question: public void addListener ( FSMouseListener l ) { if ( l == null ) { return ; } if ( ! _handlers . containsKey ( l ) ) { _handlers . put ( l , l ) ; } if ( ! _enabled && _handlers . size ( ) > 0 ) { _panel . addMouseListener ( this ) ; _panel . addMouseMotionListener ( this ) ; _enabled = true ; } }
target:  Adds a listener to receive callbacks on mouse events.