Scala - 为什么无法覆盖父类(super class)的方法

Question

  class A
  class B extends A

  class D { def get: A = ??? }
  class E extends D { override def get: B = ??? } // OK

  class F { def set(b: B): Unit = ??? }
  class G extends F { override def set(a: A): Unit = ??? } // Compile Error, override nothing

我的问题是为什么 G 不起作用，因为: (A=>Unit) 是 (B=>Unit) 的子类型

implicitly[(A => Unit) <:< (B => Unit)]

Answer 1

原因很简单:JVM 不允许基于参数类型逆变的覆盖。它只允许基于返回类型的协方差进行覆盖。

看看here用于讨论它对 Scala 的影响。

来自 Wikipedia :

Similarly, it is type safe to allow an overriding method to accept a more general argument than the method in the base class:

Ex:

class AnimalShelter {
    void putAnimal(Animal animal) {
        ...
    }
}

class CatShelter extends AnimalShelter {
    void putAnimal(Object animal) {
        ...
    }
}

Not many object oriented languages actually allow this — C++ and Java [and also Scala] would interpret this as an unrelated method with an overloaded name.