question: protected synchronized void addTransitionListener ( Hashtable < TxState , Hashtable < TxState , Set < EventListener > > > listeners , EventListener lstnr , TxState from , TxState to ) { Hashtable < TxState , Set < EventListener > > lstnrs = listeners . get ( from ) ; if ( lstnrs == null ) lstnrs = new Hashtable < TxState , Set < EventListener > > ( ) ; Set < EventListener > tolstnrs = lstnrs . get ( to ) ; if ( tolstnrs == null ) tolstnrs = new HashSet < EventListener > ( ) ; if ( ! tolstnrs . contains ( lstnr ) ) tolstnrs . add ( lstnr ) ; lstnrs . put ( to , tolstnrs ) ; listeners . put ( from , lstnrs ) ; }
target:  Help function for adding transition listeners.