Márton Braun
May 08, 2018
350

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

May 08, 2018

Transcript

1. TOP 10 KOTLIN STACK OVERFLOW QUESTIONS Márton Szabolcs Braun zsmb.co

zsmb13 [email protected]

which one?

2, 3, 4, 5)
9. Array<Int> vs IntArray Array<Int> Integer[] IntArray val arrayOfInts: Array<Int> =

arrayOf(1, 2, 3, 4, 5)
10. Array<Int> vs IntArray Array<Int> Integer[] IntArray int[] val arrayOfInts: Array<Int>

= arrayOf(1, 2, 3, 4, 5)
11. 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)

15. Array<Int> vs IntArray • IntArray  Avoids boxing  Easier

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

to initialize • Array<Int> val intArray = IntArray(10) val arrayOfInts = Array<Int>(5) { i -> i * 2 }
17. 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 }
18. 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)

which one?

24. 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)

26. val people: List<Person> = getPeople() val allowedEntrance = people .filter

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

{ it.age >= 21 } .map { it.name } .take(5) Iterable vs Sequence

49. 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()
50. 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()

52. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

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

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

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

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

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

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

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

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

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

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

= people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()
63. 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()
64. Iterable vs Sequence val people: List<Person> = getPeople() val allowedEntrance

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

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

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

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

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

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

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

= people.asSequence() .filter { it.age >= 21 } .map { it.name } .take(5) .toList()

75. Iterable vs Sequence • Iterable  Use by default 

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

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

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

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

Usually the best choice • Sequence  To handle an infinite number of elements  For huge collections, judiciously • Stream
80. 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

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

for (i in 0..args.lastIndex) { println(args[i]) }
85. 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]) }
86. 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]) }
87. 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) }
88. 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) }
89. 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) }
90. 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) }

94. SAM conversions public interface OnClickListener { void onClick(Button button); }

public class Button { public void setListener(OnClickListener listener) { ... } }

96. SAM conversions button.setListener(object : OnClickListener { override fun onClick(button: Button?)

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

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

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

{ println("Clicked!") } }) button.setListener { button: Button? -> println("Clicked!") }

101. SAM conversions #1 button.setListener { object : OnClickListener { override

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

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

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

Button?) { val listener = object : OnClickListener { override fun onClick(button: Button) { println("Clicked!") } }) } })
105. 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(...) } })
106. SAM conversions #1 button.setListener(object : OnClickListener { override fun onClick(button:

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

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

fun onClick(button: Button) { println("Clicked!") } }) } button.setListener { fun onClick(button: Button) { println("Clicked!") } }

error?

}

}
112. SAM conversions #2 public interface OnClickListener { boolean onClick(Button button);

} button.setListener { println("Clicked!") }
113. 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
114. SAM conversions #2 public interface OnClickListener { boolean onClick(Button button);

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

} button.setListener { println("Clicked!") true }

in Kotlin?
117. Replacing statics class Foo { companion object { fun x()

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

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

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

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

{ ... } } fun y() { ... } } object Foo { fun x() { ... } } fun x() { ... } Foo.x() Foo.x() x()
122. 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
123. 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

working?
125. Smart casts on mutable properties class Dog(var toy: Toy? =

null) { fun play() { if (toy != null) { toy.chew() } } }
126. 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
127. Smart casts on mutable properties class Dog(val toy: Toy? =

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

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

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

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

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

null) { fun play() { toy?.let { it.chew() it.chew() it.chew() } } }

136. null!! class Episode { var airdate: Date = null!! }

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

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

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

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

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

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

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

fun getWeekDay(date: Date?): WeekDay { return date!!.getWeekDay() } Date

}

148. Platform types in overrides How do I choose the right

type when overriding Java methods?

button); }
151. 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}") } }
152. 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}") } }
153. 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}") } }
154. 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}") } }

names?

157. Import aliases package com.example.code fun indent(str: String) : String {

... } package com.example.square fun indent(str: String) : String { ... }
158. 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")
159. Import aliases package com.example.code fun String.indent() : String { ...

} package com.example.square fun String.indent() : String { ... }
160. 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()
161. 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()
162. Import aliases import java.util.Date as UtilDate import java.sql.Date as SqlDate

fun matches(date1: UtilDate, date2: SqlDate): Boolean { ... }