Mixed projects: Java + Kotlin. Svetlana Isakova

Transcription

1 Mixed projects: Java + Kotlin Svetlana Isakova

2 Compilation of a mixed project *.kt kotlinc *.class *.jar *.java javac *.class

3 Nullability

4 Nullability Type =? Java Kotlin

5 Nullability + Type = + Type = Type Java Kotlin

6 Java type without annotation has unknown nullability public class Session { String getdescription() { return null; } }

7 Explicit type val session = Session() val description: String? = session.description println(description!!.length) KNPE val description1: String = session.description println(description1.length) IllegalStateException: session.description must not be null

8 Not-null types class A { fun x(s: List<String>) { println(s) } } private fun y(s: String) { println(s) } is compiled to public final class A { public final void x(@notnull List<String> s) { Intrinsics.checkParameterIsNotNull(s, "s"); System.out.println(s); } private final void y(string s) { System.out.println(s); } }

9 Platform type Type = Type OR Type? Java Kotlin notation in error messages: Type!

10 Inferred type val session = Session() val description = session.description println(description.length) NPE println(description?.length) val i: Int = session.description Error: Type mismatch: inferred type is String! but Int was expected

11 Prefer explicit Kotlin types when working with Java

12 Basic types

13 Primitive & wrapper types Java Kotlin Java Kotlin int Int java.lang.integer Int? double Double java.lang.double Double? boolean Boolean java.lang.boolean Boolean?

14 Generic arguments Kotlin Java List<Int> List<Integer>

15 Arrays of primitive types Kotlin Java Array<Int> Integer[] IntArray int[]

16 Any? Kotlin Java Any? java.lang.object

17 Unit fun f(): Unit { /*...*/ } fun f() { /*...*/ }

18 Unit as generic argument interface Processor<T> { fun process(): T } class NoResultProcessor : Processor<Unit> { override fun process() { // do stuff } }

19 Unit Kotlin Java Unit void Foo<Unit> Foo<Unit>

20 Nothing fun fail(message: String): Nothing { } throw IllegalStateException(message) : Nothing? val n = null

21 Nothing Kotlin Nothing List<Nothing> Java void List //raw type

22 Any & Nothing types Any Int String List<T> Parent MutableList<T> Child Nothing

23 Type hierarchy Any? Any Int? String? List<T>? Parent? MutableList<T>? Int String List<T> Parent Child? MutableList<T> Nothing? Child Nothing

24 Collections

25 List & MutableList Two interfaces declared in kotlin kotlin.list package MutableList extends List kotlin.mutablelist

26 Read-only!= immutable Read-only interface just lacks mutating (1, 2, 3) methods The actual list can be changed by another reference list mutablelist

27 Under the hood fun getnames(): List<String> fun getnames(): MutableList<String> Both functions are compiled to: java.util.list<string> getnames();

28 List in Kotlin At runtime is a good old java.util.list

29 Iterable MutableIterable Collection MutableCollection List MutableList Set MutableSet ArrayList HashSet

30 Platform type Type = Type OR Type? List = List OR MutableList Java Kotlin

31 Collection as platform type (Mutable)List<Int!>

32 Annotations

33 @JvmName // Data.kt fun foo() { } public final class DataKt { public static void foo() { } fun foo() { } public final class FooUtils { public static final void foo() { } }

34 @JvmName fun List<String>.filterValid(): List<String> fun List<Int>.filterValid(): List<Int> Error: platform declaration clash: declarations have the same JVM signature (filtervalid(ljava/util/list;)ljava/util/list;)

35 @JvmName fun List<String>.filterValid(): fun List<Int>.filterValid(): List<Int>

36 fun f(a: String, b: Int = 0, c: String = "abc") { //... } // Java void f(string a, int b, String c) { } void f(string a, int b) { } void f(string a) { }

37 fun foo() { throw IOException() }

38 Checked exceptions fun foo() { throw IOException() } Error: Exception java.io.ioexception // Java is never thrown in body try { of corresponding try statement DemoKt.foo(); } catch (IOException e) { //... }

39 Checked fun foo() { throw IOException() } // Java try { DemoKt.foo(); } catch (IOException e) { //... }

40 @JvmStatic class C { companion object fun foo() {} fun bar() {} } } // Java C.foo(); C.Companion.bar();

41 @JvmStatic object Obj fun foo() {} fun bar() {} } // Java Obj.foo(); Obj.bar(); Obj.INSTANCE.bar(); Obj.INSTANCE.foo();

42 Have a nice Kotlin!