在Java中，为什么超类方法不能从子类实例访问受保护的或私有的方法/变量？ [英] In Java, why can't a super-class method access protected or private methods/variables from a sub-class instance?

问题描述

让我们从另一个行为开始：即使您将方法/变量声明为私有，同一个类的另一个实例也可以访问它。没关系，我可以忍受它。我将这些类称为私有而不是实例私有。

Let's start with another behavior: even if you declare a method/variable as private, another instance of the same class can access it. That's OK I can live with it. I call these class-private and not instance-private.

现在问题部分：
例如，在运行时我希望能够检查所有此类中的字符串变量不为空，如果它们为null，则应将其更改为字符串NULL。

Now the question part: For example, at runtime I want to be able to check that all String variables in this class are not null, and if they are null they should be changed to the string "NULL".

我可以使用反射来运行变量并获取它们的值。但是如果我扩展我的类并添加私有甚至保护变量，我的基类就无法访问它们。在我可以使用它们之前，我必须对变量 setAccessible 。

I can run through the variables using reflection and get their values. But if I extend my class and add private or even protected variables my base class can't access them. I have to setAccessible on the variables before I can use them.

所以请向我解释为什么基础 - class（超类）无法从其子类访问私有/受保护变量。它是它的子类，所以我不明白。这背后的想法是什么？

So please explain to me why the base-class (super-class) can't access private/protected variables from its sub-class. It is its sub-class, so I don't get it. What's the idea behind this?

我知道超类不应该知道它的子类，但在我的例子中它是有意义的，不是吗？

I know that the super-class should not know about its sub-classes, but in my example it makes sense, no?

这是因为我不能或不应该以这种方式限制我的子类？

Is it because I can't or shouldn't restrict my sub-classes in this way?

更新：
根据答案，我还想知道：为什么不从同一个类访问另一个实例的私有变量被视为违反封装？

Update: Based on the answers, I want to know also: Why isn't accessing another instance's private variables from the same class considered a violation of encapsulation?

推荐答案

这很简单，因为它违反了封装。另一堂课不应该进入你的班级，并且不能搞乱你的东西，即使你把这个课程概括。例如，车辆如何了解汽车的任何信息？基类的重点是提供子类，但是像过度保护的父类一样，你建议的内容太多了。

It's as simple as it's a violation of encapsulation. Another class should not be able to reach into your class and be messing around with things, even if you generalize that class. How does a Vehicle know anything about a Car for example? The whole point of a base class is to provide for the sub-classes, but like an over-protective parent, what you're suggesting would be too much.

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