Top 10 Kotlin Stack Overflow Questions (Kotlin Budapest User Group meetup - May)

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.

Transcript

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

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2. Why do this talk?
   

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3. Why do this talk?
   

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

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5. Array vs IntArray
   

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6. Array vs IntArray
   Array
   

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7. Array vs IntArray
   Array Integer[]
   

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

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

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

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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)
    

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12. Array vs IntArray
    

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13. Array vs IntArray
    • IntArray
    

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14. Array vs IntArray
    • IntArray
     Avoids boxing
    

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

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

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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 }
    

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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)
    

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

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20. Iterable vs Sequence
    

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21. Iterable vs Sequence
    Iterable
    

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22. Iterable vs Sequence
    Iterable java.lang.Iterable
    

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

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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)
    

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25. Iterable vs Sequence
    

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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46. Iterable vs Sequence
    

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47. Iterable vs Sequence
    Sequence
    

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48. Iterable vs Sequence
    Sequence
     Lazy evaluation
    

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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()
    

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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()
    

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51. Iterable vs Sequence
    

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

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

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

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

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

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

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

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

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

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

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

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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()
    

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

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

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

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

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

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

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

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

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72. Iterable vs Sequence
    Sequence
     Lazy evaluation
    

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73. Iterable vs Sequence
    Sequence
     Lazy evaluation
     Potentially infinite
    

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74. Iterable vs Sequence
    Sequence
     Lazy evaluation
     Potentially infinite
    generateSequence(1) { n -> n * 2 }
    .take(20)
    .forEach(::println)
    

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75. Iterable vs Sequence
    

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76. Iterable vs Sequence
    • Iterable
    

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77. Iterable vs Sequence
    • Iterable
     Use by default
    

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

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

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

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

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

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83. 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
    

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

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85. Iteration
    

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

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

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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])
    }
    

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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])
    }
    

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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)
    }
    

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91. 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)
    }
    

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92. 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)
    }
    

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93. 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)
    }
    

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94. Iteration with indexes
    How can I get both the current index and element?
    

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95. Iteration with indexes
    for ((index, arg) in args.withIndex()) {
    println("$index: $arg")
    }
    

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96. Iteration with indexes
    for ((index, arg) in args.withIndex()) {
    println("$index: $arg")
    }
    args.forEachIndexed { index, arg ->
    println("$index: $arg")
    }
    

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97. Iteration with indexes
    for ((index, arg) in args.withIndex()) {
    println("$index: $arg")
    }
    args.forEachIndexed { index, arg ->
    println("$index: $arg")
    }
    

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98. Iteration with indexes
    for ((index, arg) in args.withIndex()) {
    println("$index: $arg")
    }
    args.forEachIndexed { index, arg ->
    println("$index: $arg")
    }
    

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99. SAM conversions
    What are SAM conversions?
    

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100. SAM conversions
     

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

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

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103. SAM conversions
     

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

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

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

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

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

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

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

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

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

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113. 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(...)
     }
     })
     

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

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

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

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117. SAM conversions #2
     How do I reference my listener instance?
     

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118. SAM conversions #2
     • What if I need to reference the listener instance?
     button.setListener {
     button.removeListener(this)
     }
     

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119. SAM conversions #2
     • What if I need to reference the listener instance?
     button.setListener(object: OnClickListener {
     override fun onClick(button: Button) {
     button.removeListener(this)
     }
     })
     button.setListener {
     button.removeListener(this)
     }
     

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120. SAM conversions #3
     Why am I getting a return type error?
     

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121. SAM conversions #3
     public interface OnClickListener {
     void onClick(Button button);
     }
     

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122. SAM conversions #3
     public interface OnClickListener {
     boolean onClick(Button button);
     }
     

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123. SAM conversions #3
     public interface OnClickListener {
     boolean onClick(Button button);
     }
     button.setListener {
     println("Clicked!")
     }
     

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124. SAM conversions #3
     public interface OnClickListener {
     boolean onClick(Button button);
     }
     button.setListener {
     println("Clicked!")
     }
     Kotlin: Type mismatch: inferred type is Unit but
     Boolean was expected
     

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125. SAM conversions #3
     public interface OnClickListener {
     boolean onClick(Button button);
     }
     button.setListener {
     println("Clicked!")
     true
     }
     

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126. SAM conversions #3
     public interface OnClickListener {
     boolean onClick(Button button);
     }
     button.setListener {
     println("Clicked!")
     true
     }
     

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127. SAM conversions #4
     What if my interface/method is not written in Java?
     

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128. SAM conversions #4
     • SAM constructor
     button.setListener(OnClickListener {
     println("Clicked!")
     })
     

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129. SAM conversions #4
     • SAM constructor
     button.setListener(OnClickListener {
     println("Clicked!")
     })
     Java interface Kotlin interface
     Java method SAM conversion Object expression
     Kotlin method SAM constructor Object expression
     

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130. Replacing statics #1
     How do I create static functions and variables in Kotlin?
     

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

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

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

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

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

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136. 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
     

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137. 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
     

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138. Replacing statics #2
     What about static initializers?
     

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139. Static initializers
     class X {
     companion object {
     init {
     println("Static initialization!")
     }
     }
     }
     

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

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

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142. 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
     

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

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

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

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

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

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

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149. null!!
     Why am I getting NullPointerExceptions?
     

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150. null!!
     

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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165. Platform types in overrides
     

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

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167. 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}")
     }
     }
     

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168. 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}")
     }
     }
     

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169. 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}")
     }
     }
     

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170. 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}")
     }
     }
     

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

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172. Import aliases
     

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

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174. 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")
     

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

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176. 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()
     

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177. 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()
     

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

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

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180. Kotlin vs Java for Android development
     

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181. Kotlin vs Java for Android development
     

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182. Recap
     • 1. Array vs IntArray
     • 2. Iterable vs Sequence
     • 3. Iteration (with indexes)
     • 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
     

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183. 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
     

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

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