Kotlin 常用操作符.md

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true
val any= list.any { it>8 }

//all 所有条件符合才返回true
val 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true
val 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index
val 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

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 {
    
      }

}