Re: Constructing an object from a superclass object

From:
"Mike Schilling" <mscottschilling@hotmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 18 Mar 2010 08:45:52 -0700
Message-ID:
<hnthri$1ai$1@news.eternal-september.org>
Eric Sosman wrote:

On 3/17/2010 5:33 PM, Mike Schilling wrote:

Eric Sosman wrote:

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.


If Subperson is Serializeable, you could

1. Serialize it to a ByteArrayOutputStream
2. Paying careful attention to the serialization spec, make the
changes to the resulting bytes that make it look like a serialized
Subperson instead of a serialized Person.
3. Deserailize it.


    I guess you mean "if Person is Serializable?" That is,
you're going to serialize a Person instance, twiddle the bytes,
and deserialize to a Subperson?


Yes, you're right.

    Yeah, well, but that's just a roundabout (and filthy) way of
doing the copy that the O.P. wanted to avoid.


Well, it avoids having to hard-code the names of the fields to be copied,
and I think I could write a generalized version of it that would allow
copying any object into a subclass, provided the object is serializeable and
its serialization hasn't been too customized.

Not that I recommend this, mind you ...


    "Not recommend" isn't strong enough language. You should
be using words like "abominate" and "revile" and "no jury in
the world would convict the perpetrator's killer."


That last is going a bit too far; there are many jurisdictions in which
euthanasia is still illegal.

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