文章主要给大家介绍了关于kotlin中object关键字的三种使用场景，文中通过示例代码介绍的非常详细，对大家学习或者使用kotlin具有一定的参考学习价值，需要的朋友们下面来一起学习学习吧
前言
object是Kotlin中的一个重要的关键字，也是Java中没有的。object主要有以下三种使用场景：
对象声明（Object Declaration）
伴生对象（Companion Object）
对象表达式（Object Expression）
下面就一一介绍它们所表示的含义、用法以及注意点，保证你在看完本篇之后就可以完全掌握object关键字的用法。
1. 对象声明（Object Declaration）
语法含义：将类的声明和定义该类的单例对象结合在一起（即通过object就实现了单例模式）
基本示例
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object RepositoryManager{
fun method(){
println("I'm in object declaration")
}
}
即将class关键字替换为object关键字，来声明一个类，与此同时也声明它的一个对象。只要编写这么多代码，这个类就已经是单例的了。
使用
a. 在Kotlin中：
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fun main(args: Array<String>) {
RepositoryManager.method()
}
像在Java中调用静态方法（在kotlin中没有静态方法）一样去调用其中定义的方法，但实际上是使用RepositoryManager类的单例对象去调用实例方法。如果对此还不能理解，可以看看下面对在Java中去使用的说明。
b. 在Java中：
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publicclassJavaTest {
publicstaticvoidmain(String[] args) {
RepositoryManager.INSTANCE.method();
}
}
换句话说，object declaration的类最终被编译成：一个类拥有一个静态成员来持有对自己的引用，并且这个静态成员的名称为INSTANCE，当然这个INSTANCE是单例的，故这里可以这么去使用。如果用Java代码来声明这个RepositoryManager的话，可以有如下代码：
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classRepositoryManager{
privateRepositoryManager(){}
publicstaticfinalRepositoryManager INSTANCE = newRepositoryManager();
}
4) 注意点:
尽管和普通类的声明一样，可以包含属性、方法、初始化代码块以及可以继承其他类或者实现某个接口，但是它不能包含构造器（包括主构造器以及次级构造器）
它也可以定义在一个类的内部：
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classObjectOuter {
object Inner{
fun method(){
println("I'm in inner class")
}
}
}
fun main(args: Array<String>) {
ObjectOuter.Inner.method()
}
2、伴生对象（Companion object）
在阐述伴生对象之前，首先我们要明确一点：在Kotlin中是没有static关键字的，也就是意味着没有了静态方法和静态成员。那么在kotlin中如果要想表示这种概念，取而代之的是包级别函数（package-level function）和我们这里提到的伴生对象。至于它们之间的区别，不急，我们后面再说。
语法形式：
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classA{
companion object 伴生对象名(可以省略){
//define method and field here
}
}
基本示例：
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classObjectTest {
companion object MyObjec{
val a = 20
fun method() {
println("I'm in companion object")
}
}
}
使用：
a. 在Kotlin中：
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fun main(args: Array<String>) {
//方式一
ObjectTest.MyObject.method()
println(ObjectTest.MyObject.a)
//方式二（推荐方式）
ObjectTest.method()
println(ObjectTest.a)
}
在这里请注意：在定义（定义时如果省略了伴生对象名，那么编译器会为其提供默认的名字Companion）和调用时伴生对象名是可以省略的。而且在方式二中，注意调用形式，是通过类名.方法名()的形式进行的，我们在没有生成ObjectTest类的对象时，调用了定义其内部伴生对象中定义的属性和方法，是不是类似Java中的静态方法的概念。
通过现象看本质
通过javap命令，让我们看看其生成的字节码：
注意红框中，这个MyObject成员变量的类型，是使用
MyObject这个内部类。注意它这里并没有外部类的引用，说明是以静态内部类的形式存在的。
还记得我们在前面遗留的问题：同样都可以用来替代Java中的static的概念，那么在伴生对象中定义的方法和包级别函数有什么区别呢？
先来反编译一个包含包级别函数的kt文件（或者说是类）：
可以看出，一个名叫ObjectTest2.kt文件，实际上最终会生成一个名叫ObjectTest2Kt的类，而在这个kt文件中定义的顶级函数（包级别函数）是作为这个类的静态方法的形态存在的。
那么现在可以回答遗留的问题了：实际上就是平级类（姑且称之）中的静态方法和静态内部类中的方法的区别，因为静态内部类中的方法是可以访问外部类中定义的static方法和成员的，哪怕是private的（包括私有构造器，我们常用的基于静态内部类实现的单例模式就是基于这一点），而平级类中方法是访问不到当前类中静态的private成员的。如果你觉得文字这么描述还不够直观，那么我们来看下面这一张图（盗自Kotlin in action）：
@JvmStatic注解：我们把前面定义的method方法上加上此注解，重新build工程，然后再来反编译ObjectTest和ObjectTest$MyObject这个两个类，看会有什么变化。
对于这个静态内部类而言，加与不加@JvmStatic注解其类的结构是没有变化的。但是对于目标类而言，很明显多了一个静态方法，这样我们就不难理解@JvmStatic注解的作用了：将伴生对象类中定义的实例方法和属性，添加到目标类中，并且以静态的形式存在。
对于伴生对象，最后再补充一点：一个类的伴生对象只能有一个。仔细想想也很好理解，伴生对象的名称是可以省略的。如果允许对应多个伴生对象，那么我们在多个伴生对象中都定义了一模一样的函数，在调用时到底是使用哪个伴生对象的方法呢？就会产生歧义，这样就不难理解这条语法规定了。
3、对象表达式（Object Expression）
Java的匿名内部类回顾：
在去学习对象表达式之前，我们先来回顾一下Java中的匿名内部类。
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interfaceContents {
voidabsMethod();
}
publicclassHello {
publicContents contents() {
returnnewContents() {
@Override
publicvoidabsMethod() {
System.out.println("method invoked...");
}
};
}
publicstaticvoidmain(String[] args) {
Hello hello = newHello();
hello.contents().absMethod(); //打印method invoked...
}
}
``` newContents()
这个contents()方法返回的是一个匿名内部类的对象，这个匿名内部类实现了Contents接口。这些代码很熟悉，不多说了。现在提出两个局限性问题：
a. 如果在匿名内部类中新添加了一些属性和方法，那么在外界是无法调用的
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returnnewContents() {
privateinti = 1;
publicintvalue() {
returni;
}
@Override
publicvoidabsMethod() {
System.out.println("method invoked...");
}
};
publicstaticvoidmain(String[] args) {
Hello hello = newHello();
hello.contents().absMethod();
hello.contents().value(); //Cannot resolve method 'value()'
}
当你想使用这个value方法时，编译器会报错。也好理解，就是多态的知识，父类型的引用是无法知晓子类添加方法的存在的。
b. 一个匿名内部类肯定是实现了一个接口或者是继承一个类，并且只能是一个，用数学术语说是“有且只有一个”
2) 语法形式：
object [ : 接口1,接口2,类型1, 类型2]{} //中括号中的可省略
3) 使用示例：
a. 实现一个接口或类
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interfaceAA {
fun a()
}
fun main(args: Array) {
val aa = object : AA {
override fun a() {
println("a invoked")
}
}
aa.a()
}
b. 不实现任何接口和类，并且在匿名内部类中添加方法
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fun main(args: Array) {
val obj = object {
fun a() {
println("a invoked")
}
}
obj.a() //打印：a invoked
}
从这个例子可以看出，前面我们提到的Java匿名内部类的第一个局限的地方在Kotlin中就不存在了，新添加的方法也是可以调用的
c. 实现多个接口和类
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fun main(args: Array) {
val cc = object : AA, BB() {
override fun a() {
}
override fun b() {
}
}
cc.a()
cc.b()
}
从这个例子可以看出，前面我们提到的Java匿名内部类的第二个局限性在kotlin中也不存在
4) 使用注意点：
这是Kotlin官方文档上的一段话：匿名对象只有定义成局部变量和private成员变量时，才能体现它的真实类型。如果你是将匿名对象作为public函数的返回值或者是public属性时，你只能将它看做是它的父类，当然你不指定任何类型时就当做Any看待。这时，你在匿名对象中添加的属性和方法是不能够被访问的。
再来举个例子帮助大家理解：
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https://vk.com/topic-231169012_54248950
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https://vk.com/topic-231169012_54249668
https://vk.com/topic-231169012_54249689
https://vk.com/topic-231169012_54249735
https://vk.com/topic-231169012_54249750
https://vk.com/topic-231169012_54249776
https://vk.com/topic-231169012_54249806
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https://vk.com/topic-231169012_54249928
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classMyTest {
privateval foo = object {
fun method() {
println("private")
}
}
val foo2 = object {
fun method() {
println("public")
}
}
fun m() = object {
fun method(){
println("method")
}
}
fun invoke(){
val local = object {
fun method(){
println("local")
}
}
local.method() //编译通过
foo.method() //编译通过
foo2.method() //编译通不过
m().method() //编译通不过
}
}
5) 关于在匿名对象中访问同一作用下定义的局部变量的问题：
在Java中，如果在匿名内部类中访问外部定义的局部变量，那么该局部变量必须使用final关键字进行修饰，至于为什么大家可以看我之前的一篇博文。而在Kotlin中，这条限制没有了，看下面的例子：
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var a = 1
val obj = object {
fun method() {
a++
}
}
obj.method()
println(a) //打印出2
再来解释一下：在Java中，实际上在method方法中使用的a实际上是局部变量a的一份拷贝，而不是它本身。而在Kotlin最终也是要编译成字节码供JVM去执行，所以本质上它是不会违背这一点的。那么它是怎么处理的呢？
当你访问的局部变量是val时，那么也是很Java一样，持有的是一份拷贝；而当你是一个可变变量（var）时，它的值是被存储在Ref这个类的实例成员中，Ref变量是final的，而他其中的成员变量是可以改变的。反编译后是可以看到Ref的身影的。
这里还有段有点意思的代码，给大家贴出：
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fun tryToCountButtonClicks(button: Button): Int{
var clicks = 0
button.setOnClickListener{
clicks++
}
returnclicks
}
button是个按钮，这段代码的本意上是想要统计Button被点击的次数。但是这个函数的返回值始终是0，哪怕你点击再多次。因为你对局部变量clicks值得修改是异步的，而此函数的返回值是在执行时就确定了的，就是你的值还没有被修改，函数已经返回了。如果真的想统计点击次数，可以将clicks定义成类的成员变量。
4. 对比object declaration、Companion object以及object expression的初始化时机：
a. object declaration：当第一次访问它时才初始化，是一种懒初始化
b. Companion object：当它对应的类被加载后，它才初始化，类似Java中的静态代码块
c. object expression：一旦它被执行，立马初始化
至此，关于Kotlin中的object关键字的使用就介绍完了，希望大家能有所收获~
总结
到此这篇关于kotlin中object关键字的三种使用场景的文章就介绍到这了
