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

Transcript

1. TOP 10 KOTLIN
   STACK OVERFLOW
   QUESTIONS
   Márton Szabolcs Braun
   zsmb.co zsmb13
   braun.marton@autsoft.hu
   

   View Slide

2. Why do this talk?
   

   View Slide

3. Why do this talk?
   

   View Slide

4. Array vs
   IntArray
   What’s the difference? When do I use which one?
   

   View Slide

5. Array vs IntArray
   

   View Slide

6. Array vs IntArray
   Array
   

   View Slide

7. Array vs IntArray
   Array Integer[]
   

   View Slide

8. Array vs IntArray
   Array Integer[]
   val arrayOfInts: Array = arrayOf(1, 2, 3, 4, 5)
   

   View Slide

9. Array vs IntArray
   Array Integer[]
   IntArray
   val arrayOfInts: Array = arrayOf(1, 2, 3, 4, 5)
   

   View Slide

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

    View Slide

11. Array vs IntArray
    Array Integer[]
    IntArray int[]
    val intArray: IntArray = intArrayOf(1, 2, 3, 4, 5)
    val arrayOfInts: Array = arrayOf(1, 2, 3, 4, 5)
    

    View Slide

12. Array vs IntArray
    

    View Slide

13. Array vs IntArray
    • IntArray
    

    View Slide

14. Array vs IntArray
    • IntArray
     Avoids boxing
    

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

18. Array vs IntArray
    • IntArray
     Avoids boxing
     Easier to initialize
    • Array
     Might be required by an API
     Can store nullable values
    val intArray = IntArray(10)
    val arrayOfInts = Array(5) { i -> i * 2 }
    val notPeople: Array = arrayOfNulls(13)
    

    View Slide

19. Iterable vs
    Sequence
    What’s the difference? When do I use which one?
    

    View Slide

20. Iterable vs Sequence
    

    View Slide

21. Iterable vs Sequence
    Iterable
    

    View Slide

22. Iterable vs Sequence
    Iterable java.lang.Iterable
    

    View Slide

23. Iterable vs Sequence
     Eager evaluation
    Iterable java.lang.Iterable
    

    View Slide

24. Iterable vs Sequence
     Eager evaluation
    Iterable java.lang.Iterable
    val people: List = getPeople()
    val allowedEntrance = people
    .filter { it.age >= 21 }
    .map { it.name }
    .take(5)
    

    View Slide

25. Iterable vs Sequence
    

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

46. Iterable vs Sequence
    

    View Slide

47. Iterable vs Sequence
    Sequence
    

    View Slide

48. Iterable vs Sequence
    Sequence
     Lazy evaluation
    

    View Slide

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

    View Slide

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

    View Slide

51. Iterable vs Sequence
    

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

72. Iterable vs Sequence
    

    View Slide

73. Iterable vs Sequence
    • Iterable
    

    View Slide

74. Iterable vs Sequence
    • Iterable
     Use by default
    

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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
    

    View Slide

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
    

    View Slide

81. Iteration
    How can / should I iterate over collections?
    

    View Slide

82. Iteration
    

    View Slide

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

    View Slide

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

    View Slide

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])
    }
    

    View Slide

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])
    }
    

    View Slide

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)
    }
    

    View Slide

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)
    }
    

    View Slide

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)
    }
    

    View Slide

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)
    }
    

    View Slide

91. SAM conversions
    What are SAM conversions?
    

    View Slide

92. SAM conversions
    

    View Slide

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

    View Slide

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

    View Slide

95. SAM conversions
    

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

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

    View Slide

100. SAM conversions #1
     Why are my listeners not being called?
     

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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(...)
     }
     })
     

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

109. SAM conversions #2
     Why am I getting a return type error?
     

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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
     

     View Slide

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

     View Slide

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

     View Slide

116. Replacing statics
     How do I create static functions and variables in Kotlin?
     

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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
     

     View Slide

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
     

     View Slide

124. Smart casts on
     mutable properties
     Why are smart casts not working?
     

     View Slide

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

     View Slide

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
     

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

133. null!!
     Why am I getting NullPointerExceptions?
     

     View Slide

134. null!!
     

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

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

     View Slide

146. null!!
     class Episode {
     var airdate: Date = throw KotlinNullPointerException()
     }
     

     View Slide

147. null!!
     class Episode {
     lateinit var airdate: Date
     }
     

     View Slide

148. Platform types in
     overrides
     How do I choose the right type when overriding Java methods?
     

     View Slide

149. Platform types in overrides
     

     View Slide

150. Platform types in overrides
     public interface OnClickListener {
     void onClick(Button button);
     }
     

     View Slide

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}")
     }
     }
     

     View Slide

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}")
     }
     }
     

     View Slide

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}")
     }
     }
     

     View Slide

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}")
     }
     }
     

     View Slide

155. Import aliases
     What can I do about imports with conflicting names?
     

     View Slide

156. Import aliases
     

     View Slide

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

     View Slide

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")
     

     View Slide

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

     View Slide

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()
     

     View Slide

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()
     

     View Slide

162. Import aliases
     import java.util.Date as UtilDate
     import java.sql.Date as SqlDate
     fun matches(date1: UtilDate, date2: SqlDate): Boolean {
     ...
     }
     

     View Slide

163. Kotlin vs Java for
     Android development
     Please help I just started
     

     View Slide

164. Kotlin vs Java for Android development
     

     View Slide

165. Kotlin vs Java for Android development
     

     View Slide

166. Recap
     • 1. Array 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
     

     View Slide

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
     

     View Slide

168. Photo:
     Alexey Sergeev
     Questions?
     zsmb.co zsmb13
     braun.marton@autsoft.hu
     

     View Slide