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Top 10 Kotlin Stack Overflow Questions (Google ...

Top 10 Kotlin Stack Overflow Questions (Google I/O Extended 2018 Budapest)

A selection of some of the most frequent questions about Kotlin on Stack Overflow, asked, answered and explained. Something in there for everyone, from beginners to experts.

Talk recording (Hungarian): https://youtu.be/-UMqYBoUlJc?t=4h7m38s

Márton Braun

May 08, 2018
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  1. Array<Int> vs IntArray Array<Int> Integer[] IntArray int[] val intArray: IntArray

    = intArrayOf(1, 2, 3, 4, 5) val arrayOfInts: Array<Int> = arrayOf(1, 2, 3, 4, 5)
  2. Array<Int> vs IntArray • IntArray  Avoids boxing  Easier

    to initialize val intArray = IntArray(10) val arrayOfInts = Array<Int>(5) { i -> i * 2 }
  3. Array<Int> vs IntArray • IntArray  Avoids boxing  Easier

    to initialize • Array<Int> val intArray = IntArray(10) val arrayOfInts = Array<Int>(5) { i -> i * 2 }
  4. Array<Int> vs IntArray • IntArray  Avoids boxing  Easier

    to initialize • Array<Int>  Might be required by an API val intArray = IntArray(10) val arrayOfInts = Array<Int>(5) { i -> i * 2 }
  5. Array<Int> vs IntArray • IntArray  Avoids boxing  Easier

    to initialize • Array<Int>  Might be required by an API  Can store nullable values val intArray = IntArray(10) val arrayOfInts = Array<Int>(5) { i -> i * 2 } val notPeople: Array<Person?> = arrayOfNulls<Person>(13)
  6. Iterable vs Sequence  Eager evaluation Iterable java.lang.Iterable val people:

    List<Person> = getPeople() val allowedEntrance = people .filter { it.age >= 21 } .map { it.name } .take(5)
  7. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  8. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  9. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  10. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  11. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  12. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  13. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  14. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  15. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  16. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  17. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  18. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  19. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  20. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  21. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  22. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  23. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  24. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  25. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  26. val people: List<Person> = getPeople() val allowedEntrance = people .filter

    { it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence
  27. Iterable vs Sequence Sequence  Lazy evaluation val people: List<Person>

    = getPeople() val allowedEntrance = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  28. Iterable vs Sequence Sequence  Lazy evaluation val people: List<Person>

    = getPeople() val allowedEntrance = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  29. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  30. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  31. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  32. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  33. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  34. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  35. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  36. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  37. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  38. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  39. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  40. Iterable vs Sequence val allowedEntrance = people.asSequence() .filter { it.age

    >= 21 } .map { it.name } .take(5) .toList() val people: List<Person> = getPeople() val allowedEntrance = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  41. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  42. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  43. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  44. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  45. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  46. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  47. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  48. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

    = people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
  49. Iterable vs Sequence • Iterable  Use by default 

    Usually the best choice • Sequence
  50. Iterable vs Sequence • Iterable  Use by default 

    Usually the best choice • Sequence  To handle an infinite number of elements
  51. Iterable vs Sequence • Iterable  Use by default 

    Usually the best choice • Sequence  To handle an infinite number of elements  For huge collections, judiciously
  52. Iterable vs Sequence • Iterable  Use by default 

    Usually the best choice • Sequence  To handle an infinite number of elements  For huge collections, judiciously • Stream
  53. Iterable vs Sequence • Iterable  Use by default 

    Usually the best choice • Sequence  To handle an infinite number of elements  For huge collections, judiciously • Stream  When interoperating with Java
  54. Iteration for (i in 0..args.size - 1) { println(args[i]) }

    for (i in 0..args.lastIndex) { println(args[i]) }
  55. Iteration for (i in 0..args.size - 1) { println(args[i]) }

    for (i in 0..args.lastIndex) { println(args[i]) } for (i in 0 until args.size) { println(args[i]) }
  56. Iteration for (i in 0..args.size - 1) { println(args[i]) }

    for (i in 0..args.lastIndex) { println(args[i]) } for (i in 0 until args.size) { println(args[i]) } for (i in args.indices) { println(args[i]) }
  57. Iteration for (i in 0..args.size - 1) { println(args[i]) }

    for (i in 0..args.lastIndex) { println(args[i]) } for (i in 0 until args.size) { println(args[i]) } for (i in args.indices) { println(args[i]) } for (arg in args) { println(arg) }
  58. Iteration for (i in 0..args.size - 1) { println(args[i]) }

    for (i in 0..args.lastIndex) { println(args[i]) } for (i in 0 until args.size) { println(args[i]) } for (i in args.indices) { println(args[i]) } for (arg in args) { println(arg) } args.forEach { arg -> println(arg) }
  59. Iteration for (i in 0..args.size - 1) { println(args[i]) }

    for (i in 0..args.lastIndex) { println(args[i]) } for (i in 0 until args.size) { println(args[i]) } for (i in args.indices) { println(args[i]) } for (arg in args) { println(arg) } args.forEach { arg -> println(arg) }
  60. Iteration for (i in 0..args.size - 1) { println(args[i]) }

    for (i in 0..args.lastIndex) { println(args[i]) } for (i in 0 until args.size) { println(args[i]) } for (i in args.indices) { println(args[i]) } for (arg in args) { println(arg) } args.forEach { arg -> println(arg) }
  61. SAM conversions public interface OnClickListener { void onClick(Button button); }

    public class Button { public void setListener(OnClickListener listener) { ... } }
  62. SAM conversions button.setListener(object : OnClickListener { override fun onClick(button: Button?)

    { println("Clicked!") } }) button.setListener { println("Clicked!") }
  63. SAM conversions button.setListener(object : OnClickListener { override fun onClick(button: Button?)

    { println("Clicked!") } }) button.setListener { println("Clicked!") }
  64. SAM conversions button.setListener(object : OnClickListener { override fun onClick(button: Button?)

    { println("Clicked!") } }) button.setListener { button: Button? -> println("Clicked!") }
  65. SAM conversions #1 button.setListener { object : OnClickListener { override

    fun onClick(button: Button) { println("Clicked!") } }) }
  66. SAM conversions #1 button.setListener { object : OnClickListener { override

    fun onClick(button: Button) { println("Clicked!") } }) }
  67. SAM conversions #1 button.setListener(object : OnClickListener { override fun onClick(button:

    Button?) { object : OnClickListener { override fun onClick(button: Button) { println("Clicked!") } }) } })
  68. SAM conversions #1 button.setListener(object : OnClickListener { override fun onClick(button:

    Button?) { val listener = object : OnClickListener { override fun onClick(button: Button) { println("Clicked!") } }) } })
  69. SAM conversions #1 button.setListener(object : OnClickListener { override fun onClick(button:

    Button?) { val listener = object : OnClickListener { override fun onClick(button: Button) { println("Clicked!") } }) listener.onClick(...) } })
  70. SAM conversions #1 button.setListener(object : OnClickListener { override fun onClick(button:

    Button?) { object : OnClickListener { override fun onClick(button: Button) { println("Clicked!") } }) } })
  71. SAM conversions #1 button.setListener { object : OnClickListener { override

    fun onClick(button: Button) { println("Clicked!") } }) }
  72. SAM conversions #1 button.setListener { object : OnClickListener { override

    fun onClick(button: Button) { println("Clicked!") } }) } button.setListener { fun onClick(button: Button) { println("Clicked!") } }
  73. SAM conversions #2 public interface OnClickListener { boolean onClick(Button button);

    } button.setListener { println("Clicked!") } Kotlin: Type mismatch: inferred type is Unit but Boolean was expected
  74. Replacing statics class Foo { companion object { fun x()

    { ... } } fun y() { ... } } Foo.x()
  75. Replacing statics class Foo { companion object { fun x()

    { ... } } fun y() { ... } } object Foo { fun x() { ... } } Foo.x() Foo.x()
  76. Replacing statics class Foo { companion object { fun x()

    { ... } } fun y() { ... } } object Foo { fun x() { ... } } fun x() { ... } Foo.x() Foo.x() x()
  77. Replacing statics class Foo { companion object { fun x()

    { ... } } fun y() { ... } } object Foo { fun x() { ... } } fun x() { ... } Foo.x() Foo.x() x()
  78. Function declaration reference Declaration Kotlin usage Java usage Companion object

    Foo.f() Foo.Companion.f(); Companion object with @JvmStatic Foo.f() Foo.f(); Object Foo.f() Foo.INSTANCE.f(); Object with @JvmStatic Foo.f() Foo.f(); Top level function f() UtilKt.f(); Top level function with @JvmName* f() Util.f(); * With the @JvmName annotation on the file use-site target
  79. Variable declaration reference Declaration Kotlin usage Java usage Companion object

    X.x X.Companion.getX(); Companion object with @JvmStatic X.x X.getX(); Companion object with @JvmField X.x X.x; Companion object with const X.x X.x; Object X.x X.INSTANCE.getX(); Object with @JvmStatic X.x X.getX(); Object with @JvmField X.x X.x; Object with const X.x X.x; Top level variable x ConstKt.getX(); Top level variable with @JvmField x ConstKt.x; Top level variable with const x ConstKt.x; Top level variable with @JvmName* x Const.getX(); Top level variable with @JvmName* and @JvmField x Const.x; Top level variable with @JvmName* and const x Const.x; * With the @JvmName annotation on the file use-site target
  80. Smart casts on mutable properties class Dog(var toy: Toy? =

    null) { fun play() { if (toy != null) { toy.chew() } } }
  81. Smart casts on mutable properties class Dog(var toy: Toy? =

    null) { fun play() { if (toy != null) { toy.chew() } } } Kotlin: Smart cast to 'Toy' is impossible, because 'toy' is a mutable property that could have been changed by this time
  82. Smart casts on mutable properties class Dog(val toy: Toy? =

    null) { fun play() { if (toy != null) { toy.chew() } } }
  83. Smart casts on mutable properties class Dog(var toy: Toy? =

    null) { fun play() { if (toy != null) { toy.chew() } } }
  84. Smart casts on mutable properties class Dog(var toy: Toy? =

    null) { fun play() { val _toy = toy if (_toy != null) { _toy.chew() } } }
  85. Smart casts on mutable properties class Dog(var toy: Toy? =

    null) { fun play() { toy?.let { it.chew() } } }
  86. Smart casts on mutable properties class Dog(var toy: Toy? =

    null) { fun play() { toy?.let { it.chew() it.chew() it.chew() } } }
  87. null!! class Episode { var airdate: Date = null!! }

    fun getWeekDay(date: Date?): WeekDay { return date!!.getWeekDay() }
  88. null!! class Episode { var airdate: Date = null!! }

    fun getWeekDay(date: Date?): WeekDay { return date!!.getWeekDay() } Date?
  89. null!! class Episode { var airdate: Date = null!! }

    fun getWeekDay(date: Date?): WeekDay { return date!!.getWeekDay() } WeekDay Date?
  90. null!! class Episode { var airdate: Date = null!! }

    fun getWeekDay(date: Date?): WeekDay { return date!!.getWeekDay() } Date WeekDay Date?
  91. null!! class Episode { var airdate: Date = null!! }

    fun getWeekDay(date: Date?): WeekDay { return date!!.getWeekDay() } Date
  92. null!! class Episode { var airdate: Date = null!! }

    fun getWeekDay(date: Date?): WeekDay { return date!!.getWeekDay() } Date date as Date
  93. null!! class Episode { var airdate: Date = null!! }

    fun getWeekDay(date: Date?): WeekDay { return date!!.getWeekDay() } Date date as Date throw NullPointerException()
  94. null!! class Episode { var airdate: Date = null!! }

    fun getWeekDay(date: Date?): WeekDay { return date!!.getWeekDay() } Date
  95. Platform types in overrides How do I choose the right

    type when overriding Java methods?
  96. Platform types in overrides public interface OnClickListener { void onClick(Button

    button); } class KtListener : OnClickListener { override fun onClick(button: Button?): Unit { val name = button?.name ?: "Unknown button" println("Clicked ${name}") } }
  97. Platform types in overrides public interface OnClickListener { void onClick(Button

    button); } class KtListener : OnClickListener { override fun onClick(button: Button?): Unit { val name = button?.name ?: "Unknown button" println("Clicked ${name}") } }
  98. Platform types in overrides public interface OnClickListener { void onClick(Button

    button); } class KtListener : OnClickListener { override fun onClick(button: Button?): Unit { val name = button?.name ?: "Unknown button" println("Clicked ${name}") } }
  99. Platform types in overrides public interface OnClickListener { void onClick(Button

    button); } class KtListener : OnClickListener { override fun onClick(button: Button): Unit { val name = button.name println("Clicked ${name}") } }
  100. Import aliases package com.example.code fun indent(str: String) : String {

    ... } package com.example.square fun indent(str: String) : String { ... }
  101. Import aliases package com.example.code fun indent(str: String) : String {

    ... } package com.example.square fun indent(str: String) : String { ... } import com.example.code.indent indent("hello") com.example.square.indent("world")
  102. Import aliases package com.example.code fun String.indent() : String { ...

    } package com.example.square fun String.indent() : String { ... }
  103. Import aliases package com.example.code fun String.indent() : String { ...

    } package com.example.square fun String.indent() : String { ... } import com.example.code.indent "hello".indent() "world".indent()
  104. Import aliases package com.example.code fun String.indent() : String { ...

    } package com.example.square fun String.indent() : String { ... } import com.example.code.indent as indent4 import com.example.square.indent as indent2 "hello".indent4() "world".indent2()
  105. Import aliases import java.util.Date as UtilDate import java.sql.Date as SqlDate

    fun matches(date1: UtilDate, date2: SqlDate): Boolean { ... }
  106. Recap • 1. Array<Int> vs IntArray • 2. Iterable vs

    Sequence • 3. Iteration • 4. SAM conversions • 5. Replacing statics • 6. Smart casts on mutable properties • 7. null!! • 8. Platform types in overrides • 9. Import aliases • 10. Kotlin vs Java for Android development
  107. References • Top 10 Kotlin Stack Overflow questions, article series

     https://zsmb.co/top-10-kotlin-stack-overflow-questions-1/ • Kotlin tag on Stack Overflow  https://stackoverflow.com/questions/tagged/kotlin