public void guardar(ObjectContainer db) {
    Persona p = null;
    ObjectSet<Persona> set =
    db.queryByExample(new Persona(null, this.dni, 0));
    if(set.hasNext()){
     p = set.next();
      p.dni = this.dni;
      p.edat = this.edat; p.nom = this.nom;
     db.store(p);
    }
    else { db.store(this); }
    db.commit();