Kotlin 变量名前的 Kotlin 星号运算符或扩展运算符 [英] Kotlin asterisk operator before variable name or Spread Operator in Kotlin

问题描述

我想知道 Kotlin 中在变量名之前星号的作用.我在 *args)="noreferrer">Spring Boot Kotlin 示例:

I want to know what exactly an asterisk does before a variable name in Kotlin. I saw this (*args) in a Spring boot Kotlin example:

@SpringBootApplication
open class Application {

    @Bean
    open fun init(repository: CustomerRepository) = CommandLineRunner {
        repository.save(Customer("Jack", "Bauer"))
        repository.save(Customer("Chloe", "O'Brian"))
        repository.save(Customer("Kim", "Bauer"))
        repository.save(Customer("David", "Palmer"))
        repository.save(Customer("Michelle", "Dessler"))
    }
}

fun main(args: Array<String>) {
    SpringApplication.run(Application::class.java, *args)
}

推荐答案

* 运算符在 Kotlin 中被称为 Spread 运算符.

The * operator is known as the Spread Operator in Kotlin.

来自 Kotlin 参考...

当我们调用一个可变参数函数时，我们可以一个一个地传递参数，例如asList(1, 2, 3)，或者，如果我们已经有一个数组并且想要将它的内容传递给函数，我们使用扩展运算符(在数组前加上 *):

When we call a vararg-function, we can pass arguments one-by-one, e.g. asList(1, 2, 3), or, if we already have an array and want to pass its contents to the function, we use the spread operator (prefix the array with *):

它可以在将数组传递给接受 varargs 的函数之前应用于数组.

It can be applied to an Array before passing it into a function that accepts varargs.

如果你有一个接受不同数量参数的函数......

If you have a function that accepts a varied number of arguments...

fun sumOfNumbers(vararg numbers: Int): Int {
    return numbers.sum()
}

使用扩展运算符将数组的元素作为参数传递:

Use the spread operator to pass an array's elements as the arguments:

val numbers = intArrayOf(2, 3, 4)
val sum = sumOfNumbers(*numbers)
println(sum) // Prints '9'

---

注意事项:

• * 运算符也是 乘法运算符(当然).
• 该运算符只能在向函数传递参数时使用.操作的结果无法存储，因为它不产生任何价值(它纯粹语法糖).
• 该运算符一开始可能会让一些 C/C++ 程序员感到困惑，因为它看起来像是正在取消引用指针.它不是；Kotlin 没有指针的概念.
• 在调用可变参数函数时，该运算符可以在其他参数之间使用.这在此处的示例中进行了演示.
• 该运算符类似于各种函数式编程语言中的 apply 函数.

• The * operator is also the multiplication operator (of course).
• The operator can only be used when passing arguments to a function. The result of the operation cannot be stored since it yields no value (it is purely syntactic sugar).
• The operator may confuse some C/C++ programmers at first because it looks like a pointer is being de-referenced. It isn't; Kotlin has no notion of pointers.
• The operator can be used in-between other arguments when calling a vararg function. This is demonstrated in the example here.
• The operator is similar to the apply function in various functional programming languages.

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