Operator Functions

Learn how to define your own operators, which limitations Kotlin poses on such operators, and how they lead to succinct code.

Operator functions allow you to define the meaning of well-known symbols such as + and - for your own types.

Introducing Operators #

Kotlin’s operators allow calling functions using certain symbols. For instance, the infix + operator actually calls a plus function so that a + b translates to a.plus(b) under the hood.

Many operators use infix notation and therefore can be seen as an extension of infix functions. Instead of a function name, operator functions instead use an operator symbol. However, there are also operators using prefix and postfix notation:

  • Prefix: +a, -a, !a
  • Infix: a * b, a..b, a == b, …
  • Postfix: a[i], a()

Kotlin has a predefined list of possible operators you can use (in contrast to languages like Scala or Haskell).

Important Kotlin Operators #

Note: Some of the examples below require a custom operator definition to run. For instance, "Coffee" * 3 doesn’t work out of the box because Kotlin provides no such operator implementation.

Function Name Operator Example Equivalent
Arithmetic Operators
plus + 2 + 3 2.plus(3)
minus - "Kotlin" – "in" "Kotlin".minus("in")
times * "Coffee" * 2 "Coffee".times(2)
div / 7.0 / 2.0 (7.0).div(2.0)
rem % "android" % 'a' "android".rem('a')
Assignment Operators
plusAssign += x += "Island" x.plusAssign(“island”)
minusAssign -= x -= 1.06 x.minusAssign(1.06)
timesAssign *= x *= 3 x.timesAssign(3)
divAssign /= x /= 2 x.divAssign(2)
remAssign %= x %= 10 x.remAssign(10)
Miscellaneous Operators
inc ++ grade++ grade.inc()
dec -- 'c'-- 'c'.dec()
contains in "A" in list list.contains("A")
rangeTo .. now..then now.rangeTo(then)
get […] skipList[4] skipList.get(4)
set […] = … array[5] = 42 array.set(5, 42)

For a full list of all operators, check out the Kotlin documentation.

Implementing an Operator Function #

You can define the meaning of any of the allowed operators for any of your own types. The declaration is similar as for infix functions, except that you use the operator modifier:

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