Plus and Minus Operators

In Kotlin, plus (+) and minus (-) operators are defined for collections.

They take a collection as the first operand; the second operand can be either an element or another collection. The return value is a new read-only collection:

The result of plus contains the elements from the original collection and from the second operand.
The result of minus contains the elements of the original collection except the elements from the second operand.
- If it's an element, minus removes its first occurrence; if it's a collection, all occurrences of its elements are removed.

val numbers = listOf("one", "two", "three", "four")

val plusList = numbers + "five"
val minusList = numbers - listOf("three", "four")
println(plusList)
println(minusList)

// Output:
[one, two, three, four, five]
[one, two]

The augmented assignment operators plusAssign (+=) and minusAssign (-=) are also defined for collections.

However, for read-only collections, they actually use the plus or minus operators and try to assign the result to the same variable.

Thus, they are available only on var read-only collections. For mutable collections, they modify the collection if it's a val.

Last modified: 29 March 2024