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 3println(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: triangleprintln("The first item in the list is: ${readOnlyShapes.first()}") // The first item in the list is: triangleprintln("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: appleprintln("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<String, Int>。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 150println(juiceMenu) // {apple=100, kiwi=190, orange=100, coconut=150}juiceMenu.put("apple", 200) // 修改"coconut"的值成 200println(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")) // trueprintln("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
}