Operator Overloading

Kotlin lets types define what operators like +, [], and in mean, by implementing specially-named operator functions.

Arithmetic operators

data class Vector(val x: Int, val y: Int) {
    operator fun plus(other: Vector) = Vector(x + other.x, y + other.y)
    operator fun times(scale: Int) = Vector(x * scale, y * scale)
}

Vector(1, 2) + Vector(3, 4)    // Vector(4, 6)
Vector(1, 2) * 3               // Vector(3, 6)

Operator	Function
a + b	a.plus(b)
a - b	a.minus(b)
a * b	a.times(b)
a in b	b.contains(a)
a[i]	a.get(i)
a[i] = v	a.set(i, v)
a()	a.invoke()

Indexing

class Grid(val width: Int, val height: Int) {
    private val cells = IntArray(width * height)
    operator fun get(x: Int, y: Int) = cells[y * width + x]
    operator fun set(x: Int, y: Int, value: Int) { cells[y * width + x] = value }
}

val grid = Grid(3, 3)
grid[1, 1] = 5
grid[1, 1]              // 5

invoke

Make an object callable like a function:

class Adder(val amount: Int) {
    operator fun invoke(x: Int) = x + amount
}

val addFive = Adder(5)
addFive(10)            // 15

Comparison with compareTo

Implementing Comparable (which defines compareTo) enables <, >, <=, >=, and sorting:

data class Money(val cents: Int) : Comparable<Money> {
    override fun compareTo(other: Money) = cents.compareTo(other.cents)
}

Money(100) < Money(250)   // true

Destructuring with componentN

data classes generate component1(), component2(), … so they can be destructured. Any type defining these operator functions can too:

val (x, y) = Vector(3, 4)   // x = 3, y = 4

Overload operators only when the meaning is obvious. + on a Vector is clear; + doing something surprising hurts readability.

Exercises

1. Add a minus operator to Vector.

2. Give Money a plus operator and a compareTo, then sort a list of Money.

   Solution

   data class Money(val cents: Int) : Comparable<Money> {
       operator fun plus(other: Money) = Money(cents + other.cents)
       override fun compareTo(other: Money) = cents.compareTo(other.cents)
   }
   

</details>

This solution is in examples/core/part5/ and is tested by CI.

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