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scala - 如何访问类参数？

问题描述

根据《Scala 编程》一书，第 304 页

class Queue[T] private (leading: List[T], trailing: List[T]){
  private def mirror =
    if(leading.isEmpty) new Queue(trailing.reverse, Nil)
    else this
  def head = mirror.leading.head
  def tail = {
    val q = mirror
    new Queue(q.leading.tail, q.trailing)
  }
  def append[T](x: T)={
    new Queue(leading, x::trailing)
  }
}

我收到以下错误

Error:value leading is not a member of Queue[T]
def head = mirror.leading.head

相同的错误trailing。 mirror是类的对象Queue吗？为什么它不能访问它的类参数？

标签： scala

解决方案

确实leading，并且trailing是Queue类的成员，但默认情况下它们是私有的。要使它们公开添加val注释：

前：

scala> class Queue[T] private (leading: List[T], trailing: List[T]){
     |   private def mirror =
     |     if(leading.isEmpty) new Queue(trailing.reverse, Nil)
     |     else this
     |   def head = mirror.leading.head
     |   def tail = {
     |     val q = mirror
     |     new Queue(q.leading.tail, q.trailing)
     |   }
     |   def append[T](x: T)={
     |     new Queue(leading, x::trailing)
     |   }
     | }
<console>:15: error: value leading is not a member of Queue[T]
         def head = mirror.leading.head
                           ^
<console>:18: error: value leading is not a member of Queue[T]
           new Queue(q.leading.tail, q.trailing)
                       ^
<console>:18: error: value trailing is not a member of Queue[T]
           new Queue(q.leading.tail, q.trailing)
                                       ^

后： 

scala> class Queue[T] private (val leading: List[T], val trailing: List[T]){
     |   private def mirror =
     |     if(leading.isEmpty) new Queue(trailing.reverse, Nil)
     |     else this
     |   def head = mirror.leading.head
     |   def tail = {
     |     val q = mirror
     |     new Queue(q.leading.tail, q.trailing)
     |   }
     |   def append[T](x: T)={
     |     new Queue(leading, x::trailing)
     |   }
     | }
defined class Queue

您可以在此处详细了解构造函数和 getter 。相关点是：

没有val或的参数var是私有值，仅在类中可见。

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