4. Kotlin Types [kotlin-workshop]

Part of https://github.com/jetBrains/kotlin-workshop.

Covers:
- Basic types (primitives)
- Nullable types & Java
- Collections types & Java
- Variance

Recording of this talk (excluding variance):
https://www.youtube.com/watch?list=PLQ176FUIyIUY6UK1cgVsbdPYA3X5WLam5&v=Uizh2WlJtnk

Transcript

1. Kotlin Types: Exposed
2. Agenda …types…
3. Agenda • Basic types • Nullable types • Collection types &
4. fun foo(): Int = 1 fun bar(): Int? = 1 Types under the hood
5. Under the hood?
6. Kotlin Bytecode
7. Kotlin Bytecode
8. Kotlin Bytecode
9. public static final int foo() { return 1; } @Nullable public static final Integer bar() { return Integer.valueOf(1); } Decompiled Java code
10. fun foo(): Int = 1 public static final int foo() { return 1; } Correspondence between Kotlin and Java types Kotlin code: Decompiled Java code:
11. public static final Integer bar() { return 1; } Correspondence between Kotlin and Java types Kotlin code: Decompiled Java code: fun bar(): Int? = 1
12. Kotlin Java Int int Int? java.lang.Integer Correspondence between Kotlin and Java types
13. Basic types
14. Primitive & wrapper types Kotlin Java Int int Double double Boolean boolean Kotlin Java Int? java.lang.Integer Double? java.lang.Double Boolean? java.lang.Boolean
15. Generic arguments Kotlin Java List<Int> List<Integer>
16. Arrays Kotlin Java Array<Int> Integer[]
17. Arrays of primitive types Kotlin Java Array<Int> Integer[] IntArray int[]
18. String Kotlin Java kotlin.String java.lang.String
19. "one.two.".replace(".".toRegex(), "*") kotlin.String hides some confusing methods "one.two.".replaceAll(".", "*") ******** "one.two.".replace(".", "*") one*two*
20. Any Kotlin Java Any java.lang.Object
21. User Any Any
22. Any Int Any User
23. Boxing under the hood log(2017) fun log(any: Any) { println("Value: $any") } the value is autoboxed
24. No boxing now log(2017) fun log(any: Any) { println("Value: $any") } fun log(i: Int) { println("Value: $i") }
25. Unit vs Nothing vs void ?
26. Unit instead of void No meaningful value is returned fun f(): Unit { /*...*/ } Two equivalent syntactic forms: fun f() { /*...*/ }
27. Unit Kotlin Java Unit void
28. Nothing is different to Unit/void fun fail(message: String): Nothing {
 throw IllegalStateException(message)
 } It means “this function never returns”
29. “a type that allows only one value and thus can hold no information” Unit Nothing “a type that has no values” “the function completes successfully” “the function never completes”
30. Any & Nothing types Int Any Unit Nothing
31. Expressions of Nothing type inline fun TODO(reason: String): Nothing = throw NotImplementedError("An operation is not implemented: $reason") val answer: Int = if (timeHasPassed()) { 42 } else { TODO("Needs to be done") }
32. fun greetPerson(person: Person) { val name: String = person.name ?: throw IllegalArgumentException("Name is unspecified") println("Hello, $name!") } Expressions of Nothing type
33. fun greetPerson(person: Person) { val name = person.name ?: fail("Name is unspecified") println("Hello, $name!") } Expressions of Nothing type
34. fun greetPerson(person: Person) { val name = person.name ?: return println("Hello, $name!") } Expressions of Nothing type
35. val answer = if (timeHasPassed()) { 42 } else { fail("Not ready") } fun fail(message: String) {
 throw IllegalStateException(message)
 } Let’s say, fail function returns Unit Int Unit Any : Unit
36. val answer: Any = if (timeHasPassed()) { 42 } else { fail("Not ready") } fun fail(message: String) {
 throw IllegalStateException(message)
 } Int Any Unit Let’s say, fail function returns Unit
37. val answer = if (timeHasPassed()) { 42 } else { fail("Not ready") } fun fail(message: String): Nothing {
 throw IllegalStateException(message)
 } Now, fail returns Nothing
38. val answer = if (timeHasPassed()) { 42 } else { fail("Not ready") } fun fail(message: String): Nothing {
 throw IllegalStateException(message)
 } Int Nothing Now, fail returns Nothing
39. val answer: Int = if (timeHasPassed()) { 42 } else { fail("Not ready") } fun fail(message: String): Nothing {
 throw IllegalStateException(message)
 } Nothing Int Now, fail returns Nothing
40. Nothing Kotlin Java Nothing void because we don’t have Nothing at JVM
41. Any & Nothing types Int Any User Nothing
42. Type hierarchy Any Nothing User Any? Nothing? Int? User? Int
43. Type hierarchy Any Nothing User Any? Nothing? Int? User? Int ?
44. Type of null var user = null user = User("svtk") val users = mutableListOf(null) users.add(User("svtk")) Error: Type mismatch: inferred type is User but Nothing? was expected Error: Type mismatch: inferred type is User but Nothing? was expected
45. Type of null var user: Nothing? = null user = User("svtk") val users: List<Nothing?> = mutableListOf(null) users.add(User("svtk"))
46. Specify types explicitly var user: User? = null user = User("svtk") val users: List<User?> = mutableListOf(null) users.add(User("svtk")) ✓ ✓
47. Nullable types Java &
48. Nullable Types Under the Hood @Nullable, @NotNull annotations
49. @Nullable @NotNull Type Type? Type Type Nullability annotations
50. Nullability & Java Type behaves like regular Java type ?
51. Platform type Type Type! type of “unknown” nullability notation, not syntax type that came from Java
52. A bit of history…
53. The safest approach would be… Type Type? We tried, but it didn’t work well
54. Type? then the code looks like Type And it doesn’t really work with generics !! !! !! !! !! !! !! !! !! !! !! !! If
55. Platform type Type Type! type of “unknown” nullability type that came from Java
56. Pick two • Null-safety • Convenience • Java Interop
57. Pick two • Null-safety • Convenience • Java Interop Thanks to Dr. Ross Tate of Platform types
58. Platform type in error message public class Session {
 public String getDescription()
 } val session = Session()
 val description: Boolean = session.description Compiler error: Type mismatch: inferred type is String! but Boolean was expected
59. Using Java from Kotlin val session = Session()
 val description = session.description
 println(description.length) NullPointerException! public class Session {
 public String getDescription() {
 return null;
 }
 }
60. How to still prevent NPEs? • Annotate your Java types • Specify types explicitly
61. How to still prevent NPEs? • Annotate your Java types • Specify types explicitly
62. Type @NotNull @Nullable Type Annotate your Java types
63. Different annotations are supported @Nullable @NotNull JetBrains @Nullable @NonNull Android @Nullable @CheckForNull JSR-305 @Nullable @CheckForNull FindBugs @NonNull Lombok ...
64. Using Java from Kotlin public class Session { @Nullable String getDescription() { return null; } } val session = Session() val description = session.description println(description.length) compiler error
65. Type @NotNull @Nullable Type Annotate your Java types - You can specify @NotNull as default, and annotate only @Nullable types - All of them???
66. Non-null by default (JSR-305) @ParametersAreNonnullByDefault @MyNonnullByDefault
67. @MyNonnullApi @javax.annotation.Nonnull @TypeQualifierDefault(ElementType.PARAMETER, ...) annotation class MyNonnullByDefault @MyNonnullByDefault package mypackage; package-info.java
68. @MyNonnullByDefault public class Session { public void setDescription(String description) { this.description = description; } } @NonNull by default val session = Session() session.setDescription(null) Warning: Expected type doesn’t accept nulls in Java, but the value may be null in Kotlin
69. Make it an error compileKotlin { kotlinOptions { freeCompilerArgs += "-Xjsr305=strict" } } build.gradle
70. @MyNonnullByDefault public class Session { public void setDescription(String description) { this.description = description; } } @NonNull by default val session = Session() session.setDescription(null) Error: Null can not be a value of a non-null type String
71. How to still prevent NPEs? • Annotate your Java types • Specify types explicitly
72. Specify types explicitly val session = Session() val description: String? = session.description println(description?.length) public class Session {
 public String getDescription() {
 return null;
 }
 } ✓
73. IllegalStateException: session.description must not be null val session = Session() val description: String = session.description public class Session {
 public String getDescription() {
 return null;
 }
 } Specify types explicitly
74. Intrinsic checks val session = Session() val description: String = session.description println(description)
75. val session = Session() val description: String = session.description println(description) Intrinsic checks is generated by the compiler throws an exception if session.description is null Intrinsics.checkExpressionValueIsNotNull( description, "session.description");
76. public fun foo(s: String) { } Intrinsic checks Intrinsics.checkParameterIsNotNull(s, "s");
77. How to still prevent NPEs? • Annotate your Java types • Specify types explicitly
78. Nullable platform types: summary Good compromise between safety and convenience
79. Collections
80. List & MutableList kotlin.List kotlin.MutableList • Two interfaces declared in kotlin package • MutableList extends List
81. Read-only immutable (1, 2, 3) • Read-only interface just lacks mutating methods • The actual list can be changed by another reference list mutableList
82. val mutableList = mutableListOf(1, 2, 3) val list: List<Int> = mutableList println(list) // [1, 2, 3] (1, 2, 3) list mutableList (1, 2, 3, 4) mutableList.add(4) println(list) // [1, 2, 3, 4] Read-only immutable
83. Under the hood fun getNames(): List<String>
 fun getNames(): MutableList<String> 
 java.util.List<String> getNames(); Both functions are compiled to:
84. Kotlin uses java.util.ArrayList under the hood kotlin.MutableList java.util.ArrayList kotlin.List
85. Platform type for Java collection collection type of “unknown” mutability List<String> (Mutable)List<String!> notation, not syntax type that came from Java
86. Read-only interfaces improve API object Shop { private val customers = mutableListOf<Customer>() fun getCustomers(): List<Customer> = customers } val customers = Shop.getCustomers() customers.add() you can’t shoot yourself in the foot any longer
87. Platform types: summary Good compromise between safety and convenience
88. Variance
89. Java wildcards public interface Stream<T> {
 <R> Stream<R> map( Function<? super T, ? extends R> mapper);
 }
90. interface Function1<in P, out R> {
 operator fun invoke(p: P): R
 } (Animal) -> Int (Cat) -> Any Animal Cat Int Any in/out on the declaration-site
91. Just (T) -> R on the call-site inline fun <T, R> Iterable<T>.map(
 transform: (T) -> R): List<R> Function1<T, R>
92. List & MutableList MutableList<String> MutableList<Any> String Any ✗ List<String> List<Any>
93. Copyright © 2017 https://github.com/JetBrains/kotlin-workshop