嵌套類：

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)
  }
}

有時會遇到這種情況：

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會報錯，因為方法只能接收h.Student不能接收h2.Student。

解決方法有2個：

1. 類型投影

import scala.collection.mutable.ArrayBuffer

class 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. 伴生對象

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)
    
    
  }
}

嵌套類要訪問外部類有2種方式：

1. 外部類.this

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

2. “自身類型”

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

快學Scala 第八課 （嵌套類）