I'm not saying the B1 and B2 instances should extend the same instance of A, but that they should be materialized from the same A record in the database. B1 and B2 would have to be different objects obviously. I think this could be quite possible in OJB.
But let me ask the question a different way - suppose I have an instance of A that already exists. Now I want to create a B1 instance that is associated with the already existing A instance. So I don't want a new A record to be created, only new B1 record that "extends" the pre-existing A record. Is that possible?
Thank you for your input.
Matt
Gerhard Grosse wrote:
Hi Matt,
Excuse me to jump in here, but I believe what you have in mind is not
possible in Java and even less with OJB. If your B1 and B2 instance
should 'contain' the same A instance as base class, they are identical
objects, which would require a language that supports multiple
inheritance (and virtual base classes). You could do that in in C++
but not in Java.
What you can do in Java (and OJB) is declare an interface I_A implemented by A, B1 and B2. I_A just declares all public methods of A and in B1 and B2 you implement this interface by delegation. That is in B1 and B2 you have a reference to a A object and all I_A methods on B1 and B2 are just routed to this reference. That way distinct instances of B1 and B2 can easily share a common A reference.
HTH, Gerhard
On Tue, 21 Oct 2003 00:27:02 -0400, Matt Mastrangelo <[EMAIL PROTECTED]> wrote:
Chris,
Just to clarify -- are you saying it's not possible to create the User instance without also creating a new Person instance? Maybe my example was not too clear. Let me re-explain:
I have a base class A with two subclasses, B1 and B2. I have already created an instance of B1 and stored it to the database, which means that an A record and a B1 record were inserted. I now want to create an instance of B2 that extends from the same A record as B1. Is this possible?
Thanks again.
Matt
[EMAIL PROTECTED] wrote:
Matt,
I believe since the Person and Student object exist it will set there values to null except the personId. Because in the User instance you are storing allthe other fields are null (if you set them to null when you declare them) except the loginName. So, ojb will know that person and student 001 exist and will do and update instead of an insert.
You should create them on the first store. Or do not have user extend Student, instead have a 1:1 relationship. Depends on your requirments i guess.
-chris worley
I'm trying to figure out how to store a new persisted object that extends a pre-existing object.
For example, I have 3 classes: Person, Student, and User. Student and User extend Person. A Student object already exists, and I now want to create a User object that inherits from the same Person as the Student object. Is this possible? Here's a code example:
User user = new User(); user.setLoginName("matt"); user.setPersonOid("001"); // A pre-existing person
broker.beginTransaction(); broker.store(user); broker.commitTransaction();
The result of the above code is a NEW empty person object being created with a newly generated OID (via a sequence). How can I associate the new User with a pre-existing Person?
Thanks again.
Matt
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