“ : :”在 Kotlin 是什么意思？

我刚来科特林
我用这个代码开启了另一个活动:

startActivity(Intent(this,IntroAndLang::class.java))

当前活动和目标活动都是在 Kotlin 编写的

我不明白为什么在 IntroAndLang::class.java没有单一的 :而不是 ::

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最佳答案

As stated in the docs this is a class reference:

Class References: The most basic reflection feature is getting the runtime reference to a Kotlin class. To obtain the reference to a statically known Kotlin class, you can use the class literal syntax:

val c = MyClass::class
//The reference is a value of type KClass.

Note that a Kotlin class reference is not the same as a Java class reference. To obtain a Java class reference, use the .java property on a KClass instance.

It’s also the syntax for method references as in this simple example:

list.forEach(::println)

It refers to println defined in Kotlin Standard library.

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:: converts a Kotlin function into a lambda.

Let's say you have a function that looks like this:

fun printSquare(a: Int) = println(a * 2)

And you have a class that takes a lambda as a 2nd argument:

class MyClass(var someOtherVar: Any, var printSquare: (Int) -> Unit) {
        

fun doTheSquare(i: Int) {
printSquare(i)
}
}

How do you pass the printSquare function into MyClass? If you try the following, it wont work:

  MyClass("someObject", printSquare) //printSquare is not a LAMBDA, it's a function so it gives compile error of wrong argument

So how do we CONVERT printSquare into a lambda so we can pass it around? Use the :: notation.

MyClass("someObject",::printSquare) //now compiler does not complain since it's expecting a lambda and we have indeed converted the `printSquare` FUNCTION into a LAMBDA.

Also, please note that this is implied... meaning this::printSquare is the same as ::printSquare. So if the printSquare function was in another class, like a Presenter, then you could convert it to lambda like this:

Presenter::printSquare

UPDATE:

Also this works with constructors. If you want to create the constructor of a class and then convert it to a lambda, it is done like this:

(x, y) -> MyClass::new

this translates to MyClass(x, y) in Kotlin.

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:: is used for Reflection in kotlin

  1. Class Reference val myClass = MyClass::class
  2. Function Reference this::isEmpty
  3. Property Reference ::someVal.isInitialized
  4. Constructor Reference ::MyClass

For detailed reading Official Documentation

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Since kotlin 1.1, in addition to class, function, property and constructor references as stated above, '::' can also be used to obtain the bound references to all of the above.

For instance, using '::class' could be used to get the exact class of a particular object despite the type of the receiver as below...

val widget: Widget = ...
assert(widget is GoodWidget) { "Bad widget: ${widget::class.qualifiedName}" }

widget::class returns the exact class of the object 'widget' as either 'GoodWidget' or 'BadWidget' despite the type of the receiver expression (i.e 'Widget' as declared initially)

More info at https://kotlinlang.org/docs/reference/reflection.html


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