1.LINQ-风格
val words = "A long time ago in a galaxy far far away".split(" ")
val shortWords = mutableListOf()
words.getShortWordsTo(shortWords, 3)
println(shortWords)//[ago, in, far, far]
getShortWordsTo
fun List.getShortWordsTo(shortWords: MutableList, maxLength: Int) {
this.filterTo(shortWords) { it.length <= maxLength }
val articles = setOf("a", "A", "an", "An", "the", "The")
shortWords -= articles //removeAll
}
上面逻辑看起来似乎不太直观
LINQ:
"A long time ago in a galaxy far far away"
.split(" ")
.filter { it.length <= 3 }
.toMutableList()
.let {
val articles = setOf("a", "A", "an", "An", "the", "The")
it.removeAll(articles)
it
}
.run { println(this) }
apply代替let let返回block() 而apply返回this
"A long time ago in a galaxy far far away"
.split(" ")
.filter { it.length <= 3 }
.toMutableList()
.apply {
removeAll(setOf("a", "A", "an", "An", "the", "The"))
}
.joinToString(separator = "-", prefix = "{", postfix = "}")
.run { println(this) }
2.kotlin标准库中内置的Iterable的扩展函数
准备容器数据
data class Product(val name: String, val price: Double, val isChecked: Boolean)
fun main() {
var list = ArrayList()
repeat(5) {
list.add(Product("gaom$it", it.toDouble(), it >= 2))
}
}
forEachIndexed
//带游标遍历 解构
list.forEachIndexed { index, (name, _, isChecked) -> println("$index={$name, $isChecked}") }
any/all/count
println(list.any { !it.isChecked })//任何item没有选中 , 则全选按钮置为 -未选中状态
println(list.all { it.isChecked })//所有item选中 , 则全选按钮置为 -选中状态
println(list.count { it.isChecked })// 选中状态商品数量
计算所选商品价格
println(
list
.filter { it.isChecked }
.map { it.price }//转换类型
// .reduce{} //reduce与fold类似,但是没有初始值
.fold(.0) { total, next -> total + next }
//fold 折叠 计算init为初始值 可传入任意类型
)
计算所选商品价格
println(list
.filter { it.isChecked }
.sumByDouble { it.price })
partition 按条件分割trueList,falseList
list.partition { it.isChecked }
.run {
//first list
println(first.map { it.price }
.fold(.0) { total, next -> total + next })
//second list
println(second.map { it.price }
.fold(.0) { total, next -> total + next }
)
}
操作符 & 中缀
//操作符 -=
val aList = mutableListOf(1, 2, 3)
val bList = mutableListOf(1, 2, 3, 4)
bList -= aList
aList + listOf(1, 2)
//中缀 zip
listOf(1, 2).zip(listOf(7, 8))//(1, 7) (2, 8)
listOf(1, 2) zip listOf(7, 8)
list转换成map associate函数
val numbers = listOf("one", "two", "three", "four")
println(numbers.associateWith { value -> value.length })//key默认是集合中的元素 , 要求返回一个value
//{one=3, two=3, three=5, four=4}
println(numbers.associateBy { key -> key.first().toUpperCase() })//value默认是集合中的元素 , 要求返回一个key
//{O=one, T=three, F=four}
println(
numbers.associateBy(
keySelector = { it.first().toUpperCase() },
valueTransform = { it.length })
)
//{O=3, T=5, F=4}
val names = listOf("Alice Adams", "Brian Brown", "Clara Campbell")
println(names.associate { name -> name.split(" ").let { it[0] to it[1] } })
//{Alice=Adams, Brian=Brown, Clara=Campbell}
去重
val distinctList = mutableListOf(0, 1, 2, 2, 3, 4, 4)
println(distinctList.distinct()) // [0, 1, 2, 3, 4]
//根据指定元素 而不是条件!!! it%2==0
println(distinctList.distinctBy { it })
distinctBy源码
public inline fun Iterable.distinctBy(selector: (T) -> K): List {
val set = HashSet()
val list = ArrayList()
for (e in this) {
val key = selector(e)
if (set.add(key))//HashSet作用就是去重复数据
list.add(e)
}
return list
}
产生的最大值的原始元素
val listMaxBy = listOf(1, 2, 2, 4)
println("maxBy=${listMaxBy.maxBy { -it }}")//maxBy=1
none
val listMaxBy = listOf(1, 2, 2, 4)
println("none=${listMaxBy.none { it==5 }}")//true
orEmpty
val listMaxBy = listOf(1, 2, 2, 4)
println("orEmpty=${listMaxBy.orEmpty()}")//orEmpty=[1, 2, 2, 4]
orEmpty源码
public inline fun List?.orEmpty(): List = this ?: emptyList()
slice 相当于 Java subList
val listMaxBy = listOf(1, 2, 2, 4)
println("slice1=${listMaxBy.slice(1..3)}")//slice1=[2, 2, 4]
println("slice2=${listMaxBy.slice(listOf(1,2))}")//slice2=[2, 2]
take 舍弃目标index之后的数据
val listMaxBy = listOf(1, 2, 2, 4)
println("take=${listMaxBy.take(3)}")//take=[1, 2, 2]
flatten 平铺
val numberSets1 = listOf(setOf(1, 2, 3), setOf(4, 5, 6), setOf(7, 8))
println("flatten=${numberSets1.flatten()}")//[1, 2, 3, 4, 5, 6, 7, 8]
rangeTo
val numbersToList = (1..100).toList()
joinToString
val numbersToList = (1..100).toList()
println("numbersToList=${numbersToList.joinToString(limit = 10, truncated = "<...>")}")
//1, 2, 3, 4, 5, 6, 7, 8, 9, 10, <...>
sequence 惰性序列操作
println(people.asSequence()
.map { it.name } //中间操作
.filter { it.startsWith("A") } //中间操作
.toList()) //末端操作
//找到第一个平方大于3的数，输出它的平方
println(listOf(1, 2, 3, 4).asSequence()
.map { it * it }
.find { it > 3 })
takeWhile
val list = listOf(1, 2, 3, 4, 5, 6, 7, 9)
list.takeWhile { it <= 3 }.forEach(::println)
解构声明
val (c, ca) = "param=car=c".split("=").reversed()
println("c=$c")
println("ca=$ca")
foreach的坑
fun forEach0() {
val list = listOf(1, 2, 3, 4)
list.forEach {
println("it=$it")
if (it == 3) return //break不能在这里使用
}
println("Hello") //如果这样return, 将会执行不到这里
//>>>it=1
//>>>it=2
//>>>it=3
}
fun forEach1() {
val list = listOf(1, 2, 3, 4)
list.forEach block@{
println("it=$it")
if (it == 3) return@block //手动加标记 //lambda依然执行
}
println("Hello")
//>>>it=1
//>>>it=2
//>>>it=3
//>>>it=4
//>>>Hello
}
fun forEach2() {
val list = listOf(1, 2, 3, 4)
list.forEach {
println("it=$it")
if (it == 3) return@forEach // forEach做为标记 //lambda依然执行
}
println("Hello")
//>>>it=1
//>>>it=2
//>>>it=3
//>>>it=4
//>>>Hello
}
fun forEach3() {
val list = listOf(1, 2, 3, 4)
list.let {
it.forEach {
println("it=$it")
if (it == 3) return@let //let方法做为标记 其他作用域函数皆可
}
}
println("Hello")
//>>>it=1
//>>>it=2
//>>>it=3
//>>>Hello
}
fun forEach4() {
val list = listOf(1, 2, 3, 4)
fun a() {
list.forEach {
println("it=$it")
if (it == 3) return@a //声明一个函数
}
}
a()
println("Hello")
//>>>it=1
//>>>it=2
//>>>it=3
//>>>Hello
}