集合

1、使用數(shù)組

arrayOf ,并使用Array類(lèi)中的屬性與方法來(lái)處理他們內(nèi)部的值
java中：

String [] strings = new String[4];
strings[0] = "an";
strings[1] = "array";
strings[2] = "of";
strings[3] = "strings";
//或者
strings = "an array of strings".split(" ");

kotlin中：

val strings = arrayOf("this","is","an","array","of","strings")

arrayOfNulls創(chuàng)建僅包含空值的數(shù)組，必須指定類(lèi)型

val nullStringArray = arrayOfNulls<String>(5)//一個(gè)僅僅包含空值的數(shù)組

Array類(lèi)

val squares = Array(5){i -> (i*i).toString()}
//結(jié)果為{"0","1","4","9","16"}的數(shù)組

withIndex訪問(wèn)數(shù)組

val strings = arrayOf("this","is","an","array","of","strings")
for((index,value) in strings.withIndex()){
    pringln("Index $index maps to $value")
}

Index 0 maps to this
Index 1 maps to is
Index 2 maps to an
Index 3 maps to array
Index 4 maps to of
Index 5 maps to strings

2、創(chuàng)建集合

不可變的集合：listOf、setOf、mapOf
可變版本：mutableListOf、mutableSetOf、mutableMapOf

var numList = listOf(3,1,4,1,5,9)
var numSet = setOf(3,1,4,1,5,9)//numSet.size == 5
var map = mapOf(1 to "one",2 to "two",3 to "three")

var numList = mutableListOf(3,1,4,1,5,9)
var numSet = mutableSetOf(3,1,4,1,5,9)
var map = mutableMapOf(1 to "one",2 to "two",3 to "three")

3、為已存在的集合創(chuàng)建只讀視圖

如何為現(xiàn)有可變list/map/set創(chuàng)建只讀版本。
將值賦值給List、Set、Map即可

val muNums = mutableListOf(3,1,4,1,5,9)

val onlyReadNum :List<Int> = muNums .toList()

該方法創(chuàng)建了一個(gè)單獨(dú)的對(duì)象，內(nèi)容相同但不代表是相同對(duì)象。
如果想要一個(gè)相同對(duì)象的只讀視圖，將可變list賦值給一個(gè)List類(lèi)的引用即可：

val readOnlySameList :List<Int> = muNums 

如果指向的list被修改，只讀視圖也會(huì)顯示修改后的值。

4、從集合構(gòu)建map

如何通過(guò)每個(gè)鍵生成，鍵值關(guān)聯(lián)起來(lái)的map？
associateWith函數(shù)

val keys = 'a'..'f'
val map = key.associateWith{it to it.toString().repeat(5).capitalize()}
println(map)

{a=(a, Aaaaa), b=(b, Bbbbb), c=(c, Ccccc), d=(d, Ddddd), e=(e, Eeeee), f=(f, Fffff)}
//如果是老版本的kotlin輸出如下
{a=Aaaaa, b=Bbbbb, c=Ccccc, d=Ddddd, e=Eeeee, f=Fffff}

在kotlin 1.3中的associateWith簡(jiǎn)化了代碼，生成了String值，而不是生成Pair<Char,String>

val keys = 'a'..'f'
val map = key.associateWith{it.toString().repeat(5).capitalize()}
println(map)

{a=Aaaaa, b=Bbbbb, c=Ccccc, d=Ddddd, e=Eeeee, f=Fffff}

5、當(dāng)集合為空的時(shí)候返回默認(rèn)值

使用ifEmpty 和ifBlank

data class Product(val name:String,
                  val price:Double,
                   val onSale:Boolean = false)
//     val products = listOf(pA,pB,pC)
val products = listOfNotNull<Product>()
println(nameOfP1(products))

 fun nameOfP1(products :List<Product>) = 
        products.filter{it.onSale}
            .map{it.name}
            .joinToString(separator = ",")
            
fun nameOfP2(products :List<Product>) = 
        products.filter{it.onSale}
            .map{it.name}
            .ifEmpty{listOf("none")}  //默認(rèn)空集合
            .joinToString(separator = ",")
            
fun nameOfP3(products :List<Product>) = 
        products.filter{it.onSale}
            .map{it.name}
            .joinToString(separator = ",")
            .ifEmpty{"none"}  //默認(rèn)空字符串

1結(jié)果只是換行了
23結(jié)果會(huì)輸出none

6、將變量限制在給定區(qū)間

給定一個(gè)值，在區(qū)間內(nèi)返回本身，在區(qū)間外返回最小值和最大值。
使用coerceIn函數(shù)，并傳入最小值和最大值。

    val range = 3..8
    println(5.coerceIn(range))  //5
    println(5.coerceIn(3,8))    //5
    
    println(1.coerceIn(range))  //3
    println(1.coerceIn(3,8))    //3

7、處理集合中的窗口

chunked：將集合切分為相同部分
windowed：將塊沿集合滑動(dòng)給定間隔的塊

chunked，切分集合為一個(gè)包含list的list，每個(gè)列表都具有給定大小或更小

fun main() {
    val range = 0..10
    val chunked = range.chunked(3)
    println("chunked="+chunked)

//chunked=[[0, 1, 2], [3, 4, 5], [6, 7, 8], [9, 10]]

    val chunked2 = range.chunked(3){it.average()}
    println("chunked2="+chunked2)
//chunked2=[1.0, 4.0, 7.0, 9.5]
}

chunked函數(shù)其實(shí)是windowed函數(shù)的特殊情況
windowed函數(shù)接收3個(gè)函數(shù) size（每個(gè)窗口的元素?cái)?shù)量）、step（每一步向前移動(dòng)的元素?cái)?shù)，默認(rèn)1）、partialwindows（默認(rèn)false，如果沒(méi)有足夠數(shù)量，是否保留最后一部分）

chunked(size:Int) ->return windowed(size,size,true)

8、解構(gòu)list

解構(gòu)是通過(guò)將對(duì)象的值分配給變量的集合來(lái)從中提取值的過(guò)程。

val list = listOf("a","b","c","d","e","f","g")
val (a,b,c,d,e) = list
println("$a $b $c $d $e")
//a b c d e

列表創(chuàng)建的前五個(gè)元素已經(jīng)分配給了相同名稱(chēng)變量。之所以可行是因?yàn)長(zhǎng)ist類(lèi)具有名為componentN的標(biāo)準(zhǔn)庫(kù)中定義的擴(kuò)展函數(shù)，其中N從1到5.

9、將多個(gè)屬性排序

sortedWith / compareBy

data class Golfer(val score:Int,val first:String,val last:String)

fun main() {   
    val golfers = listOf(
        Golfer(70,"jack","nicklaus"),
        Golfer(68,"tom","watson"),
        Golfer(68,"bubba","watson"),
        Golfer(70,"tiger","woods"),
        Golfer(68,"ty","webb"),
    )

    //score > lastname > first name
    val sorted = golfers.sortedWith(
        compareBy({it.score},{it.last},{it.first})
    )
    sorted.forEach{println(it)}
}
->
Golfer(score=68, first=bubba, last=watson)
Golfer(score=68, first=tom, last=watson)
Golfer(score=68, first=ty, last=webb)
Golfer(score=70, first=jack, last=nicklaus)
Golfer(score=70, first=tiger, last=woods)

另一種方式是使用thenBy

val comparator = compareBy<Golfer>(Golfer::score)
    .thenBy(Golfer::last)
    .thenBy(Golfer::first)

golfers.sortedWith(comparator )
    .forEach(::printLn)

10、自定義迭代器

11、根據(jù)類(lèi)型過(guò)濾集合

filterIsInstance / filterIsInstanceTo

kotlin集合中包含一個(gè)filter的擴(kuò)展函數(shù)，接受一個(gè)可以用來(lái)提取滿足任何布爾條件的元素謂詞作為參數(shù)

val list = listOf("a",LocalDate.now(),3,1,4,"b")
val strings = list.filter{it is String}

for(s in strings){
    s.length//不編譯，類(lèi)型被刪除
}

盡管過(guò)濾有效，但推定類(lèi)型為L(zhǎng)ist<Any>因此不會(huì)轉(zhuǎn)換為String.
可以添加filterIsInstance

val list = listOf("a",LocalDate.now(),3,1,4,"b")

val all= list.filterIsInstance<Any>()
val strings= list.filterIsInstance<String>()
val ints= list.filterIsInstance<Int>()
val datas= list.filterIsInstance<LocalDate::class.java>()

all -> list
strings ->"a","b"
ints - > 1,3,4
dates -> LocalDate.now()

filterIsInstance 返回類(lèi)型是List<R>
filterIsInstanceTo返回類(lèi)型是MutableCollection<inR>

val list = listOf("a",LocalDate.now(),3,1,4,"b")

val all= list.filterIsInstanceTo(mutableListOf())
val strings= list.filterIsInstanceTo(mutableListOf<String>())
val ints= list.filterIsInstanceTo(mutableListOf<Int>())
val datas= list.filterIsInstanceTo(mutableListOf<LocalDate>())

12、在數(shù)列中創(chuàng)建區(qū)間

kotlin使用雙點(diǎn)操作符可以創(chuàng)建區(qū)間如1..5,創(chuàng)建一個(gè)IntRange。kotlin的區(qū)間都是閉區(qū)間，即包含兩個(gè)端點(diǎn)。

import java.time.LocalDate
fun main() {
    val started = LocalDate.now()
    val mid = started.plusDays(3)
    val end = started.plusDays(5)
    
    val dataRange = started..end

    println(started in dataRange)//t
    println(mid in dataRange)//t
    println(end in dataRange)//t
    println(started.minusDays(1) in dataRange)//f
}

一旦嘗試在區(qū)間進(jìn)行迭代，會(huì)有編譯錯(cuò)誤

for(date in dateRange) println(it)//編譯錯(cuò)誤

問(wèn)題在于區(qū)間而不在于數(shù)列，自定義數(shù)列實(shí)現(xiàn)了iterable接口
需要自定義Iterator，接口覆蓋next和hasNext