Kotlin系列之常用操作符

前言：

本篇文章来介绍下Kotlin的常用操作符。kotlin相对于java来说是一门十分简洁的语言，这其中操作符起了很大的作用，这些操作符和Rxjava的类似，如果你熟悉Rxjava，那么这些操作符学起来就得心应手很多，毕竟这些操作符还是挺简单的说。。。

常用操作符大致可以分为

• 总数操作符
• 过滤操作符
• 映射操作符
• 元素操作符
• 生产操作符
• 顺序操作符

首先我想说的是，学习这些操作符应该从以下几个方面进行
- 敲：在开发工具里面敲这些代码
- 看：点击去看看操作符的源码
- 跑：亲自跑一下代码，看看运行的结果
- 思：综合思考这些操作符的意义，加深理解

- 总数操作符

private val list= listOf<Int>(0,1,2,3,4,5,6,7,8,9)//any 只要有一个符合就返回trueval any= list.any { it>8 }//all 所有条件符合才返回trueval all= list.all { it>0 }//count 返回符合条件的数目val count= list.count { it>5 }//none 如果没有任何元素与给定的函数匹配，则返回trueval none= list.none{it>10}//fold 在一个初始值的基础上 从第一项到最后一项通过 一个函数操作 所有的元素。//下面是初始值4 每项进行累加val fold= list.fold(4){total,next->total+next}//foldRight与fold一样，但是顺序是从最后一项到第一项。注意与fold的区别，参数位置调过来了val foldRight=list.foldRight(4) { next, total -> total + next }//reduce 从第一项到最后一项通过 一个函数操作 所有的元素，相对于fold，没有初始值//reduceRight 是从后到前val reduce= list.reduce { acc, i -> acc+i }//forEach 遍历每个元素并且进行操作val foreach= list.forEach { println(it) }//forEachIndexed 与foreach相同，但是可以得到indexval forEachIndexed= list.forEachIndexed { index, value -> println("$index -> $value")  }//max 返回最大的值，如果没有则返回null min同val max=list.max()//maxBy 根据指定的函数返回最大值 minBy同val maxBy=list.maxBy { -it }//sumBy 每项经过函数转换后的和val sumBy=list.sumBy { it+9 }

- 过滤操作符

private val list= listOf<Int>(0,1,2,3,4,5,6,5,4,3,2,1,0)/** * drop 返回包含去掉前n个元素的所有元素的列表 * Returns a list containing all elements except first [n] elements. * 返回[4, 5, 6, 5, 4, 3, 2, 1, 0] */val drop= list.drop(4)/** * dropwhile 根据特定的函数 从第一项开始 直到不满足条件后返回 列表 * Returns a list containing all elements except first elements that satisfy the given [predicate]. * 返回[0, 1, 2, 3, 4, 5, 6, 5, 4, 3, 2, 1, 0] */val dropwhile=list.dropWhile {it > 1 }/** * dropLastWhile 返回根据特定的函数 从最后一项开始 直到不满足条件后返回 列表 * Returns a list containing all elements except last elements that satisfy the given [predicate]. * 返回[0, 1, 2, 3, 4, 5, 6, 5, 4, 3, 2, 1, 0] */val dropLastWhile= list.dropLastWhile { it>4 }/** *filter 返回所有符合给定函数条件的元素。 * Returns a list containing only elements matching the given [predicate]. * [5, 6, 5] */val filter=list.filter { it>4 }/** * filterNot 返回所有不符合给定函数条件的元素 * Returns a list containing all elements not matching the given [predicate]. * [0, 1, 2, 3, 4, 4, 3, 2, 1, 0] */val filterNot=list.filterNot { it>4 }/** * filterNotNull 返回非null元素 * Returns a list containing all elements that are not `null`. * [0, 1, 2, 3, 4, 5, 6, 5, 4, 3, 2, 1, 0] */val filterNotNull= list.filterNotNull()/** * 返回满足该ranger的元素集合 * Returns a list containing elements at indices in the specified [indices] range. * [0, 1, 2, 3, 4, 5, 6] */val slice= list.slice(0..6)/** * listOf(0,4,7)是集合list的坐标 * Returns a list containing elements at specified [indices]. * [0, 4, 5] */val slice2= list.slice(listOf(0,4,7))/** *返回前n项 * Returns a list containing first [n] elements. * [0, 1, 2, 3] */val take= list.take(4)/** * 返回后n项 * Returns a list containing last [n] elements. * [3, 2, 1, 0] */val takeLast= list.takeLast(4)/** * 从第一项开始判断，直到不符合就返回，返回符合的前几项数据 * Returns a list containing first elements satisfying the given [predicate]. * [] */val takeWhile= list.takeWhile { it>4 }

- 映射操作符

private val list= listOf(0,1,2,3,4,5,4,3,2,1,0,-1)/** * 返回满足条件的集合 * Returns a list containing the results of applying the given [transform] function * to each element in the original collection. * [false, false, false, true, true, true, true, true, false, false, false, false] */val map=list.map { it>2 }/** * 返回特定函数后的集合，参数是Iterable类型， * Returns a single list of all elements yielded from results of [transform] * function being invoked on each element of original collection. * [0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 4, 5, 3, 4, 2, 3, 1, 2, 0, 1, -1, 0] */val flatMap=list.flatMap { listOf(it, it + 1) }/** * 根据函数将集合分组，返回map类型对象 * Groups elements of the original collection by the key returned by the given [keySelector] function * applied to each element and returns a map where each group key is associated with a list of corresponding elements. * * The returned map preserves the entry iteration order of the keys produced from the original collection. * {false=[0, 1, 2, 3, 3, 2, 1, 0, -1], true=[4, 5, 4]} * * @sample samples.collections.Collections.Transformations.groupBy */val groupBy=list.groupBy {value-> value>3 }/** * 返回一个集合，通过 角标和值 来生成 * Returns a list containing the results of applying the given [transform] function * to each element and its index in the original collection. * @param [transform] function that takes the index of an element and the element itself * and returns the result of the transform applied to the element. * [0, 1, 2, 3, 4, 5, 4, 3, 2, 1, 0, -1] */val mapIndexed=list.mapIndexed { index, value ->value}/** * 返回一个每一个非null元素根据给定的函数转换所组成的List * Returns a list containing only the non-null results of applying the given [transform] function * to each element in the original collection. * [0, 2, 4, 6, 8, 10, 8, 6, 4, 2, 0, -2] */val mapNotNull=list.mapNotNull { it*2 }

- 元素操作符

private val list= listOf(0,1,2,3,4,5,6,4,3,2,1,0,-1)//如果指定元素可以在集合中找到，则返回true。val contains=list.contains(2)/** * 返回给定index对应的元素，如果index数组越界则会 抛出IndexOutOfBoundsException * Returns an element at the given [index] or throws an [IndexOutOfBoundsException] if the [index] is out of bounds of this collection. * 2 */val elementAt=list.elementAt(2)/** * 返回给定index对应的元素，如果index数组越界则会根据给定函数返回默认值,第二个参数default，lamdba表达式 * Returns an element at the given [index] or the result of calling the [defaultValue] * function if the [index] is out of bounds of this collection. * 2 */val elementAtOrElse=list.elementAtOrElse(2){"error"}/** * 返回给定index对应的元素，如果index数组越界则会 返回null * Returns an element at the given [index] or `null` if the [index] is out of bounds of this list. * null */val elementAtOrNull=list.elementAtOrNull(19)/** * Returns first element. * @throws [NoSuchElementException] if the list is empty. * 0 */val first=list.first()/** * 返回符合给定函数条件的第一个元素,没有回抛异常 * Returns the first element matching the given [predicate]. * @throws [NoSuchElementException] if no such element is found. * 4 */val first2=list.first { it>3 }/** * 返回符合给定函数条件的第一个元素，如果没有符合则返回null * Returns the first element matching the given [predicate], or `null` if element was not found. * null */val firstOrNull=list.firstOrNull { it>9 }/** * 返回指定元素的第一个index，如果不存在，则返回-1 * Returns the index of the first occurrence of the specified element in the list, or -1 if the specified * element is not contained in the list. * 3 */val indexOf=list.indexOf(3)/** * 返回第一个符合给定函数条件的元素的index，如果没有符合则返回-1 * Returns index of the first element matching the given [predicate], or -1 if the list does not contain such element. * 0 */val indexOfFirst=list.indexOfFirst { it%3==0 }/** * 返回最后一个符合给定函数条件的元素的index，如果没有符合则返回-1 * Returns index of the last element matching the given [predicate], or -1 if the list does not contain such element. * 11 */val indexOfLast=list.indexOfLast { it%3==0 }/** * 返回符合给定函数条件的最后一个元素,没有抛异常 * Returns the last element matching the given [predicate]. * @throws [NoSuchElementException] if no such element is found. * 6 */val last=list.last { it>4 }/** * 返回指定元素的最后一个index，如果不存在，则返回-1 * Returns the index of the last occurrence of the specified element in the list, or -1 if the specified * element is not contained in the list. * 8 */val lastIndexOf=list.lastIndexOf(3)/** * 返回符合给定函数条件的最后一个元素，如果没有符合则返回null * Returns the last element matching the given [predicate], or `null` if no such element was found. * null */val lastOrNull=list.lastOrNull { it>8 }/** * 返回符合给定函数的单个元素，如果没有符合或者超过一个，则抛出异常 * Returns the single element matching the given [predicate], or throws exception if there is no or more than one matching element. * 6 */val single=list.single { it>5 }/** * 返回符合给定函数的单个元素，如果没有符合或者超过一个，则返回null * Returns the single element matching the given [predicate], or `null` if element was not found or more than one element was found. * null */val singleOrNull=list.singleOrNull { it>8 }

- 生产操作符

private val list1= listOf(0,1,2,3,4,5)private val list2= listOf(4,5,2,1,5)/** * Returns a list of pairs built from elements of both collections with same indexes. List has length of shortest collection. * [(0, 4), (1, 5), (2, 2), (3, 1), (4, 5)] */val zip= list1.zip(list2)/** * Returns a list of values built from elements of both collections with same indexes using provided [transform]. List has length of shortest collection. * [4, 6, 4, 4, 9] */val zip2= list1.zip(list2){it1,it2->it1+it2}//[(0, 0), (1, 1), (2, 2), (3, 3), (4, 4), (5, 5)]val zip4= list1.zip(list1+list2)//[0, 2, 4, 6, 8, 10]val zip3= list1.zip(list1+list2){it1,it2->it1+it2}/** * Returns a pair of lists, where * *first* list is built from the first values of each pair from this collection, * *second* list is built from the second values of each pair from this collection. * ([1, 3, 7], [3, 4, 8]) */val unzip= listOf(Pair(1,3),Pair(3,4),Pair(7,8)).unzip()/** * 把一个给定的集合分割成两个，第一个集合是由原集合每一项元素匹配给定函数条件返回true的元素组成， * 第二个集合是由原集合每一项元素匹配给定函数条件返回false的元素组成 * Splits the original collection into pair of lists, * where *first* list contains elements for which [predicate] yielded `true`, * while *second* list contains elements for which [predicate] yielded `false`. * ([0, 2, 4], [1, 3, 5])  value */val partition= list1.partition { it%2==0 }/** * 返回一个包含原集合和给定集合中所有元素的集合，因为函数的名字原因，我们可以使用+操作符。 * Returns a list containing all elements of the original collection and then all elements of the given [elements] sequence. * [0, 1, 2, 3, 4, 5, 4, 5, 2, 1, 5] */val plus=list1.plus(list2)

- 顺序操作符

private val list= listOf(1,8,9,4,5,6,4,5,3,8,5)/** * 返回一个与指定list相反顺序的list * Returns a list with elements in reversed order. */val reverse=list.reversed()/** * 返回一个自然排序后的list * Returns a list of all elements sorted according to their natural sort order. */val sort=list.sorted()/** * 返回一个根据指定函数排序后的list * Returns a list of all elements sorted according to natural sort order of the value returned by specified [selector] function. */val sortBy=list.sortedBy { it>3 }/** * 返回一个降序排序后的List * Returns a list of all elements sorted descending according to their natural sort order. */val sortDescending=list.sortedDescending()/** * 返回一个根据指定函数降序排序后的list * Returns a list of all elements sorted descending according to natural sort order of the value returned by specified [selector] function. */val sortDescendingBy=list.sortedByDescending { it>4 }

再来看看下面几个更加常用和功能更加强大的操作符

data class Person(val name:String="wy",val age:Int=10)//let  fun function(){      val  person=Person()      person.let {          person.name          person.age    }    person.apply {        name        age    }    with(person){        name        age    }      with(person){          person.apply {              name              age          }      }      BufferedReader(FileReader("build.gradle")).use {      }}

以上几个操作符我并不想做太多的解释，相信如果你掌握了前面的几类操作符之后，再查看这些操作符的源码后肯定能快速掌握，毕竟这些真的不是什么晦涩难懂的东西。