CODEWITHSUNDEEP
operatorskotlin
Gergely Lukacsy
Gergely Lukacsy

Reputation: 3074

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

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?

Upvotes: 80

Views: 51724

Answers (3)

smo0f
smo0f

Reputation: 812

From the Kotlin docs:

->

Upvotes: 5

donturner
donturner

Reputation: 19146

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.

Upvotes: 25

miensol
miensol

Reputation: 41638

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:

Upvotes: 86

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