Cloning objects in Java [3 questions]

Question

will the clone method of Asub be called by doing this? Or is Asub deep cloned properly? If not, is there a way to propery deep clone Asub through this kind of method?

Answer 1

First of all, note that the clone() interface is broken, thus should not be used in new code. It is better to implement copy constructor(s) instead.

However, if you really need to do it, the proper way is for TopMost to implement Cloneable. Why? Says Effective Java 2nd Edition, Item 11:

So what does Cloneable do, given that it contains no methods? It determines the behavior of Object’s protected clone implementation: if a class implements Cloneable, Object’s clone method returns a field-by-field copy of the object; otherwise it throws CloneNotSupportedException.This is a highly atypical use of interfaces and not one to be emulated. Normally, implementing an interface says something about what a class can do for its clients. In the case of Cloneable, it modifies the behavior of a protected method on a superclass.

Moreover, Asub.clone should be declared public, not protected - otherwise you can't call it from the outside world. Also, if you are using Java5 or above, it is legal and desirable for Asub.clone to return Asub, not Object (and similarly for its superclasses).

You don't show any members in the classes - the implementations of clone in the various classes can be a whole lot different depending on the types of members in that class. Namely, if a class has any mutable members, you need to carefully deep copy all of them, otherwise you end up with distinct objects sharing their internal state.

However, supposing your classes have only primitive or immutable fields, the cloning works as expected, although you have a lot of unnecessary clone methods in your abstract classes, all of which simply call super.clone() - you may be better off with Asub.clone() only.

As a side note, if Top a = (Top) super.clone() is not a typo, you introduce a dependency from base class to derived class, which is not a good idea.