https://www.youtube.com/watch?v=QIRJKIDLZgU Effective Java는 효율적이고 잘 설계된 Java 프로그램을 구현하기 위한 지침들을 소개하는 책입니다. Kotlin에서는 이 지침들이 잘 적용될 수 있는 설계를 갖추고 있는데요, Kotlin Reference 문서에서는 설명을 뒷받침하기 위해 이 책을 인용하기도 합니다. 세션에서는 "이펙티브 자바" 책의 규칙들을 짚어가면서, Kotlin에 어떻게 반영되었는지, 혹은 어떻게 적용될수 있는지에 대해 공유합니다.

2. •
• 4
•
•

3. Kotlin Night

4. Effective Kotlin
Effective Java?🤔

5. Kotlin Java ?

6. ( )
Kotlin ,
.
,
.
- Andrey
Q. Kotlin .

7. .
.
- Andrey
Q. JVM
?

8. , , .
.
- Hadi hariri
Q. ?

9. Kotlin ,
.
Kotlin

10. Effective Kotlin
Effective Java?🤔

11. Effective Java
3rd Edition
Java
•
•
•

12. 1.
2.
3. private
4. private
5.
6.
….

13. Kotlin
!
Effective Java
?

14. • Effecive Java Kotlin
• Effective Kotlin !
Effecive Java Kotlin

15. Item 3

16. public class NutritionFacts {
private final int servingSize; !// (mL, 1 )
private final int servings; !// ( , n )
private final int calories; !// (1 )
private final int fat; !// (g/1 )
private final int sodium; !// (mg/1 )
private final int carbohydrate; !// (g/1 )
}

17. public NutritionFacts(int servingSize, int servings) {
this(servingSize, servings, 0);
}
public NutritionFacts(int servingSize, int servings,
int calories) {
this(servingSize, servings, calories, 0);
}
public NutritionFacts(int servingSize, int servings,
int calories, int fat,
int sodium, int carbohydrate) {
this.servingSize = servingSize;
this.servings = servings;
this.calories = calories;
this.fat = fat;
this.sodium = sodium;
this.carbohydrate = carbohydrate;
}

18. public static void main(String[] args) {
NutritionFacts cocaCola =
new NutritionFacts(240, 8, 100, 0, 35, 27);
}
•
•

19. public static void main(String[] args) {
NutritionFacts cocaCola = new NutritionFacts();
cocaCola.setServingSize(240);
cocaCola.setServings(8);
cocaCola.setCalories(100);
cocaCola.setSodium(35);
cocaCola.setCarbohydrate(27);
}
•
•

20. public class NutritionFacts {
public static class Builder {
!//
}
public static void main(String[] args) {
NutritionFacts cocaCola = new NutritionFacts
.Builder(240, 8)
.calories(100)
.sodium(35)
.carbohydrate(27)
.build();
}
}

21. public static class Builder {
!//
private final int servingSize;
private final int servings;
!// - .
private int calories = 0;
private int fat = 0;
private int sodium = 0;
private int carbohydrate = 0;
public Builder(int servingSize, int servings) {
this.servingSize = servingSize;
this.servings = servings;
}
public Builder calories(int val) {
calories = val;
return this;
}
!//
}
Boilerplate 😵

22. class NutritionFacts(
private val servingSize: Int,
private val servings: Int,
private val calories: Int = 0,
private val fat: Int = 0,
private val sodium: Int = 0,
private val carbohydrate: Int = 0
)
Default arguments

23. val nutritionFacts = NutritionFacts(
servingSize = 240,
servings = 8,
calories = 100,
fat = 0,
sodium = 35,
carbohydrate = 27
)
Named arguments

24. Named arguments
Default arguments
Item 3

25. private
Item 3

26. •
•
.

27. public class Elvis {
public static final Elvis INSTANCE = new Elvis();
private Elvis() { }
public void leaveTheBuilding() {
System.out.println("Whoa baby, I'm outta here!");
}
public static void main(String[] args) {
Elvis elvis = Elvis.INSTANCE;
elvis.leaveTheBuilding();
}
}

28. object Elvis {
fun leaveTheBuilding() {
println("Whoa baby, I'm outta here!");
}
}
fun main() {
Elvis.leaveTheBuilding()
}

29. Decompile ?
public final class Elvis {
public static final Elvis INSTANCE;
public final void leaveTheBuilding() {
String var1 = "Whoa baby, I'm outta here!";
System.out.println(var1);
}
private Elvis() {
}
static {
Elvis var0 = new Elvis();
INSTANCE = var0;
}
}
object Elvis {
fun leaveTheBuilding() {
println("Whoa baby, I'm outta here!");
}
}
fun main() {
Elvis.leaveTheBuilding()
}

30. object
Item 3
private

31. private
Item 4

32. class NoConstructor private constructor()
val noConstructor = NoConstructor()
Compile Error!

33. ?
.
( )

34. class NoConstructor private constructor() {
val number = 1
fun printHello() {
println("Hello!")
}
}

35. val number = 1
fun printHello() {
println("Hello!")
}

36. ,
Top Level function
Item 4
private

37. Item 18

38. • - ,
•
• InstrumentedHashSet
• HashSet

39. class InstrumentedHashSet<E> : HashSet<E> {
var addCount = 0
private set
override fun add(element: E): Boolean {
addCount!++
return super.add(element)
}
override fun addAll(elements: Collection<E>): Boolean {
addCount += elements.size
return super.addAll(elements)
}
}

40. fun main() {
val s = InstrumentedHashSet<String>()
s.addAll(List.of("1", "2", “3"))
println(s.addCount)
}
“6” 🤔
3 “3”

41. public boolean addAll(Collection<? extends E> c) {
boolean modified = false;
for (E e : c)
if (add(e))
modified = true;
return modified;
}
HashSet AbstractSet

42. override fun add(element: E): Boolean {
addCount!++
return super.add(element)
}
override fun addAll(elements: Collection<E>): Boolean {
addCount += elements.size
return super.addAll(elements)
}
addAll elements ,
super.addAll add

43. open class ForwardingSet<E>(
private val s: MutableSet<E>
) : MutableSet<E> {
override fun add(element: E): Boolean {
return s.add(element)
}
override fun addAll(elements: Collection<E>): Boolean {
return s.addAll(elements)
}
clear, iterator, remove, removeAll, ………
}

44. class InstrumentedHashSet<E> : ForwardingSet<E>(HashSet()) {
!//
var addCount = 0
private set
override fun add(element: E): Boolean {
addCount!++
return super.add(element)
}
override fun addAll(elements: Collection<E>): Boolean {
addCount += elements.size
return super.addAll(elements)
}
} “3” !

45. override fun add(element: E): Boolean {
addCount!++
return super.add(element)
}
override fun addAll(elements: Collection<E>): Boolean {
addCount += elements.size
return super.addAll(elements)
}
super.addAll, ForwardingSet addAll
InstrumentedHashSet add .

46. • .
.
•
.
.
.

47. open class ForwardingSet<E>(
private val s: MutableSet<E>
) : MutableSet<E> {
override fun add(element: E): Boolean {
return s.add(element)
}
override fun addAll(elements: Collection<E>): Boolean {
return s.addAll(elements)
}
clear, iterator, remove, removeAll, ………
}

48. open class ForwardingSet<E>(
private val s: MutableSet<E>
) : MutableSet<E> by s

49. class InstrumentedHashSet<E>(
private val s: MutableSet<E>
) : MutableSet<E> by s {
var addCount = 0
private set
override fun add(element: E): Boolean {
addCount!++
return s.add(element)
}
override fun addAll(elements: Collection<E>): Boolean {
addCount += elements.size
return s.addAll(elements)
}
}

50. .
Kotlin (Deleagte )
by .
Item 18

51. .
Item 19

52. •
,
•
.
•
.

53. class Parent { }
class Child extends Parent { }

54. final class Parent { }
class Child extends Parent { }

55. class Parent
class Child : Parent()

56. open class Parent
class Child : Parent()

58. .
.
Item 19
.

59. public public
Item 16

60. Public
public class Point {
public double x;
public double y;
}

61. public class Point {
private double x;
private double y;
public double getX() { return x; }
public double getY() { return y; }
public void setX(double x) { this.x = x; }
public void setY(double y) { this.y = y; }
}

62. Public
public class Point {
public double x;
public double y;
}
• API
.
• .
• .
• .

63. class Point {
var x : Double = 0.0
var y : Double = 0.0
}

64. class Point {
var x : Double = 0.0
get() {
println("x ")
return field
}
var y : Double = 0.0
get() {
println("y ")
return field
}
}

65. Kotlin Getter, Setter
.
Item 16
public public

66. @Override
Item 40

67. public class Monogram {
private final char character;
public Monogram(char character) {
this.character = character;
}
public boolean equals(Monogram m) {
return character !== m.character;
}
public static void main(String[] args) {
Set<Monogram> set = new HashSet<Monogram>();
set.add(new Monogram('a'));
set.add(new Monogram('a'));
System.out.println(set.size());
}
1 “2”
equals ,
!
@Override ?

68. public class Monogram {
private final char character;
public Monogram(char character) {
this.character = character;
}
@Override
public boolean equals(Monogram m) {
return character !== m.character;
}
public static void main(String[] args) {
Set<Monogram> set = new HashSet<Monogram>();
set.add(new Monogram('a'));
set.add(new Monogram('a'));
System.out.println(set.size());
}
}
Compile error
Object
!🧐

69. public class Monogram {
@Override
public boolean equals(Object o) {
if (!(o instanceof Monogram)) return false;
Monogram monogram = (Monogram) o;
return character !== monogram.character;
}
public static void main(String[] args) {
Set<Monogram> set = new HashSet<Monogram>();
set.add(new Monogram('a'));
set.add(new Monogram('a'));
System.out.println(set.size());
}
} “1”

70. class Monogram(private val character: Char) {
override fun equals(other: Any?): Boolean {
return if(other is Monogram) {
character !== other.character
} else {
false
}
}
}
Compile error

71. Java @Override
.
Kotlin
override .
Item 40
@Override

72. Item 17

73. ( )
•
• .
•
.
• ( )
•
•

74. • Java
private char character = 'a';
private final char character = 'a';
• Kotlin
private var character: Char = 'a'
private val character: Char = ‘a'

75. final VS val ?
• final - ,
• val, var -
• ( ) ,
• , val

76. Kotlin List vs Java List(Kotlin MuableList)
• Kotlin List (add, addAll)
- Read Only
• Mutable , List
List
.

77. .
Kotlin .
Item 17

78. Item 61

79. •
• int, double, boolean
•
• String, List
• .
• - Integer, Double, Boolean

80. vs
•
.
• null .
•
.

81. Kotlin ?
• Kotlin .(int, boolean)
• ,
(Nullable, Generics )
.

82. private static final int a = 3;
@Nullable
private static final Integer b = 3;
val a: Int = 3
val b: Int? = 3
Decompile

83. Kotlin ,
.
Item 61

84. equals
equals hashCode
toString
Item 10, 11, 12

85. Java Object equals, Kotlin Any equals
• (reflexivity) : null x , x.equals(x) true .
• (symmertry): null x, y , x.equals(y) true
y.equals(x) true .
• (transitivity): null x, y, z x.equals(y) true
, y.equals(z) true x.equals(z) true .
• (consistency): null x, y , x.equals(y)
true false .
• null- : null x , x.equals(null) false .

86. Java Object equals, Kotlin Any equals
• (reflexivity) : null x , x.equals(x) true .
• (symmertry): null x, y , x.equals(y) true
y.equals(x) true .
• (transitivity): null x, y, z x.equals(y) true
, y.equals(z) true x.equals(z) true .
• (consistency): null x, y , x.equals(y)
true false .
• null- : null x , x.equals(null) false .
data class MyData(val text: String)

87. Java Hashcode
• equals ,
hashCode
. , .
• equals(Object) , hashCode
.
• equals(Object) , hashCode
.
.

88. HashCode (?)
• int result c . c 2.a .
equals . 10 ).
• f .
• c .
• , Type.hashCode(f) . Type .
• equals equals , hashCode
. , (canonical representation)
hashCode . null 0 ( 0 .)
• , .
, 2.b . (0 ) .
Arrays.hashCode .
• 2.a c result . .
result = 31 * result + c;
• result .

89. HashCode (?)
• int result c . c 2.a .
equals . 10 ).
• f .
• c .
• , Type.hashCode(f) . Type .
• equals equals , hashCode
. , (canonical representation)
hashCode . null 0 ( 0 .)
• , .
, 2.b . (0 ) .
Arrays.hashCode .
• 2.a c result . .
result = 31 * result + c;
• result .
data class MyData(val text: String)

90. toString
• toString
.
• toString
.

91. data class Student(
val name: String,
val score: Int
)
fun main() {
val student = Student(" ", 42)
println(student)
}
Student(name= , score=42)

92. Data class equals, hashCode,
toString .
Item 10, 11, 12
equals
equals hashCode
toString

93. Item 77

94. try {
!//
} catch (Exception e) {
e.printStackTrace();
}

95. ?
try {
!//
} catch (Exception e) {
e.printStackTrace();
}
•
•
•
?🤔

96. Checked Exception
• try - catch , throw Exception
Appendable append(CharSequence csq) throws IOException;
•
•
•
•

97. Kotlin ?
• Checked Exception
• - ,
“ ”

98. try {
!//
} catch (Exception e) {
e.printStackTrace();
}
“ ”
UncheckedException

99. Kotlin CheckedException .
.
Item 77

100. Item 83

101. •
• .
• ,
• Kotlin Delegated Properties(by lazy)

102. val number by lazy {
expensiveFunction()
}

103. fun <T> lazy(
mode: LazyThreadSafetyMode,
initializer: () !-> T
)
public enum class LazyThreadSafetyMode {
SYNCHRONIZED,
PUBLICATION,
NONE,
}

104. val number by lazy(LazyThreadSafetyMode.NONE) {
expensiveFunction()
}

105. Kotlin Deleagted Properties(by lazy)
.
.
Item 83

106. API
Item 31

107. List<? extends Number> numbers;
if(condition) {
numbers = new ArrayList<Int>();
} else {
numbers = new ArrayList<Double>();
}
val numbers: List<Number> = if(condition) {
ArrayList<Int>()
} else {
ArrayList<Double>()
}

108. Declaration-site variance,
Use-site variance
API
Item 31
API

109. !🥳