Kotlin, beyond the basics

Talk given at 360AnDev (https://360andev.com/) 2018 in Denver Colorado, USA.

Video: https://www.youtube.com/watch?v=Ot2DOKztu38

Features 3 important topics for intermediate Kotlin developers - adapting to the functional programming paradigm, learning more about functions and lambdas, and learning about Kotlin-Java interoperability

Transcript

1. Kotlin,
   beyond the basics
   

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2. segunfamisa
   segunfamisa.com
   

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3. Intro
   

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4. Imperative vs Functional Programming
   

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5. Imperative vs Functional Programming
   Functions in Kotlin - lambdas, stdlib functions, etc
   

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6. Imperative vs Functional Programming
   Functions in Kotlin - lambdas, stdlib functions, etc
   Java & Kotlin Interop
   

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7. Imperative vs Functional
   programming
   

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8. Functional == actions are composed into
   functions and usually reads as “what is done”
   Imperative == structured in steps of execution
   and usually reads as “how it’s done”
   Imperative == structured in steps of execution
   and usually reads as “how it’s done”
   

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9. Functional == actions are composed into
   functions and usually reads as “what is done”
   Imperative == structured in steps of execution
   and usually reads as “how it’s done”
   Functional == actions are composed into
   functions and usually reads as “what is done”
   Imperative == structured in steps of execution
   and usually reads as “how it’s done”
   

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10. // Java - print even numbers
    for (int i = start; i < end; i++) {
    if (i % 2 == 0) {
    System.out.println(i);
    }
    }
    

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11. // Java - print even numbers
    for (int i = start; i < end; i++) {
    if (i % 2 == 0) {
    System.out.println(i);
    }
    }
    // Kotlin - print even numbers
    (start until end)
    .filter { it % 2 == 0 }
    .map { println(it) }
    

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12. Imperative Functional
    

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13. Imperative Functional
    Immutability isn’t
    encouraged at language
    level
    Encourages immutability
    

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14. Imperative Functional
    Immutability isn’t
    encouraged at language
    level
    Encourages immutability
    Instances of classes,
    structures, objects are
    first class citizens
    Functions are first class
    citizens
    

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15. Imperative Functional
    Immutability isn’t
    encouraged at language
    level
    Encourages immutability
    Instances of classes,
    structures, objects are
    first class citizens
    Functions are first class
    citizens
    Loops, method calls,
    conditionals
    Function calls
    

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16. Imperative Functional
    Immutability isn’t
    encouraged at language
    level
    Encourages immutability
    Instances of classes,
    structures, objects are
    first class citizens
    Functions are first class
    citizens
    Loops, method calls,
    conditionals
    Function calls
    Side-effects allowed in
    functions
    Pure functions
    

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17. functions in Kotlin
    

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18. Functions are first class citizens
    

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19. Functions are first class citizens
    They can:
    ● be stored in a variable - just like other types
    

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20. Functions are first class citizens
    They can:
    ● be stored in a variable - just like other types
    ● take another function as parameter
    

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21. Functions are first class citizens
    They can:
    ● be stored in a variable - just like other types
    ● take another function as parameter
    ● return a function
    

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22. Functions are first class citizens
    They can:
    ● be stored in a variable - just like other types
    ● take another function as parameter
    ● return a function
    a.k.a higher order functions
    

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23. Lambdas & higher order
    functions
    

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24. Lambdas & higher order functions
    fun atLeastAndroidP(action: () -> Unit) {
    ...
    }
    

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25. Lambdas & higher order functions
    fun atLeastAndroidP(action: () -> Unit) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
    action()
    }
    }
    

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26. Lambdas & higher order functions
    // Usage:
    atLeastAndroidP {
    // do something on Android P
    }
    fun atLeastAndroidP(action: () -> Unit) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
    action()
    }
    }
    

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27. Lambdas & higher order functions
    fun atLeastAndroidP(action: () -> Unit) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
    action()
    }
    }
    Lambda
    // Usage:
    atLeastAndroidP {
    // do something on Android P
    }
    

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28. Lambdas & higher order functions
    fun atLeastAndroidP(action: () -> Unit) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
    action()
    }
    }
    Higher order function
    // Usage:
    atLeastAndroidP {
    // do something on Android P
    }
    

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29. Lambdas & higher order functions
    // Usage:
    atLeastAndroidP {
    // do something on Android P
    }
    fun atLeastAndroidP(action: () -> Unit) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
    action()
    }
    }
    

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30. Lambdas & higher order functions
    // Usage:
    atLeastAndroidP {
    // do something on Android P
    }
    fun atLeastAndroidP(action: () -> Unit) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
    action()
    }
    } Tip: Take a look at the generated
    code to understand what’s happening
    under the hood
    

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31. Lambdas & higher order functions
    // Generated Java code
    public final void atLeastAndroidP(@NotNull Function0 action) {
    if (VERSION.SDK_INT > 28) {
    action.invoke();
    }
    }
    fun atLeastAndroidP(action: () -> Unit) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
    action()
    }
    }
    

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32. Lambdas & higher order functions
    // Generated Java code
    public final void atLeastAndroidP(@NotNull Function0 action) {
    if (VERSION.SDK_INT > 28) {
    action.invoke();
    }
    }
    Every lambda is an object!
    Not great for performance
    fun atLeastAndroidP(action: () -> Unit) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
    action()
    }
    }
    

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33. () -> Unit Function0
    (Int) -> Boolean Function1
    (Int, String) -> Boolean Function2
    Every lambda corresponds to a FunctionN interface -
    where N is the number of params in the lambda
    

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34. Lambdas & higher order functions
    // FunctionN interfaces used by Kotlin
    interface Function0 : Function
    interface Function1 : Function
    interface Function2 : Function
    ...
    interface Function22 : Function {
    /** Invokes the function with the specified arguments. */
    public operator fun invoke(p1: P1, p2: P2...p22: P22): R
    }
    

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35. Improving lambdas’ performance - the inline keyword
    // Kotlin
    fun atLeastAndroidP(action: () -> Unit) {...}
    

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36. Improving lambdas’ performance - the inline keyword
    // Kotlin
    inline fun atLeastAndroidP(action: () -> Unit) {...}
    

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37. Improving lambdas’ performance - the inline keyword
    // Kotlin
    inline fun atLeastAndroidP(action: () -> Unit) {...}
    // Usage:
    fun doSomethingOnP() {
    atLeastAndroidP { println("I'm on Android P") }
    }
    

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38. Improving lambdas’ performance - the inline keyword
    // Kotlin
    inline fun atLeastAndroidP(action: () -> Unit) {...}
    // Usage:
    fun doSomethingOnP() {
    atLeastAndroidP { println("I'm on Android P") }
    }
    // Generated Java code
    public final void doSomethingOnP() {
    if (VERSION.SDK_INT > 28) {
    String var0 = "I'm on Android P";
    System.out.println(var0);
    }
    }
    

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39. Improving lambdas’ performance - the inline keyword
    // Kotlin
    inline fun atLeastAndroidP(action: () -> Unit) {...}
    // Usage:
    fun doSomethingOnP() {
    atLeastAndroidP { println("I'm on Android P") }
    }
    // Generated Java code
    public final void doSomethingOnP() {
    if (VERSION.SDK_INT > 28) {
    String var0 = "I'm on Android P";
    System.out.println(var0);
    }
    }
    Method body is copied to the
    call site
    

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40. Other good-to-know concepts when working with lambdas:
    

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41. noinline keyword - used to mark lambdas in an inline
    function that we don’t want to inline
    Other good-to-know concepts when working with lambdas:
    

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42. noinline keyword - used to mark lambdas in an inline
    function that we don’t want to inline
    Other good-to-know concepts when working with lambdas:
    inline fun atLeastAndroidP(action: () -> Unit, noinline fallback: () -> Unit) {
    if (Build.VERSION.SDK_INT > Build.VERSION_CODES.P) {
    action()
    } else {
    fallback()
    }
    }
    

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43. noinline keyword - used to mark other lambda params in an
    inline function that we don’t want to inline
    Other good-to-know concepts when working with lambdas:
    inline fun atLeastAndroidP(action: () -> Unit, noinline fallback: () -> Unit) {
    if (Build.VERSION.SDK_INT > Build.VERSION_CODES.P) {
    action()
    } else {
    fallback()
    }
    }
    the `fallback` lambda is not inlined
    

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44. non-local returns - typically non-local returns are not
    allowed within lambdas unless the function is inlined.
    Other good-to-know concepts when working with lambdas:
    

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45. non-local returns - typically non-local returns are not
    allowed within lambdas unless the function is inlined.
    fun useNormalLambda() {
    normalLambda {
    ...
    return@normalLambda // works fine (local return)
    }
    }
    Other good-to-know concepts when working with lambdas:
    

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46. non-local returns - typically non-local returns are not
    allowed within lambdas unless the function is inlined.
    fun useNormalLambda() {
    normalLambda {
    ...
    return@normalLambda // works fine (local return)
    return // compile error (non-local return)
    }
    }
    Other good-to-know concepts when working with lambdas:
    

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47. non-local returns - typically non-local returns are not
    allowed within lambdas unless the function is inlined.
    Other good-to-know concepts when working with lambdas:
    fun useNormalLambda() {
    normalLambda {
    ...
    return@normalLambda // works fine (local return)
    return // compile error (non-local return
    }
    }
    fun useInlineLambda() {
    inlineLambda {
    return // works fine. Returns from useInlineLambda
    }
    }
    

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48. crossinline keyword - used when the lambdas are not used
    directly in the inline function
    Other good-to-know concepts when working with lambdas:
    

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49. crossinline keyword - used when the lambdas are not used
    directly in the inline function
    Other good-to-know concepts when working with lambdas:
    inline fun View.waitForLayout(crossinline action: () -> Unit) = with(viewTreeObserver) {
    addOnGlobalLayoutListener(object : ViewTreeObserver.OnGlobalLayoutListener {
    override fun onGlobalLayout() {
    this@waitForLayout.viewTreeObserver.removeOnGlobalLayoutListener(this)
    action()
    }
    })
    }
    

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50. crossinline keyword - used when the lambdas are not used
    directly in the inline function
    Other good-to-know concepts when working with lambdas:
    inline fun View.waitForLayout(crossinline action: () -> Unit) = with(viewTreeObserver) {
    addOnGlobalLayoutListener(object : ViewTreeObserver.OnGlobalLayoutListener {
    override fun onGlobalLayout() {
    this@waitForLayout.viewTreeObserver.removeOnGlobalLayoutListener(this)
    action()
    }
    })
    }
    Non-local returns are not allowed
    within the lambda
    

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51. Standard library functions
    let, run, apply, also
    

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52. Standard library functions
    Some of these functions are also referred to as Scoping
    functions
    

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53. let - Standard.kt
    public inline fun T.let(block: (T) -> R): R = block(this)
    

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54. let - Standard.kt
    public inline fun T.let(block: (T) -> R): R = block(this)
    Lambda function
    

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55. let - Standard.kt
    public inline fun T.let(block: (T) -> R): R = block(this)
    Receiver
    

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56. let - Standard.kt
    public inline fun T.let(block: (T) -> R): R = block(this)
    // Sample usage
    client?.email
    ?.let {
    mailer.sendMessage(it, message)
    }
    

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57. let - Standard.kt
    // Sample usage
    client?.email
    ?.let {
    mailer.sendMessage(it, message)
    }
    public inline fun T.let(block: (T) -> R): R = block(this)
    

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58. let - Standard.kt
    // Sample usage
    client?.email
    ?.let {
    mailer.sendMessage(it, message)
    }
    // Or
    client?.email
    ?.let { email ->
    mailer.sendMessage(email, message)
    }
    public inline fun T.let(block: (T) -> R): R = block(this)
    

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59. run - Standard.kt
    public inline fun T.run(block: T.() -> R): R = block()
    Runs the block of code and returns
    the last line in the lambda
    

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60. run - Standard.kt
    public inline fun T.run(block: T.() -> R): R = block()
    Receiver
    

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61. run - Standard.kt
    public inline fun T.run(block: T.() -> R): R = block()
    Function literal with
    receiver
    

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62. run - Standard.kt
    public inline fun T.run(block: T.() -> R): R = block()
    // Sample usage
    fun performTransformation(input: String) : String {...}
    val transformedName = name.run {
    println("Transforming name...")
    val transformedString = performTransformation(this)
    transformedString
    }
    

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63. run - Standard.kt
    public inline fun T.run(block: T.() -> R): R = block()
    // Sample usage
    fun performTransformation(input: String) : String {...}
    val transformedName = name.run {
    println("Transforming name...")
    val transformedString = performTransformation(this)
    transformedString
    }
    

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64. run - Standard.kt
    public inline fun T.run(block: T.() -> R): R = block()
    // Sample usage
    fun performTransformation(input: String) : String {...}
    val transformedName = name.run {
    println("Transforming name...")
    val transformedString = performTransformation(this)
    transformedString
    }
    

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65. run - Standard.kt
    public inline fun T.run(block: T.() -> R): R = block()
    // Sample usage
    fun performTransformation(input: String) : String {...}
    val transformedName = name.run {
    println("Transforming name...")
    val transformedString = performTransformation(this)
    transformedString
    } Receiver is accessed as
    “this”, and exposes the inner
    members of the receiver class
    

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66. run - Standard.kt
    public inline fun T.run(block: T.() -> R): R = block()
    // Sample usage
    fun performTransformation(input: String) : String {...}
    val transformedName = name.run {
    println("Transforming name...")
    val transformedString = performTransformation(this)
    transformedString
    }
    transformedString is
    returned from the lambda
    

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67. also - Standard.kt
    inline fun T.also(block: (T) -> Unit): T { block(this); return this }
    

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68. also - Standard.kt
    inline fun T.also(block: (T) -> Unit): T { block(this); return this }
    // Sample usage
    val dev = Developer().also {
    it.name = "Segun"
    it.lovesCatVideos = true
    it.stack = "Android"
    }
    

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69. also - Standard.kt
    inline fun T.also(block: (T) -> Unit): T { block(this); return this }
    The receiver is accessed as “it”
    // Sample usage
    val dev = Developer().also {
    it.name = "Segun"
    it.lovesCatVideos = true
    it.stack = "Android"
    }
    

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70. also - Standard.kt
    inline fun T.also(block: (T) -> Unit): T { block(this); return this }
    // Sample usage
    val dev = Developer().also {
    it.name = "Segun"
    it.lovesCatVideos = true
    it.stack = "Android"
    }
    // OR
    val dev = Developer().also { developer ->
    developer.name = "Segun"
    developer.lovesCatVideos = true
    developer.stack = "Android"
    }
    

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71. apply - Standard.kt
    inline fun T.apply(block: T.() -> Unit): T { block(); return this }
    

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72. apply - Standard.kt
    inline fun T.apply(block: T.() -> Unit): T { block(); return this }
    // Sample usage
    val dev = Developer().apply {
    this.name = "Segun"
    this.lovesCatVideos = true
    this.stack = "Android"
    }
    

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73. apply - Standard.kt
    inline fun T.apply(block: T.() -> Unit): T { block(); return this }
    // Sample usage
    val dev = Developer().apply {
    this.name = "Segun"
    this.lovesCatVideos = true
    this.stack = "Android"
    }
    Exposes the receiver as “this”
    

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74. Things to consider
    

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75. Things to consider
    ● Avoid overusing these scoping functions - readability can
    degrade quickly
    

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76. Things to consider
    ● Avoid overusing these scoping functions - readability can
    degrade quickly
    ● Consider renaming it to something better when possible
    

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77. Things to consider
    ● Avoid overusing these scoping functions - readability can
    degrade quickly
    ● Consider renaming it to something better when possible
    ● More stdlib functions
    ○ takeIf, takeUnless, with, etc
    

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78. stdlib functions in action
    

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79. // Java code
    public void sendMessageToClient(
    @Nullable Client client,
    @Nullable String message,
    @NotNull Mailer mailer
    ) {
    if (client == null || message == null) return;
    PersonalInfo personalInfo = client.getPersonalInfo();
    if (personalInfo == null) return;
    String email = personalInfo.getEmail();
    if (email == null) return;
    mailer.sendMessage(email, message);
    }
    stdlib functions in action
    

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80. // Java code
    public void sendMessageToClient(
    @Nullable Client client,
    @Nullable String message,
    @NotNull Mailer mailer
    ) {
    if (client == null || message == null) return;
    PersonalInfo personalInfo = client.getPersonalInfo();
    if (personalInfo == null) return;
    String email = personalInfo.getEmail();
    if (email == null) return;
    mailer.sendMessage(email, message);
    }
    // Kotlin equivalent
    fun sendMessageToClient(
    client: Client?,
    message: String?,
    mailer: Mailer
    ) {
    client?.takeIf { message != null }
    ?.personalInfo
    ?.email
    ?.let { email ->
    mailer.sendMessage(email, message!!)
    }
    }
    stdlib functions in action
    

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81. // Java code
    public void sendMessageToClient(
    @Nullable Client client,
    @Nullable String message,
    @NotNull Mailer mailer
    ) {
    if (client == null || message == null) return;
    PersonalInfo personalInfo = client.getPersonalInfo();
    if (personalInfo == null) return;
    String email = personalInfo.getEmail();
    if (email == null) return;
    mailer.sendMessage(email, message);
    }
    // Kotlin equivalent
    fun sendMessageToClient(
    client: Client?,
    message: String?,
    mailer: Mailer
    ) {
    client?.takeIf { message != null }
    ?.personalInfo
    ?.email
    ?.let { email ->
    mailer.sendMessage(email, message!!)
    }
    }
    stdlib functions in action
    

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82. // Java code
    public void sendMessageToClient(
    @Nullable Client client,
    @Nullable String message,
    @NotNull Mailer mailer
    ) {
    if (client == null || message == null) return;
    PersonalInfo personalInfo = client.getPersonalInfo();
    if (personalInfo == null) return;
    String email = personalInfo.getEmail();
    if (email == null) return;
    mailer.sendMessage(email, message);
    }
    // Kotlin equivalent
    fun sendMessageToClient(
    client: Client?,
    message: String?,
    mailer: Mailer
    ) {
    client?.takeIf { message != null }
    ?.personalInfo
    ?.email
    ?.let { email ->
    mailer.sendMessage(email, message!!)
    }
    }
    stdlib functions in action
    

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83. // Java code
    public void sendMessageToClient(
    @Nullable Client client,
    @Nullable String message,
    @NotNull Mailer mailer
    ) {
    if (client == null || message == null) return;
    PersonalInfo personalInfo = client.getPersonalInfo();
    if (personalInfo == null) return;
    String email = personalInfo.getEmail();
    if (email == null) return;
    mailer.sendMessage(email, message);
    }
    // Kotlin equivalent
    fun sendMessageToClient(
    client: Client?,
    message: String?,
    mailer: Mailer
    ) {
    client?.takeIf { message != null }
    ?.personalInfo
    ?.email
    ?.let { email ->
    mailer.sendMessage(email, message!!)
    }
    }
    stdlib functions in action
    Functional, readable, concise
    

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84. Java & Kotlin Interoperability
    ...a tale of 2 islands
    

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85. Java & Kotlin Interoperability
    Important because:
    ● Android APIs are written in Java
    

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86. Java & Kotlin Interoperability
    Important because:
    ● Android APIs are written in Java
    ● Kotlin & Java may be mixed in the same codebase
    

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87. - nullability
    Java & Kotlin Interoperability
    

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88. String!
    String?
    What do these mean when you think about nullability &
    types?
    String
    Pop
    

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89. String!
    String?
    String
    Non-nullable
    type
    Nullable
    type
    Platform type
    Kotlin compiler has
    no idea whether
    it’s nullable or
    not
    

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90. types are very important when considering
    interoperability between Java & Kotlin
    

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91. String String? String
    or
    

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92. // Calling Java from Kotlin
    // Compiles fine. Safe in runtime
    val nameLength = myObject.name.length
    // Java
    @NotNull
    public String getName() {
    return "Segun";
    }
    ...
    

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93. // Calling Java from Kotlin
    // Compiler error. Need safe access
    val nameLength = myObject.name.length
    // Java
    @Nullable
    public String getName() {
    return null;
    }
    ...
    

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94. // Calling Java from Kotlin
    // Compiler error. Need safe access
    val nameLength = myObject.name.length
    // Compiles fine. Safe in runtime
    val nameLength = myObject.name?.length
    // Java
    @Nullable
    public String getName() {
    return null;
    }
    ...
    

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95. // Calling Java from Kotlin
    // Compiles fine. NPE in runtime
    val nameLength = myObject.name.length
    // Java
    public String getName() {
    return null;
    }
    ...
    

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96. // Java
    public String getName() {
    return null;
    }
    ...
    // Calling Java from Kotlin
    // Compiles fine. NPE in runtime
    val nameLength = myObject.name.length
    // Compiles fine. Safe in runtime
    val nameLength = myObject.name?.length
    

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97. @Nullable
    String getName() {..}
    Kotlin compiler treats as nullable
    “?”
    @NotNull
    String getName() {..}
    Kotlin compiler treats as
    non-nullable
    String getName() {..} Kotlin compiler has no clue. Treats
    as platform type
    

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98. - calling Kotlin from Java
    Java & Kotlin Interoperability
    

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99. Kotlin package-level functions
    

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100. Kotlin package-level functions
     Functions can be defined in a file directly without being in
     any class or object.
     // InteropDemo.kt
     fun getName() = "Segun"
     

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101. Kotlin package-level functions
     Functions can be defined in a file directly without being in
     any class or object.
     // InteropDemo.kt
     fun getName() = "Segun"
     // Calling from Java
     InteropDemoKt.getName();
     

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102. Kotlin package-level functions
     @JvmName annotation helps to tell Kotlin compiler what the
     generated Java class name should be when consumed from Java
     

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103. Kotlin package-level functions
     // InteropDemo.kt
     @file:JvmName("InteropDemo")
     fun getName() = "Segun"
     @JvmName annotation helps to tell Kotlin compiler what the
     generated Java class name should be when consumed from Java
     

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104. Kotlin package-level functions
     // InteropDemo.kt
     @file:JvmName("InteropDemo")
     fun getName() = "Segun"
     // Calling from Java
     InteropDemo.getName();
     @JvmName annotation helps to tell Kotlin compiler what the
     generated Java class name should be when consumed from Java
     

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105. @JvmName
     The annotation can also be applied to functions
     

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106. @JvmName
     // Kotlin code
     @JvmName("getInteropName")
     fun getName() = "Segun"
     The annotation can also be applied to functions
     

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107. @JvmName
     // Kotlin code
     @JvmName("getInteropName")
     fun getName() = "Segun"
     // Consumed from Java
     demo.getInteropName();
     The annotation can also be applied to functions
     

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108. @JvmName
     The annotation can also be applied to functions
     // Kotlin code
     @JvmName("getInteropName")
     fun getName() = "Segun"
     // Consumed from Java
     demo.getInteropName();
     // Consumed from Kotlin
     demo.getName()
     

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109. @JvmStatic
     Kotlin does not have static methods.
     Instead, it has objects, companion objects
     

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110. @JvmStatic
     // Kotlin Object
     object StringUtils {
     fun upperCase(text: String) = text.toUpperCase()
     }
     

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111. @JvmStatic
     // Kotlin Object
     object StringUtils {
     fun upperCase(text: String) = text.toUpperCase()
     }
     // Calling from Java
     String upperCaseName = StringUtils.INSTANCE.upperCase("Segun");
     

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112. @JvmStatic
     // Kotlin Object
     object StringUtils {
     + @JvmStatic
     fun upperCase(text: String) = text.toUpperCase()
     }
     // Calling from Java
     String upperCaseName = StringUtils.INSTANCE.upperCase("Segun");
     

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113. @JvmStatic
     // Kotlin Object
     object StringUtils {
     + @JvmStatic
     fun upperCase(text: String) = text.toUpperCase()
     }
     // Calling from Java
     String upperCaseName = StringUtils.INSTANCE.upperCase("Segun");
     String upperCaseName = StringUtils.upperCase("Segun");
     

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114. @JvmStatic - Static providers in Dagger
     Specially useful when working with Dagger static Providers
     on Android
     

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115. @Module
     abstract class RepositoryModule {
     @Binds
     abstract fun bindRepo(repository: Repository): IRepository
     companion object {
     // static provider
     @Provides
     fun provideApiService: ApiService = ...
     }
     }
     @JvmStatic - Static providers in Dagger
     

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116. @Module
     abstract class RepositoryModule {
     @Binds
     abstract fun bindRepo(repository: Repository): IRepository
     companion object {
     // static provider
     @Provides
     fun provideApiService: ApiService = ...
     }
     }
     Won’t compile.
     Dagger error.
     Why?
     @JvmStatic - Static providers in Dagger
     

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117. @JvmStatic - Static providers in Dagger
     // Generated Java class
     @Module
     public abstract class RepositoryModule {
     ...
     @Binds
     @NotNull
     public abstract IRepository bindRepo(@NotNull Repository var1);
     public static final class Companion {
     @Provides
     @NotNull
     public final ApiService providesApiService() {...}
     }
     }
     

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118. @JvmStatic - Static providers in Dagger
     // Generated Java class
     @Module
     public abstract class RepositoryModule {
     ...
     @Binds
     @NotNull
     public abstract IRepository bindRepo(@NotNull Repository var1);
     public static final class Companion {
     @Provides
     @NotNull
     public final ApiService providesApiService() {...}
     }
     }
     No @Module
     

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119. @JvmStatic - Static providers in Dagger
     // Generated Java class
     @Module
     public abstract class RepositoryModule {
     ...
     @Binds
     @NotNull
     public abstract IRepository bindRepo(@NotNull Repository var1);
     public static final class Companion {
     @Provides
     @NotNull
     public final ApiService providesApiService() {...}
     }
     }
     No @Module
     @Provides method is generated as a
     member of the Companion class
     

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120. @JvmStatic - Static providers in Dagger
     // Generated Java class
     @Module
     public abstract class RepositoryModule {
     ...
     @Binds
     @NotNull
     public abstract IRepository bindRepo(@NotNull Repository var1);
     public static final class Companion {
     @Provides
     @NotNull
     public final ApiService providesApiService() {...}
     }
     }
     No @Module
     @Provides method is generated as a
     member of the Companion class
     Dagger is unable to provide this
     dependency
     

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121. @JvmStatic - Static providers in Dagger
     @Module
     abstract class RepositoryModule {
     @Binds
     abstract fun bindRepo(repository: Repository): IRepository
     + @Module
     companion object {
     // static provider
     @Provides
     + @JvmStatic
     fun provideApiService: ApiService = ...
     }
     }
     Fixed by adding @Module and @JvmStatic annotations
     

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122. @JvmStatic - Static providers in Dagger
     Fixed by adding @Module and @JvmStatic annotations
     // Generated Java class with fixes
     @Module
     public abstract class RepositoryModule {
     @Binds
     public abstract IRepository bindRepos(@NotNull Repository var1);
     @Provides
     @JvmStatic
     public static final ApiService providesApiService() {...}
     @Module
     public static final class Companion {
     @Provides
     @JvmStatic
     public final ApiService providesApiService() {...}
     ...
     }
     }
     Kotlin compiler generates a
     static method within the
     RepositoryModule class
     

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123. @JvmStatic - Static providers in Dagger
     Fixed by adding @Module and @JvmStatic annotations
     // Generated Java class with fixes
     @Module
     public abstract class RepositoryModule {
     @Binds
     public abstract IRepository bindRepos(@NotNull Repository var1);
     @Provides
     @JvmStatic
     public static final ApiService providesApiService() {...}
     @Module
     public static final class Companion {
     @Provides
     @JvmStatic
     public final ApiService providesApiService() {...}
     ...
     }
     }
     Dagger will see this module,
     but generated code will not be
     used.
     RepositoryModule.Companion
     module isn’t registered
     

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124. @JvmOverloads
     

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125. @JvmOverloads
     // Kotlin class with default values
     class Point(val x: Int = 0, val y: Int = 0)
     

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126. @JvmOverloads
     // Kotlin class with default values
     class Point(val x: Int = 0, val y: Int = 0)
     // Calling from Java
     Point p1 = new Point();
     Point p2 = new Point(1, 3);
     

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127. @JvmOverloads
     // Kotlin class with default values
     class Point(val x: Int = 0, val y: Int = 0)
     // Calling from Java
     Point p1 = new Point();
     Point p2 = new Point(1, 3); Specify none or all params
     

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128. @JvmOverloads
     // Kotlin class with default values
     class Point @JvmOverloads constructor(val x: Int = 0, val y: Int = 0)
     

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129. @JvmOverloads
     // Kotlin class with default values
     class Point @JvmOverloads constructor(val x: Int = 0, val y: Int = 0)
     // Generated Java code
     public final class Point {
     // Overloaded constructors
     public Point() {...}
     public Point(int x) {...}
     public Point(int x, int y) {...}
     }
     

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130. @JvmOverloads
     // Calling from Java
     Point p1 = new Point();
     Point p2 = new Point(1);
     Point p3 = new Point(1, 3);
     // Kotlin class with default values
     class Point @JvmOverloads constructor(val x: Int = 0, val y: Int = 0)
     // Generated Java code
     public final class Point {
     // Overloaded constructors
     public Point() {...}
     public Point(int x) {...}
     public Point(int x, int y) {...}
     }
     

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131. @JvmOverloads
     Also works on other functions that are not constructors
     

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132. @Jvm* annotations
     There are more:
     @JvmField
     @JvmSuppressWildcards
     @JvmMultifileClass... etc
     

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133. Resources
     KotlinConf videos
     #Kotlin on twitter
     Kotlinweekly.net
     kotlinlang.org
     

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134. Thank you
     segunfamisa
     segunfamisa.com
     

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