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

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     
}