Re: Constructing an object from a superclass object
On 3/17/2010 3:04 PM, Michael Preminger wrote:
Hello!
I have an application where I use different types of persons.
Sometimes I need to convert a general Person (superclass) into a
specific one(subclass)
Is there a way to do it per constructor of the subclass, like:
public Subperson (Person person)
without explicitly copying the member values from the super object
and destroying it later?
Or do I need to do it outside the class, like
Subperson sp = (Subperson) person ?
I would prefer the former, if it was possible.
Since the Subperson has attributes that an ordinary Person
lacks, there's no way to transform a plain Person into a
Subperson.
You can, as in your constructor example, build a brand-new
Subperson and copy (or derive) some of its data from a separate
Person object. But the Person object does not "become" a
Subperson thereby; the two objects have independent existences.
On the other hand, if you start with a Subperson and want
to view it/him/her as a plain Person, that's easy: a Subperson
already *is* a Person, and has all a Person's attributes. And
if you have a Person reference that happens to point to an
actual Subperson instance (viewing it only as a Person), you
can "down-cast" to a Subperson again:
Person p = new Subperson("John Galt");
// do Person-like things with p
// can't get at Subperson-like things via p
This works because a Subperson *is* a Person.
Subperson s = (Subperson)p;
// now s and p refer to the same object, but
// s can access its Subpersonhood in addition
// to its Personhood
This works because the Subperson is still a Subperson, even
though you viewed it as merely a plain Person for a while.
Person p2 = new Person("Archibald Leach");
// do Person-like things with p2
Subperson s2 = (Subperson)p2; // ClassCastException!
This fails because a plain Person is *not* a Subperson.
--
Eric Sosman
esosman@ieee-dot-org.invalid