C++-like friend class mechanism in Java
friend, java
Solution
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
}
Problem
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.