check

s?.toUpperCase() if (s != null) s.toUpperCase() else null

Chaining safe calls together

In the real world, a field of one class may be a nullable type, and that class may have another nullable type, and so
on. For example, in the following code Order has the nullable field customer ; the Customer type has a nullable field
address ; and Address has a nullable field street2 :

class Order (
var customer: Customer?,
var pizzas: List<Pizza>

99
Safe-call operator

class Customer (
var name: String,
var address: Address?
)

class Address (
var street1: String,
var street2: String?,
var city: String,
var state: String,
var zip: String
)

class Pizza(
//...
)

A great thing about the safe-call operator is that it can be chained together so you can access the street2 field like
this:

val street2Address = order.customer?.address?.street2

That syntax can be read as, “Reach inside the order instance and if the customer field is not null, and its address
field is not null, give me the street2 value (which may be null). However, if customer or address are null, give me a
null value.”

It’s important to know that street2Address may still be null, but the key here is that this code won’t throw a
NullPointerException if customer , address , or street2 is null.

Complete code example

Here’s a larger example of using the safe-call operator:

class Order (
var customer: Customer?,
var pizzas: List<Pizza>
)

class Customer (
var name: String,
var address: Address?
)

class Address (
var street1: String,
var street2: String?,
var city: String,
var state: String,
var zip: String
)

class Pizza(
//...
)

fun main(args: Array<String>) {

val address = Address(
"123 Main",
null,
"Talkeetna",

100
Safe-call operator

"Alaska",
"99676"
)

// customer has no address

var customer = Customer("Al", null)
val order = Order(customer, emptyList<Pizza>())
println("test1: ${order.customer?.address?.street2}") //null

// customer has address but no 'street2'

customer.address = address
println("test2: ${order.customer?.address?.street2}") //null

Null object pattern

Depending on your needs, don’t forget about the Null Object Pattern. Here’s a link to a Scala null object pattern
example I put together.

101
Elvis operator

The Elvis Operator ?:

The Safe-Call operator shown in the previous lesson is nice in many situations where a variable may contain a null
value, and you’re okay with ending up with another null value when you use it in an expression. But what about those
times where you want to handle that null value and return something besides null ? That’s where the so-called
“Elvis” Operator comes in.

I don’t see it myself, but I’ve read that if you turn your head sideways the ?: characters look a little like Elvis.
The official name for this operator is the “Null Coalescing Operator,” and I think “Elvis Operator” is just easier to
remember.

Key points
?: comes after a nullable type instance and can be read as “or else” or “if that’s null do this”

?. returns null, but ?: lets you return some other value besides null

Examples
In the previous lesson I returned String? from my toUpper function. I do that because the function accepts a
nullable string, and may therefore return a null value:

fun toUpper(s: String?): String? = s?.toUpperCase()

---------

The Elvis operator — ?: — is nice because it lets you easily return something besides null :

fun toUpper(s: String?): String = s?.toUpperCase() ?: "DUDE!"

-------- --

Here’s how that function works:

>>> toUpper("foo")
FOO

>>> toUpper(null)
DUDE!

The use of the Safe-Call and Elvis operators in this example is equivalent to this code:

fun toUpper(s: String?): String {

if (s != null) {
return s.toUpperCase()
} else {
return "DUDE!"
}
}

Discussion

102
Elvis operator

The Option class in the Scala programming language has a method called getOrElse , and the Optional class in
Java has an orElse method. I find that the Elvis operator reminds me of these methods. It’s basically a way of saying
“or else,” which you can see in this example:

fun toUpper(s: String?): String = s?.toUpperCase() ?: "DUDE!"

---------------- ----------------
if s is not null "or else" return
return this this

You can also think of the Elvis operator as being an operator named ifThatsNullReturnThis :

fun toUpper(s: String?): String = s?.toUpperCase() ifThatsNullReturnThis: "DUDE!"

Another example
Here’s another example you can use for testing:

class Person (
var firstName: String,
var mi: String?,
var lastName: String
)

fun main(args: Array<String>) {

var p: Person? = null
p = Person("Alvin", null, "Alexander")
val mi = p?.mi ?: "<blank>"
println("mi = $mi") //prints "mi = <blank>"
}

103
let operator

The let Operator

The let operator is an interesting construct that lets you run an algorithm on a variable inside a closure. When it’s
combined with the Safe-Call operator you can think of the approach as, “If the value exists, run this algorithm with the
value.” This is best shown by example.

let by itself

To get started, let’s see how let works by itself — without the Safe-Call operator. Here’s an example:

> val name = "foo"

> name.let { println("`it` is $it") }

`it` is foo
kotlin.Unit

TODO: IS "closure" THE CORRECT NAME FOR THIS?

The first line of the REPL output shows that the println statement is executed, and the value of it is the string
"foo" . This is how let works; it makes the value of the variable it’s applied to available inside the closure. The

value in the closure is available through the variable it , but you can also use the “named parameter” approach, if
you prefer:

// named parameter
name.let { s -> println("`s` is $s") }
- --

That code works just like the previous it example.

let has a return value

Before you move on, notice that the second line of output in the Kotlin REPL is this:

kotlin.Unit

This line is printed because let has a return value. The previous example just printed to STDOUT, so there is no
return value, so the REPL shows kotlin.Unit . Here’s an example that has a more useful return value:

> "foo".let { it.length }

3

In this example it contains the string "foo" , and "foo".length is 3 , which is the value returned. In the real world
you’ll often assign the result from let to a variable, like this:

> val len = "foo".let { it.length }

> len
3

Using let with the Safe-Call operator

104
let operator

Now that you’ve seen how to use let by itself, let’s look at how to use it in combination with the safe-call operator.

Given a String? value which may or may not be null:

val name: String? = null

//name might be changed later ...

You can print the value, if it exists:

name?.let { println("The name is $it") }

If name is null you won’t see any output; let is smart enough to know that the value is null, so the closure won’t be
executed. But if it happens to contain a value, such as the string "Al" , you’ll see output like this:

"The name is Al"

The advantage of let

Using let with the Safe-Call operator is the same as writing this code:

if (name != null) {
// do something with `name`
}

The advantage of let is that it’s more concise than constantly writing if statements like that.

A space before let

Note that if you prefer, you can also write that expression like this, with space before let :

name?. let { println("The name is $it") }

Some people find that approach more readable.

Lambda syntax and let

In a related note, you can write a lambda expression with let using a named parameter, like this:

// named param
toUpper(name)?.let { s -> println("Hello $s") }
- --

You can also write the lambda expression as I’ve shown previous in this lesson, with the special it variable name:

// `it`
toUpper(name)?.let { println("Hello $it") }
---

Key points
if the value is null, let does nothing

105
let operator

if the value is not null, let runs your algorithm

let provides its own function scope and can be called on any value (NerdGuide)

TODO
difference(s) between let , run , and apply

from my tests, let seems to create access to it , run does not

per book, "let returns the value of the closure"
per my test, apply doesn’t create an it reference
how let works:

"foo".let { println(it) } foo

106
!! operator

The “Force” Operator ( !! )

Kotlin also includes an operator generally known as the Force Operator. It lets you convert a nullable type to its non-
nullable equivalent, such as converting a String? to a String , and it should only be used when you are 100%
certain that a nullable type isn’t null.

The official name of this operator is the “Non-null assertion operator.”

Examples
If you’re 100% certain that a nullable type doesn’t contain a null value, you can convert it to its non-nullable
equivalent, like this:

> val s1: String? = "Christina"

> val s2: String = s1!!

> s2
Christina

However, beware that if the nullable type does contain a null value you’ll get a null pointer exception during this
process:

> val s1: String? = null

> val s2: String = s1!!

kotlin.KotlinNullPointerException

Check for null before using !!

Getting a null pointer exception defeats the entire purpose of using nullable types, so the typical way of using the !!
operator is to test that the nullable variable isn’t null before doing the assignment, like this:

var nullableName: String? = null

// nullableName may be reassigned ...

var name = ""

if (nullableName != null) {
name = nullableName!!
}

Key points
A few key points about the Force operator:

!! gives you a way to convert a nullable type to its non-nullable equivalent

Be careful, it will throw a kotlin.KotlinNullPointerException if the value is null

And one note about when you should use the Force operator:

107
!! operator

Rarely!!

In general, there are better ways to use nullable types rather than converting them all to their non-nullable equivalent.

108
Nullability example

A Nullability Example
TODO: Show one or more examples of all of the operators working together.

109
Nullability summary

Nullability Summary
Examples of working with nullable types:

> val s: String? = "Rocky" //nullable type

> s?.length //safe-call operator

5

> s?.length ?: 0 //elvis operator

5

> s!!.length //force operator

5

More:

// Nullable type
var s: String? = null

// Safe-call operator
val street2Address = order.customer?.address?.street2

// Elvis operator (1)

fun toUpper(s: String?): String = s?.toUpperCase() ?: "DUDE!"

// Elvis operator (2)

var p: Person? = null
p = Person("Alvin", null, "Alexander")
val mi = p?.mi ?: "<blank>"
println("mi = $mi")

// Force operator (1)

> val s1: String? = "Christina"
> val s2: String = s1!!
> s2
Christina

// Force operator (2)

> val s1: String? = null
> val s2: String = s1!!
kotlin.KotlinNullPointerException

Nullability and collections (TODO)

listOfNotNull("a", "b", null) //[a, b]
listOf("a", "b", null) //[a, b, null]

110
Collections

Introduction to Kotlin Collections

Kotlin collections classes are the same as Java collections, but you can create them differently, and they also have an
API that has been enhanced with Kotlin’s extension functions.

Key: Mutable vs immutable collections

One important thing to know about Kotlin collections classes is that they’re split into two categories:

Mutable classes — elements can be added and removed

Immutable classes — their elements can’t be changed

In general I’ve found that my code is simpler if I prefer the immutable collections classes. That is, I always use an
immutable class unless there’s a compelling reason not to.

111
Array

Array
Arrays are created with various arrayOf functions. Here are two ways to create arrays (implicit and explicit syntax):

val x = arrayOf(1,2,3)
val x: Array<Int> = arrayOf(1,2,3)

val y = arrayOf("a", "b", "c")

val y: Array<String> = arrayOf("a", "b", "c")

Array creation functions

Kotlin has these array-creation functions that are listed in kotlin.Library.kt:

fun <reified @PureReifiable T> arrayOfNulls(size: Int): Array<T?>

fun doubleArrayOf(vararg elements: Double): DoubleArray
fun floatArrayOf(vararg elements: Float): FloatArray
fun longArrayOf(vararg elements: Long): LongArray
fun intArrayOf(vararg elements: Int): IntArray
fun charArrayOf(vararg elements: Char): CharArray
fun shortArrayOf(vararg elements: Short): ShortArray
fun byteArrayOf(vararg elements: Byte): ByteArray
fun booleanArrayOf(vararg elements: Boolean): BooleanArray

Warning: Array is not the same as IntArray

Note: arrayOf(1,2,3) and intArrayOf(1,2,3) are not the same:

> val b: Array<Int> = arrayOf(1,2,3)

> b.javaClass
class [Ljava.lang.Integer;
> b.javaClass.name
[Ljava.lang.Integer;
> b.javaClass.kotlin
class kotlin.Array

> val c: IntArray = intArrayOf(1,2,3)

> c.javaClass
class [I
> c.javaClass.name
[I
> c.javaClass.kotlin
class kotlin.IntArray

TODO: Briefly discuss this.

112
List

The Kotlin List Class

This chapter introduces Kotlin lists. Some issues are:

Mutable vs. immutable

Handling null values
Different types of lists

Note: This chapter has a number of TODO items, but it currently shows how to create a variety of different list
types.

Creation functions
Use these functions to create lists in Kotlin:

Function Type
arrayListOf ArrayList<T>

emptyList List<T>

listOf List<T>

listOfNotNull List<T>

mutableListOf MutableList<T>

Examples:

val list = listOf(1, 2, 3)

val list = arrayListOf(1, 2, 3)
val list = mutableListOf("a", "b", "c")

// empty lists
val list = listOf<Int>()
val list = arrayListOf<Double>()
val list = mutableListOf<String>()
val list: List<Int> = emptyList()

// nullability (implicit or explicit)

val list = listOf("a", null) // [a, null]
val list: List<String?> = listOf("a", null) // [a, null]

val list = arrayListOf("a", null) // [a, null]

val list = mutableListOf("a", null) // [a, null]
val list = listOfNotNull<String>("a", null) // [a]
val list = listOfNotNull("a", null) // [a]

// comparison: listOf vs listOfNotNull

val list = listOf("a", null) // [a, null]
val list = listOfNotNull("a", null) // [a]

Per the Kotlin docs, the difference between listOf and listOfNotNull :

listOf : Returns an immutable list containing only the specified object element.

listOfNotNull : Returns a new read-only list either of single given element, if it is not null, or empty list if the

element is null.

Understanding casting

113
List

Some examples of casting and explicitly declaring list types:

val a: List<Int> = listOf(1,2,3)

val b: Collection<Int> = listOf(1,2,3)
val c: Iterable<Int> = listOf(1,2,3)

val a: MutableList<Int> = mutableListOf(1,2,3)

val b: List<Int> = mutableListOf(1,2,3)
val c: MutableCollection<Int> = mutableListOf(1,2,3)
val d: Collection<Int> = mutableListOf(1,2,3)
val e: Iterable<Int> = mutableListOf(1,2,3)
val f: MutableIterable<Int> = mutableListOf(1,2,3)

val a: ArrayList<Int> = arrayListOf(1,2,3)

val b: AbstractList<Int> = arrayListOf(1,2,3)
val c: MutableList<Int> = arrayListOf(1,2,3)
val d: AbstractCollection<Int> = arrayListOf(1,2,3)
val e: MutableCollection<Int> = arrayListOf(1,2,3)
val f: Collection<Int> = arrayListOf(1,2,3)
val g: MutableIterable<Int> = arrayListOf(1,2,3)
val h: Iterable<Int> = arrayListOf(1,2,3)
val i: List<Int> = arrayListOf(1,2,3)

114
Map

The Kotlin Map Class

This chapter introduces maps in Kotlin.

Kotlin Map functions

Use these functions to create Maps in Kotlin:

Function Type
mapOf Map<K,V>

hashMapOf HashMap<K, V>

linkedMapOf LinkedHashMap<K, V>

sortedMapOf SortedMap<K, V>

mutableMapOf MutableMap<K, V>

Kotlin also has linkedStringMapOf and stringMapOf functions for JavaScript.

Examples:

val map = mapOf("b" to 2, "a" to 1) // {b=2, a=1}

val map = hashMapOf("b" to 2, "a" to 1) // {a=1, b=2}
val map = linkedMapOf("b" to 2, "a" to 1) // {b=2, a=1}
val map = sortedMapOf("b" to 2, "a" to 1) // {a=1, b=2}
val map = mutableMapOf("b" to 2, "a" to 1) // {b=2, a=1}

Iterating over a map

Iterating over a map with for :

val map = mapOf("a" to 1, "b" to 2, "c" to 3)

for ((k,v) in map) {

println("value of $k is $v")
}

// output
value of a is 1
value of b is 2
value of c is 3

115
Set

Set
This chapter contains examples of how to create and use sets in Kotlin.

Creating sets
Use these functions to create sets:

Function Type
setOf Set<T>

hashSetOf HashSet<T>

linkedSetOf LinkedHashSet<T>

sortedSetOf TreeSet<T>

mutableSetOf MutableSet<T>

Kotlin also has linkedStringSetOf and stringSetOf functions for JavaScript.

Examples:

val set = setOf(3, 5, 1) // [3, 5, 1]

val set = hashSetOf(3, 5, 1) // [5, 1, 3]
val set = linkedSetOf(3, 5, 1) // [3, 5, 1]
val set = sortedSetOf(3, 5, 1) // [1, 3, 5]
val set = mutableSetOf(3, 5, 1) // [3, 5, 1]

TODO
Need examples of:

adding elements
updating elements
removing elements
clearing the set
iterating over (for-loop)

116
Sequence methods

Collections Methods
Kotlin collections classes have many methods on them — many. To help make your life easier, this lesson shares
examples for the most common methods that are available to sequential collections classes.

Note: This lesson is very much a work in progress.

Filtering methods
binarySearch, distinct, distinctBy, drop, dropWhile, dropLast, dropLastWhile, elementAt, elementAtOrElse,
elementAtOrNull, filter, filterIndexed, filterIsInstance, find, findLast, first, firstOrNull, get, getOrElse, indexOf,
indexOfFirst, indexOfLast, intersect, last, lastIndexOf, lastOrNull, orEmpty, single, singleOrNull, take, takeWhile,
takeLast, takeLastWhile, union

Transformers
associate, flatten, flatMap, intersect, map, mapNotNull, mapIndexed, mapIndexedNotNull, reversed, slice, sorted,
sortedByDescending, sortedWith, union, unzip, zip

Aggregators
fold, foldIndexed, foldRight, foldRightIndexed, reduce, reduceIndexed, reduceRight, reduceRightIndexed

Grouping
groupBy, groupByTo, groupingBy, partition

Statistics
average, count, max, maxBy, maxWith, min, minBy, minWith, sum, sumBy

Information about the collection

all, any, contains, containsAll, none, forEach, forEachIndexed, isEmpty, isNotEmpty, onEach

Examples
The following examples use these lists:

val a = listOf(10, 20, 30, 40, 10)

val names = listOf("joel", "ed", "chris", "maurice")

Here are the examples:

a.any{it > 20} //true

117
Sequence methods

a.contains(10) //true
a.count() //5
a.count{it > 10} //3
a.distinct() //[10, 20, 30, 40]
a.distinctBy()
a.drop(1) //[20, 30, 40, 10]
a.drop(2) //[30, 40, 10]
a.dropLast(1) //[10, 20, 30, 40]
a.dropLast(2) //[10, 20, 30]
a.dropWhile{it < 30} //[30, 40, 10]
a.dropLastWhile{it != 30} //[10, 20, 30]
a.filter{it != 10} //[20, 30, 40]
a.find{it != 10} //20
a.first() //10
a.first{}
a.firstOrNull() //TODO
a.fold(0){acc, x -> acc+x} //110 (sum function)
a.forEach{println(it)} //prints out the list values
a.getOrElse(0){0} //10
a.getOrElse(1){0} //20
a.getOrElse(11){0} //0
TODO: better groupBy
a.groupBy({it}, {it+1}) //{10=[11, 11], 20=[21], 30=[31], 40=[41]}
a.indexOf(10) //0
a.indexOf(30) //2
a.indexOfFirst()
a.indexOfLast()
a.intersect()
a.isEmpty() //false
a.isNotEmpty() //true
a.last() //10
a.last{}
a.lastIndexOf()
a.lastOrNull()
a.map{it + 1} //[11, 21, 31, 41, 11]
a.map{it * 2} //[20, 40, 60, 80, 20]
a.max() //40
a.maxBy{it + 3} //40
maxWith(TODO)
a.min() //10
a.minBy{it + 3} //10
minWith(TODO)
none
a.onEach{println(it)} //prints each element and returns
//a copy of the list
orEmpty
a.partition{it >10} //([20, 30, 40], [10, 10])
a.reduce{acc, x -> acc+x} //110 (sum function)
a.slice(0..2) //[10, 20, 30]
a.slice(1..2) //[20, 30]
a.sorted() //[10, 10, 20, 30, 40]
a.sortedBy{it} //[10, 10, 20, 30, 40]
names.sortedBy{it.length} //[ed, joel, chris, maurice]
a.sortedWith()
a.sum() //110
a.sumBy{it + 1} //115
a.take(1) //[10]
a.take(2) //[10, 20]
a.takeLast(1) //[10]
a.takeLast(2) //[40, 10]
a.takeLastWhile{it < 40} //[10]
a.takeWhile{it < 40} //[10, 20, 30]
a.union(names) //[10, 20, 30, 40, joel, ed, chris, maurice]
a.zip(names) //[(10, joel), (20, ed), (30, chris), (40, maurice)]
names.zip(a) //[(joel, 10), (ed, 20), (chris, 30), (maurice, 40)]

118
Sequence methods

Convert a list to a String

Convert a list/array to a String :

val nums = listOf(1,2,3,4,5)

> nums.joinToString()
1, 2, 3, 4, 5

> nums.joinToString(
separator = ", ",
prefix = "[",
postfix = "]",
limit = 3,
truncated = "there’s more ..."
)
[1, 2, 3, there’s more ...]

119
Map methods

Common Map Methods

This lesson is TODO.

120
Miscellaneous

TODO
This chapter is TODO.

121
A Swing example

Kotlin and Swing

Kotlin works with Java Swing classes like JFrame , JTextArea , etc., very easily. Here’s an example of a Kotlin
application that opens a JFrame , adds a few components to it, and then displays it:

import java.awt.BorderLayout
import java.awt.Dimension
import javax.swing.JFrame
import javax.swing.JScrollPane
import javax.swing.JTextArea

fun main(args: Array<String>) {

val textArea = JTextArea()

textArea.setText("Hello, Kotlin/Swing world")
val scrollPane = JScrollPane(textArea)

val frame = JFrame("Hello, Kotlin/Swing")

frame.getContentPane().add(scrollPane, BorderLayout.CENTER)
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
frame.setSize(Dimension(600, 400))
frame.setLocationRelativeTo(null)
frame.setVisible(true)

To see how that code works, save it to a file named KotlinSwing.kt, then compile it:

$ kotlinc KotlinSwing.kt -include-runtime -d KotlinSwing.jar

then execute the jar file with this java command:

$ java -jar KotlinSwing.jar

122
Build tools

Kotlin Build Tools

TODO: Ant
TODO: Gradle
TODO: Maven
TODO: other

123
Idioms

Kotlin Idioms (TODO)

Don’t use null
Don’t throw exceptions
Use nullability
Pure functions and EOP

124
An OOP example

A Kotlin OOP Example

This lesson is TODO.

125
Command line

Kotlin at the Command Line

This lesson demonstrates examples of how to use Kotlin commands at the command line, including kotlinc .

Compiling a Kotlin file

$ kotlinc Hello.kt

That command creates a file named HelloKt.class.

Compiling to an executable Jar file

Compiling to an executable Jar file:

$ kotlinc Hello.kt -include-runtime -d Hello.jar

Then execute the jar file with this java command:

$ java -jar Hello.jar

Hello, world

126
Android

Android
This section of the book will contain Kotlin syntax examples that can help in building Android applications.

FloatingActionButton lambda
This code:

fab.setOnClickListener(object : View.OnClickListener {
override fun onClick(view: View) {
save("Note1.txt")
}
})

can be written like this:

fab.setOnClickListener { save("Note1.txt") }

127
Contributors

Contributors
The initial version of this book is created by Alvin Alexander, who has previously written these books:

Scala Cookbook
Functional Programming, Simplified
Hello, Scala
A Survival Guide for New Consultants
How I Sold My Business: A Personal Diary

128
License

License
Everything in this book — with the exception of the book cover — is released under the Attribution-ShareAlike 4.0
International license. The license is described in full detail below and at that URL.

For those viewing a print version of this book, here are those URLs:

Alvin Alexander’s website: https://alvinalexander.com

The Kotlin Quick Reference website: http://kotlin-quick-reference.com
The Attribution-ShareAlike 4.0 International license: https://creativecommons.org/licenses/by-sa/4.0/)

The cover image

If you want to fork the project that’s fine, but it doesn’t make sense to have the same cover image on different books,
so that’s one reason why the cover image is not open source. That, and I paid a designer to create it and a stock
image company for part of the artwork. So, if you fork the project, create your own cover.

I wouldn’t include the image in this repository if I didn’t have to, but Gitbook requires it.

If you’re interested in a good designer, Alex Blokowsky on 99designs.com created this cover image for me.

The license
The text of the Attribution-ShareAlike 4.0 International license follows.

Attribution-ShareAlike 4.0 International

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recipient of the Licensed Material automatically
receives an offer from the Licensor to exercise the
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b. Additional offer from the Licensor -- Adapted Material.

Every recipient of Adapted Material from You
automatically receives an offer from the Licensor to
exercise the Licensed Rights in the Adapted Material
under the conditions of the Adapter's License You apply.

c. No downstream restrictions. You may not offer or impose

any additional or different terms or conditions on, or
apply any Effective Technological Measures to, the
Licensed Material if doing so restricts exercise of the
Licensed Rights by any recipient of the Licensed
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6. No endorsement. Nothing in this Public License constitutes or

may be construed as permission to assert or imply that You
are, or that Your use of the Licensed Material is, connected
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the Licensor or others designated to receive attribution as
provided in Section 3(a)(1)(A)(i).

b. Other rights.

1. Moral rights, such as the right of integrity, are not

licensed under this Public License, nor are publicity,
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the extent possible, the Licensor waives and/or agrees not to
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2. Patent and trademark rights are not licensed under this

Public License.

3. To the extent possible, the Licensor waives any right to

collect royalties from You for the exercise of the Licensed
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under any voluntary or waivable statutory or compulsory
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Section 3 -- License Conditions.

Your exercise of the Licensed Rights is expressly made subject to the

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a. Attribution.

1. If You Share the Licensed Material (including in modified

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a. retain the following if it is supplied by the Licensor

with the Licensed Material:

i. identification of the creator(s) of the Licensed

Material and any others designated to receive
attribution, in any reasonable manner requested by

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the Licensor (including by pseudonym if

designated);

ii. a copyright notice;

iii. a notice that refers to this Public License;

iv. a notice that refers to the disclaimer of

warranties;

v. a URI or hyperlink to the Licensed Material to the

extent reasonably practicable;

b. indicate if You modified the Licensed Material and

retain an indication of any previous modifications; and

c. indicate the Licensed Material is licensed under this

Public License, and include the text of, or the URI or
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2. You may satisfy the conditions in Section 3(a)(1) in any

reasonable manner based on the medium, means, and context in
which You Share the Licensed Material. For example, it may be
reasonable to satisfy the conditions by providing a URI or
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3. If requested by the Licensor, You must remove any of the

information required by Section 3(a)(1)(A) to the extent
reasonably practicable.

b. ShareAlike.

In addition to the conditions in Section 3(a), if You Share

Adapted Material You produce, the following conditions also apply.

1. The Adapter's License You apply must be a Creative Commons

license with the same License Elements, this version or
later, or a BY-SA Compatible License.

2. You must include the text of, or the URI or hyperlink to, the
Adapter's License You apply. You may satisfy this condition
in any reasonable manner based on the medium, means, and
context in which You Share Adapted Material.

3. You may not offer or impose any additional or different terms

or conditions on, or apply any Effective Technological
Measures to, Adapted Material that restrict exercise of the
rights granted under the Adapter's License You apply.

Section 4 -- Sui Generis Database Rights.

Where the Licensed Rights include Sui Generis Database Rights that
apply to Your use of the Licensed Material:

a. for the avoidance of doubt, Section 2(a)(1) grants You the right
to extract, reuse, reproduce, and Share all or a substantial
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b. if You include all or a substantial portion of the database

contents in a database in which You have Sui Generis Database
Rights, then the database in which You have Sui Generis Database
Rights (but not its individual contents) is Adapted Material,

including for purposes of Section 3(b); and

c. You must comply with the conditions in Section 3(a) if You Share
all or a substantial portion of the contents of the database.

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For the avoidance of doubt, this Section 4 supplements and does not
replace Your obligations under this Public License where the Licensed
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Section 5 -- Disclaimer of Warranties and Limitation of Liability.

a. UNLESS OTHERWISE SEPARATELY UNDERTAKEN BY THE LICENSOR, TO THE

EXTENT POSSIBLE, THE LICENSOR OFFERS THE LICENSED MATERIAL AS-IS
AND AS-AVAILABLE, AND MAKES NO REPRESENTATIONS OR WARRANTIES OF
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WARRANTIES OF TITLE, MERCHANTABILITY, FITNESS FOR A PARTICULAR
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ACCURACY, OR THE PRESENCE OR ABSENCE OF ERRORS, WHETHER OR NOT
KNOWN OR DISCOVERABLE. WHERE DISCLAIMERS OF WARRANTIES ARE NOT
ALLOWED IN FULL OR IN PART, THIS DISCLAIMER MAY NOT APPLY TO YOU.

b. TO THE EXTENT POSSIBLE, IN NO EVENT WILL THE LICENSOR BE LIABLE

TO YOU ON ANY LEGAL THEORY (INCLUDING, WITHOUT LIMITATION,
NEGLIGENCE) OR OTHERWISE FOR ANY DIRECT, SPECIAL, INDIRECT,
INCIDENTAL, CONSEQUENTIAL, PUNITIVE, EXEMPLARY, OR OTHER LOSSES,
COSTS, EXPENSES, OR DAMAGES ARISING OUT OF THIS PUBLIC LICENSE OR
USE OF THE LICENSED MATERIAL, EVEN IF THE LICENSOR HAS BEEN
ADVISED OF THE POSSIBILITY OF SUCH LOSSES, COSTS, EXPENSES, OR
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c. The disclaimer of warranties and limitation of liability provided

above shall be interpreted in a manner that, to the extent
possible, most closely approximates an absolute disclaimer and
waiver of all liability.

Section 6 -- Term and Termination.

a. This Public License applies for the term of the Copyright and
Similar Rights licensed here. However, if You fail to comply with
this Public License, then Your rights under this Public License
terminate automatically.

b. Where Your right to use the Licensed Material has terminated under
Section 6(a), it reinstates:

1. automatically as of the date the violation is cured, provided

it is cured within 30 days of Your discovery of the
violation; or

2. upon express reinstatement by the Licensor.

For the avoidance of doubt, this Section 6(b) does not affect any
right the Licensor may have to seek remedies for Your violations
of this Public License.

c. For the avoidance of doubt, the Licensor may also offer the
Licensed Material under separate terms or conditions or stop
distributing the Licensed Material at any time; however, doing so
will not terminate this Public License.

d. Sections 1, 5, 6, 7, and 8 survive termination of this Public

License.

Section 7 -- Other Terms and Conditions.

a. The Licensor shall not be bound by any additional or different

terms or conditions communicated by You unless expressly agreed.

b. Any arrangements, understandings, or agreements regarding the

134
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Licensed Material not stated herein are separate from and

independent of the terms and conditions of this Public License.

Section 8 -- Interpretation.

a. For the avoidance of doubt, this Public License does not, and
shall not be interpreted to, reduce, limit, restrict, or impose
conditions on any use of the Licensed Material that could lawfully
be made without permission under this Public License.

b. To the extent possible, if any provision of this Public License is

deemed unenforceable, it shall be automatically reformed to the
minimum extent necessary to make it enforceable. If the provision
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without affecting the enforceability of the remaining terms and
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c. No term or condition of this Public License will be waived and no

failure to comply consented to unless expressly agreed to by the
Licensor.

d. Nothing in this Public License constitutes or may be interpreted

as a limitation upon, or waiver of, any privileges and immunities
that apply to the Licensor or You, including from the legal
processes of any jurisdiction or authority.

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Creative Commons may be contacted at creativecommons.org.

135
About

About This Book

Kotlin Quick Reference is created by Alvin Alexander, author of:

Scala Cookbook
Functional Programming, Simplified
Hello, Scala

Gratitude: Many thanks to the creators of the Kotlin programming language for creating such a wonderful
programming language. Additional thanks to the creators of GitBook, who made it easy to create this book online and
in multiple other electronic formats.

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