An Introduction to Kotlin

@ToddGinsberg An Introduction To Kotlin GSO/Triad JUG 2021-03-11 Todd Ginsberg
Research Architect

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

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

@ToddGinsberg Agenda - What Is Kotlin?

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

- Syntax & Features

- Syntax & Features - Summary

- Syntax & Features - Summary - Questions and Possibly Answers

@ToddGinsberg Who is this Guy? Todd Ginsberg

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

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

@ToddGinsberg One More Thing... I

@ToddGinsberg What Is Kotlin?

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

Released under Apache 2.0 license

Released under Apache 2.0 license Designed as a general purpose language

Released under Apache 2.0 license Designed as a general purpose language • Targets Java bytecode down to JVM 6

Released under Apache 2.0 license Designed as a general purpose language • Targets Java bytecode down to JVM 6 • Targets ECMAScript 5.1

Released under Apache 2.0 license Designed as a general purpose language • Targets Java bytecode down to JVM 6 • Targets ECMAScript 5.1 • Targets other native platforms

@ToddGinsberg Major Features 100% Interoperable with Java

@ToddGinsberg Major Features 100% Interoperable with Java Designed to avoid
entire classes of defects

entire classes of defects Lots of small improvements that add up

entire classes of defects Lots of small improvements that add up Far less code to accomplish the same task in Java - Less cognitive load on developers

@ToddGinsberg Major Features Null-safe

@ToddGinsberg Major Features Null-safe 215,000+ issues on GitHub!

@ToddGinsberg Major Features Null-safe 215,000+ issues on GitHub! 17,275 duplicates
on Stack Overﬂow!

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

@ToddGinsberg Major Features Positive Community

@ToddGinsberg Major Features Positive Community Even on Stack Overﬂow!

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

@ToddGinsberg Am I Alone?

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

@ToddGinsberg Supported on Android 2018 Prefer Kotlin over Java

@ToddGinsberg Supported on Android 2019 Android is now Kotlin ﬁrst!

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

Framework 5 Kotlin is an option on start.spring.io

Framework 5 Kotlin is an option on start.spring.io Spring @NotNull annotations == Better Kotlin nullability support

Framework 5 Kotlin is an option on start.spring.io Spring @NotNull annotations == Better Kotlin nullability support Comprehensive Kotlin documentation and examples

@ToddGinsberg Gradle DSL

@ToddGinsberg Syntax

@ToddGinsberg Variables and Values var place: String = "Greensboro"

@ToddGinsberg Variables and Values var place: String = "Greensboro" place
= "North Carolina" // OK!

= "North Carolina" // OK! val name: String = "Todd"

= "North Carolina" // OK! val name: String = "Todd" name = "Emma" // Compile Error!

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

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

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

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

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

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

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

= """ { "name": "Todd" } """

= """ { "name": "Todd", "age": $age } """

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

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

@ToddGinsberg Null-Safe Traversal var city: String? = "Greensboro"

@ToddGinsberg Null-Safe Traversal var city: String? = "Greensboro" // Not
allowed, might be null! city.toUpperCase()

@ToddGinsberg Null-Safe Traversal var city: String? = "Greensboro" // Not
allowed, might be null! city.toUpperCase() // Safe traversal city?.toUpperCase()

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

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

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

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

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

val lowest: Int = listOf(1, 2, 3).min()!! val lowest: Int = emptyList<Int>().min()!!

val lowest: Int = listOf(1, 2, 3).min()!! val lowest: Int = emptyList<Int>().min()!! // NullPointerException!

@ToddGinsberg Null Safety Remember the names!

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

?: == Elvis

?: == Elvis !! == Hold My Beer

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

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

@ToddGinsberg 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" }

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

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

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

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

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

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

} val c = Customer()

} val c = Customer() c.name = "Todd"

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

}

set(value) { field = value?.toUpperCase() } }

private set(value) { field = value?.toUpperCase() } }

get() { return field?.toUpperCase() } }

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

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

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

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

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

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

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

@ToddGinsberg Let’s Write a POJO! public class Person { private
String firstName; private String lastName; public Person() { } public Person(final String firstName, final String lastName) { this.firstName = firstName; this.lastName = 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; } }

@ToddGinsberg Let’s Write a POJO! import java.util.Objects; public class Person
{ private String firstName; private String lastName; public Person() { } public Person(final String firstName, final String lastName) { this.firstName = firstName; this.lastName = 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; } @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); } }

@ToddGinsberg Let’s Write a POJO! import java.util.Objects; public class Person
{ private String firstName; private String lastName; public Person() { } public Person(final String firstName, final String lastName) { this.firstName = firstName; this.lastName = 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; } @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 + '\'' + '}'; } }

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

String, val lastName: String) • Getters (and Setters for vars) as Properties

String, val lastName: String) • Getters (and Setters for vars) as Properties • toString()

String, val lastName: String) • Getters (and Setters for vars) as Properties • toString() • hashCode() and equals()

String, val lastName: String) • Getters (and Setters for vars) as Properties • toString() • hashCode() and equals() • And…

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

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

@ToddGinsberg Destructuring Data Classes val me = Person("Todd", "Ginsberg")

@ToddGinsberg Destructuring Data Classes val me = Person("Todd", "Ginsberg") val
(first, last) = me // first == Todd // last == Ginsberg

@ToddGinsberg Destructuring Data Classes val listOfPeople = listOf( Person("Todd", "Ginsberg"),
Person("Emma", "Ginsberg") )

Person("Emma", "Ginsberg") ) listOfPeople.forEach { (first, last) -> println("Found: $first $last") }

Person("Emma", "Ginsberg") ) listOfPeople.forEach { (first, _) -> println("Found: $first") }

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

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

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

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

Int = offset + 4 random() // 4

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

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

@ToddGinsberg FUNctions - Adding Parameters fun combine(first: Int, second: Int,
third: Int = 0): Int = first + second + third

@ToddGinsberg FUNctions - Adding Parameters fun combine(first: Int, second: Int,
third: Int = 0): Int = first + second + third combine(1, 2) combine(1, 2, 3)

@ToddGinsberg FUNctions - Named Parameters fun combine(first: Int, second: Int):
Int = first + second

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

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

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

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

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

@ToddGinsberg 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)

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

by default. • Arguments are always ﬁnal.

by default. • Arguments are always final. • Functions can be defined in a file, outside of a class.

by default. • Arguments are always final. • Functions can be defined in a file, outside of a class. • Functions can be defined within another function.

by default. • Arguments are always final. • Functions can be defined in a file, outside of a class. • Functions can be defined within another function. • Kotlin supports tail recursive functions.

@ToddGinsberg What About Checked Exceptions? ?

@ToddGinsberg What About Checked Exceptions? NO

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

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

@ToddGinsberg Type Aliases fun doSomethingWithMap( ops: Map<SomeType, List<String>> ) {
ops.entries // ... }

@ToddGinsberg Type Aliases typealias Operations = Map<SomeType, List<String>>

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

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

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

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

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

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

@ToddGinsberg Reiﬁed Generics inline fun <reified T> loggerOf(): Logger =
Logger.getLogger(T::class.java)

@ToddGinsberg Reiﬁed Generics inline fun <reified T> loggerOf(): Logger =
Logger.getLogger(T::class.java) // Works! val log = loggerOf<Metrics>()

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

@ToddGinsberg Summary

@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

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

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

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

less code • The code I write is more expressive and clear

less code • The code I write is more expressive and clear • I avoid whole classes of defects

less code • The code I write is more expressive and clear • I avoid whole classes of defects • Allows me to write in a more functional style

less code • The code I write is more expressive and clear • I avoid whole classes of defects • Allows me to write in a more functional style • Plays well with the tools I use (Spring, IDEA, Gradle)

less code • The code I write is more expressive and clear • I avoid whole classes of defects • Allows me to write in a more functional style • Plays well with the tools I use (Spring, IDEA, Gradle) • Writing Kotlin, for me, is more fun

@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)

@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) Kotlin

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

An Introduction to Kotlin

An Introduction to Kotlin

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