Why Effective Java Still Matters to Kotlin Developers

Effective Java by Joshua Bloch profoundly influenced the design of Kotlin yet by exploring a selection of "items" from the book you will see how it is still just as relevant to Kotlin developers today when we aim to write Effective Kotlin code. For example, consider how data classes implement equals, and you can see that it doesn't follow the rules suggested and as such is not always optimal.

droidcon London 2018
https://skillsmatter.com/skillscasts/12708-why-effective-java-still-matters-to-kotlin-developers

Transcript

1. Why Effective Java Still Matters to Kotlin Developers Matt Dolan info@mattdolan.com
2. Kotlin influencer
3. Read on medium
4. Effective Java - Item 2 Consider a builder when faced with many constructor parameters
5. Step-by-step construction AlertDialog.Builder(context) .setTitle("Hello") .setMessage("droidCon 2018") .setPositiveButton("Ok") { _, _ !" } .create()
6. data class Person( val id: Int?, val name: String, val dateOfBirth: Date?, val address: Address? ) Person(null, "Matt Dolan", null, null)
7. Named parameters data class Person( val id: Int?, val name: String, val dateOfBirth: Date?, val address: Address? ) Person( id = null, name = "Matt Dolan", dateOfBirth = null, address = null)
8. Default parameters data class Person( val id: Int? = null, val name: String, val dateOfBirth: Date? = null, val address: Address? = null ) Person( name = "Matt Dolan”)
9. Java interoperability
10. Java interoperability data class Person @JvmOverloads constructor( val id: Int? = null, val name: String, val dateOfBirth: Date? = null, val address: Address? = null )
11. Telescoping constructors Person(Integer id, String name, Date dob, Address address) Person(Integer id, String name, Date dob) Person(Integer id, String name) Person(String name)
12. @JvmOverloads is an anti-pattern
13. “Client code is much easier to read and write with builders than with telescoping constructors” –Joshua Bloch
14. Kotlin DSL val me = person("Matt Dolan") { address("60 Sloane Avenue") { secondLine = "London" } }
15. Kotlin DSL ≍ Type-safe builders
16. “Start with Builders Immutability with a side of Java-friendliness!” –Aaron Sarazan - Next Level DSLs - KotlinConf18
17. class Builder(private val name: String) { private var id: Int? = null private var dateOfBirth: Date? = null private var address: Address? = null fun setId(id: Int) = apply { this.id = id } fun setDateOfBirth(dateOfBirth: Date) = apply { this.dateOfBirth = dateOfBirth } fun setAddress(address: Address) = apply { this.address = address } fun build() = Person(id, name, dateOfBirth, address) }
18. val me = person("Matt Dolan") { address("60 Sloane Avenue") { secondLine = "London" } } fun person( name: String, init: Person.Builder.() !" Unit = {} ) = Person.Builder(name).apply(init).build()
19. val me = person("Matt Dolan") { address("60 Sloane Avenue") { secondLine = "London" } } fun person( name: String, init: Person.Builder.() !" Unit = {} ) = Person.Builder(name).apply(init).build()
20. val me = person("Matt Dolan") { address("60 Sloane Avenue") { secondLine = "London" } } fun person( name: String, init: Person.Builder.() !" Unit = {} ) = Person.Builder(name).apply(init).build()
21. val me = person("Matt Dolan") { address("60 Sloane Avenue") { secondLine = "London" } } @JvmSynthetic fun person( name: String, init: Person.Builder.() !" Unit = {} ) = Person.Builder(name).apply(init).build()
22. val me = person("Matt Dolan") { address("60 Sloane Avenue") { secondLine = "London" } } @JvmSynthetic fun Person.Builder.address( firstLine: String, init: Address.Builder.() !" Unit = {} ) = Address.Builder(firstLine).apply(init).build() .let(#$setAddress)
23. val me = person("Matt Dolan") { address("60 Sloane Avenue") { address("50 Sloane Avenue") { address("40 Sloane Avenue") { secondLine = "London" } } } }
24. val me = person("Matt Dolan") { address("60 Sloane Avenue") { address("50 Sloane Avenue") { address("40 Sloane Avenue") { secondLine = "London" } } } }
25. @DslMarker annotation class PersonDsl data class Person(…) { @PersonDsl class Builder(private val name: String) { … } }
26. Pro tips for Builders • Start with a Builder when creating Kotlin DSLs • Add a @DSLMarker to your Builders
27. … for Java interoperability • Avoid @JvmOverloads unless only the very last parameter is optional • Use a Builder with more than one optional parameter • Add @JvmSynthetic to Kotlin only functions
28. Effective Java - Item 3 Enforce the singleton property with a private constructor or an enum type
29. “A singleton is simply a class that is instantiated exactly once” – Design Patterns: Elements of Reusable Object-Oriented Software
30. object Bart { void eatMyShorts() { … } } Bart.eatMyShorts()
31. public class Bart { public static final Bart INSTANCE = new Bart(); private Bart() { … } public void eatMyShorts() { … } }
32. Reflection val anotherBart = Bart#$class.java.getDeclaredConstructor().run { isAccessible = true newInstance() }
33. Serializable object Bart : Serializable { … }
34. Serializable object Bart : Serializable { fun readResolve(): Any = Bart fun eatMyShorts() { … } }
35. Enum enum class Bart { INSTANCE; fun eatMyShorts() { … } } Bart.INSTANCE.eatMyShorts()
36. public class Bart { public static final Bart INSTANCE = new Bart(); private Bart() { … } public void eatMyShorts() { … } }
37. object Bart { val expensiveObject by lazy { … } }
38. Utility classes
39. object ArrayUtils { fun parallelSorted(array: IntArray): IntArray { return array.copyOf().apply { Arrays.parallelSort(this) } } }
40. object ArrayUtils { fun parallelSorted(array: IntArray): IntArray { return array.copyOf().apply { Arrays.parallelSort(this) } } } val unsorted = intArrayOf(1, 2, 3, 4, 5) val sorted = ArrayUtils.parallelSorted(unsorted)
41. fun IntArray.parallelSorted() = copyOf().apply { Arrays.parallelSort(this) }
42. fun IntArray.parallelSorted() = copyOf().apply { Arrays.parallelSort(this) } val unsorted = intArrayOf(1, 2, 3, 4, 5) val sorted = unsorted.parallelSorted()
43. @file:JvmName("ArrayUtils") fun IntArray.parallelSorted() = copyOf().apply { Arrays.parallelSort(this) }
44. Pro tips for Singleton • Prefer top-level extension functions over utility classes • Use object declaration for singletons • Use lazy for lazy initialisation
45. Effective Java - Item 6 Avoid creating unnecessary objects
46. fun String.isSignedInt() = matches(Regex("[+-]?%&d+"))
47. Nullable types data class Nullable( val a: Int, val b: Int? )
48. Nullable types data class Nullable( val a: Int, val b: Int? ) public final class Nullable { private final int a; @Nullable private final Integer b; public Nullable(int a, @Nullable Integer b) { … } … }
49. Collections val numbers = listOf(1, 2, 3, 4, 5)
50. Collections val numbers = listOf(1, 2, 3, 4, 5) List numbers = CollectionsKt.listOf( new Integer[]{1, 2, 3, 4, 5});
51. IntArray SparseIntArray
52. Lambda val factor = 2 val awesomeAlgorithm = { a: Int, b: Int !" a + b * factor } val result = awesomeAlgorithm(1, 2)
53. Lambda final int factor = 2; Function2 awesomeAlgorithm = (Function2)(new Function2() { '( $FF: synthetic method '( $FF: bridge method public Object invoke(Object var1, Object var2) { return this.invoke(((Number)var1).intValue(), ((Number)var2).intValue()); } public final int invoke(int a, int b) { return a + b * factor; } }); int result = ((Number)awesomeAlgorithm.invoke(1, 2)).intValue();
54. Lambda final int factor = 2; Function2 awesomeAlgorithm = (Function2)(new Function2() { '( $FF: synthetic method '( $FF: bridge method public Object invoke(Object var1, Object var2) { return this.invoke(((Number)var1).intValue(), ((Number)var2).intValue()); } public final int invoke(int a, int b) { return a + b * factor; } }); int result = ((Number)awesomeAlgorithm.invoke(1, 2)).intValue(); 1 new
55. Lambda final int factor = 2; Function2 awesomeAlgorithm = (Function2)(new Function2() { '( $FF: synthetic method '( $FF: bridge method public Object invoke(Object var1, Object var2) { return this.invoke(((Number)var1).intValue(), ((Number)var2).intValue()); } public final int invoke(int a, int b) { return a + b * factor; } }); int result = ((Number)awesomeAlgorithm.invoke(1, 2)).intValue(); 1
56. Lambda final int factor = 2; Function2 awesomeAlgorithm = (Function2)(new Function2() { '( $FF: synthetic method '( $FF: bridge method public Object invoke(Object var1, Object var2) { return this.invoke(((Number)var1).intValue(), ((Number)var2).intValue()); } public final int invoke(int a, int b) { return a + b * factor; } }); int result = ((Number)awesomeAlgorithm.invoke(1, 2)).intValue(); 2 Boxed Unboxed
57. Lambda final int factor = 2; Function2 awesomeAlgorithm = (Function2)(new Function2() { '( $FF: synthetic method '( $FF: bridge method public Object invoke(Object var1, Object var2) { return this.invoke(((Number)var1).intValue(), ((Number)var2).intValue()); } public final int invoke(int a, int b) { return a + b * factor; } }); int result = ((Number)awesomeAlgorithm.invoke(1, 2)).intValue(); 3 Unboxed Boxed
58. Lambda final int factor = 2; Function2 awesomeAlgorithm = (Function2)(new Function2() { '( $FF: synthetic method '( $FF: bridge method public Object invoke(Object var1, Object var2) { return this.invoke(((Number)var1).intValue(), ((Number)var2).intValue()); } public final int invoke(int a, int b) { return a + b * factor; } }); int result = ((Number)awesomeAlgorithm.invoke(1, 2)).intValue(); 4 Unboxed Boxed
59. Lambda val factor = 2 val awesomeAlgorithm = { a: Int, b: Int !" a + b * factor } val result = awesomeAlgorithm(1, 2) 4 objects created
60. Lambda inline fun awesome(value: Int, algorithm: (Int) !" Int) = algorithm(value)
61. Kotlin 1.3 - Inline Class inline class Days(private val value: Int) { val hours: Int get() = value * 24 }
62. Kotlin 1.3 - Inline Class inline class Days(private val value: Int) { val hours: Int get() = value * 24 } val a = Days(1) val b = Days(2) println(a )* b)
63. Kotlin 1.3 - Inline Class inline class Days(val value: Int) { val hours: Int get() = value * 24 } val a = Days(1) val b = Days(2) '( No boxing println(a.value )* b.value)
64. “numbers are physically stored as JVM primitive types, unless we need a nullable number reference or generics are involved” – Kotlin Documentation: Basic Types
65. Pro tips for unnecessary objects • inline lambdas and classes • Be careful with primitive vs reference types
66. Effective Java - Item 10 Obey the general contract when overriding equals
67. data class Person( val firstName: String, val lastName: String, val age: Int )
68. public boolean equals(@Nullable Object var1) { if (this +, var1) { if (var1 instanceof Person) { Person var2 = (Person)var1; return (areEqual(firstName, var2.firstName) -. areEqual(lastName, var2.lastName) -. age )* var2.age); } return false; } return true; }
69. “For best performance, you should first compare fields that are more likely to differ, less expensive to compare, or, ideally, both” –Joshua Bloch
70. Pro tips for equals • equals is generated in parameter order • Don’t optimise early
71. Effective Java - Item 11 Always override hashCode when you override equals
72. data class PersonWithUniqueId( val id: Int, val firstName: String, val lastName: String )
73. data class PersonWithUniqueId( val id: Int, val firstName: String, val lastName: String ) public int hashCode() { return (this.id * 31 + (this.firstName/0hashCode() 12 0)) * 31 + (this.lastName/0hashCode() 12 0); }
74. data class PersonWithUniqueId( val id: Int, val firstName: String, val lastName: String ) { override fun hashCode() = id.hashCode() }
75. data class PersonWithUniqueId( val id: Int, val firstName: String, val lastName: String ) { override fun hashCode() = id.hashCode() } Integer.valueOf(this.id).hashCode()
76. Pro tips for hashcode • Be aware that it may do too much • Don’t optimise early
77. Effective Java - Item 13 Override clone judiciously
78. “A better approach to object copying is to provide a copy constructor or copy factory” –Joshua Bloch
79. data class Object(val list: List<Int>) val list = listOf(1) val alice = Object(list) val bob = alice.copy() println(alice.list) '( [1] println(bob.list) '( [1]
80. data class Object(val list: List<Int>) val list = mutableListOf(1) val alice = Object(list) val bob = alice.copy() list.add(2) println(alice.list) '( [1, 2] println(bob.list) '( [1, 2]
81. data class Object(val list: List<Int>) val list = mutableListOf(1) val alice = Object(Collections.unmodifiableList(list))
82. Pro tips for copy • val is read only not immutable
83. Follow me on medium medium.com/@appmattus/