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

1. TOP 10 KOTLIN STACK OVERFLOW QUESTIONS Márton Szabolcs Braun zsmb.co zsmb13 braun.marton@autsoft.hu
2. Why do this talk?
3. Why do this talk?
4. Array<Int> vs IntArray What’s the difference? When do I use which one?
5. Array<Int> vs IntArray
6. Array<Int> vs IntArray Array<Int>
7. Array<Int> vs IntArray Array<Int> Integer[]
8. Array<Int> vs IntArray Array<Int> Integer[] val arrayOfInts: Array<Int> = arrayOf(1, 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)
12. Array<Int> vs IntArray
13. Array<Int> vs IntArray • IntArray
14. Array<Int> vs IntArray • IntArray  Avoids boxing
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)
19. Iterable vs Sequence What’s the difference? When do I use which one?
20. Iterable vs Sequence
21. Iterable vs Sequence Iterable
22. Iterable vs Sequence Iterable java.lang.Iterable
23. Iterable vs Sequence  Eager evaluation Iterable java.lang.Iterable
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)
25. 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. 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
46. Iterable vs Sequence
47. Iterable vs Sequence Sequence
48. Iterable vs Sequence Sequence  Lazy evaluation
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()
51. Iterable vs Sequence
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()
72. Iterable vs Sequence
73. Iterable vs Sequence • Iterable
74. Iterable vs Sequence • Iterable  Use by default
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
81. Iteration How can / should I iterate over collections?
82. Iteration
83. Iteration for (i in 0..args.size - 1) { println(args[i]) }
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) }
91. SAM conversions What are SAM conversions?
92. SAM conversions
93. SAM conversions public interface OnClickListener { void onClick(Button button); }
94. SAM conversions public interface OnClickListener { void onClick(Button button); } public class Button { public void setListener(OnClickListener listener) { ... } }
95. SAM conversions
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!") }
100. SAM conversions #1 Why are my listeners not being called?
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!") } }
109. SAM conversions #2 Why am I getting a return type error?
110. SAM conversions #2 public interface OnClickListener { void onClick(Button button); }
111. SAM conversions #2 public interface OnClickListener { boolean onClick(Button button); }
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 }
116. Replacing statics How do I create static functions and variables 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
124. Smart casts on mutable properties Why are smart casts not 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() } } }
133. null!! Why am I getting NullPointerExceptions?
134. null!!
135. null!! class Episode { var airdate: Date = null!! }
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
144. null!! class Episode { var airdate: Date = null!! }
145. null!! class Episode { var airdate: Date = null!! }
146. null!! class Episode { var airdate: Date = throw KotlinNullPointerException() }
147. null!! class Episode { lateinit var airdate: Date }
148. Platform types in overrides How do I choose the right type when overriding Java methods?
149. Platform types in overrides
150. Platform types in overrides public interface OnClickListener { void onClick(Button 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}") } }
155. Import aliases What can I do about imports with conflicting names?
156. Import aliases
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 { ... }
163. Kotlin vs Java for Android development Please help I just started
164. Kotlin vs Java for Android development
165. Kotlin vs Java for Android development
166. 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
167. 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
168. Photo: Alexey Sergeev Questions? zsmb.co zsmb13 braun.marton@autsoft.hu