在这个使用泛型的例子中，asInstanceOf有什么问题吗？

Question

Consider this (somewhat contrived) example:

abstract class Obj[A, B] {
    def id: Long
    def parent: B
}

abstract class TopLevel[A] extends Obj[A, A] {
    def parent: A = this.asInstanceOf[A] // How terrible is this?
}

abstract class AbsChild[A, B] extends Obj[A, B] {
    def parent: B
}

case class Top(id: Long) extends TopLevel[Top]

case class Child(id: Long, parent: Top) extends AbsChild[Child, Top]

To paint a better picture, imagine AbsChild as some kind of directory on a file system, and TopLevel as the physical drive that an AbsChild belongs to. So parent doesn't actually refer to the direct parent of the object (like the directory that contains it), but rather a reference to the top level object in the tree.

In some applications, I'm going to be dealing with a List[Obj[A, B]] , where it isn't immediately known what Obj is. In this case, it would be nice for even a TopLevel to have a parent , which should just return a reference to itself. And herein lies my question.

Defining def parent: A = this for TopLevel doesn't work:

<console>:14: error: type mismatch;
 found   : TopLevel.this.type (with underlying type TopLevel[A])
 required: A

But def parent: A = this.asInstanceOf[A] does, and seems to function correctly in practice.

scala> val top = Top(1)
top: Top = Top(1)

scala> val child = Child(1, top)
child: Child = Child(1,Top(1))

scala> top.parent
res0: Top = Top(1)

scala> child.parent
res1: Top = Top(1)

But is this really okay? Using asInstanceOf[A] feels incredibly dirty, and leaves me wondering if it will fail somehow with a ClassCastException .

Answer 1

You would get a ClassCastException if you had a subclass of TopLevel[A] that wasn't an A . To avoid the need to cast you should use a self type (I'm not sure if that's the correct name):

abstract class TopLevel[A] extends Obj[A, A] {
  this: A =>
    def parent: A = this
}