Même si cela fait un peu plus de 2 ans maintenant, et a eu beaucoup de bonnes réponses, je constate qu'il manque d'autres façons d'obtenir des champs Kotlin "statiques". Voici un exemple de guide pour l' staticinteropérabilité Kotlin-Java :
Scénario 1: Création d'une méthode statique dans Kotlin pour Java
Kotlin
@file:JvmName("KotlinClass") //This provides a name for this file, so it's not defaulted as [KotlinClassKt] in Java
package com.frybits
class KotlinClass {
companion object {
//This annotation tells Java classes to treat this method as if it was a static to [KotlinClass]
@JvmStatic
fun foo(): Int = 1
//Without it, you would have to use [KotlinClass.Companion.bar()] to use this method.
fun bar(): Int = 2
}
}
Java
package com.frybits;
class JavaClass {
void someFunction() {
println(KotlinClass.foo()); //Prints "1"
println(KotlinClass.Companion.bar()); //Prints "2". This is the only way to use [bar()] in Java.
println(KotlinClass.Companion.foo()); //To show that [Companion] is still the holder of the function [foo()]
}
//Because I'm way to lazy to keep typing [System.out], but I still want this to be compilable.
void println(Object o) {
System.out.println(o);
}
}
La réponse de Michael Anderson fournit plus de profondeur que cela et devrait certainement être référencée pour ce scénario.
Ce scénario suivant gère la création de champs statiques dans Kotlin afin que Java n'ait pas à continuer d'appeler KotlinClass.foo()dans les cas où vous ne voulez pas de fonction statique.
Scénario 2: création d'une variable statique dans Kotlin pour Java
Kotlin
@file:JvmName("KotlinClass") //This provides a name for this file, so it's not defaulted as [KotlinClassKt] in Java
package com.frybits
class KotlinClass {
companion object {
//This annotation tells Kotlin to not generate the getter/setter functions in Java. Instead, this variable should be accessed directly
//Also, this is similar to [@JvmStatic], in which it tells Java to treat this as a static variable to [KotlinClass].
@JvmField
var foo: Int = 1
//If you want something akin to [final static], and the value is a primitive or a String, you can use the keyword [const] instead
//No annotation is needed to make this a field of [KotlinClass]. If the declaration is a non-primitive/non-String, use @JvmField instead
const val dog: Int = 1
//This will be treated as a member of the [Companion] object only. It generates the getter/setters for it.
var bar: Int = 2
//We can still use [@JvmStatic] for 'var' variables, but it generates getter/setters as functions of KotlinClass
//If we use 'val' instead, it only generates a getter function
@JvmStatic
var cat: Int = 9
}
}
Java
package com.frybits;
class JavaClass {
void someFunction() {
//Example using @JvmField
println(KotlinClass.foo); //Prints "1"
KotlinClass.foo = 3;
//Example using 'const val'
println(KotlinClass.dog); //Prints "1". Notice the lack of a getter function
//Example of not using either @JvmField, @JvmStatic, or 'const val'
println(KotlinClass.Companion.getBar()); //Prints "2"
KotlinClass.Companion.setBar(3); //The setter for [bar]
//Example of using @JvmStatic instead of @JvmField
println(KotlinClass.getCat());
KotlinClass.setCat(0);
}
void println(Object o) {
System.out.println(o);
}
}
L'une des grandes fonctionnalités de Kotlin est que vous pouvez créer des fonctions et des variables de haut niveau. Cela le rend idéal pour créer des listes "sans classe" de champs et fonctions constants, qui à leur tour peuvent être utilisés comme staticfonctions / champs en Java.
Scénario 3: Accès aux champs et fonctions de niveau supérieur dans Kotlin à partir de Java
Kotlin
//In this example, the file name is "KSample.kt". If this annotation wasn't provided, all functions and fields would have to accessed
//using the name [KSampleKt.foo()] to utilize them in Java. Make life easier for yourself, and name this something more simple
@file:JvmName("KotlinUtils")
package com.frybits
//This can be called from Java as [KotlinUtils.TAG]. This is a final static variable
const val TAG = "You're it!"
//Since this is a top level variable and not part of a companion object, there's no need to annotate this as "static" to access in Java.
//However, this can only be utilized using getter/setter functions
var foo = 1
//This lets us use direct access now
@JvmField
var bar = 2
//Since this is calculated at runtime, it can't be a constant, but it is still a final static variable. Can't use "const" here.
val GENERATED_VAL:Long = "123".toLong()
//Again, no need for @JvmStatic, since this is not part of a companion object
fun doSomethingAwesome() {
println("Everything is awesome!")
}
Java
package com.frybits;
class JavaClass {
void someFunction() {
println(KotlinUtils.TAG); //Example of printing [TAG]
//Example of not using @JvmField.
println(KotlinUtils.getFoo()); //Prints "1"
KotlinUtils.setFoo(3);
//Example using @JvmField
println(KotlinUtils.bar); //Prints "2". Notice the lack of a getter function
KotlinUtils.bar = 3;
//Since this is a top level variable, no need for annotations to use this
//But it looks awkward without the @JvmField
println(KotlinUtils.getGENERATED_VAL());
//This is how accessing a top level function looks like
KotlinUtils.doSomethingAwesome();
}
void println(Object o) {
System.out.println(o);
}
}
Une autre mention notable qui peut être utilisée en Java comme champs "statiques" est les objectclasses Kotlin . Ce sont des classes singleton à paramètre zéro qui sont instanciées paresseusement lors de la première utilisation. Plus d'informations à leur sujet peuvent être trouvées ici: https://kotlinlang.org/docs/reference/object-declarations.html#object-declarations
Cependant, pour accéder au singleton, un INSTANCEobjet spécial est créé, qui est tout aussi encombrant à gérer qu'il l' Companionest. Voici comment utiliser des annotations pour lui donner une staticsensation de pureté en Java:
Scénario 4: utiliser des objectclasses
Kotlin
@file:JvmName("KotlinClass")
//This provides a name for this file, so it's not defaulted as [KotlinClassKt] in Java
package com.frybits
object KotlinClass { //No need for the 'class' keyword here.
//Direct access to this variable
const val foo: Int = 1
//Tells Java this can be accessed directly from [KotlinClass]
@JvmStatic
var cat: Int = 9
//Just a function that returns the class name
@JvmStatic
fun getCustomClassName(): String = this::class.java.simpleName + "boo!"
//Getter/Setter access to this variable, but isn't accessible directly from [KotlinClass]
var bar: Int = 2
fun someOtherFunction() = "What is 'INSTANCE'?"
}
Java
package com.frybits;
class JavaClass {
void someFunction() {
println(KotlinClass.foo); //Direct read of [foo] in [KotlinClass] singleton
println(KotlinClass.getCat()); //Getter of [cat]
KotlinClass.setCat(0); //Setter of [cat]
println(KotlinClass.getCustomClassName()); //Example of using a function of this 'object' class
println(KotlinClass.INSTANCE.getBar()); //This is what the singleton would look like without using annotations
KotlinClass.INSTANCE.setBar(23);
println(KotlinClass.INSTANCE.someOtherFunction()); //Accessing a function in the object class without using annotations
}
void println(Object o) {
System.out.println(o);
}
}