mutableListOf、mutableMapOf：可変コレクションの構築

変更が必要な場合

mutableListOf

val list = mutableListOf("a", "b", "c")
list.add("d")
list.add(0, "z")  // insert at index 0
list.remove("b")
println(list)  // [z, a, c, d]

mutableSetOf

val set = mutableSetOf(1, 2, 3)
set.add(4)
set.add(2)  // already exists, no change
set.remove(1)
println(set)  // [2, 3, 4]

mutableMapOf

val map = mutableMapOf("a" to 1, "b" to 2)
map["c"] = 3
map["a"] = 10      // update
map.remove("b")
println(map)  // {a=10, c=3}

addAll、removeAll、retainAll

val nums = mutableListOf(1, 2, 3)
nums.addAll(listOf(4, 5, 6))
println(nums)  // [1, 2, 3, 4, 5, 6]
nums.removeAll { it % 2 == 0 }
println(nums)  // [1, 3, 5]

clearとreplaceAll

val list = mutableListOf(1, 2, 3, 4)
list.replaceAll { it * 2 }
println(list)  // [2, 4, 6, 8]
list.clear()
println(list)  // []

buildListによるリストの構築

val squares = buildList {
    for (i in 1..5) add(i * i)
}
println(squares)  // [1, 4, 9, 16, 25]

buildMapとbuildSet

val map = buildMap {
    put("en", "English")
    put("de", "German")
    put("tr", "Turkish")
}
println(map["tr"])  // Turkish

ArrayList、LinkedList：型の明示

val arrayList = ArrayList<String>()
arrayList.add("x")
val deque = ArrayDeque<Int>()
deque.addFirst(1)
deque.addLast(2)

Mapの蓄積に使うgetOrPut

val groups = mutableMapOf<String, MutableList<Int>>()
for (i in 1..6) {
    val key = if (i % 2 == 0) "even" else "odd"
    groups.getOrPut(key) { mutableListOf() }.add(i)
}
println(groups)

可変リストの並べ替え

val nums = mutableListOf(3, 1, 4, 1, 5, 9)
nums.sort()
println(nums)  // [1, 1, 3, 4, 5, 9]
nums.sortDescending()
println(nums)  // [9, 5, 4, 3, 1, 1]

確認問題

まとめ