2：kotlin集合（Collections）

集合有助于数据分组，方便后续操作

集合类型	说明
Lists	有序的可重复的集合
Sets	无序的不可重复的集合
Maps	键值对映射集合，键唯一，且一个键只能映射到一个值

每个集合类型都可以是可变的或者只读的

List

List按照添加的顺序存储内容，并允许重复

存储的内容通常称作元素

List类型

创建只读List使用listOf()方法

创建可变List使用mutableListOf()方法

在创建列表时，Kotlin可以推断存储项的类型。可以在列表声明后的尖括号<>中添加类型来显式声明类型

fun main() {
    val readOnlyShapes = listOf("triangle", "square", "circle")
    println(readOnlyShapes) // [triangle, square, circle]

    val shapes: MutableList<String> = mutableListOf("triangle", "square", "circle")
    println(shapes) // [triangle, square, circle]
    shapes[2] = "circle2"
    // shapes[3] = "circle3"   // Index 3 out of bounds for length 3
    println(shapes) // [triangle, square, circle2]
}

操作MutableList索引不能超过初始长度

为了防止不必要的修改，你可以通过将可变列表赋值给List来获得它们的只读视图

fun main() {
    val shapes: MutableList<String> = mutableListOf("triangle", "square", "circle")
    val shapesLocked: List<String> = shapes
}

这种方式通常叫做铸造（casting）

List常用方法

查看List方法

获取元素

fun main() {
   val readOnlyShapes = listOf("triangle", "square", "circle")
   println("The first item in the list is: ${readOnlyShapes[0]}")  // The first item in the list is: triangle
   println("The first item in the list is: ${readOnlyShapes.first()}") // The first item in the list is: triangle
   println("The last item in the list is: ${readOnlyShapes.last()}")   // The last item in the list is: circle
}

获取长度

fun main() {
    val readOnlyShapes = listOf("triangle", "square", "circle")
    println("This list has ${readOnlyShapes.count()} items") // This list has 3 items
}   

判断是否包含某一元素

fun main() {
    val readOnlyShapes = listOf("triangle", "square", "circle")
    println("circle" in readOnlyShapes) // true
}

新增和删除

fun main() {
    val shapes: MutableList<String> = mutableListOf("triangle", "square", "circle")
    // 添加 "pentagon"
    shapes.add("pentagon")
    println(shapes) // [triangle, square, circle, pentagon]
    
    // 删除第一个 "pentagon"
    shapes.remove("pentagon")
    println(shapes) // [triangle, square, circle]
}

Set

Set集合中存储的数据无序并且不能重复

Set类型

创建只读Set使用setOf()方法

创建可变Set使用MutableList()方法

fun main() {
    val readOnlyFruit = setOf("apple", "banana", "cherry", "cherry")
    val fruit: MutableSet<String> = mutableSetOf("apple", "banana", "cherry", "cherry")
}

Set常用方法

获取元素

因为Set为无序集合，所以不能通过索引获取集合元素

fun main() {
    val set = setOf("apple", "banana", "cherry", "cherry")
    println("The first item in the set is: ${set.first()}") // The first item in the set is: apple
    println("The last item in the set is: ${set.last()}") //  The last item in the set is: cherry
}

Map

Map以键值对的形式存储数据。你可以通过引用键来访问值

键是唯一的，如果插入重复键则会覆盖之前的值

Map类型

要创建只读Map，使用mapOf()函数
要创建可变地图MutableMap，使用mutableMapOf()函数

在创建Map时，kotlin可以推断出存储的元素类型。要显式声明类型，可以在Map声明后的尖括号<>中添加键和值的类型。例如：MutableMap。键的类型为String，值的类型为Int

fun main() {
    val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
    println(readOnlyJuiceMenu)  // {apple=100, kiwi=190, orange=100}
    
    val juiceMenu: MutableMap<String, Int> = mutableMapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
    println(juiceMenu)  // {apple=100, kiwi=190, orange=100}
}

常用方法

获取数据

使用键获取值

fun main() {
    val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
    println(readOnlyJuiceMenu["apple"]) // 100
}

获取所有的键或者值

fun main() {
    val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
    println(readOnlyJuiceMenu.keys) // [apple, kiwi, orange]
    println(readOnlyJuiceMenu.values)   // [100, 190, 100]
}

获取键值对数量

fun main() {
    val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
    println(readOnlyJuiceMenu.count()) // 3
}

修改/删除

fun main() {
    val juiceMenu: MutableMap<String, Int> = mutableMapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
    juiceMenu.put("coconut", 150) // 添加键"coconut"和值 150
    println(juiceMenu) // {apple=100, kiwi=190, orange=100, coconut=150}
    juiceMenu.put("apple", 200) // 修改"coconut"的值成 200
    println(juiceMenu) // {apple=200, kiwi=190, orange=100, coconut=150}

    juiceMenu.remove("orange")    // 删除"orange"
    println(juiceMenu)  // {apple=200, kiwi=190, coconut=150}
}

是否包含某元素

是否包含某个键

fun main() {
    val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
    println(readOnlyJuiceMenu.containsKey("kiwi"))  // true
    println("orange" in readOnlyJuiceMenu.keys) // true
}

是否包含某个值

fun main() {
    val readOnlyJuiceMenu = mapOf("apple" to 100, "kiwi" to 190, "orange" to 100)
    println(200 in readOnlyJuiceMenu.values)  // false
}