这一节用个小例子来理解下密封类及let、run、apply、also、with之间的区别

Java有枚举，Kotlin也有枚举，但多了一个密封类，密封类就是子类有限的类

/**
 * 密封类
 */
sealed class Language{

    class Java : Language(){
        fun println(){
            println("${javaClass.name}")
        }
    }

    class Kotlin : Language(){
        fun println(){
            println("${javaClass.name}")
        }
    }

    class Python : Language(){
        fun println(){
            println("${javaClass.name}")
        }
    }

    class Swift : Language(){
        fun println(){
            println("${javaClass.name}")
        }
    }
}

定义一个类

class Programmer(val name: String,val age: Int,val language: Language){
    override fun toString(): String {
        return "$name - $age - ${language.javaClass.simpleName} engineer"
    }

    fun work(){
        println("$name codeing")
    }
}

看看 let run apply also with 有什么区别

fun main(args: Array<String>) {
    println("Hello World")

    //fun <T, R> T.let(block: (T) -> R): R = block(this)
    //可以使用it关键字返回实体，单入参返回最后一行
    val programmer1 = Programmer("John",25,Language.Kotlin())
    val ss = programmer1?.let {
        println(it.toString())
        it.work()
    }


    //fun <T, R> T.run(block: T.() -> R): R = block()
    //可以使用this关键字返回实体，单入参返回最后一行
    val programmer2 = Programmer("Aluxe",28,Language.Python())
    programmer2?.run {
        println(toString())
        this.work()
    }


    //fun <T> T.apply(block: T.() -> Unit): T { block(); return this }
    //可以使用this关键字返回实体，单入参无返回。方法体返回this
    val programmer3 = Programmer("Alice",21,Language.Swift())
    programmer3?.apply {
        println(this.toString())
    }.work()



    //fun <T> T.also(block: (T) -> Unit): T { block(this); return this }
    //可以使用it关键字返回实体，单入参无返回。方法体返回this
    val programmer4 = Programmer("Chen",31,Language.Java())
    programmer4?.also {
        println(it.toString())
    }.work()



    //fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()
    //可以使用this关键字返回实体，接收2个入参，第2个入参返回最后一行。
    val programmer5 = Programmer("Smith",19,Language.Python())
    with(programmer5){
        println(this.toString())
        work()
    }
}