在使用类型参数或泛型时想要运行时多态性？

Question

下面是关于"animal eats food"的故事，还有猫吃鱼

class Food
abstract class Animal {
  type F
  def eat(food: F)
}
class Fish extends Food
class Cat extends Animal {
  type F = Fish
  def eat(fish: F) {
    println("eat " + fish.getClass.getSimpleName)
  }
}

(new Cat).eat(new Fish) //eat Fish

val animal: Animal = new Cat
animal.eat(new Fish) //error: type mismatch

现在我已经使用了抽象类型成员（或类型参数），我失去 runtime polymorphism（最后一行）。（使用基本类型 Animal 来键入任意子类型和运行没有问题。）

否则我可以从 Animal 中删除类型参数并在 Cat 中进行类型检查：

abstract class Animal {
  def eat(food: Food)
}
class Cat extends Animal {
  def eat(food: Food) {
    food match {
      case fish: Fish => println("eat" + fish)
      case _ => throw new IllegalArgumentException("I only eat fish")
    }
  }
}

但我想为他们打字更好。
那么我可以在使用类型参数/泛型时保留运行时间多态性吗？

Answer 1

我不确定这是否是最佳方法，但它似乎有效。您可以允许将任何种类的食物传递给动物冒着 class 抛出异常的风险，并在发生时进行处理。

class Food
abstract class Animal {
  type SuitableFood <: Food
  def eat(food: SuitableFood)

  def eatWithExceptionHandling(food: Food) {
    try {
      eat(food.asInstanceOf[SuitableFood])
    }
    catch {
      case ex: ClassCastException => println("Wrong food")
    }
  }
}

class Grass extends Food
class Fish extends Food

class Cow extends Animal {
  type SuitableFood = Grass
  override def eat(food: Grass) {
    println("Cow eating grass")
  }
}

class Cat extends Animal {
  type SuitableFood = Fish
  override def eat(food: Fish) {
    println("Cat eating fish")
  }
}

val (cow, cat, grass ,fish) = (new Cow, new Cat, new Grass, new Fish)

cow.eat(grass) //"Cow eating grass"
cat.eat(fish) //"Cat eating fish"
//cow.eat(fish) // error: type mismatch
cow.eatWithExceptionHandling(grass) //"Cow eating grass"
cow.eatWithExceptionHandling(fish) //"Wrong food"

Answer 2

Now that I have used abstract-type member (or type-parameter) , I lose the runtime polymorphism (at last line). (that using base type Animal to typing arbitrary subtypes and run with no problem.)

不，您可以用其子类型替换 Animal（在这种情况下，并非总是如此！），但反之则不行。如果可以的话，您也可以使用 Object，因为它也是 Cat:

的基本类型

val object: Object = new Cat
object.eat(new Fish)

希望您明白为什么这不应该编译。

或者你可以换一种说法：应该

val animal: Animal = makeAnAnimal() 
animal.eat(new Fish)

编译？如果您认为答案是 "yes"，请考虑 makeAnAnimal() 可以 return 一个 Cow。

在使用类型参数或泛型时想要运行时多态性？

Want runtime polymorphism while using type parameter or generic?

scala

generics

polymorphism

abstract-type