difference between protected in C++ and protected in Java

In C++, protected members cannot be accessed outside the class and its derived classes using obj.membername

#include <QtCore>
//#include "windows.h"

//#include <QApplication>

class A
{
public:
    void fun()
    {
        qDebug()<<"a";
        A a;
        a.aa=123;
        this->aa=123;
        A::aa=123;
    }
protected:
     int aa;

};

class B
{
public:
    void fun()
    {
        A a;
        a.aa=123; //illegal
    }
};

class AA:public A
{
    void fun()
    {
        //A*pa=new A;
        A::aa=123;
        this->aa=123;
        aa=123;
        AA aa;
        aa.aa=123;

        A a;
        a.aa=123; //illegal
    }
};

int main(int argc, char *argv[])
{
    A a;
   // a.a=123;
    a.aa=123; //illegal
   // A::aa=123;
    return 0;
}

Protected members can be visited in the derived classes using the member name directly, parentclass::membername, this->membername, subclassobj.membername, but cannot be accessed using parentclassobj.membername.

Protected members in Java behavior differently due to the existence of package. They are like public members for non-derived classes in the same package. They can be accessed using obj.membername, while this is not true in C++. For non-derived classes in different packages, they are not accessible. In derived classes in the same package, they can be accessed using parentclassobj.membername(which is not true in C++), subclassobj.membername, membername itself. For derived classes that are not in the same package, they can be accessed by membername, subclassobj.membername, but cannot be accessed using parentclassobj.membername. So the cross-package access for protected members in Java keeps the same as in C++.

//App.java
package fruit;

public class App 
{
  String a="aa";
  protected void fun()
  {
    System.out.println("in App");
  }
}

class App1
{
  String a="aa";
  void fun()
  {
    App app=new App();
    app.fun();
    
    System.out.println("in App1");
  }
}

class App2 extends App
{
  String a="aa";
  void fun1()
  {
    App app=new App();
    app.fun();
    fun();
    
    App2 app2=new App2();
    app2.fun();
    System.out.println("in App2");
  }
}

//Test.java
import fruit.App;
class Test extends App
{
  String a="aa";
  public static void main(String[] args)
  {
    App app=new App();
    //app.fun();
    //fun();
    System.out.println("hello");
  }
  void fun2(/*App app*/)
  {
    fun();
    Test app=new Test();
    app.fun();
    App ap=new App();
    ap.fun();//illegal
  }
}

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