An Introduction to Kotlin

@ToddGinsberg
An Introduction
To Kotlin
Utah Java Users Group
2020-04-16
Todd Ginsberg
Principal Software Developer

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@ToddGinsberg
What’s in a Name?
By Editors of Open Street Map - Open Street Map, CC

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@ToddGinsberg
What’s in a Name?
Kotlin Island
By Editors of Open Street Map - Open Street Map, CC

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@ToddGinsberg
Agenda
- What Is Kotlin?

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@ToddGinsberg
Agenda
- What Is Kotlin?
- Am I Alone?

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@ToddGinsberg
Agenda
- What Is Kotlin?
- Am I Alone?
- Syntax & Features

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@ToddGinsberg
Agenda
- What Is Kotlin?
- Am I Alone?
- Syntax & Features
- Summary

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@ToddGinsberg
Agenda
- What Is Kotlin?
- Am I Alone?
- Syntax & Features
- Summary
- Questions and Possibly Answers

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@ToddGinsberg
Who is this Guy?
Todd Ginsberg

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@ToddGinsberg
Who is this Guy?
Todd Ginsberg
Just moved to Raleigh from Chicago

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@ToddGinsberg
Who is this Guy?
Todd Ginsberg
Just moved to Raleigh from Chicago
Principal Developer at Netspend
- A payments company in Austin, TX!
- We love Java and Kotlin!

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@ToddGinsberg
One More Thing...
I

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@ToddGinsberg
What Is Kotlin?

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@ToddGinsberg
What Is Kotlin?
Statically typed language, developed by JetBrains

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What Is Kotlin?
Released under Apache 2.0 license

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@ToddGinsberg
What Is Kotlin?
Designed as a general purpose language

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@ToddGinsberg
What Is Kotlin?
● Targets JVM 6 or 8 bytecode

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What Is Kotlin?
● Targets ECMAScript 5.1

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What Is Kotlin?
● Targets ECMAScript 5.1
● Targets other native platforms thanks to LLVM

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Major Features
100% Interoperable with Java

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Major Features
Designed to avoid entire classes of defects

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Major Features
Lots of small improvements that add up

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@ToddGinsberg
Major Features
Lots of small improvements that add up
Far less code to accomplish the same task in Java
- Less cognitive load on developers

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Major Features
Null-safe

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Major Features
Null-safe
199,000+ issues on GitHub!

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@ToddGinsberg
Major Features
Null-safe
199,000+ issues on GitHub!
15,650 duplicates on Stack Overﬂow!

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@ToddGinsberg
Major Features
Pragmatic Improvements
No shame in copying language features that
make developers more productive

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@ToddGinsberg
Major Features
Positive Community

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@ToddGinsberg
Major Features
Positive Community
Even on Stack Overﬂow!

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@ToddGinsberg
Major Features
Positive Community
Even on Stack Overﬂow!
RTFM!

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@ToddGinsberg
Am I Alone?

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@ToddGinsberg
Supported on Android
2017
It should work,
give it a try!

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@ToddGinsberg
2018
Prefer Kotlin
over Java

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@ToddGinsberg
2019
Android is now
Kotlin ﬁrst!

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@ToddGinsberg
Spring Framework Support
Kotlin is fully supported since Spring Framework 5

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Kotlin is an option on start.spring.io

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Spring @NotNull annotations == Better Kotlin nullability support

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@ToddGinsberg
Spring @NotNull annotations == Better Kotlin nullability support
Comprehensive Kotlin documentation and examples

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@ToddGinsberg
Gradle DSL

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@ToddGinsberg
Syntax

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Variables and Values
var place: String = "Salt Lake City"

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place = "UT" // OK!

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place = "UT" // OK!
val name: String = "Todd"

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place = "UT" // OK!
val name: String = "Todd"
name = "Emma" // Compile Error!

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Type Inference
val d: Int = 2
val text: String = "Todd has $d doughnuts"

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Type Inference
val d = 2
val text = "Todd has $d doughnuts"

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Equality
val name1 = "EXAMPLE"
val name2 = "example"

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Equality
// Structural Equality
name1 == name2.toUpperCase() // True!

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Equality
// Structural Equality
name1 == name2.toUpperCase() // True!
// Referential Equality
name1 === name2 // False!

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Raw Strings
val json = "{\n\"name\": \"Todd\"\n}"

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Raw Strings
val json = """{ "name": "Todd" }"""

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Raw Strings
val json =
"""
{
"name": "Todd"
}
"""

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Null Safety
// Guaranteed to never be null
var name: String = "Todd"

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Null Safety
// Guaranteed to never be null
var name: String = "Todd"
// May be null
var salary: Int? = null

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Null-Safe Traversal
var city: String? = "Salt Lake City"

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Null-Safe Traversal
// Not allowed, might be null!
city.toUpperCase()

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Null-Safe Traversal
// Not allowed, might be null!
city.toUpperCase()
// Safe traversal
city?.toUpperCase()

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Elvis
val lowest : Int? = listOf(1, 2, 3).min()

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Elvis
val lowest : Int? = listOf(1, 2, 3).min()
val lowest : Int = listOf(1, 2, 3).min() ?: 0

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Combine Safe-Traversal and Elvis
println(
city?.toUpperCase() ?: ”UNKNOWN”
)

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Manual Override
val lowest: Int? = listOf(1, 2, 3).min()

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Manual Override
val lowest: Int = listOf(1, 2, 3).min()!!

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Manual Override
val lowest: Int = emptyList().min()!!

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Manual Override
val lowest: Int = emptyList().min()!!
// NullPointerException!

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Null Safety
Remember the names!

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Null Safety
Remember the names!
?. == Safe Traversal

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Null Safety
Remember the names!
?: == Elvis

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Null Safety
Remember the names!
?: == Elvis
!! == Hold My Beer

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Expressions - if
val status = if (code == 42) {
"Success"
} else {
"Fail"
}

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Expressions - try/catch
val number = try {
code.toInt()
} catch (e: NumberFormatException) {
0
}

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Expressions - when
val result = when (x) {
0 -> "x is 0”
in 1..10 -> "x is between 1 and 10"
in someSet -> "x is in someSet"
is SomeType -> "x is an instance of SomeType"
parseString(s) -> "the same as parseString"
else -> "x doesn't match anything"
}

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@ToddGinsberg
Smart Casting
when (x) {
is Int -> print(x % 2 == 0)
is String -> print(x.length + 1)
is IntArray -> print(x.sum())
}

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@ToddGinsberg
Smart Casting
if(x != null) {
println(x.toString())
}

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Classes
class Entity : SomeInterface {
// ...
}

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Classes - Inheritance
open class Entity : SomeInterface {
// ...
}
class Customer : Entity() {
// ...
}

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Properties
public class Customer {
private String name;
public String getName() {
return name;
}
public void setName(final String name) {
this.name = name;
}
}

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Properties
class Customer {
var name: String? = null
}

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Properties
class Customer {
}
val c = Customer()

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Properties
class Customer {
}
val c = Customer()
c.name = "Todd"

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Properties
class Customer {
}
val c = Customer()
c.name = "Todd"
println("My name is ${c.name}")

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Properties
class Customer {
}

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Properties
class Customer {
set(value) {
field = value?.toUpperCase()
}
}

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@ToddGinsberg
Properties
class Customer {
private set(value) {
field = value?.toUpperCase()
}
}

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@ToddGinsberg
Properties
class Customer {
get() {
return field?.toUpperCase()
}
}

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Property Delegation
class Country {
val gdp: Int = slowCalculation()
}

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@ToddGinsberg
Property Delegation
class Country {
val gdp: Int by lazy {
slowCalculation()
}
}

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@ToddGinsberg
Class Delegation
// Java
class MyJavaImpl implements HugeInterface {
private final HugeInterface backing;
public MyJavaImpl(HugeInterface hi) {
backing = hi;
}
// IMPLEMENT EVERYTHING
}

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@ToddGinsberg
Class Delegation
// Java
class MyJavaImpl extends SomeImplementation {
// Implement Some
}

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@ToddGinsberg
Class Delegation
// Kotlin
class MyImpl(h: HugeInterface) : HugeInterface by h {
// Implement Some
}

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Sealed Classes
sealed class Message
class StartServer(val name: String) : Message()
class StopServer(val name: String) : Message()
object CountServers : Message()

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Sealed Classes
val response = when(msg) {
is StartServer -> startServer(msg.name)
is StopServer -> stopServer(msg.name)
is CountServers -> countServers()
}

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@ToddGinsberg
Let’s Write a POJO!
public class Person {
public String firstName;
public String lastName;
}

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@ToddGinsberg
private String firstName;
private String lastName;
public String getFirstName() {
return firstName;
}
public void setFirstName(final String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(final String lastName) {
this.lastName = lastName;
}
}

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public Person() { }
public Person(final String firstName, final String lastName) {
}
return firstName;
}
}
return lastName;
}
}
}

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@ToddGinsberg
import java.util.Objects;
public Person() { }
}
return firstName;
}
}
return lastName;
}
}
@Override
public boolean equals(final Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
final Person person = (Person) o;
return Objects.equals(firstName, person.firstName) &&
Objects.equals(lastName, person.lastName);
}
@Override
public int hashCode() {
return Objects.hash(firstName, lastName);
}
}

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@ToddGinsberg
import java.util.Objects;
public Person() { }
}
return firstName;
}
}
return lastName;
}
}
@Override
public boolean equals(final Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
final Person person = (Person) o;
return Objects.equals(firstName, person.firstName) &&
Objects.equals(lastName, person.lastName);
}
@Override
public int hashCode() {
return Objects.hash(firstName, lastName);
}
@Override
public String toString() {
return "Person{" +
"firstName='" + firstName + '\'' +
", lastName='" + lastName + '\'' +
'}';
}
}

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@ToddGinsberg
Data Classes to the Rescue!
data class Person(val firstName: String,
val lastName: String)

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@ToddGinsberg
● Getters (and Setters for vars) as Properties

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● toString()

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● toString()
● hashCode() and equals()

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@ToddGinsberg
● toString()
● hashCode() and equals()
● And…

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Copying Data Classes
val me = Person("Todd", "Ginsberg")

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Copying Data Classes
val emma = me.copy(firstName = "Emma")
// Person(”Emma", "Ginsberg")

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Destructuring Data Classes
val (first, last) = me
// first == “Todd” last == “Ginsberg”

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@ToddGinsberg
FUNctions!
fun generateRandomNumber(): Int {
return 4
}

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FUNctions!
return 4
}
fun generateRandomNumber(): Int = 4

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FUNctions!
return 4
}
fun generateRandomNumber(): Int = 4
fun generateRandomNumber() = 4

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FUNctions - Default Values
fun random(offset: Int = 0): Int =
offset + 4

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offset + 4
random() // 4

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offset + 4
random() // 4
random(1) // 5

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FUNctions - Adding Parameters
fun combine(first: Int,
second: Int): Int =
first + second

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second: Int,
third: Int = 0): Int =
first + second + third

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second: Int,
third: Int = 0): Int =
first + second + third
combine(1, 2)
combine(1, 2, 3)

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FUNctions - Named Parameters
second: Int): Int =
first + second

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second: Int): Int =
first + second
combine(1, 2) // 3

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second: Int): Int =
first + second
combine(first = 1, second = 2) // 3

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second: Int): Int =
first + second
combine(second = 2, first = 1) // 3

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FUNction Names
@Test
fun replicantsHaveAFourYearLifespan() {
// ...
}

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FUNction Names
@Test
fun `Replicants have a four year lifespan!`() {
// ...
}

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Operator Overloading
Limited to predeﬁned operators!
Expression Translated to
a + b a.plus(b)
a - b a.minus(b)
a * b a.times(b)
a / b a.div(b)
a % b a.rem(b)
a..b a.rangeTo(b)

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a in b b.contains(a)
a !in b !b.contains(a)

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a[i] a.get(i)
a[i] = b a.set(i, b)

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a += b a.plusAssign(b)
a -= b a.minusAssign(b)
a *= b a.timesAssign(b)
a /= b a.divAssign(b)
a %= b a.remAssign(b)

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@ToddGinsberg
a > b a.compareTo(b) > 0
a < b a.compareTo(b) < 0
a >= b a.compareTo(b) >= 0
a <= b a.compareTo(b) <= 0

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Extension FUNctions
// Java
public static boolean isEven(int i) {
return i % 2 == 0;
}

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Extension FUNctions
// Kotlin
fun Int.isEven(): Boolean =
this % 2 == 0

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Extension FUNctions
// Kotlin
fun Int.isEven(): Boolean =
this % 2 == 0
2.isEven() // True!

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The use Extension
// Java
try (Connection conn = getConnection()) {
// ...
}

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The use Extension
// Kotlin
getConnection().use { conn ->
// ...
}

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The apply Extension
// Expression and Statements
val p = Person()
p.name = "Dr. Robert Hume"
p.age = 47

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The apply Extension
// Single Expression
val p = Person().apply {
name = "Dr. Robert Hume"
age = 47
}

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The apply Extension
// Single Expression
val p = Person().apply {
name = "Dr. Robert Hume"
age = 47
address = Address().apply {
line1 = "5 Tall Cedar Rd."
…
}
}

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Higher-Order Functions
fun measureTimeMillis(block: () -> Unit): Long {
val start = System.currentTimeMillis()
block()
return System.currentTimeMillis() - start
}

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Higher-Order Functions
fun measureTimeMillis(block: () -> Unit): Long {
val start = System.currentTimeMillis()
block()
return System.currentTimeMillis() - start
}
val time = measureTimeMillis {
someSlowQuery()
}

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@ToddGinsberg
One More Thing on FUNctions
● Functions are ﬁnal by default.

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● Arguments are always ﬁnal.

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● Functions can be deﬁned in a ﬁle, outside of a class.

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● Functions can be deﬁned within another function.

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● Functions can be deﬁned within another function.
● Kotlin supports tail recursive functions.

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What About Checked Exceptions?
?

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What About Checked Exceptions?
NO

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Lambdas
listOf(1, 2, 3, 4)
.filter { x -> x % 2 == 0 }
.map { y -> y * 2 }
// List[4, 8]

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@ToddGinsberg
Lambdas
listOf(1, 2, 3, 4)
.filter { it % 2 == 0 }
.map { it * 2 }
// List[4, 8]

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Type Aliases
fun doSomethingWithMap(
ops: Map>
) {
ops.entries // ...
}

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@ToddGinsberg
Type Aliases
typealias Operations = Map>
fun doSomethingWithMap(ops: Operations) {
ops.entries // …
}

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@ToddGinsberg
Import Aliases
import java.util.Date
import java.sql.Date

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@ToddGinsberg
Import Aliases
import java.util.Date as UtilDate
import java.sql.Date as SqlDate

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@ToddGinsberg
Import Aliases
import java.util.Date as SqlDate
import java.sql.Date as UtilDate

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@ToddGinsberg
Reiﬁed Generics
// Ugly
val log = Logger.getLogger(Metrics::class.java)
// What I want
val log = loggerOf()

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Reiﬁed Generics
// Thanks type erasure :(
fun loggerOf(): Logger =
Logger.getLogger(T::class.java)

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Reiﬁed Generics
inline fun loggerOf(): Logger =

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Reiﬁed Generics
inline fun loggerOf(): Logger =
// Works!
val log = loggerOf()

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@ToddGinsberg
So Many Things Left Out! :(
- Companion objects
- Massive STDLIB replaces your util classes
- Inline classes
- Unsigned integers
- Multiplatform
- Coroutines
- DSL support
- Contracts
- Inﬁx functions

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@ToddGinsberg
Summary

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@ToddGinsberg
Make “Bad” Choices Explicit or Impossible
Fixed in Kotlin:
- Singleton support built in
- Override keyword mandatory
- Properties over ﬁelds with getter/setter
- Mutability is minimized (val + collections)
- Inheritance prohibited by default
- Builders are easy with default values
- No checked exceptions
- Structural equality is the same everywhere
- Delegation support makes composition easier

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@ToddGinsberg
Why Do I Use Kotlin?
● I write far less code

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● The code I write is more expressive and clear

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● I avoid whole classes of defects

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@ToddGinsberg
● Allows me to write in a more functional style

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@ToddGinsberg
● Plays well with the tools I use (Spring, IDEA, Gradle)

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@ToddGinsberg
● Plays well with the tools I use (Spring, IDEA, Gradle)
● Writing Kotlin, for me, is more fun

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@ToddGinsberg
Quantiﬁed Fun!
% of developers who are developing with the
language or technology but have not
expressed interest in continuing to do so
(Stack Overflow Developer Survey 2019)

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@ToddGinsberg
Quantiﬁed Fun!
% of developers who are developing with the
language or technology but have not
expressed interest in continuing to do so
(Stack Overflow Developer Survey 2019)
Kotlin

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@ToddGinsberg
Want to Learn More?
https://play.kotlinlang.org

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@ToddGinsberg
Want to Learn More?
https://www.coursera.org/learn/kotlin-for-java-developers

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#springone
@s1p
@ToddGinsberg
[email protected]
https://todd.ginsberg.com
Thank You!

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An Introduction to Kotlin

An Introduction to Kotlin

Todd Ginsberg

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