Kotlin 中的箭头(“->")运算符有什么作用? [英] What does the arrow ("->") operator do in Kotlin?

问题描述

可能有点宽泛的问题，但官方文档甚至没有提到作为独立实体的箭头运算符(或语言结构，我不知道哪个短语更准确).

Probably a little bit broad question, but the official documentation doesn't even mentioning the arrow operator (or language construct, I don't know which phrase is more accurate) as an independent entity.

最明显的用途是when条件语句，它用于将表达式分配给特定条件:

The most obvious use is the when conditional statement, where it is used to assign an expression to a specific condition:

  val greet = when(args[0]) {
    "Appul" -> "howdy!"
    "Orang" -> "wazzup?"
    "Banan" -> "bonjur!"
    else    -> "hi!"
  }

  println(args[0] +" greets you: ""+ greet +""")

其他用途是什么，它们有什么作用?Kotlin 中箭头操作符有一般含义吗?

What are the other uses, and what are they do? Is there a general meaning of the arrow operator in Kotlin?

推荐答案

-> 是 Kotlin 语法的一部分(类似于 Java 的 lambda 表达式语法) 并且可以在 3 个上下文中使用:

The -> is part of Kotlin's syntax (similar to Java's lambda expressions syntax) and can be used in 3 contexts:

• when 将匹配/条件"部分与结果/执行"块分开的表达式

• when expressions where it separates "matching/condition" part from "result/execution" block

 val greet = when(args[0]) {
   "Apple", "Orange" -> "fruit"
   is Number -> "How many?"
   else    -> "hi!"
 }

将参数与函数体分开的 lambda 表达式

lambda expressions where it separates parameters from function body

  val lambda = { a:String -> "hi!" }
  items.filter { element -> element == "search"  }

将参数类型与结果类型分开的函数类型，例如比较器

  fun <T> sort(comparator:(T,T) -> Int){
  }

有关 Kotlin 语法的详细信息在文档中 尤其是:

Details about Kotlin grammar are in the documentation in particular:

• functionType
• functionLiteral
• whenEntry

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