Scala method clone in class Object cannot be accessed in T Access to protected method clone not permitted because

问题

I've got troubles with cloning in Scala. Is this possible to clone an object of an arbitrary type T? Something like this:

import collection.immutable.Stack

object Tester extends App {
  trait Grand[T <: Cloneable] {
    val stack = Stack[T]()
    val h: T

    def snapshot() {
      stack push h.clone().asInstanceOf[T]
    }
  }
}

however it throws:
scala: method clone in class Object cannot be accessed in T
Access to protected method clone not permitted because prefix type T does not conform to trait Grand in object Tester where the access take place

What goes wrong there?

回答1:

I was advised on this question. In such a situation the best approach is to use structural typing:

trait Grand[T <: {def cloneObject: T}]

so that the user code might be the following:

case class Person(name: String) {
  def cloneObject = copy()
}

object Roll extends App with Grand[Person] {
...
}

回答2:

clone() is from java.lang.Object not from java.lang.Cloneable. Cloneable is just a tagging-interface.

What you are trying probably does not work because implementing Cloneable does not force the implementor to override the protected Object.clone() with a public one.

see http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#clone() and http://docs.oracle.com/javase/7/docs/api/java/lang/Cloneable.html

ln conclusion: No it is not possible to clone an object of an arbitrary type T1. You could however clone any object of type T2 where T2 is bound by a type with a public override of clone(). Your example fails because Object.clone() is protected, i.e. can only be called from within a subclass.

来源：https://stackoverflow.com/questions/15860647/scala-method-clone-in-class-object-cannot-be-accessed-in-t-access-to-protected-m