code: static final Object getObjectId(Object obj) {
          if (obj == null) {
              throw new NullPointerException();
          }
          if (!(obj instanceof ZooPC)) {
          	//non-peristent class
          	return null;
          }
          long id = Session.getObjectId(obj);
          //non-persistent object --> null
          return id >= 0 ? id : null;
      }
docs: Returns <code>null</code> if the object is transient or not
  persistent capable.
  @param obj
  @return <code>null</code> if the object is transient or not
  persistent capable.