Java Kotlin 中的私有构造函数
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Private constructor in Kotlin
提问by Marvin
In Java it's possible to hide a class' main constructor by making it privateand then accessing it via a publicstaticmethod inside that class:
在 Java 中，可以通过创建类的主构造函数private然后通过publicstatic该类中的方法访问它来隐藏类的主构造函数：
public final class Foo {
/* Public static method */
public static final Foo constructorA() {
// do stuff
return new Foo(someData);
}
private final Data someData;
/* Main constructor */
private Foo(final Data someData) {
Objects.requireNonNull(someData);
this.someData = someData;
}
// ...
}
How can the same be reached with Kotlin without separating the class into a publicinterface and a privateimplementation? Making a constructor privateleads to it not being accessible from outside the class, not even from the same file.
在不将类分为public接口和private实现的情况下，如何使用 Kotlin实现相同的功能？创建一个构造函数private会导致它不能从类外部访问，甚至不能从同一个文件访问。
采纳答案by rafal
You can even do something more similar to "emulating" usage of public constructor while having private constructor.
您甚至可以在拥有私有构造函数的同时做一些更类似于“模拟”公共构造函数用法的事情。
class Foo private constructor(val someData: Data) {
companion object {
operator fun invoke(): Foo {
// do stuff
return Foo(someData)
}
}
}
//usage
Foo() //even though it looks like constructor, it is a function call
回答by Marvin
This is possible using a companion object:
这可以使用伴随对象：
class Foo private constructor(val someData: Data) {
companion object {
fun constructorA(): Foo {
// do stuff
return Foo(someData)
}
}
// ...
}
Methods inside the companion object can be reached just like if they were members of the surrounding class (e.g. Foo.constructorA())
可以访问伴随对象中的方法，就像它们是周围类的成员一样（例如Foo.constructorA()）
回答by Prasanth
See kotlin documentation here:
请参阅此处的 kotlin 文档：
https://kotlinlang.org/docs/reference/classes.html#constructors
https://kotlinlang.org/docs/reference/classes.html#constructors
https://kotlinlang.org/docs/reference/visibility-modifiers.html#constructors
https://kotlinlang.org/docs/reference/visibility-modifiers.html#constructors
class DontCreateMe private constructor () { /*...*/ }
回答by Abhinandan Chada
This is the Answer
class Hide private constructor(val someData: Data) {
}
By declaring the constructor private, we can hiding constructor.
回答by Omar
I've been coding in Java for a few years, and learning now Kotlin as a new language. From what I found while trying to make a Singleton without being able to call the constructor was to make a primary constructor in the class as private declared, and then calling your function (in my case the singleton) from a companion object like this.
我已经用 Java 编码几年了，现在正在学习 Kotlin 作为一种新语言。从我在尝试创建一个不能调用构造函数的单例时发现的是，在类中创建一个主构造函数作为私有声明，然后从这样的伴随对象调用你的函数（在我的情况下是单例）。
open class Singleton private constructor() {
fun produceSomeChicken(): String {
return "CHICKEN"
}
companion object {
private var INSTANCE : Singleton? = null
fun getInstance(): Singleton {
if (INSTANCE == null) {
println("IS NULL")
INSTANCE = Singleton()
println("INSTANCE CREATED")
}
return INSTANCE as Singleton
}
}}
and then calling the function from anywhere you want like that for example
然后从任何你想要的地方调用该函数，例如
println(Singleton.getInstance().produceSomeChicken()) //instance created without calling constructor
//println(Singleton().produceSomeChicken()) //is not possible to call, prevented by using private constructor (gives back compile error)
Eventhough the question is old, I still hope it helps some of you! You're welcome
尽管这个问题很老，但我仍然希望它对你们中的一些人有所帮助！别客气
