kotlin内置函数：Apply、also、run、let、with的用法

悠悠 2023-10-05 23:55 6阅读 0赞

## 区分kotlin内置函数 ##

### apply、also、run、let、with ###

#### 1、从返回值角度区分 ####

*  apply、also返回值是 对象
 *  run、let返回最后一行代码的 结果值，指定的 return 返回值

#### 2、从内置函数内内使用的对象名称 ####

*  apply、run、with使用this
 *  also、let使用it

使用`this`可以在内置函数内省略不写直接调用对象的公有成员变量，`it`必须填上不可省略

--------------------

### 例子 ###

用Kotlin写了一个Person类进行测试

class Person() {
        
    
        var name: String? = null
        var address: String? = null
        var age: Int = -1
    
        private fun initDef() {
        
            name = "Mr.Li";
            address = "China"
            age = 24
        }
    
        init {
        
            initDef()
        }
    
        fun setRealName(realName: String) {
        
            this.name = realName
        }
    
        fun setPantName(pantName: String) {
        
            this.name = pantName
        }
    
    
        override fun toString(): String {
        
            return "name : $name || address : $address || age : $age "
        }
    }
    
    
    
    fun main(args: Array<String>) {
        
        val man = Person()
        println("created -> ${
          man.toString()}")
    
        val applyMan = man.apply {
        
            this.age = 18
            address = "ShangHai" // this可以省略不写
            setPantName("Lmm")
        }
        println("apply -> ${
          applyMan.toString()}")
    
        val alsoMan = man.also {
        
            it.address = "China ShangHai" // it 则不行
            it.setRealName("LiMM")
        }
        println("also -> ${
          alsoMan.toString()}")
    
        val runMan = man.run {
        
            age = 30
            setPantName("Kuye")
            this// this 作为最后的返回值
        }
        println("run -> ${
          runMan.toString()}")
        println("runMan Type -> ${
          runMan.javaClass.typeName}")
    
        val runManName = man.run {
        
            age = 30
            setPantName("Kuye")
            name// name 作为最后的返回值
        }
        println("runManName -> $runManName")
        println("runManName Type -> ${
          runManName?.javaClass?.typeName}")
    
        val runManReturn: String = man.run {
        
            age = 30
            setPantName("Kuye")
            name//
            return@run "runManReturn" //直接用Return指定一个返回值  这里可以是表达式、成员函数的返回等等
        }
        println("runManReturn -> $runManReturn")
        println("runManReturn Type -> ${
          runManReturn.javaClass.typeName}")
    
        val letMan = man.let {
        
            it.address = "China GUANGXI"
            it.setRealName("LiMI")
            it// it 作为返回值
        }
        println("letMan -> ${
          letMan.toString()}")
        println("letMan Type -> ${
          letMan.javaClass.typeName}")
    
        val letManReturn: String = man.run {
        
            age = 30
            setRealName("Kuye")
            name//
            return@run "letManReturn" //直接用Return指定一个返回值  这里可以是表达式、成员函数的返回等等
        }
        println("letManReturn -> $letManReturn")
        println("letManReturn Type -> ${
          letManReturn.javaClass.typeName}")
    
        with(man) {
        
            // this可以省略不写
            setPantName("LMM")
            this.address = "China SHANGHAI"
            age = 15
        }
    
        println("with man  -> $man")
    }

### 输出结果： ###

created -> name : Mr.Li || address : China || age : 24 
    apply -> name : Lmm || address : ShangHai || age : 18 
    also -> name : LiMM || address : China ShangHai || age : 18 
        
    run -> name : Kuye || address : China ShangHai || age : 30 
    runMan Type -> infunc.Person
        
    runManName -> Kuye
    runManName Type -> java.lang.String
        
    runManReturn -> runManReturn
    runManReturn Type -> java.lang.String
        
    letMan -> name : LiMI || address : China GUANGXI || age : 30 
    letMan Type -> infunc.Person
        
    letManReturn -> letManReturn
    letManReturn Type -> java.lang.String
        
    with man  -> name : LMM || address : China SHANGHAI || age : 15