Java中的受保护引用

参见英文答案 > Understanding java’s protected modifier????????????????????????????????????6个
>???????????? What is the difference between public,protected,package-private and private in Java?????????????????????????????????????24个
我有三个班：

package pac;

public class A {
    protected A a;  
    protected final int i = 10;
}

public class B extends A {

    void foo() {
        A a = new A();
        int b = a.a.i;  //compiles fine
    }
}

package another.pac;

public class C extends A {

    void foo() {
        A a = new A();
        int b = a.a.i;  //Does not compile. a.a is inaccessible
    }
}

为什么我们不能从被放入另一个包中访问受保护的成员,但我们可以从同一个包中访问？它们都是一个的子类,因此应该允许访问.

JLS 6.6.2.1说：

If the access is by a field access expression E.Id,or a method
invocation expression E.Id(…),or a method reference expression E ::
Id,where E is a Primary expression (§15.8),then the access is
permitted if and only if the type of E is S or a subclass of S.

C级令人满意.怎么了？

public class C extends A {

    void foo() {
       int b = i;  
    }
}