List與MutableList

唯讀List

語法

val 變數: List<類型> = listOf(物件, 物件, 物件..)

只能讀取，不能修改。

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val names: List<String> = listOf("Tom","Jack","lucy")
for (name in names) {
    println(name)
}

Tom
Jack
lucy

建立可讀寫MutableList

建立MutableList

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val mutableList = mutableListOf<String>("Alice", "Bill", "Gina")

建立空的MutableList

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val mutableList:MutableList<Int> = mutableListOf()

MutableList新增刪除修改

新增

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mutableList.add("Ray")
// 指定插入的索引位置
mutableList.add(1,"Tina")

刪除

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mutableList.remove("Alice")
// 指定刪除索引位置
mutableList.removeAt(0)

刪除前判斷有存在才刪。

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val mutableList = mutableListOf<String>("Alice", "Bill", "Gina")
mutableList.removeIf{it.contains("Alice") }
println(mutableList)

[Bill, Gina]

清空

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mutableList.clear()
println(mutableList)

[]

安全取值

get(索引)可以取得元素的值，但搭配以下二種更安全。

語法

無此元素，就傳回預設值
集合變數.getOrElse(索引) { "預設值"}

無此元素傳回null，使用貓王運算子?:，印出預設值
集合變數s.getOrNull(索引) ?: "預設值"

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val namelist = listOf<String>("Mary", "Amy", "Jery")
println("======== list data ============")
println(namelist.get(0))
println(namelist.getOrElse(5) { "no data"})
println(namelist.getOrNull(5) ?: "no data")

Mary
no data
no data

索引取值

語法

${變數[索引]}

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val namelist = listOf<String>("Mary", "Amy", "Jery")
println("list[0] = ${namelist[0]}")
println("list[1] = ${namelist[1]}")
println("list[2] = ${namelist[2]}")
println("list first = ${namelist.first()}")
println("list last = ${namelist.last()}")

list[0] = Mary
list[1] = Amy
list[2] = Jery

修改集合

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val mutableList = mutableListOf<String>("Alice", "Bill", "Gina")
mutableList[0] = "Gee"
println(mutableList[0])

Gee

mutator

能用在MutableList，Array無法使用。

語法

+= 新增元素
-= 刪除元素

新增

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val mutableList = mutableListOf<String>("Alice", "Bill", "Gina")
mutableList += "Momo"
mutableList += "Yoyo"
println(mutableList)

[Alice, Bill, Gina, Momo, Yoyo]

刪除

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val mutableList = mutableListOf<String>("Alice", "Bill", "Gina")
mutableList += "Momo"
mutableList += "Yoyo"
println(mutableList)
mutableList -= "Momo"
mutableList -= "Yoyo"
println(mutableList)

[Alice, Bill, Gina]

List與MutableList互轉

List轉MutableList

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val namelist = listOf<String>("Mary", "Amy", "Jery")
namelist.toMutableList()

MutableList轉List

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mutableList.toList()

first()

取出第一個元素。

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println("list first = ${namelist.first()}")

list first = Mary

last()

取出最後一個元素。

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println("list last = ${namelist.last()}")

list last = Jery

size

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val namelist = listOf<String>("Mary", "Amy", "Jery")
println("list size = ${namelist.size}")

list size = 3

contains

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val namelist = listOf<String>("Mary", "Amy", "Jery")
println("list contains = ${namelist.contains("Amy")}")

list contains = true

in

與contains功能一樣，都有包含的意思。

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val namelist = listOf<String>("Mary", "Amy", "Jery")
println("Mary" in namelist)

true

empty

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val mutableList:MutableList<Int> = mutableListOf()
if (mutableList.isEmpty())
    println("empty")
if (mutableList.isNotEmpty())
    println("not empty")

empty

distinct()去掉重覆

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val mutableList1 = mutableListOf<String>("Alice", "Bill", "Gina", "Alice")
println(mutableList1)
val distinctList  = mutableList1.distinct()
println(distinctList)

[Alice, Bill, Gina, Alice]
[Alice, Bill, Gina]

訪問集合中所有元素

for裡面的i變數，前面不會有val或var，也不會有變數類型。

其它建立List的方式

public inline fun <T> List(size: Int, init: (Int) -> T): List<T>

建立大小為3，裡面的值為0 .. (size - 1)，並把它印出來，以下沒有建立List，因為沒有傳回值。

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List(3) {
  println(it)
}

0
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以下有傳回值，就會建立不可變的List。

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val list = List(3) {index: Int -> index }
println(list)

[0, 1, 2]

Lambda{}花括號內最後一行就是要組成的格式，$it就是index索引。

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val list = List(100) {
  "Item $it"
}
println(list)

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val list = mutableListOf<String>()
list.addAll(List(500) {"Item $it"})

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