Understanding Object.clone() in Java

Question

I know that using this cloning mechanism is not a good idea (since it is 'broken' as some authors suggest), although I need help understanding how it works. We're given the following class hierarchy:

class N implements Cloneable{
    protected int num;
    public N clone() throws CloneNotSupportedException{
        return (N)super.clone();
    }
}

class M extends N{
    protected String str;
    public M clone() throws CloneNotSupportedException{
        M obj = (M)super.clone();
        obj.setString(new String(this.str));
        return obj;
    }
    void setString(String str){
        this.str = str;
    }
}

Since N extends Object, how does the super.clone() return an instance of N? super.clone() is actually Object.clone() which returns a reference to an object of class Object. Why are we then able to cast it to N? N has a member num that is not in the class Object. How does the default behavior actually manage to automatically clone this variable (since it has no record of it in class Object)?

Also, the same goes for M. In M.clone() we're casting an object from class N (returned by super.clone()) to an object of class M. I know that all of this is valid, yet I do not understand why.

Answer 1

Object#clone is a native method that makes a low-level binary copy of your object, thus producing another instance of the same class. Therefore it is safe to downcast.

Note that this is the only way to have a polymorphic cloning method.

Answer 2

Technically, Object.clone() is a native method:

protected native Object clone() throws CloneNotSupportedException;

And the JVM internally knows how big the current object is, and what the type of the object is. So, it can create the appropriate object as a bit-wise copy and return a reference to it.

Answer 3

1. Since N extends Object, how does the super.clone() return an instance of N?
2. super.clone() is actually Object.clone() which returns a reference to an object of class Object. Why are we then able to cast it to N?
3. N has a member num that is not in the class Object. How does the
   default behavior actually manage to automatically clone this variable (since it has no record of it in class Object)?

Answers :

1. At runtime, you are inside an instance of N class calling its parent clone method. Consider it the exact same thing as if you had overrided it in your N class. The Object class is a native, and abstract object. When you call N.toString() in fact you call the first toString method the JVM finds in N's hierarchy.
2. Same here, remember you are in an instance of N class.
3. it does not : How do I copy an object in Java?

Answer 4

You can also use the XStream object to clone your objects, like this :

public static <T> T cloneObject(
    T object) {
    XStream xstream = new XStream();
    return (T) xstream.fromXML(xstream.toXML(object));
  }