Collections

Collections are where Kotlin really shines. Lists, sets, and maps come in two flavors — read-only and mutable — and they're packed with powerful operations that let you transform data in elegant ways.

Lists

listOf creates a read-only list. mutableListOf gives you one you can add to or remove from.

fun main() {
  val fruits = listOf("Apple", "Banana", "Cherry")
  val mutable = mutableListOf("Apple", "Banana")

  mutable.add("Cherry")
  mutable.remove("Apple")

  println(fruits)
  println(mutable)
  println(fruits[1])
}
    

Access elements with square brackets or use .get(). Remember: read-only means you can't change the list, but if the items are mutable objects themselves, they can still be modified.

Sets and Maps

Sets hold unique values. Maps hold key-value pairs.

fun main() {
  val uniqueNums = setOf(1, 2, 3, 3, 2)
  println(uniqueNums)

  val scores = mapOf("Alice" to 95, "Bob" to 87)
  println(scores["Alice"])
  println(scores.getOrDefault("Eve", 0))
}
    

Sets automatically deduplicate — notice the repeated values disappear. Maps use to infix function for creating key-value pairs.

Common operations

This is where Kotlin collections really flex. filter, map, and forEach let you process data in one clean chain.

fun main() {
  val numbers = listOf(1, 2, 3, 4, 5, 6)

  val evens = numbers.filter { it % 2 == 0 }
  val doubled = numbers.map { it * 2 }
  val sum = numbers.reduce { acc, n -> acc + n }

  println("Evens: $evens")
  println("Doubled: $doubled")
  println("Sum: $sum")

  numbers.forEach { print("$it ") }
}
    

filter keeps items matching a condition. map transforms each item. forEach does something with each item. reduce combines them into a single value.