C++-like friend class mechanism in Java

Question

Do you know how can I make an object changeable only inside a special class? In this example I want the object PrivateObject to be only changable (incrementable) inside the Box class, nowhere else. Is there a way to achieve this?

public class Box { 

    private PrivateObject prv;

    public void setPrivateObject(PrivateObject p){
        prv = p;
    }

    public void changeValue(){
        prv.increment();

    }

}


public class PrivateObject {
    private value;
    public increment(){
        value++;
    }

}

PrivateObject priv = new PrivateObject ();
Box box = new Box();
box.setPPrivateObject(priv);
box.changevalue();
priv.increment(); // I don't want it to be changeable outside the Box class!

In C++, I would make all the PrivateObject properties and methods private, and the declare the Box class as a friend for the PrivateObject class.

Answer 1

If PrivateObject is strongly related to Box why not make it an inner class inside Box?

class Box { 

    public static class PrivateObject {
       private value;

       private increment(){
         value++;
       }
    }

    private PrivateObject prv;

    public void setPrivateObject(PrivateObject p){
        prv = p;
    }

    public void changeValue(){
        prv.increment();
    }       
}

Now you cannot call increment from outside Box:

public static void main(String args[]) {
    Box.PrivateObject obj = new Box.PrivateObject();
    Box b = new Box();
    b.setPrivateObject(obj);
    obj.increment(); // not visible     
}