Enumeration e = mot.VPersonnes.elements();
while(e.hasMoreElements())
{
     Object obj;
     obj = e.nextElement();
     jCBPers.addItem( ( (obj.getClass()) obj.GetIdentifiant()).GetID() );
     //jCBPers.addItem( ( ( (PersonnelScientifique) e.nextElement()).GetIdentifiant()).GetID() ); //It's working with STATIC CASTING !!!!
            
}

The sub-class of Persons (the class that are in the vector) IMPLEMENT Interface "EstIdentifiable".
The differents Persons have 1 "ID". 
ID is an instance of the class "Matricule".
Matricule IMPLEMENT Interface "Identifiant"
And 

interface EstIdentifiable got this method : 
public String GetIdentifiant();

and 
interface Identifiant got this method:
public String GetID();



When i do : 
String ID = aRichPerson.GetIdentifiant().GetID();

The id is correctly returned !

In my example, I need to CAST the object "obj" in the correct Class to reach the GetIdentifiant().GetID() methods.