In Java, why can't a super-class method access protected or private methods/variables from a sub-class instance?

Question

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.

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".

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.

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?

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

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

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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?

Answer 1

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.