val list = ArrayList<String>()
list.add("苹果")
list.add("香蕉")
list.add("橘子")
这种初始化集合的方式比较繁琐,在Kotlin当中给我们提供了一个listOf方法可以简化集合的初始化
val list = listOf<String>("苹果", "香蕉", "橘子")使用Kotlin当中的for-in方法对集合进行遍历
fun main() {
val list = listOf<String>("苹果", "香蕉", "橘子")
for (fruit in list) {
println(fruit)
}
}需要注意的是使用listOf方法初始化的集合是一种不可变的集合,意思就是使用listOf初始化的集合只能用来读取不能用来添加,修改或者删除
那么我们要是想要快速初始化一个集合,又想要可变可以使用mutableListOf方法进行初始化
fun main() {
val list = mutableListOf<String>("苹果", "香蕉", "橘子")
list.add("哈密瓜")
for (fruit in list) {
println(fruit)
}
}
Set集合地用户几乎和List一样,只是将创建集合地方式换成了setOf和mutableSetOf
val set = setOf<String>("香蕉", "橘子", "苹果")set集合存放的元素是不可重复的
Map集合的用法在Kotlin当中也比较多,和java比较类似的是下面这一种
val map = HashMap<String, Int>()
map.put("香蕉", 1)
map.put("西瓜", 2)
但是在Kotlin当中还有一种类似于数组下标的写法
val map = HashMap<String, Int>() map["香蕉"] = 1 map["西瓜"] = 2
而在map中读取一条写法
map.get("香蕉")
//或者还可以是这样(推荐)
map["香蕉"]
当然上面这些都不是最简单的写法,在Kotlin当中给Map也提供了mapOf和mutableMapOf两个函数进行快速的初始化
fun main() {
val map = mapOf<String, Int>("香蕉" to 1, "西瓜" to 2)
for ((fruit, number) in map) {
println("fruit is " + fruit + "number is " + number)
}
}
在一个水果集合当中,找到单词最长那个水果
写法一:
val list = listOf<String>("Apple", "Banana", "Orange", "Pear")
var maxLengthFruit = ""
for (fruit in list) {
if (fruit.length > maxLengthFruit.length) {
maxLengthFruit = fruit
}
}
println("max length fruit is " + maxLengthFruit)
写法二:(使用函数式API写法)
val list = listOf<String>("Apple", "Banana", "Orange", "Pear")
val maxLengthFruit = list.maxBy { it.length }
println("max length fruit is " + maxLengthFruit)
{参数名1: 参数类型, 参数名2: 参数类型 -> 函数体}
val list = listOf<String>("Apple", "Banana", "Orange", "Pear")
//定义一个lambda表达式
val lambda = {fruit: String -> fruit.length}
//然后将这个lambda表达式作为参数传递给maxBy函数,maxBy函数的作用就是遍历集合,根据参数条件来找到最大值
val maxLengthFruit = list.maxBy(lambda)
val maxLengthFruit = list.maxBy({<!--{cke_protected}{C}%3C!%2D%2D%20%2D%2D%3E-->fruit: String -> fruit.length})val maxLengthFruit = list.maxBy(){<!--{cke_protected}{C}%3C!%2D%2D%20%2D%2D%3E-->fruit: String -> fruit.length}val maxLengthFruit = list.maxBy{<!--{cke_protected}{C}%3C!%2D%2D%20%2D%2D%3E-->fruit: String -> fruit.length}val maxLengthFruit = list.maxBy{<!--{cke_protected}{C}%3C!%2D%2D%20%2D%2D%3E-->fruit -> fruit.length}val maxLenght = list.maxBy{<!--{cke_protected}{C}%3C!%2D%2D%20%2D%2D%3E-->it.length} val list = listOf<String>("Apple", "Banana")
val newList = list.map { it.toUpperCase() }
for (fruit in newList) {
println(fruit)
}
val list = listOf<String>("Apple", "Banana", "Orange", "Pear", "Grape")
val newList = list.filter { it.length <= 5 }
.map { it.toUpperCase() }
for (fruit in newList) {
println(fruit)
}
val list = listOf<String>("Apple", "Banana", "Orange", "Pear", "Grape")
val anyResult = list.any {it.length <= 5}
val allResult = list.all { it.length <= 5 }
println("anyResult is" + anyResult + ", allResult is" + allResult)
