为什么不能将父类分配给子类类型的变量? [英] Why can't I assign a parent class to a variable of subclass type?

问题描述

为什么子类的引用变量不能指向父类的对象?即

Why reference variable of child class can't point to object of parent? i.e

Child obj = new Parent();

但是我们可以反之亦然 请用内存视图(堆)回答

However we can do vice versa Kindly answer with memory view (heap)

推荐答案

没有理由与内存有关.这要简单得多.子类可以通过添加新方法来扩展其超类的行为.虽然没有给出，但超类具有其子类的所有方法.请看以下示例:

There is no reason which has something to do with the memory. It's much more simple. A subclass can extend the behaviour of its superclass by adding new methods. While it is not given, that a superclass has all the methods of its subclasses. Take the following example:

public class Parent {
    public void parentMethod() {}
}

public class Child extends Parent {
    public void childMethod() {}
}

现在让我们考虑一下，如果将Parent的实例分配给类型为Child的变量，将会发生什么情况.

Now let's think about what would happen if you could assign an instance of Parent to a variable of type Child.

Child c = new Parent(); //compiler error

由于c的类型为Child，因此可以调用方法childMethod().但是，由于它实际上是一个Parent实例，没有这种方法，因此将导致编译器或运行时问题(取决于检查的完成时间).

Since c is of type Child, it is allowed to invoke the method childMethod(). But since it's really a Parent instance, which does not have this method, this would cause either compiler or runtime problems (depending on when the check is done).

相反，这没问题，因为您不能通过扩展类来删除方法.

The other way round is no problem, since you can't remove methods by extending a class.

Parent p = new Child(); //allowed

Child是Parent的子类，因此继承了parentMethod().因此，您可以节省调用该方法.

Child is a subclass of Parent and thus inherits the parentMethod(). So you can invoke this method savely.

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