public void addListener(final ProjectorListener projectorListener){
  if (listeners == null) {
    listeners=new ArrayList<ProjectorListener>();
  }
  listeners.add(projectorListener);
}

public void addListener ( final ProjectorListener projectorListener ) { if ( listeners == null ) { listeners = new ArrayList < ProjectorListener > ( ) ; } listeners . add ( projectorListener ) ; }
add a projector listener .
