受保护的为什么不能在不同的包子类中访问？

Question

Error: m1() has protected access in A

When I try to using superclass reference variable ( A obj=new B() )

This is a first class in package pkg1;

package pkg1;
public class A {
    protected void m1() {
        System.out.println("protected method");
    }
}

This is a second class which is in another package pkg2 importing pkg1;

package pkg2;

import pkg1.A;
public class B extends A {
    @Override
    protected void m1() {
        System.out.println("override m1");
    }

    public static void main(String ar[]) {
        A obj=new B();
        obj.m1();
    }
}

Answer 1

I feel like you're still slightly confused with why protected m1() isn't visible.

You understand that main is a method in B and B is a subclass of A , therefore you feel like it should be accessible.

The key is obj is cast to be interpreted of type A . Then you try to call instance method m1 from static context in B (see this link above for why this is important). By class A definition, m1 is accessible through:

1. Class definition itself on a given instance of itself.

//example 
package pkg1;

public class C {
    protected void m1() {
        System.out.println("protected method");
    }

   public static void callM1(C anInstanceOfC){
        anInstanceOfC.m1();
   }
}

package pkg2;

import pkg1.C;
public class D extends C {
    @Override
    public void m1() {
        System.out.println("override m1");
    }

    public static void main(String ar[]) {
        C obj=new D();
        C.callM1(obj);//output would be "override m1"
    }
}

2. Instance methods (of course) of the class within itself and of subclasses.

//example 
package pkg1;

public class C {
    protected void m1() {
        System.out.println("protected method");
    }

    public void m2() {
        m1();//call to instance method m1
    }

   public static void callM1(C anInstanceOfC){
        anInstanceOfC.m2();
   }
}

package pkg2;

import pkg1.C;
public class D extends C {
    @Override
    public void m1() {
        System.out.println("override m1");
    }

    public void m3() {
        //super.m2()
        m2();//call to superclass method
    }

    public static void main(String ar[]) {
        D obj=new D();
        D.m3(); // output "override m1"
    }
}

Answer 2

protected keyword means that you can access protected-defined data from child classes. In example, you try to access protected data from non-child static context. You should probably try this:

package pkg2;

import pkg1.A;
public class B extends A {
    @Override
    public void m1() {
        System.out.println("override m1");
    }

    public static void main(String ar[]) {
        B obj=new B();
        obj.m1();
    }
}

Answer 3

The access level of m1() is protected. Protected methods can only be accessed by subclasses or by other classes in the same package . From your static 'main' method, you can not call a protected instance method of a class in a different package. If class B was in the same package as class A, then you will have no error and your code would work fine.