Difficultity with abstract mapping

[Marcus Young]

Hi,
I have just been working an issue related to mapping abstract classes.  
I've read quite a lot about it but I'm still unclear.  The structure I 
have is as follows:

Abstract Class A which can hold a collection of  'A' objects.
Classes B and C which extend A
Each class is mapped to a separate table.
I am  hoping to be able to retreive records in such a way as to 'build' 
an 'A' class object including the objects collection which may include 
objects of class B and C.   So far I have been working with 'super' and 
'extents' - but to be honest I'm not really sure how to get it working.  
Most commonly I'm getting exceptions of "InstantiationException". -  
this appears to be related to the abstract 'A' class?  I can resolve 
this particular exception by using a concrete class, however I 
particularly wanted to use an abstract class.  Any assistance would be 
greatly appreciated.

I end up with something like this:
public abstract class A extends TransferObject
{  
   protected Long _id;
}

public class B extends A
{
   private String _name;
   private String _description;
*}
*public class C extends A
{
   private String _name;
   private String _description;
*}
*   <class-descriptor
     class="com.eclipsej.permission.A" table="A">
        <field-descriptor
           name="_id"
           column="ID"
           jdbc-type="BIGINT"
           primarykey="true"
           autoincrement="true"
        />
        <!-- extent-class class-ref="com.eclipsej.user.B" / -->
   </class-descriptor> 
 
<!-- Defnitions for com.eclipsej.user.B -->
  <class-descriptor class="com.eclipsej.user.B" table="roles">
     <field-descriptor name="_id" column="ID" jdbc-type="BIGINT" 
primarykey="true" autoincrement="true"/>
     <field-descriptor name="_name" column="NAME" jdbc-type="VARCHAR"/>
     <field-descriptor name="_description" column="DESCRIPTION" 
jdbc-type="VARCHAR"/>    
     <reference-descriptor name="super" 
class-ref="com.eclipsej.permission.A"
        auto-retreive="true" auto-update="true" auto-delete="true">
        <foreignkey field-ref="_id"/>
     </reference-descriptor>  
  </class-descriptor>*
*

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