public void addHexListener(final IHexPanelListener listener){
  Preconditions.checkNotNull(listener,"Error: Listener can't be null");
  if (!m_listeners.contains(listener)) {
    m_listeners.add(listener);
  }
}

public void addHexListener ( final IHexPanelListener listener ) { Preconditions . checkNotNull ( listener , "Error: Listener can't be null" ) ; if ( ! m_listeners . contains ( listener ) ) { m_listeners . add ( listener ) ; } }
adds a new event listener to the list of event listeners .
