1 前言

        序列（Sequence）是 Kotlin 中为方便操作集合及其元素而定制的接口，是一个延迟获取数据的集合，只有需要元素时才会生产元素。在处理大量数据时，序列可以显著地提升性能。

        Sequence 类似 Java 中的 Stream，详见 → Stream 总结。Sequence 有中间操作和终端操作，如下。

• 中间操作：每次操作返回一个新的 Sequence 对象（主要操作有：filter、distinct、drop、take、sorted、sortedDescending、map、mapIndexed、minus 等）。
• 终端操作：每次操作返回一个值或集合，每个序列只能进行一次终端操作（主要操作有：forEach、joinToString、min、max、count、sum、average、any、first、last、reduce 等）。

2 Sequence 的创建

2.1 sequenceOf

var sequence = sequenceOf(1, 2, 3)

2.2 asSequence

        1）IntRange

var intRange = 1..10
var sequence = intRange.asSequence()

         2）Array

var array = arrayOf(1, 2, 3)
var sequence = array.asSequence()

        3）List

var list = listOf("AA", "BBB", "CC")
var sequence = list.asSequence()

        4）Set

var set = setOf("AA", "BBB", "CC")
var sequence = set.asSequence()

2.3 BufferedReader

val reader = BufferedReader(FileReader("G:\\stream.txt"))
val sequence = reader.lineSequence()

2.4 generateSequence

var sequence = generateSequence(arrayOf(1, 1)) { arr ->arrayOf(arr[1], arr[0] + arr[1])}.take(10).map { arr -> arr[0] }
// 1, 1, 2, 3, 5, 8, 13, 21, 34, 55
println(sequence.joinToString())

3 Sequence 的中间操作

3.1 主要接口

// 过滤
public fun <T> Sequence<T>.filter(predicate: (T) -> Boolean): Sequence<T>
// 去重
public fun <T> Sequence<T>.distinct(): Sequence<T>
// 丢弃前 n 个元素
public fun <T> Sequence<T>.drop(n: Int): Sequence<T>
// 截取前 n 个元素
public fun <T> Sequence<T>.take(n: Int): Sequence<T>
// 排序(升序)
public fun <T : Comparable<T>> Sequence<T>.sorted(): Sequence<T>
// 排序(降序)
public fun <T : Comparable<T>> Sequence<T>.sortedDescending(): Sequence<T>
// 映射(T -> R)
public fun <T, R> Sequence<T>.map(transform: (T) -> R): Sequence<R>
// 映射(index, T -> R)
public fun <T, R> Sequence<T>.mapIndexed(transform: (index: Int, T) -> R): Sequence<R>
// 删除序列中第一个 element
public operator fun <T> Sequence<T>.minus(element: T): Sequence<T>

3.2 案例

fun main() {var sequence = sequenceOf(4, 9, 1, 8, 5, 5, 7, 3, 6, 2)sequence.filter { e -> e > 2 && e < 8 } // 4 5 5 7 3 6.distinct() // 4 5 7 3 6.drop(1) // 5 7 3 6.take(3) // 5 7 3.sorted() // 3 5 7.map { e -> e * e } // 9 25 49.forEach(::println)
}

4 Sequence 的终端操作

4.1 主要接口

        1）统计函数

// 最小值
public fun <T : Comparable<T>> Sequence<T>.min(): T?
// 最大值
public fun <T : Comparable<T>> Sequence<T>.max(): T?
// 元素个数
public fun <T> Sequence<T>.count(): Int
// 求和
public fun Sequence<Int>.sum(): Int
// 求平均值
public fun Sequence<Int>.average(): Double
// 序列中是否有元素
public fun <T> Sequence<T>.any(): Boolean
// 获取第一个元素
public fun <T> Sequence<T>.first(): T
// 获取最后一个元素
public fun <T> Sequence<T>.last(): T

        2）遍历元素

// 遍历元素
public inline fun <T> Sequence<T>.forEach(action: (T) -> Unit): Unit

        3）拼接元素

// 转换为字符串
public fun <T> Sequence<T>.joinToString(separator: CharSequence = ", ", prefix: CharSequence = "", postfix: CharSequence = "", limit: Int = -1, truncated: CharSequence = "...", transform: ((T) -> CharSequence)? = null): String

         4）规约运算

// 规约运算，定义运算 o, result = ((((e1 o e2)) o e3) o e4) o ...
public inline fun <S, T : S> Sequence<T>.reduce(operation: (acc: S, T) -> S): S

        5）集合转换

public fun <T> Sequence<T>.toList(): List<T>
public fun <T> Sequence<T>.toMutableList(): MutableList<T>
public fun <T> Sequence<T>.toSet(): Set<T>
public fun <T> Sequence<T>.toMutableSet(): MutableSet<T>
public fun <T> Sequence<T>.toHashSet(): HashSet<T>
public fun <T : Comparable<T>> Sequence<T>.toSortedSet(): java.util.SortedSet<T>

4.2 案例

        1）统计函数

fun main() {var sequence = sequenceOf(1, 3, 5)var min = sequence.min() // 1var max = sequence.max() // 5var count = sequence.count() // 3var sum = sequence.sum() // 9var avg = sequence.average() // 3var hasElement = sequence.any() // truevar first = sequence.first() // 1var last = sequence.last() // 5
}

        2）遍历元素

fun main() {var sequence = sequenceOf(1, 3, 5)sequence.forEach(::println) // 1、3、5
}

        3）拼接元素

fun main() {var sequence = sequenceOf(1, 3, 5)var str = sequence.joinToString(",", "[", "]")
}

        4）规约运算

fun main() {var sequence = sequenceOf(1, 3, 5)var sum = sequence.reduce(Integer::sum) // 9// 1*1-3*3=-8, (-8)*(-8)-5*5=39var res = sequence.reduce { e1, e2 -> e1 * e1 - e2 * e2 } // 39
}

        5）集合转换

fun main() {val sequence = sequenceOf(1, 2, 3)var list = sequence.toList()var mutableList = sequence.toMutableList()var set = sequence.toSet()var mutableSet = sequence.toMutableSet()var hashSet = sequence.toHashSet()var sortedSet = sequence.toSortedSet()
}