Kotlin 集合常用API

collections-diagram.png

       //只读,没有add,remove等方法
        val list1 = listOf<String>("one", "two", "three", "four", "five", "six")
        val set1 = setOf<String>("one", "two", "three", "four", "five", "six")
        val map1 = mapOf<String, Int>("one" to 1, "two" to 2)
        //可写  mutable
        val map2 = mutableMapOf<String, Int>().apply { this["one"] = 1;this["two"] = 2 }

        //空集合
        val emptyList = emptyList<String>()
        val emptySet = emptySet<String>()
        val emptyMap = emptyMap<String, Int>()

        //具体类型集合
        val arrayList: ArrayList<String>? = arrayListOf()

        //复制集合,副本和源数据不关联,删除副本集合数据,不影响源数据
        val copyList = arrayList?.toList()

        //集合遍历

        for ((key, value) in map1) {
            println("key = $key value = $value")
        }
        //API 24
//        map1.forEach { key, value -> println("key = $key value = $value") }

        val iterator = list1.iterator()
//        list1.listIterator()
        while (iterator.hasNext()) {
            val next = iterator.next()
        }

        for (index in list1.indices) {
            println("index = $index")
        }
        for (msg in list1) {
            println("msg = $msg")
        }
        list1.forEach { println(it) }

        //区间
        for (index in 0..10) {
            //[0,10]
            println("index = $index")
        }
        for (index in 0 until 10) {
            //[0,10)
            println("index = $index")
        }
        for (index in 0 until 10 step 2) {
            //[0,10) ,相当于 i=i+2 , 默认i++
            println("index = $index")
        }
        for (index in 10 downTo 1) {
            //[10,1] 倒序
            println("index = $index")
        }
        //map
        list1.mapIndexed { index, s -> println("index = $index msg = $s") }
        list1.mapIndexedNotNull { index, s -> println("index = $index msg = $s") }
        map1.mapKeys { println("key = ${it.key.toUpperCase()}") }
        map1.mapValues { println("key = ${it.value}") }

        //zip
        val colors = listOf("red", "brown", "grey")
        val animals = listOf("fox", "bear", "wolf")
        println(colors.zip(animals) { color, animal -> "The ${animal.capitalize()} is $color" })
        //[The Fox is red, The Bear is brown, The Wolf is grey]

        val numberPairs = listOf("one" to 1, "two" to 2, "three" to 3, "four" to 4)
        println(numberPairs.unzip())
        //([one, two, three, four], [1, 2, 3, 4])

        //joinToString()
        val numbers = listOf("one", "two", "three", "four")

        println(numbers)
        println(numbers.joinToString())

        val listString = StringBuffer("The list of numbers: ")
        numbers.joinTo(listString)
        println(listString)
        //[one, two, three, four]
        //one, two, three, four
        //The list of numbers: one, two, three, four

        val numbers2 = listOf("one", "two", "three", "four")
        println(numbers2.joinToString(separator = " | ", prefix = "start: ", postfix = ": end"))
        //start: one | two | three | four: end

        val numbers3 = (1..100).toList()
        println(numbers3.joinToString(limit = 10, truncated = "<...>"))
        //1, 2, 3, 4, 5, 6, 7, 8, 9, 10, <...>

        val numbers4 = listOf("one", "two", "three", "four")
        println(numbers4.joinToString { "Element: ${it.toUpperCase()}" })
        //Element: ONE, Element: TWO, Element: THREE, Element: FOUR

        //过滤
        val numbers5 = listOf("one", "two", "three", "four")

        val filteredIdx = numbers5.filterIndexed { index, s -> (index != 0) && (s.length < 5) }
        val filteredNot = numbers5.filterNot { it.length <= 3 }
        //[two, four]   filteredIdx
        //[three, four]  filteredNot 长度不大于3

        val numbers6 = listOf("one", "two", "three", "four")
        val (match, rest) = numbers6.partition { it.length > 3 }
        //[three, four]
        //[one, two]

        val numbers7 = listOf("one", "two", "three", "four")

        println(numbers7.any { it.endsWith("e") })//true,任何一个值以e结尾
        println(numbers7.none { it.endsWith("a") })//true,没有一个值以a结尾
        println(numbers7.all { it.endsWith("e") })//false,所有值都是以e结尾

        //分组
        val numbers8 = listOf("one", "two", "three", "four", "five")
        println(numbers8.groupBy(keySelector = { it.first() }, valueTransform = { it.toUpperCase() }))
        //{o=[ONE], t=[TWO, THREE], f=[FOUR, FIVE]}
        val numbers9 = listOf("one", "two", "three", "four", "five", "six")
        println(numbers9.groupingBy { it.first() }.eachCount())
        //{o=1, t=2, f=2, s=1}

        //取集合一部分
        //slice
        val numbers10 = listOf("one", "two", "three", "four", "five", "six")
        println(numbers10.slice(1..3))//[two, three, four]
        println(numbers10.slice(0..4 step 2))//[one, three, five]
        println(numbers10.slice(setOf(3, 5, 0)))//[four, six, one]

        //take drop
        val numbers11= listOf("one", "two", "three", "four", "five", "six")
        println(numbers11.take(3))//[one, two, three] 前三个  takeWhile 进行过滤
        println(numbers11.takeLast(3))//[four, five, six] 后三个
        println(numbers11.drop(1))//[two, three, four, five, six] 放弃第一个
        println(numbers11.dropLast(5))//[one] 从后往前数,放弃后面5个

        //chunked
        val numbers12 = (0..13).toList()
        println(numbers12.chunked(3))
        //[[0, 1, 2], [3, 4, 5], [6, 7, 8], [9, 10, 11], [12, 13]]
        println(numbers12.chunked(3) { it.sum() })
        //[3, 12, 21, 30, 25]

        //windowed
        val numbers13 = listOf("one", "two", "three", "four", "five")
        println(numbers13.windowed(3))
        //[[one, two, three], [two, three, four], [three, four, five]]

        //zip
        println(numbers.zipWithNext())
        //[(one, two), (two, three), (three, four), (four, five)]

        //取单个元素
        val numbers14 = listOf("one", "two", "three", "four", "five")
        println(numbers.elementAt(3))//four,会抛出数组角标越界
        println(numbers14.elementAtOrNull(5))//null
        println(numbers14.elementAtOrElse(5) { index -> "The value for index $index is undefined"})
        //The value for index 5 is undefined
        //随机获取元素
        println(numbers.random())
        //是否存在 in contains()

        //排序,根据首字母自然排序
        val numbers15 = listOf("one", "two", "three", "four")
        println("Sorted ascending: ${numbers15.sorted()}")
        //Sorted ascending: [four, one, three, two]

        //自然排序倒序
        println("Sorted descending: ${numbers15.sortedDescending()}")
        //Sorted descending: [two, three, one, four]

        //自定义排序
        val sortedNumbers = numbers15.sortedBy { it.length }
        println("Sorted by length ascending: $sortedNumbers")
        //Sorted by length ascending: [one, two, four, three]
        val sortedByLast = numbers.sortedByDescending { it.last() }
        println("Sorted by the last letter descending: $sortedByLast")
        //Sorted by the last letter descending: [four, two, one, three] 根据尾字母倒序

        //倒序
        println(numbers.reversed())//[four, three, two, one]
        //随机排序
        println(numbers.shuffled())

        val numbers16 = mutableListOf("one", "two", "three", "four")
        val reversedNumbers = numbers16.asReversed()
        println(reversedNumbers)//[four, three, two, one]
        numbers16.add("five")
        println(reversedNumbers)//[five, four, three, two, one]

        //聚合操作
        val numbers17 = listOf(6, 42, 10, 4)

        println("Count: ${numbers17.count()}")//Count: 4
        println("Max: ${numbers17.max()}")//Max: 42
        println("Min: ${numbers17.min()}")//Min: 4
        println("Average: ${numbers17.average()}")//Average: 15.5
        println("Sum: ${numbers17.sum()}")//Sum: 62

        val numbers18 = listOf(5, 42, 10, 4)
        val min3Remainder = numbers18.minBy { it % 3 }
        println(min3Remainder)//42

        val strings = listOf("one", "two", "three", "four")
        val longestString = strings.maxWith(compareBy { it.length })
        println(longestString)//three

        val numbers19 = listOf(5, 42, 10, 4)
        println(numbers19.sumBy { it * 2 })//122
        println(numbers19.sumByDouble { it.toDouble() / 2 })//30.5