父类可以看到子类的受保护变量吗？

Question

I'm very fuzzy on how inheritance works and just want to make sure that I'm looking in the right direction.

From my understanding I understand that

1. public variables are accessible by all packages,
2. default within the single package,
3. private within only the class,
4. and protected with sub-classes.

I know that child classes can see parent class's protected variables.

My question: Does it work the other way around?

Answer 1

Does it work the other way around?

No, not if they're not in the same package. According to the table in the Java access control tutorial , protected exposes the member to subclasses and other classes in the same package, not superclasses:

\n                  Access Levels \n+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ \n|  Modifier Class Package Subclass World | \n|  public YYYY | \n|  protected YYYN | \n|  no modifier YYNN | \n|  private YNNN | \n+−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−+ \n

And from the JLS :

6.6.2. Details on protected Access

A protected member or constructor of an object may be accessed from outside the package in which it is declared only by code that is responsible for the implementation of that object.

The superclass isn't responsible for the implementation of the subclass object.

You can test it yourself:

b/Base.java :

package b;

import a.Example;

public class Base {
    public static void showAnswer(Example e) {
        System.out.println(e.answer); // 
    }
}

a/Example.java :

package a;

import b.Base;

public class Example extends Base
{
    protected int answer = 42;

    public static void main(String[] args)
    {
        Example e = new Example();
        Base.showAnswer(e);
    }
}

Trying to compile that fails with:

\n./b/Base.java:7: error: answer has protected access in Example \n        System.out.println(e.answer);  //  \n                            ^ \n1 error \n

Answer 2

If you think about it, what you are asking does not make too much sense. Consider this:

public class A {
    public void doSomething() {
        // This is not actually possible!!!
        bField = 12;
    }
}

public class B extends A {
    protected int bField;
}

public class C extends A {
     // ... whatever except bField...
}

...
A aInstance = new C();
// What should happen here? a C does not have a bField!!!
aInstance.doSomething();

Answer 3

As you see its possible from subclasses not superclasses

You can read offical documentation here

Answer 4

Does it work the other way around?

No unless they are in the same package