I'd like to see the code to do this. Can you show me some code?
dan
Joel Crisp wrote:
> Hi
>
> You use the EJB to store a field containing the real polymorphic object.
>
> Easy really ;-) Think EJB == Persitance helper
>
> Joel
>
> dan benanav wrote:
> >
> > I have been looking into the possibility of using EJB beans for a
> > variety of projects. It seems to me that for real life production
> > applications EJB is very difficult to use and to learn. Sure the simple
> > examples are easy and work well. In principle, EJB sounds great. The
> > idea of having a standard server and freeing the programmer from system
> > level programming issues sounds great. It seems to me that the reality
> > of EJB is that the programmer must still consider system level issues
> > and that he or she can no longer use traditional object oriented
> > techniques.
> >
> > I''ll back this up with a concrete example. This example actually is
> > required for a real application but I have taken out some of the details
> > and changed some of the names. I know that similar things have been
> > discussed in other forms in various newsgroups and other EJB docs. I
> > have yet to see a good answer yet. The point is that using typical OO
> > programming this is easy, but using EJB I am forced to think of other
> > approaches. In this example we have to represent a certain kind of
> > graph called a PGraph. PGraphs have two kinds of nodes, ANodes and
> > TNodes. Both PGraphs, ANodes and TNodes have to be persistent. ANodes
> > are connected to TNodes and TNodes are connected to ANodes. The PGraph
> > has a distinguished ANode called the starting node. All ANodes and
> > TNodes added to the graph must be reachable from the starting ANode.
> >
> > Using an object oriented approach I would create classes similar to what
> > is shown below. I have left out some details, like error checking and
> > exceptions.
> >
> > public class PGraph
> > {
> > private ANode myStartANode;
> > private HashSet myAnodes;
> > private HashSet myTnodes;
> > public void PGraph(Anode a)
> > {
> > myStartANode = a;
> > myAnodes.add(a);
> > }
> >
> > public void connect(ANode a, TNode t) throws NoSuchElementException
> > {
> > if(!myAnodes.contains(a))
> > {
> > throw new NoSuchElementException();
> > }
> > a.addConnectionTo(t);
> > myTNodes.add(t);
> > }
> >
> > public void connect(TNode t, aNode a) throws NoSuchElementException
> > {
> > if(!myTNodes.contains(t))
> > {
> > throw new NoSuchElementException();
> > }
> > t.addConnectionTo(a);
> > myANodes.add(a);
> > }
> > public Collection getAnodes()
> > {
> > return myAnodes;
> > }
> >
> > }
> >
> > public class ANode
> > {
> > private Vector myTNodes;
> > public void addConnectionTo(TNode t)
> > {
> > myTNodes.addElement(t);
> > }
> > }
> >
> > public class TNode
> > {
> > private Vector myANodes;
> > public void addConnectionTo(ANode a)
> > {
> > myANodes.addElement(a);
> > }
> > }
> >
> > Now another important requirement is inheritance. I now want to define
> > something called ACNode and TRNode. ACNode inherits from ANode and
> > defines a number of new methods and TRNode inherits from TNode and
> > defines a number of new methods. From there I will inherit from TRNodes
> > and ACNodes to define many new classes. A PGraph will often end up
> > containing mixture of several types of TRNodes and ACNodes
> > (Polymorphism).
> >
> > Using traditional object oriented programming representing this problem
> > is easy and I can freely use inheritance and standard classes like
> > Vector, HashSet etc... I can also use inheritance and refer to other
> > objects without difficulty.
> >
> > The application I am considering needs to make the PGraph instances and
> > the ACNode and TRNode instances persistent. The method calls on
> > PGraph, ACNode and TRNode need to be synchronized since more than one
> > thread can call those methods at the same time. And of course the calls
> > will be remote calls. Adding those features will take take time. I may
> > also want to cache PGraphs and ACNodes and TRNodes for efficiency. That
> > is where EJB should help. But it seems that in order to achieve those
> > results I need to rethink the representations of PGraph, ACNodes and
> > TRNodes. It appears that I am no longer free to use classes like
> > Vector, HashSet etc.. Or if I choose to use those structures I will need
> > to do Bean Managed Persistence. And even in that case I don't think
> > this example will work correctly. For example, when I call getAnodes()
> > will I get a Collection of polymorphic types? That is essential for
> > this problem.
> >
> > Bottom line is that I am not sure that I shouldn't just implement the
> > persistence, caching and multi-threading issues myself, because the EJB
> > approach would force a different almost unnatural implementation and
> > representation of the problem.
> >
> > How about it? Can anyone come up with an EJB (1.0 or 1.1) approach to
> > this problem?
> >
> > dan
> >
> > ===========================================================================
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> > of the message "signoff EJB-INTEREST". For general help, send email to
> > [EMAIL PROTECTED] and include in the body of the message "help".
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