Android_Kotlin_Lambda编程

 集合的创建与遍历

list集合

val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape")    //不可变集合

listOf

fun main() {

val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape")

for (fruit in list) {

println(fruit)

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}

 mutableListOf

fun main() {

val list = mutableListOf("Apple", "Banana", "Orange", "Pear", "Grape")    //可变集合

list.add("Watermelon")

for (fruit in list) {

println(fruit)

}

}

set集合

Set 集合中是不可以存放重复元素的，如果存放了多个相同的元素，只会保留其中一 份

setOf(可变) 和 mutableSetOf(不可变)

val set = setOf("Apple", "Banana", "Orange", "Pear", "Grape")

for (fruit in set) {

println(fruit)

}

map集合（键值对）

val map = HashMap<String, Int>()

map.put("Apple", 1)

map.put("Banana", 2)

map.put("Orange", 3)

map.put("Pear", 4)

map.put("Grape", 5)

map["Apple"] = 1    //添加键值对

val number = map["Apple"]     //读取键值对

val map = HashMap<String, Int>()

map["Apple"] = 1

map["Banana"] = 2

map["Orange"] = 3

map["Pear"] = 4

map["Grape"] = 5

 mapof()（不可变）

val map = mapOf("Apple" to 1, "Banana" to 2, "Orange" to 3, "Pear" to 4, "Grape" to 5)

fun main() {

val map = mapOf("Apple" to 1, "Banana" to 2, "Orange" to 3, "Pear" to 4, "Grape" to 5)

for ((fruit, number) in map) {

println("fruit is " + fruit + ", number is " + number)

}

}

集合的函数式API

找到单词最长的水果

val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")

var maxLengthFruit = ""

for (fruit in list) {

if (fruit.length > maxLengthFruit.length) {

maxLengthFruit = fruit

}

}

println("max length fruit is " + maxLengthFruit)

val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")

val maxLengthFruit = list.maxBy { it.length }   //it表示list集合里面每个单词的长度

println("max length fruit is " + maxLengthFruit)

lambda表达式语法结构

{参数名1: 参数类型, 参数名2: 参数类型 -> 函数体} 

val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")

val lambda = { fruit: String -> fruit.length }

val maxLengthFruit = list.maxBy(lambda)   //maxBy 函数实质上就是接收了一个 Lambda 参数而

将单词都变成大写形式

fun main() {

val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")

val newList = list.map { it.toUpperCase() }

for (fruit in newList) {

println(fruit)

}

}

filter函数

对单词长度进行过滤，只保留5个字母以下的单词

fun main() {

val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")

val newList = list.filter { it.length <= 5 }

.map { it.toUpperCase() }

for (fruit in newList) {

println(fruit)

}

}

any和all函数

any 函数用于判断集合中是否至少存在一个元素满足指定条件，

all 函数用于判断集合中是否所有元素都满足指定条件

返回true或者false

fun main() {

val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")

val anyResult = list.any { it.length <= 5 }

val allResult = list.all { it.length <= 5 }

println("anyResult is " + anyResult + ", allResult is " + allResult)

}