为什么从孩子到父母的强制转换是不可能的
我有一个动物类，其中有一个孩子狗。
class Animal {
public void makeSound() {
System.out.println("Animal sound");
}
}
class Dog extends Animal {
@Override
public void makeSound() {
super.makeSound();
}
}
public static void main(String[] args) {
Dog dog = (Dog) new Animal(); // It compiles, but throws runtime exception.
dog.makeSound();
}
在这里，如果我们将子对象转换为父对象，它将编译，但会抛出运行时异常 - ClassCastException。当然，可能存在子级具有父级没有的字段或方法的情况。这就是为什么我们不能将 Dog 转换为 Animal。而且，不存在 IS-A 关系。但是，如果父类和子类只有相同的字段和方法，为什么强制转换是不可能的？
我在采访中被问到这个问题）
谢谢。
I have a class Animal which has one child Dog.
class Animal {
public void makeSound() {
System.out.println("Animal sound");
}
}
class Dog extends Animal {
@Override
public void makeSound() {
super.makeSound();
}
}
public static void main(String[] args) {
Dog dog = (Dog) new Animal(); // It compiles, but throws runtime exception.
dog.makeSound();
}
Here if we will cast child to parent, it will compile, but it will throw runtime exception - ClassCastException. Of course, there can be a case when the child will have a field or method which the parent doesn't. That's why we can't cast Dog to Animal. Also, there's no IS-A relationship. But why the cast is not possible, in case parent and child classes have only the same fields and methods?
I've got asked this question in the interview)
Thank you.
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当编译器发现您尝试将 Animal 对象转换为 Dog 时，它“知道”Dog 在层次结构中位于 Animal 之下，因此该动物可能是狗，因此它允许这样做。然而，在运行时，事实证明它只是一个动物，而不是一个狗，所以转换失败。
这是一个奇怪的情况，在阅读程序时很明显动物只是一个动物而不是狗，因为它只是用“new Animal()”创建的，但编译器不知道这一点——它只是知道它是一种动物。编译器很聪明，但他们不可能知道一切。我希望这是有道理的。
When the compiler sees that you are trying to cast the Animal object into a Dog, it "knows" that Dog is underneath Animal in the hierarchy, so it is possible that the animal is a dog, so it allows it. However, at runtime, it turns out that it is just an Animal, and is not a Dog, so the cast fails.
This is a weird case in which it is obvious in reading the program that the Animal is just an Animal and not a Dog because it was just created with "new Animal()", but the compiler doesn't know that--it just knows that it is an Animal. Compilers are smart, but they can't know everything. I hope that makes sense.