Kotlin 集合List 、Set、Map操作汇总

一、list 转成已逗号等分割的String

 val numbers = listOf("one", "two", "three", "four")
 println(numbers.joinToString(separator = " | ", prefix = "start: ", postfix = ": end"))

start: one | two | three | four: end

二、划分

  val numbers = listOf("one","two","three","four")
  val (match,rest)=numbers.partition {
     it.length>3 }
  println(match)
  println(rest)

[three, four]
[one, two]

三、 加减操作符

 var numbers = listOf(Person("张三",12), Person("李四",10))
    val plusList = numbers + Person("王五",12)
    numbers +=Person("王五",12)
    val minusList = numbers - Person("张三",12)
    println(plusList)
    println(numbers)
    println(minusList)

[Person(name=张三, age=12), Person(name=李四, age=10), Person(name=王五, age=12)]
[Person(name=张三, age=12), Person(name=李四, age=10), Person(name=王五, age=12)]
[Person(name=李四, age=10), Person(name=王五, age=12)]

四、分组

   val numbers = listOf("one", "two", "three", "four", "five")
    println(numbers.groupBy {
     it.first().uppercase() })
    println(numbers.groupBy(keySelector = {
    it.first()}, valueTransform = {
    it.uppercase()}))
    println(numbers.groupingBy {
     it.first() }.eachCount())

{O=[one], T=[two, three], F=[four, five]}
{o=[ONE], t=[TWO, THREE], f=[FOUR, FIVE]}
{o=1, t=2, f=2}

五、取集合的一部分

    val numbers = listOf("one", "two", "three", "four", "five", "six")
    println(numbers.takeWhile {
     !it.startsWith('f') })
    println(numbers.takeLastWhile {
     it != "three" })
    println(numbers.dropWhile {
     it.length == 3 })
    println(numbers.dropLastWhile {
     it.contains('i') })

[one, two, three]
[four, five, six]
[three, four, five, six]
[one, two, three, four]

六、Chunked分块

    val numbers = (0..13).toList()
    println(numbers.chunked(3))
    println(numbers.chunked(3) {
     it.sum() })

[[0, 1, 2], [3, 4, 5], [6, 7, 8], [9, 10, 11], [12, 13]]
[3, 12, 21, 30, 25]

七、按条件取单个元素

 val numbers = listOf("one", "two", "three", "four", "five", "six")
 //会发生异常
 println(numbers.first {
     it.length > 6 })
 //意见使用这个
 println(numbers.firstOrNull {
     it.length > 6 })

Exception in thread "main" java.util.NoSuchElementException: Collection contains no element matching the predicate.

八、随机取元素

  val numbers = listOf(1, 2, 3, 4)
  println(numbers.random())

九、 检测元素存在与否 contains() 和 in

 val numbers = listOf("one", "two", "three", "four", "five", "six")
    println(numbers.contains("four"))
    println("zero" in numbers)

    println(numbers.containsAll(listOf("four", "two")))
    println(numbers.containsAll(listOf("one", "zero")))

true
false
true
false

十、isEmpty() 和 isNotEmpty()

 val numbers = listOf("one", "two", "three", "four", "five", "six")
    println(numbers.isEmpty())
    println(numbers.isNotEmpty())

    val empty = emptyList<String>()
    println(empty.isEmpty())
    println(empty.isNotEmpty())

false
true
true
false

十一、排序

fun main(args: Array<String>) {
    
    val lengthComparator = Comparator {
     str1: String, str2: String -> str1.length - str2.length }
    println(listOf("aaa", "bb", "c").sortedWith(lengthComparator))
    println(Version(1, 2) > Version(1, 3))
    println(Version(2, 0) > Version(1, 5))
    println(listOf("aaa", "bb", "c").sortedWith(compareBy {
     it.length }))

}

class Version(val major: Int, val minor: Int) : Comparable<Version> {
    
    override fun compareTo(other: Version): Int = when {
    
        this.major != other.major -> this.major compareTo other.major
        this.minor != other.minor -> this.minor compareTo other.minor
        else -> 0
    }
}

[c, bb, aaa]
false
true
[c, bb, aaa]

十二、聚合操作

val numbers = listOf(5, 2, 10, 4)

    println("Count: ${numbers.count()}")
    println("Max: ${numbers.maxOrNull()}")
    println("Min: ${numbers.minOrNull()}")
    println("Average: ${numbers.average()}")
    println("Sum: ${numbers.sum()}")

    val simpleSum = numbers.reduce {
     sum, element -> sum + element }
    println(simpleSum)
    val sumDoubled = numbers.fold(0) {
     sum, element -> sum + element * 2 }
    println(sumDoubled)

Count: 4
Max: 10
Min: 2
Average: 5.25
Sum: 21
21
4

十三、删除元素

    val numbers = mutableListOf(1, 2, 3, 4, 3)
    numbers.remove(3)                    // 删除了第一个 `3`
    println(numbers)
    numbers.remove(5)                    // 什么都没删除
    println(numbers)

[1, 2, 4, 3]
[1, 2, 4, 3]

十四、按索引取元素

 val numbers = listOf(1, 2, 3, 4)
    println(numbers.get(0))
    println(numbers[0])
    //numbers.get(5)                         // exception!
    println(numbers.getOrNull(5))             // null
    println(numbers.getOrElse(5, {
    it}))        // 5

1
1
null
5

十五、Comparator 二分搜索 排序

fun main(args: Array<String>) {
    
    val numbers = mutableListOf("one", "two", "three", "four")
    numbers.sort()
    println(numbers)
    println(numbers.binarySearch("two"))  // 3
    println(numbers.binarySearch("z")) // -5
    println(numbers.binarySearch("two", 0, 2))  // -3
    val productList = listOf(
        Person("WebStorm", 49),
        Person("AppCode", 49),
        Person("DotTrace", 129),
        Person("ReSharper", 14)
    )

    println(
        productList.binarySearch(
            Person("AppCode", 99),
            compareBy<Person> {
     it.age }.thenBy {
     it.name })
    )
    println(  productList.sortedWith(compareBy<Person> {
     it.age }.thenBy {
     it.name }))
}

data class Person(var name: String, var age: Int)

[four, one, three, two]
3
-5
-3
1
[Person(name=ReSharper, age=14), Person(name=AppCode, age=49), Person(name=WebStorm, age=49), Person(name=DotTrace, age=129)]

十六、Set 交集

    val numbers = setOf("one", "two", "three")

    println(numbers union setOf("four", "five"))
    println(setOf("four", "five") union numbers)

    println(numbers intersect setOf("two", "one"))
    println(numbers subtract setOf("three", "four"))
    println(numbers subtract setOf("four", "three")) // 相同的输出

[one, two, three, four, five]
[four, five, one, two, three]
[one, two]
[one, two]
[one, two]

Map

参考连接: https://book.kotlincn.net/text/collections-overview.html