【kotlin】Kotlin笔记16-高阶函数(函数类型)

た入场券 2022-11-17 11:27 140阅读 0赞

### Kotlin笔记16-高阶函数 ###

**10.1 高阶函数**

*  **定义高阶函数**

<table> 
 <thead> 
  <tr> 
   <th>序号</th> 
   <th align="left">Tips</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>1</td> 
   <td align="left">接受Lambda参数的函数称为具有函数式编程风格的API</td> 
  </tr> 
  <tr> 
   <td>2</td> 
   <td align="left">借助高阶函数定义自己的函数式API</td> 
  </tr> 
  <tr> 
   <td>定义</td> 
   <td align="left">如果一个函数接收另一个函数作为参数,或者返回值的类型是另一个函数,那么该函数称为高阶函数</td> 
  </tr> 
  <tr> 
   <td>新概念</td> 
   <td align="left">函数类型</td> 
  </tr> 
  <tr> 
   <td>作用</td> 
   <td align="left">高阶函数允许让函数类型的参数决定函数的执行逻辑</td> 
  </tr> 
 </tbody> 
</table>

`函数类型的基本语法规则:`

(String, Int) -> Unit

<table> 
 <thead> 
  <tr> 
   <th>-&gt;左边</th> 
   <th>-&gt;右边</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>声明函数接收的参数</td> 
   <td>声明返回值类型</td> 
  </tr> 
  <tr> 
   <td>参数之间 , 逗号隔开</td> 
   <td>无返回值就使用Unit=void(Java)</td> 
  </tr> 
 </tbody> 
</table>

`将上述函数添加到参数声明或返回值上,那么他就是高阶函数:`

fun example(func: (String, Int) -> Unit) {
    	func("hello", 123)
    }

`Example:`

fun num1AndNum2(num1: Int, num2: Int, operation: (Int, Int) -> Int): Int {
        return operation(num1, num2)
    }

fun plus(num1: Int, num2: Int): Int {
        return num1 + num2
    }
    
    fun minus(num1: Int, num2: Int): Int {
        return num1 - num2
    }

`用法:`

fun main() {
        val num1=100
        val num2=80
        val result1= num1AndNum2(num1,num2,::plus)
        val result2= num1AndNum2(num1,num2,::minus)
        println("result1 is $result1")
        println("result2 is $result2")
    }

`结果:`  
![在这里插入图片描述][20210409135734758.png]  
*Kotlin支持Lambda表达式,匿名函数,成员引用来调用高阶函数*  
`用法1-lambda表达式:`

val result1 = num1AndNum2(num1, num2) { n1, n2 ->
    	n1 + n2
    }
    val result2 = num1AndNum2(num1, num2) { n1, n2 ->
    	n1 - n2
    }

*小试牛刀*  
`仿apply:`

fun StringBuilder.build(block: StringBuilder.() -> Unit): StringBuilder {
        block()
        return this
    }

1.  给StringBuilder定义了一个build扩展函数,接受函数类型参数,返回值也是StringBuilder
2.  调用build函数时传入Lambda表达式自动拥有Stringbuilder的上下文,同时拥有apply的实现方式

`apply实现Example:`

fun eatFruits() {
        val list= listOf("Apple","Banana","Orange","Orange","Pear","Grape")
        val result=StringBuilder().apply {
            append("Start eating fruits.\n")
            for (fruit in list){
                append("$fruit ")
            }
            append("\nAte all fruits.")
        }
        println(result.toString())
    }

`apply实现Example:`

fun eatFruits1(){
        val list= listOf("Apple","Banana","Orange","Orange","Pear","Grape")
        val result=StringBuilder().build {
            append("Start eating fruits.\n")
            for (fruit in list){
                append("$fruit ")
            }
            append("\nAte all fruits.")
        }
        println(result.toString())
    }

[20210409135734758.png]: /images/20221022/3d411c0988644ae78ee667e0437e3059.png