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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.
fun main() {
    val numbers = listOf("one", "two", "three", "four")

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

For the details on plus and minus operators for maps, see Map Specific Operations. 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. For more details see Collection Write Operations.