code: public void addDataModelListener(DataModelListener listener) {
  
          if (listener == null) {
              throw new NullPointerException();
          }
          if (listeners == null) {
              //noinspection CollectionWithoutInitialCapacity
              listeners = new ArrayList<DataModelListener>();
          }
          listeners.add(listener);
  
      }
docs: <p>Add a new {@link DataModelListener} to the set interested in
  notifications from this {@link DataModel}.</p>
  
  @param listener The new {@link DataModelListener} to be registered
  
  @throws NullPointerException if <code>listener</code>
  is <code>null</code>