【Kotlin】Kotlin笔记7-Lambda编程2(List,Map的函数式API)

矫情吗；* 2022-11-16 05:39 157阅读 0赞

### Kotlin笔记7-Lambda编程2-List,Map的函数式API ###

**4.2 Lambda编程2**

*  **Lambda表达式**

`Example:`

//list
    //最长水果单词
    fun findLongestFruit() {
        val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
        var maxLengthFruit = ""
        for (fruit in list) {
            if (fruit.length > maxLengthFruit.length)
                maxLengthFruit = fruit
        }
        println("max length fruit is $maxLengthFruit")
    }

*集合的函数式API*

fun findLongestFruit1() {
        //优化
        val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
        val maxLengthFruit = list.maxByOrNull { it.length }
        println("max length fruit is $maxLengthFruit")
    }

**Lambda表达式的语法结构**

{ 参数名1: 参数类型, 参数名2: 参数类型 -> 函数体 }

<table> 
 <thead> 
  <tr> 
   <th>序号</th> 
   <th>Tips</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>1</td> 
   <td>最后一行会作为Lambda的返回值</td> 
  </tr> 
 </tbody> 
</table>

`推演过程:`

val lambda = { fruit: String -> fruit.length }
    val maxLengthFruit = list.maxByOrNull(lambda)

`直接传入:`

val maxLengthFruit = list.maxByOrNull({ fruit: String -> fruit.length })

`移到括号外:`

val maxLengthFruit1 = list.maxByOrNull() { fruit: String -> fruit.length }

`lambda是函数唯一参数,省略括号:`

val maxLengthFruit2 = list.maxByOrNull { fruit: String -> fruit.length }

`类型推导:`

val maxLengthFruit3 = list.maxByOrNull { fruit -> fruit.length }

`唯一参数,it代替:`

> **it:** 唯一参数,it代替

val maxLengthFruit4 = list.maxByOrNull { it.length }

*  **Map函数API**

<table> 
 <thead> 
  <tr> 
   <th>序号</th> 
   <th>Tips</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>1</td> 
   <td>将集合的每个元素映射成一个另外的值</td> 
  </tr> 
 </tbody> 
</table>

`Example:`

fun getUpperFruits() {
        val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
        val newlist = list.map { it.toUpperCase() }
        for (fruit in newlist) {
            println(fruit)
        }
    }

> **filter:**

`Example:`

fun filterFruits() {
        val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
        val newlist = list.filter { it.length < 5 }
            .map { it.toUpperCase() }
        for (fruit in newlist) {
            println(fruit)
        }
    }

> **any:** 是否存在一个元素满足指定条件

val anyResult = list.any { it.length <= 5 }

> **all:** 是否所有元素都满足指定条件

val allResult = list.all { it.length <= 5 }

`Example:`

//any&all
    fun anyAndAll() {
        val list = listOf("Apple", "Banana", "Orange", "Pear", "Grape", "Watermelon")
        val anyResult = list.any { it.length <= 5 }
        val allResult = list.all { it.length <= 5 }
        println("anyResult is $anyResult, allResult is $allResult")
    }

![在这里插入图片描述][20210406143657114.png]

[20210406143657114.png]: /images/20221022/f591448622254a95bd6a5259986aaa56.png