嵌套类：

class Human {  class Student{    val age = 10  }}object ClassDemo {  def main(args: Array[String]): Unit = {    val h = new Human    val s = new h.Student    println(s.age)  }}

有时会遇到这种情况：

class Human {  class Student {    def addS(s: Student) = {      val ab = new ArrayBuffer[Student]()      ab += s    }  }}object ClassDemo {  def main(args: Array[String]): Unit = {    val h = new Human    val h2 = new Human    val s = new h.Student    val s2 = new h2.Student    s.addS(s2)  }}

以上addS会报错，因为方法只能接收h.Student不能接收h2.Student。

解决方法有2个：

1. 类型投影

import scala.collection.mutable.ArrayBufferclass Human {  class Student {    def addS(s: Human#Student) = {      val ab = new ArrayBuffer[Human#Student]()      ab += s    }  }}object ClassDemo {  def main(args: Array[String]): Unit = {    val h = new Human    val h2 = new Human    val s = new h.Student    val s2 = new h2.Student    s.addS(s2)  }}

2. 伴生对象

object Human {  class Student {  }}class Human {    def addS(s: Human.Student) = {      val ab = new ArrayBuffer[Human.Student]()      ab += s    }}object ClassDemo {  def main(args: Array[String]): Unit = {        val h = new Human    val s = new Human.Student    val s2 = new Human.Student        h.addS(s)    h.addS(s2)          }}

嵌套类要访问外部类有2种方式：

1. 外部类.this

class Human {  val name = "Sky"  class Student {    println(Human.this.name)    def addS(s: Student) = {      val ab = new ArrayBuffer[Student]()      ab += s    }  }}

2. “自身类型”

class Human {  outter =>  class Student {    println(outter.name)    def addS(s: Student) = {      val ab = new ArrayBuffer[Student]()      ab += s    }  }    val name = "Sky"}