question: public void addSessionEventURL ( String url , SessionID sid ) { Set < SessionID > sids = sessionEventURLs . get ( url ) ; if ( sids == null ) { sids = Collections . newSetFromMap ( new ConcurrentHashMap < SessionID , Boolean > ( ) ) ; Set < SessionID > previousValue = sessionEventURLs . putIfAbsent ( url , sids ) ; if ( previousValue != null ) { sids = previousValue ; } } if ( sids . add ( sid ) ) { notifyPersistenceManager ( ) ; } }
target:  Adds a listener for the associated session ID.