What does the arrow (“->”) operator do in Kotlin?

问题

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.

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 +"\"")

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

回答1:

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

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!"
 }

lambda expressions where it separates parameters from function body

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

function types where it separates parameters types from result type e.g. comparator
```
  fun <T> sort(comparator:(T,T) -> Int){
  }
```

Details about Kotlin grammar are in the documentation in particular:

functionType
functionLiteral
whenEntry

回答2:

The -> is a separator. It is special symbol used to separate code with different purposes. It can be used to:

Separate the parameters and body of a lambda expression
```
val sum = { x: Int, y: Int -> x + y }
```
Separate the parameters and return type declaration in a function type
```
(R, T) -> R
```

Separate the condition and body of a when expression branch

when (x) {
    0, 1 -> print("x == 0 or x == 1")
    else -> print("otherwise")
}

Here it is in the documentation.

回答3:

From the Kotlin docs:

->

separates the parameters and body of a lambda expression

separates the parameters and return type declaration in a function type

separates the condition and body of a when expression branch

来源：https://stackoverflow.com/questions/42646016/what-does-the-arrow-operator-do-in-kotlin

标签

operators

kotlin