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object-declarations.md

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有时候需要创建一个对某个类做了轻微改动的类的对象，而不用为之显式声明新的子类。 Kotlin 可以用对象表达式与对象声明处理这种情况。

对象表达式

Object expressions create objects of anonymous classes, that is, classes that aren't explicitly declared with the class declaration. Such classes are useful for one-time use. You can define them from scratch, inherit from existing classes, or implement interfaces. Instances of anonymous classes are also called anonymous objects because they are defined by an expression, not a name.

Creating anonymous objects from scratch

Object expressions start with the object keyword.

If you just need an object that doesn't have any nontrivial supertypes, write its members in curly braces after object:

fun main() {
//sampleStart
    val helloWorld = object {
        val hello = "Hello"
        val world = "World"
        // object expressions extend Any, so `override` is required on `toString()`
        override fun toString() = "$hello $world"
    }
//sampleEnd
    print(helloWorld)
}

{kotlin-runnable="true"}

Inheriting anonymous objects from supertypes

如需创建一个继承自某个（或某些）类型的匿名类的对象， specify this type after object and a colon (:). Then implement or override the members of this class as if you were inheriting from it:

window.addMouseListener(object : MouseAdapter() {
    override fun mouseClicked(e: MouseEvent) { /*……*/ }

    override fun mouseEntered(e: MouseEvent) { /*……*/ }
})

如果超类型有一个构造函数，那么传递适当的构造函数参数给它。 多个超类型可以由跟在冒号后面的逗号分隔的列表指定：

open class A(x: Int) {
    public open val y: Int = x
}

interface B { /*……*/ }

val ab: A = object : A(1), B {
    override val y = 15
}

Using anonymous objects as return and value types

When an anonymous object is used as a type of a local or private but not inline declaration (function or property), all its members are accessible via this function or property:

class C {
    private fun getObject() = object {
        val x: String = "x"
    }

    fun printX() {
        println(getObject().x)
    }
}

If this function or property is public or private inline, its actual type is:

  • Any if the anonymous object doesn't have a declared supertype
  • The declared supertype of the anonymous object, if there is exactly one such type
  • The explicitly declared type if there is more than one declared supertype

In all these cases, members added in the anonymous object are not accessible. Overridden members are accessible if they are declared in the actual type of the function or property:

interface A {
    fun funFromA() {}
}
interface B

class C {
    // The return type is Any. x is not accessible
    fun getObject() = object {
        val x: String = "x"
    }

    // The return type is A; x is not accessible
    fun getObjectA() = object: A {
        override fun funFromA() {}
        val x: String = "x"
    }

    // The return type is B; funFromA() and x are not accessible
    fun getObjectB(): B = object: A, B { // explicit return type is required
        override fun funFromA() {}
        val x: String = "x"
    }
}

Accessing variables from anonymous objects

对象表达式中的代码可以访问来自包含它的作用域的变量：

fun countClicks(window: JComponent) {
    var clickCount = 0
    var enterCount = 0

    window.addMouseListener(object : MouseAdapter() {
        override fun mouseClicked(e: MouseEvent) {
            clickCount++
        }

        override fun mouseEntered(e: MouseEvent) {
            enterCount++
        }
    })
    // ……
}

对象声明

{id="object-declarations-overview"}

单例模式在一些场景中很有用， 而 Kotlin 使单例声明变得很容易：

object DataProviderManager {
    fun registerDataProvider(provider: DataProvider) {
        // ……
    }

    val allDataProviders: Collection<DataProvider>
        get() = // ……
}

这称为对象声明。并且它总是在 object 关键字后跟一个名称。 就像变量声明一样，对象声明不是一个表达式，不能用在赋值语句的右边。

对象声明的初始化过程是线程安全的并且在首次访问时进行。

如需引用该对象，直接使用其名称即可：

DataProviderManager.registerDataProvider(……)

这些对象可以有超类型：

object DefaultListener : MouseAdapter() {
    override fun mouseClicked(e: MouseEvent) { …… }

    override fun mouseEntered(e: MouseEvent) { …… }
}

对象声明不能在局部作用域（即不能直接嵌套在函数内部），但是它们可以嵌套到其他对象声明或非内部类中。

{type="note"}

Data objects

Data object declarations is an Experimental feature. It may be dropped or changed at any time. Opt-in is required with the compilerOptions.languageVersion.set(KotlinVersion.KOTLIN_1_9) compiler option.

{type="note"}

When printing a plain object declaration in Kotlin, you'll notice that its string representation contains both its name and the hash of the object:

object MyObject

fun main() {
    println(MyObject) // MyObject@1f32e575
}

Just like data classes, you can mark your object declaration with the data modifier to get a nicely formatted string representation without having to manually provide an implementation for its toString function:

data object MyObject

fun main() {
    println(MyObject) // MyObject
}

Sealed class hierarchies are a particularly good fit for data object declarations, since they allow you to maintain symmetry with any data classes you might have defined alongside the object:

sealed class ReadResult {
    data class Number(val value: Int): ReadResult()
    data class Text(val value: String): ReadResult()
    data object EndOfFile: ReadResult()
}

fun main() {
    println(ReadResult.Number(1)) // Number(value=1)
    println(ReadResult.Text("Foo")) // Text(value=Foo)
    println(ReadResult.EndOfFile) // EndOfFile
}

伴生对象

类内部的对象声明可以用 companion 关键字标记：

class MyClass {
    companion object Factory {
        fun create(): MyClass = MyClass()
    }
}

该伴生对象的成员可通过只使用类名作为限定符来调用：

val instance = MyClass.create()

可以省略伴生对象的名称，在这种情况下将使用名称 Companion：

class MyClass {
    companion object { }
}

val x = MyClass.Companion

Class members can access the private members of the corresponding companion object.

其自身所用的类的名称（不是另一个名称的限定符）可用作对该类的伴生对象 （无论是否具名）的引用：

class MyClass1 {
    companion object Named { }
}

val x = MyClass1

class MyClass2 {
    companion object { }
}

val y = MyClass2

请注意，即使伴生对象的成员看起来像其他语言的静态成员，在运行时他们仍然是真实对象的实例成员，而且，例如还可以实现接口：

interface Factory<T> {
    fun create(): T
}

class MyClass {
    companion object : Factory<MyClass> {
        override fun create(): MyClass = MyClass()
    }
}

val f: Factory<MyClass> = MyClass

当然，在 JVM 平台，如果使用 @JvmStatic 注解，你可以将伴生对象的成员生成为真正的静态方法和字段。更详细信息请参见Java 互操作性一节 。

对象表达式和对象声明之间的语义差异

对象表达式和对象声明之间有一个重要的语义差别：

  • 对象表达式是在使用他们的地方立即执行（及初始化）的。
  • 对象声明是在第一次被访问到时延迟初始化的。
  • 伴生对象的初始化是在相应的类被加载（解析）时，与 Java 静态初始化器的语义相匹配 。