scala - 为什么在 scala 中不能用抽象成员覆盖具体成员？

Question

例如，在以下代码中:

class Animal
class Dog extends Animal
trait Base {
  def a: Animal = new Dog
}
trait Deri extends Base {
  override val a: Dog
}

出现以下错误:

error: overriding value a in trait Deri of type Dog; method a in trait Base of type => Animal needs to be a stable, immutable value; (Note that value a in trait Deri of type Dog is abstract, and is therefore overridden by concrete method a in trait Base of type => Animal)

我想知道，因为我已经用 override 显式修改了 Deri 中的 a，而 Scala 是否选择相反的方式(如错误消息中所示，用 Base 中的内容覆盖 Deri 中的 a )？

Answer 1

根据Scala Spec ，具体的定义总是覆盖抽象的定义。

This definition also determines the overriding relationships between matching members of a class C and its parents. First, a concrete definition always overrides an abstract definition. Second, for definitions M and M' which are both concrete or both abstract, M overrides M′ if M appears in a class that precedes (in the linearization of C) the class in which M′ is defined.

因此，要使其编译，您必须确保抽象方法可以被具体方法覆盖。更改德里:

trait Deri extends Base { override def a:Animal }

或更改基础

trait Base { val a: Dog = new Dog }