用子类参数覆盖子类方法？ [英] Overriding subclass methods with subclassed arguments?

问题描述

当子类覆盖时，如何强制基本方法接受相同的特定子类实例？

How can I force base methods to take in the same specific subclass instance when overriden by a subclass?

ie：

abstract class Animal {
  def mateWith(that: Animal)
}

class Cow extends Animal {
  override def mateWith...?
}

逻辑上，牛应该只能 mateWith 另一个 Cow 。但是，如果我执行覆盖def mateWith（that：Cow），这实际上并没有覆盖基类方法（我想要它，因为我想强制执行它存在于子类中。

Logically, a Cow should only be able to mateWith another Cow. However, if I do override def mateWith(that: Cow), this doesn't actually override the base class method (which I want it to, since I want to enforce its existence in the subclass).

我可以检查以确保其他实例是Cow类型，如果不是则抛出异常 - 这是我最好的选择？如果我有更多动物怎么办？我将不得不重复抛出异常代码。

I could check to make sure the other instance is of type Cow, and throw an exception if it isn't - is this my best option? What if I have more animals? I would have to repeat the exception-throwing code.

推荐答案

abstract class Animal[T <: Animal[T]] {
  def mateWith(that: T)
}

class Cow extends Animal[Cow] {
  override def mateWith(that: Cow) { println("cow") }
}

class Dog extends Animal[Dog] {
  override def mateWith(that: Dog) { println("dog") }
}

并像这样使用：

scala> (new Cow).mateWith(new Cow)
cow

scala> (new Cow).mateWith(new Dog)
<console>:17: error: type mismatch;
 found   : Dog
 required: Cow
              (new Cow).mateWith(new Dog)
                                 ^

不需要抛出异常代码;类型系统在编译时为你处理它！

No exception-throwing code needed; the type system handles it for you at compile-time!

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