【kotlin】Kotlin笔记15-扩展函数和运算符重载(operator)

谁践踏了优雅 2022-11-17 05:26 259阅读 0赞

### Kotlin笔记15-扩展函数和运算符重载 ###

**9.1 延迟初始化和密封类**

*  **扩展函数**

*如何对一个字符串计数*  
`Example:`

object StringUtil{
        fun letterCount(str:String):Int{
            var count=0
            for(char in str){
                if(char.isLetter()){
                    count++
                }
            }
            return count
        }
    }
    
    fun main() {
        val str="ABC123xyz!@#"
        val count=StringUtil.letterCount(str)
    }

<table> 
 <thead> 
  <tr> 
   <th>语法</th> 
   <th>特性</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>Java</td> 
   <td>如上Java编程中最标准的实现思维</td> 
  </tr> 
  <tr> 
   <td>Kotlin</td> 
   <td>使用一种更加面向对象的思维实现-将方法添加到现有类中(扩展函数)</td> 
  </tr> 
 </tbody> 
</table>

`定义扩展函数的语法结构:`

fun ClassName.method(param1: Int, param2: Int): Int {
    	return 0
    }

`优化:`

fun String.letterCount(): Int {
        var count = 0
        for (char in this) {
            if (char.isLetter())
                count++
        }
        return count
    }

`用法:`

val count1="ABC123xyz!@#".letterCount()

<table> 
 <thead> 
  <tr> 
   <th>序号</th> 
   <th>Tips</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>1</td> 
   <td>建议向哪个类添加扩展函数,即创建一个同名文件:便于查找,以及顶层方法的特性,使他拥有全局的访问域</td> 
  </tr> 
  <tr> 
   <td>2</td> 
   <td>Kotlin的扩展函数可以使一个final类扩展任何函数,使其API变得丰富</td> 
  </tr> 
  <tr> 
   <td>3</td> 
   <td>扩展函数可以大幅提升代码质量和开发效率</td> 
  </tr> 
 </tbody> 
</table>

> **reverse():** 反串字符串函数(String)

> **capitalize():** 首字母大写函数(String)

*  **运算符重载**

<table> 
 <thead> 
  <tr> 
   <th>语言</th> 
   <th>特性</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>Java</td> 
   <td>+ - * / % ++ –</td> 
  </tr> 
  <tr> 
   <td>Kotlin</td> 
   <td>允许我们将所有的运算符,关键字进行重载,扩展他们的用法</td> 
  </tr> 
 </tbody> 
</table>

> **operator**

*+:plus(), -:minus()*  
`Example:`

class Obj {
    	operator fun plus(obj: Obj): Obj {
    		//处理相加的逻辑
    	}
    }
    
    val obj1 = Obj()
    val obj2 = Obj()
    val obj3 = obj1 + obj2 /-> obj1.plus(obj2)

*让两个Menoy相加:*

class Money(val value: Int) {
        operator fun plus(money: Money): Money {
            val sum = value + money.value
            return Money(sum)
        }
    }

`用法:`

fun main() {
        val money1=Money(5)
        val money2=Money(10)
        val money3=money1+money2
        println(money3.value)
    }

`结果:`  
![在这里插入图片描述][20210408194213993.png]  
*运算符多重重载*

class Money(val value: Int) {
        operator fun plus(money: Money): Money {
            val sum = value + money.value
            return Money(sum)
        }
    
        operator fun plus(newValue: Int): Money {
            val sum = value + newValue
            return Money(sum)
        }
    }

fun main() {
        val money1 = Money(5)
        val money2 = Money(10)
        val money3 = money1 + money2
        val money4 = money3 + 20
        println(money3.value)
        println(money4.value)
    }

`结果:`  
![在这里插入图片描述][20210408194601858.png]  
*语法糖表达式和实际调用函数对照表*

<table> 
 <thead> 
  <tr> 
   <th>语法糖表达式</th> 
   <th>实际调用函数</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>a + b</td> 
   <td>a.plus(b)</td> 
  </tr> 
  <tr> 
   <td>a - b</td> 
   <td>a.minus(b)</td> 
  </tr> 
  <tr> 
   <td>a * b</td> 
   <td>a.times(b)</td> 
  </tr> 
  <tr> 
   <td>a / b</td> 
   <td>a.div(b)</td> 
  </tr> 
  <tr> 
   <td>a % b</td> 
   <td>a.rem(b)</td> 
  </tr> 
  <tr> 
   <td>a ++</td> 
   <td>a.inc()</td> 
  </tr> 
  <tr> 
   <td>a –</td> 
   <td>a.dec()</td> 
  </tr> 
  <tr> 
   <td>+a</td> 
   <td>a.unaryPlus()</td> 
  </tr> 
  <tr> 
   <td>-a</td> 
   <td>a.unaryMinus()</td> 
  </tr> 
  <tr> 
   <td>!a</td> 
   <td>a.not()</td> 
  </tr> 
  <tr> 
   <td>a == b</td> 
   <td>a.equals(b)</td> 
  </tr> 
  <tr> 
   <td>a &gt; b</td> 
   <td></td> 
  </tr> 
  <tr> 
   <td>a &lt; b</td> 
   <td></td> 
  </tr> 
  <tr> 
   <td>a &gt;= b</td> 
   <td>a.compareTo(b)</td> 
  </tr> 
  <tr> 
   <td>a &lt;= b</td> 
   <td></td> 
  </tr> 
  <tr> 
   <td>a…b</td> 
   <td>a.rangeTo(b)</td> 
  </tr> 
  <tr> 
   <td>a[b]</td> 
   <td>a.get(b)</td> 
  </tr> 
  <tr> 
   <td>a[b] = c</td> 
   <td>a.set(b,c)</td> 
  </tr> 
  <tr> 
   <td>a in b</td> 
   <td>b.contains(a)</td> 
  </tr> 
 </tbody> 
</table>

*小试牛刀-随机长度字符串生成*  
`Example:`

fun getRandomLengthString(str: String): String{
    	val n = (1..20).random()
    	val builder= StringBuilder()
    	repeat(n){
    		builder.append(str)
    	}
    	return builder.toString()
    }

`str*n:`

operator fun String.times(n:Int):String{
        val builder=StringBuilder()
        repeat(n){
            builder.append(this)
        }
        return builder.toString()
    }

`用法:`

fun main() {
        val str = "str" * 3
        println(str)
    }

`结果:`  
![在这里插入图片描述][20210409091812611.png]  
`那么,还可不可以再优化了呢`

> **repeat():** String类中将字符串重复n遍的函数

operator fun String.times(n:Int)= repeat(n)
    
    fun getRandomLength(str: String) = str*(1..20).random()

[20210408194213993.png]: /images/20221022/a9976d0ac0944f818d5e117205da7d44.png
[20210408194601858.png]: /images/20221022/43ecb9d440084efaac2751668a00e001.png
[20210409091812611.png]: /images/20221022/da5add09cfa845feba058688fca8c8d4.png