Reviewing Kotlin (Conference for Kotliners 2019)

I have been teaching our rapidly growing team of Android developers Kotlin for about a year, and for the last few months, I’ve been reviewing tens of thousands of lines of code written by almost a dozen people at times. Here’s what we’ve found out together about learning, teaching, and reviewing Kotlin. I’ll tell you what worked for us and what didn’t, so that you may be more prepared for this path than we were. I’ll also point out some of the issues that most often arose in the code while our developers were getting familiar with the language.

Recording and resources: https://zsmb.co/talks/reviewing-kotlin/

Transcript

1. Reviewing Kotlin
   Márton Braun
   zsmb.co zsmb13
   braun.marton@autsoft.hu
   

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2. This is not…
   

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3. How great code reviews are, and what’s
   the best way to do them
   This is not…
   

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4. and how to get your
   management onboard with the idea
   Why you should use Kotlin for Android
   development…
   How great code reviews are, and what’s
   the best way to do them
   This is not…
   

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5. and how to get your
   management onboard with the idea
   Why you should use Kotlin for Android
   development…
   All the awesome things that are in the
   Kotlin standard library
   How great code reviews are, and what’s
   the best way to do them
   This is not…
   

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6. Life is Great and Everything Will Be Ok,
   Kotlin is Here
   Christina Lee, Jake Wharton
   Google I/O '17
   Dissecting the stdlib
   Huyen Tue Dao
   KotlinConf 2018
   Code Review Best Practices
   Trisha Gee
   SCLConf 2018
   This is not…
   

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13. 99000 lines of Kotlin code
    5700 commits
    700 merge requests
    

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22. Breaking the habit
    

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23. Breaking the habit
    Enums
    

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24. Breaking the habit
    Enums Lambdas
    

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25. Breaking the habit
    Enums Lambdas Typechecks
    

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26. Breaking the habit
    Enums Lambdas Typechecks
    is
    as
    as?
    

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27. entries: ArrayList) {
    fun updateColors(chart: ,
    }
    PieChart
    

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28. entries: ArrayList) {
    fun updateColors(chart: ,
    val colors = ArrayList()
    }
    PieChart
    

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29. entries: ArrayList) {
    fun updateColors(chart: ,
    val colors = ArrayList()
    entries.forEach { entry ->
    colors.add(entry.data as Int)
    }
    }
    PieChart
    

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30. entries: ArrayList) {
    fun updateColors(chart: ,
    val colors = ArrayList()
    entries.forEach { entry ->
    colors.add(entry.data as Int)
    }
    chart.colors = colors
    }
    PieChart
    

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31. entries: ArrayList) {
    fun updateColors(chart: ,
    val colors = ArrayList()
    entries.forEach { entry ->
    colors.add(entry.data as Int)
    }
    chart.colors = colors
    }
    PieChart
    

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32. fun PieChart.updateColors(entries:
    val colors = ArrayList()
    entries.forEach { entry ->
    colors.add(entry.data as Int)
    }
    this.colors = colors
    }
    List) {
    Array
    

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33. fun PieChart.updateColors(entries:
    val colors = ArrayList()
    entries.forEach { entry ->
    colors.add(entry.data as Int)
    }
    this.colors = colors
    }
    List) {
    

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34. fun PieChart.updateColors(entries:
    val colors = ArrayList()
    entries.forEach { entry ->
    colors.add(entry.data as Int)
    }
    this.colors = colors
    }
    List) {
    

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35. fun PieChart.updateColors(entries: List) {
    val colors = mutableListOf()
    entries.forEach { entry ->
    colors.add(entry.data as Int)
    }
    this.colors = colors
    }
    

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36. fun PieChart.updateColors(entries: List) {
    val colors = mutableListOf()
    entries.forEach { entry ->
    colors.add(entry.data as Int)
    }
    this.colors = colors
    }
    

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37. fun PieChart.updateColors(entries: List) {
    val colors = mutableListOf()
    entries.map { entry ->
    colors.add( )
    }
    this.colors = colors
    }
    entry.data as Int
    

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38. fun PieChart.updateColors(entries: List) {
    val colors = mutableListOf()
    entries.map { entry ->
    }
    this.colors = colors
    }
    entry.data as Int
    

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39. fun PieChart.updateColors(entries: List) {
    this.colors = colors
    }
    entry.data as Int
    }
    entries.map { entry ->
    val colors =
    

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40. fun PieChart.updateColors(entries: List) {
    this.colors = colors
    }
    entry.data as Int
    }
    entries.map { entry ->
    val colors =
    

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41. fun PieChart.updateColors(entries: List) {
    this.colors
    entry.data as Int
    }
    }
    = entries.map { entry ->
    

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44. fun getExaminationResult(resultId: UUID): ExaminationResult {
    }
    

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45. fun getExaminationResult(resultId: UUID): ExaminationResult {
    return getItemGroups()
    }
    

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46. fun getExaminationResult(resultId: UUID): ExaminationResult {
    return getItemGroups()
    }
    .map { it.toExaminationResult() }
    

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47. fun getExaminationResult(resultId: UUID): ExaminationResult {
    return getItemGroups()
    }
    .map { it.toExaminationResult() }
    .first { it.id == resultId }
    

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48. fun getExaminationResult(resultId: UUID): ExaminationResult {
    return getItemGroups()
    }
    .map { it.toExaminationResult() }
    .first { it.id == resultId }
    

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49. fun getExaminationResult(resultId: UUID): ExaminationResult {
    return getItemGroups()
    }
    .map { it.toExaminationResult() }
    .first { it.id == resultId }
    

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50. fun getExaminationResult(resultId: UUID): ExaminationResult {
    return getItemGroups()
    }
    .first { it.id == resultId }
    .map { it }
    .toExaminationResult()
    

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51. fun getExaminationResult(resultId: UUID): ExaminationResult {
    return getItemGroups()
    }
    .first { it.id == resultId }
    .toExaminationResult()
    

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52. val events: List = getAllEvents()
    val upcoming = events.filter {
    it.date >
    }
    OffsetDateTime.now()
    

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53. val events: List = getAllEvents()
    val upcoming = events.filter {
    it.date >
    }
    OffsetDateTime.now()
    val now =
    now
    

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54. fun startBluetoothLeScan() {
    }
    

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55. fun startBluetoothLeScan() {
    val scanner = BluetoothAdapter.getDefaultAdapter()
    .bluetoothLeScanner
    }
    

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56. fun startBluetoothLeScan() {
    val scanner = BluetoothAdapter.getDefaultAdapter()
    .bluetoothLeScanner
    /* ... */
    }
    

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57. fun startBluetoothLeScan() {
    val scanner = BluetoothAdapter.getDefaultAdapter()
    .bluetoothLeScanner
    /* ... */
    scanner
    }
    .startScan(/* ... */)
    

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58. fun startBluetoothLeScan() {
    val scanner = BluetoothAdapter.getDefaultAdapter()
    .bluetoothLeScanner
    /* ... */
    scanner?
    }
    .startScan(/* ... */)
    

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59. if (scanner != null) {
    } else {
    /* ¯\_(ツ)_/¯ */
    }
    }
    fun startBluetoothLeScan() {
    val scanner = BluetoothAdapter.getDefaultAdapter()
    .bluetoothLeScanner
    /* ... */
    scanner.startScan(/* ... */)
    

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62. data class DailyFluidConsumption(
    )
    

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63. data class DailyFluidConsumption(
    val quantity: Double,
    )
    

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64. data class DailyFluidConsumption(
    val quantity: Double,
    val recommended: Double = 4.0,
    )
    

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65. data class DailyFluidConsumption(
    val quantity: Double,
    val recommended: Double = 4.0,
    val percentage: Int =
    ((quantity / recommended) * 100)
    .roundToInt()
    .coerceIn(0..100)
    )
    

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66. data class DailyFluidConsumption(
    val quantity: Double,
    val recommended: Double = 4.0
    )
    val percentage: Int =
    ((quantity / recommended) * 100)
    .roundToInt()
    .coerceIn(0..100)
    

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67. data class DailyFluidConsumption(
    val quantity: Double,
    val recommended: Double = 4.0
    )
    val DailyFluidConsumption.percentage: Int
    ((quantity / recommended) * 100)
    .roundToInt()
    .coerceIn(0..100)
    

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68. data class DailyFluidConsumption(
    val quantity: Double,
    val recommended: Double = 4.0
    )
    val DailyFluidConsumption.percentage: Int
    get() = ((quantity / recommended) * 100)
    .roundToInt()
    .coerceIn(0..100)
    

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69. zsmb.co/data-classes-arent-that-magical
    

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70. class Ingredient(
    val id: UUID = UUID.randomUUID(),
    val name: String = "",
    val quantity: Double? = null,
    val unit: String? = null,
    val imageId: UUID? = null
    )
    

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71. class Ingredient(
    val id: UUID = UUID.randomUUID(),
    val name: String = "",
    val quantity: Double? = null,
    val unit: String? = null,
    val imageId: UUID? = null
    )
    

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72. class Ingredient(
    val id: UUID = UUID.randomUUID(),
    val name: String = "",
    val quantity: Double? = null,
    val unit: String? = null,
    val imageId: UUID? = null
    )
    

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73. class Ingredient(
    val id: UUID = UUID.randomUUID(),
    val name: String = "",
    val quantity: Double? = null,
    val unit: String? = null,
    val imageId: UUID? = null
    )
    Ingredient(id, "Pixie dust", 6.28, "teaspoon")
    

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74. class Ingredient(
    val id: UUID = UUID.randomUUID(),
    val name: String = "",
    val quantity: Double? = null,
    val unit: String? = null,
    val imageId: UUID? = null
    )
    Ingredient(id, "Pixie dust", 6.28, "teaspoon")
    

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75. class Ingredient(
    val id: UUID = UUID.randomUUID(),
    val name: String = "",
    val quantity: Double? = null,
    val unit: String? = null,
    val imageId: UUID? = null
    )
    Ingredient(id, "Pixie dust", 6.28, "teaspoon")
    

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76. class Ingredient(
    val id: UUID,
    val name: String,
    val quantity: Double?,
    val unit: String?,
    val imageId: UUID?
    )
    Ingredient(id, "Pixie dust", 6.28, "teaspoon")
    

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77. class Ingredient(
    val id: UUID,
    val name: String,
    val quantity: Double?,
    val unit: String?,
    val imageId: UUID?
    )
    Ingredient(id, "Pixie dust", 6.28, "teaspoon", null)
    

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78. id =
    name =
    quantity =
    unit =
    imageId =
    id,
    6.28,
    )
    "teaspoon",
    null
    Ingredient(
    "Pixie dust",
    class Ingredient(
    val id: UUID,
    val name: String,
    val quantity: Double?,
    val unit: String?,
    val imageId: UUID?
    )
    

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79. suspend fun
    

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80. suspend fun getValidMeasurements(
    measurements: List
    ): List {
    }
    

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81. suspend fun getValidMeasurements(
    measurements: List
    ): List {
    return measurements.filter {
    }
    }
    

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82. suspend fun getValidMeasurements(
    measurements: List
    ): List {
    return measurements.filter {
    val validator = getValidatorForType(it.type)
    }
    }
    

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83. suspend fun getValidMeasurements(
    measurements: List
    ): List {
    return measurements.filter {
    val validator = getValidatorForType(it.type)
    it.value in (validator.minValue..validator.maxValue)
    }
    }
    

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84. suspend fun getValidMeasurements(
    measurements: List
    ): List {
    return measurements.filter { measurement ->
    val validator = getValidatorForType(measurement.type)
    measurement.value in (validator.minValue..validator.maxValue)
    }
    }
    

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85. return measurements.filter { measurement ->
    val validator = getValidatorForType(measurement.type)
    measurement.value in (validator.minValue..validator.maxValue)
    }
    }
    suspend fun getValidMeasurements(
    measurements: List
    ): List {
    

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86. return measurements.filter { measurement ->
    val validator = getValidatorForType(measurement.type)
    measurement.value in (validator.minValue..validator.maxValue)
    }
    }
    suspend fun getValidMeasurements(
    measurements: List
    ): List {
    

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87. return measurements.filter { measurement ->
    val validator = getValidatorForType(measurement.type)
    measurement.value in (validator.minValue..validator.maxValue)
    }
    }
    suspend fun getValidMeasurements(
    measurements: List
    ): List {
    

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88. val validatorsByType = getAllValidators()
    suspend fun getValidMeasurements(
    measurements: List
    ): List {
    }
    return measurements.filter { measurement ->
    val validator = getValidatorForType(measurement.type)
    measurement.value in (validator.minValue..validator.maxValue)
    }
    

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89. val validatorsByType = getAllValidators().associateBy { it.type }
    suspend fun getValidMeasurements(
    measurements: List
    ): List {
    }
    return measurements.filter { measurement ->
    val validator = getValidatorForType(measurement.type)
    measurement.value in (validator.minValue..validator.maxValue)
    }
    

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90. val validatorsByType = getAllValidators()
    suspend fun getValidMeasurements(
    measurements: List
    ): List {
    }
    return measurements.filter { measurement ->
    val validator = validatorsByType.getValue(measurement.type)
    measurement.value in (validator.minValue..validator.maxValue)
    }
    .associateBy { it.type }
    

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107. fun add (x:Int,y: Int) :Int{
     return x +y
     }
     fun main() {
     println( add(2,3) )
     }
     

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108. fun add(x: Int, y: Int): Int {
     return x + y
     }
     fun main() {
     println(add(2, 3))
     }
     

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109. What did you miss about Java?
     

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110. Related talks
     • Code Review Best Practices
      Trisha Gee, SCLConf 2018
      https://www.youtube.com/watch?v=jXi8h44cbQA
     • Life is Great and Everything Will Be Ok, Kotlin is Here
      Christina Lee & Jake Wharton, Google I/O ‘17
      https://www.youtube.com/watch?v=fPzxfeDJDzY
     • Dissecting the stdlib
      Huyen Tue Dao, KotlinConf 2018
      https://www.youtube.com/watch?v=Fzt_9I733Yg
     

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111. Learning resources
     • Kotlin in Action, Dmitry Jemerov and Svetlana Isakova
      https://www.manning.com/books/kotlin-in-action
     • Coursera course, Andrey Breslav and Svetlana Isakova
      https://www.coursera.org/learn/kotlin-for-java-developers
     • O’Reilly courses, Hadi Hariri
      https://hadihariri.com/2016/11/01/oreilly-kotlin-course/
     

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112. Learning resources
     • Kotlin Bootcamp for Programmers
      https://eu.udacity.com/course/kotlin-bootcamp-for-
     programmers--ud9011
     • Developing Android Apps with Kotlin
      https://eu.udacity.com/course/developing-android-apps-with-
     kotlin--ud9012
     

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113. Further reading
     • Data classes aren’t (that) magical
      https://zsmb.co/data-classes-arent-that-magical/
     

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114. zsmb13
     zsmb.co/talks
     

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115. Questions?
     Márton Braun
     zsmb.co/talks
     zsmb13
     braun.marton@autsoft.hu
     

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