An Introduction to Kotlin

Kotlin sure has been receiving a lot of buzz lately, is there something to it? Developed by JetBrains, Kotlin has been catching on lately because of its null safety guarantees, functional nature, type inference, full interoperability with Java, cross-platform support, and ease of use with Android and Spring. Is this something you and your team should considering spending time to learn?

One of the most interesting aspects of Kotlin is its design philosophy. Bugs that exist in Java such as NullPointerExceptions are not possible in Kotlin. This mentality of preventing common bugs at the language level exists all through the Kotlin syntax and standard library. Kotlin is also far less verbose than Java, reducing the amount of code you need to read in order to understand it. It also runs on JVMs down to Java 6, without sacrificing features. So if you are in a constrained environment such as Android or an enterprise shop struggling to upgrade, Kotlin might be worth considering.

In this talk, you will learn where Kotlin came from, what its major features are, and why people are using it more and more. At the end of the talk, you will be in a position to make an educated decision about whether Kotlin is right for you!

Transcript

1. An Introduction
   To Kotlin
   Kansas City Java User Group
   2019-06-19
   Todd Ginsberg
   `
   @ToddGinsberg
   Principal Software Developer
   

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

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3. @ToddGinsberg
   But First…
   I
   Java
   

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4. @ToddGinsberg
   Who Is This Person?
   Todd Ginsberg
   Principal Software Developer @Netspend
   (a payments company in Austin, TX)
   Java developer since 1995
   Kotlin developer since 2016
   Chicago Java User Group Board Member and CFO
   

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5. @ToddGinsberg
   Agenda
   1.What is Kotlin?
   2.Am I Alone?
   3.Syntax and Code
   4.Community
   5.Summary / Wrap-up
   6.Q & A
   

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6. @ToddGinsberg
   What Is Kotlin?
   

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7. @ToddGinsberg
   What Is Kotlin?
   Statically typed language, developed by JetBrains
   Released under Apache 2.0 license
   Designed as a general purpose language
   • Targets JVM bytecode 6 through 12
   • Targets ECMAScript 5.1
   • Targets native thanks to LLVM
   

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8. @ToddGinsberg
   Major Features
   100% interoperable with Java
   Very easy to learn
   Fantastic IDE support
   Large, positive community
   Pragmatic – No shame in copying language features that
   make developers more productive
   

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9. @ToddGinsberg
   Major Features
   Designed to avoid entire classes of defects
   Null-safe
   185,000+ issues on GitHub!
   13,959 duplicates on Stack Overflow!
   Type Inference
   More than diamond operator or local var (thanks Java 10!)
   

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10. @ToddGinsberg
    Am I Alone?
    

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11. @ToddGinsberg
    Supported on Android
    Beep!
    You can use
    Kotlin now!
    2017
    

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12. @ToddGinsberg
    Supported on Android
    Beep!
    Prefer Kotlin
    over Java!
    2018
    

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13. @ToddGinsberg
    Supported on Android
    Beep!
    Android is now
    Kotlin-first!
    2019
    

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14. @ToddGinsberg
    Spring Framework Support
    Kotlin is fully supported in Spring Framework 5
    Kotlin is an option on start.spring.io
    Spring @NotNull annotations == Better Kotlin nullability support
    Comprehensive Kotlin documentation and examples
    

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15. @ToddGinsberg
    Supported in Gradle
    

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16. @ToddGinsberg
    Kotlin Adoption
    MAR
    2017 Assess
    NOV
    2017 Trial
    MAY
    2018 Adopt
    ThoughtWorks Technology Radar
    

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17. @ToddGinsberg
    Syntax
    

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18. @ToddGinsberg
    Variables and Values
    var place: String = "Chicago"
    place = "Illinois" // OK!
    val name: String = "Todd"
    name = "Emma" // Compile Error!
    

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

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20. @ToddGinsberg
    val d = 2
    val description = "Todd has $d doughnuts"
    Type Inference
    

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21. @ToddGinsberg
    Equality
    val name1 = "EXAMPLE"
    val name2 = "example"
    // Structural Equality
    name1 == name2.toUpperCase() // True!
    // Referential Equality
    name1 === name2 // False!
    // Unfortunately…
    name1 ==== name2 // Compiler Error L
    

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

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

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

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

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

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27. @ToddGinsberg
    val lowest: Int? = listOf(1, 2, 3).min()
    val lowest: Int = listOf(1, 2, 3).min()!!
    val lowest: Int = emptyList().min()!!
    // KotlinNullPointerException! L
    Manual Override
    

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28. @ToddGinsberg
    Null Safety
    Remember the names!
    ?. == Safe Traversal
    ?: == Elvis
    !! == Hold My Beer
    

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

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

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31. @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"
    }
    

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

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

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

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35. @ToddGinsberg
    Properties
    class Customer(var name: String) {
    var state: String? = null
    }
    val c = Customer("Todd")
    c.state = "IL"
    println("${c.name} from ${c.state}")
    // "Todd from IL"
    

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36. @ToddGinsberg
    Properties – Set
    class Customer(var name: String) {
    var state: String? = null
    set(value) {
    field = value?.toUpperCase()
    }
    }
    

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37. @ToddGinsberg
    Properties – Set
    class Customer(var name: String) {
    var state: String? = null
    private set(value) {
    field = value?.toUpperCase()
    }
    }
    

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38. @ToddGinsberg
    Properties – GET
    class Customer(var name: String) {
    var state: String? = null
    get() {
    return field?.toUpperCase()
    }
    }
    

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

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

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

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

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43. @ToddGinsberg
    Sealed Classes
    sealed class Message
    class StartServer(val name: String) : Message()
    class StopServer(val name: String) : Message()
    object CountServers : Message()
    

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

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

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46. @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;
    }
    }
    

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47. @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;
    }
    }
    

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48. @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);
    }
    }
    

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49. @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 + '\'' +
    '}';
    }
    }
    

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50. @ToddGinsberg
    Data Classes To The Rescue!
    data class Person(val firstName: String,
    val lastName: String)
    • Getters (and Setters for vars) as Properties
    • toString()
    • hashCode() and equals()
    • And…
    

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

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

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

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54. @ToddGinsberg
    FUNctions – Default Values
    fun random(offset: Int = 0): Int =
    offset + 4
    random() // 4
    random(1) // 5
    

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55. @ToddGinsberg
    FUNctions – Named Parameters
    fun combine(first: Int,
    second: Int): Int =
    first + second
    combine(1, 2) // 3
    combine(first = 1, second = 2) // 3
    combine(second = 2, first = 1) // 3
    

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56. @ToddGinsberg
    FUNctions – Named & Default
    fun combine(first: Int,
    second: Int,
    third: Int = 0): Int =
    first + second + third
    combine(1, 2) // 3
    combine(second = 2, first = 1, third = 3) // 6
    

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

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58. @ToddGinsberg
    Operator Overloading
    Limited overloading – cannot define your own 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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59. @ToddGinsberg
    Operator Overloading
    Limited overloading – cannot define your own operators.
    Expression Translated to
    a in b b.contains(a)
    a !in b !b.contains(a)
    

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60. @ToddGinsberg
    Operator Overloading
    Limited overloading – cannot define your own operators.
    Expression Translated to
    a[i] a.get(i)
    a[i] = b a.set(i, b)
    

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61. @ToddGinsberg
    Operator Overloading
    Limited overloading – cannot define your own operators.
    Expression Translated to
    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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62. @ToddGinsberg
    Operator Overloading
    Limited overloading – cannot define your own operators.
    Expression Translated to
    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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63. @ToddGinsberg
    fun measureTimeMillis(block: () -> Unit): Long {
    val start = System.currentTimeMillis()
    block()
    return System.currentTimeMillis() - start
    }
    val time = measureTimeMillis {
    someSlowQuery()
    }
    High Order FUNctions
    

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64. @ToddGinsberg
    Extension FUNctions
    // Java
    public static boolean isEven(int i) {
    return i % 2 == 0;
    }
    // Kotlin
    fun Int.isEven(): Boolean = this % 2 == 0
    2.isEven() // True!
    

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65. @ToddGinsberg
    The use Extension
    // Java
    try (DatabaseConnection conn = getConnection()) {
    // ...
    }
    // Kotlin
    getConnection().use { conn ->
    // ...
    }
    

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66. @ToddGinsberg
    The apply Extension
    // Expression and Statements
    val p = Person()
    p.name = "Todd"
    p.age = 21
    // Single Expression
    val p = Person().apply {
    name = "Todd"
    age = 21
    }
    

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67. @ToddGinsberg
    One More Thing On FUNctions…
    Functions are final 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.
    

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68. @ToddGinsberg
    What About Checked Exceptions?
    NO
    

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

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

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71. @ToddGinsberg
    Lambdas Are Closures
    val ints = listOf(1, 2, 3, 4)
    var sum = 0
    ints.forEach { sum += it }
    println(sum) // 10
    

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

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

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

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

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

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

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

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

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80. @ToddGinsberg
    Community
    

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81. @ToddGinsberg
    Community Resources
    https://kotlinlang.slack.com
    • 22,000+ members
    • Get answers and chat with people who work with the language
    • Welcoming and helpful
    • Language authors hang out here
    Stack Overflow
    • Actually get meaningful answers
    • Not uncommon to get many useful answers
    • Language authors here as well
    

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82. @ToddGinsberg
    Community
    KΛTEGORY funKTionale
    Λrrow
    

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83. @ToddGinsberg
    Try Kotlin!
    https://play.kotlinlang.org
    

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84. @ToddGinsberg
    Learn Kotlin!
    https://bit.ly/GoogleKotlinClass
    

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85. @ToddGinsberg
    Learn Kotlin!
    https://www.coursera.org/learn/kotlin-for-java-developers
    

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86. @ToddGinsberg
    Fun == Fun
    Kotlin
    % 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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87. @ToddGinsberg
    Summary
    

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88. @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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89. @ToddGinsberg
    Why Kotlin is Right For Me
    I write far 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.
    Writing Kotlin, for me, is more fun.
    Plays well with the tools I already use (Spring, IDEA, Gradle)
    

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90. @ToddGinsberg
    Is Kotlin is Right For You?
    

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91. Than You!
    @ToddGinsberg
    https://todd.ginsberg.com
    todd@ginsberg.com
    Anonymous Feedback
    

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