java - 为什么我允许 "direct access"到对象的 protected 字段，该对象的类是在不同的包中定义的？

Question

Main.java，主包:

package pkgs.main;
import pkgs.test.B;

// Just some method inside the "main class"
void method() {
    B b = new B();
    b.x ++;  //   <--- why is this allowed?
}

A.java，主包:

package pkgs.main;

public class A {
    protected int x;
}

B.java，测试包:

package pkgs.test;
import pkgs.main.A;

public class B extends A {
}

编辑:

看待这个问题的另一种方式如下。我将在现有示例代码中添加两行额外的代码:

Main.java，主包:

// Just some method inside the "main class"
void method() {
    B b = new B();
    b.x ++;  //   <--- why is this allowed?

    b.y ++;  //   (Additional code) Compilation ERROR, which is correct.
}

B.java，测试包:

public class B extends A {
    protected int y;  // (Additional code)  protected field;
                      // access to it is disallowed inside the 
                      // "main calling class" above.
}

Answer 1

由于 B 扩展了 A ， protected 字段也允许访问它的子级。

The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.

请优先阅读 Controlling Access to Members of a Class

Modifier    Class   Package Subclass    World
---------------------------------------------

protected   Y      Y        **Y**           N