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

问题描述

我想知道在Kotlin中，变量名之前星号到底是做什么的. 我在*args)"rel =" noreferrer> Spring boot Kotlin示例:

I want to know what exactly asterisk does before variable name in Kotlin. I saw this (*args) at 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

在Kotlin中，*运算符被称为 Spread运算符.

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

来自科林参考书.

当我们调用vararg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将其内容传递给该函数，则可以使用传播运算符(使用*前缀该数组):

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 *):

在将其传递给接受varargs的函数之前，可以将其应用于数组.

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

如果您有一个接受各种参数的函数...

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

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

您可以像这样将数组传递给它...

You can pass an array into it like so...

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

---

注释:

• *运算符也是乘法运算符(当然).
• 仅当将参数传递给函数时才能使用运算符. 操作的结果无法存储，因为它不产生任何值(它是纯语法糖).
• 运算符起初可能会使某些C/C ++程序员感到困惑，因为它看起来像是在取消引用指针.不是Kotlin 没有指针的概念.
• 在调用vararg函数时，可以在其他参数之间使用运算符.在示例此处中进行了演示.
• 运算符类似于各种功能编程语言中的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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