question: public static synchronized void registerIdentityHandler ( final IdentityHandler identityHandler ) { if ( identityHandler == null ) throw new IllegalArgumentException ( "Attempt to register null IdentityHandler" ) ; LinkedList < IdentityHandler > newList = duplicateIdentityHandlers ( ) ; newList . remove ( identityHandler ) ; newList . addFirst ( identityHandler ) ; identityHandlers = newList ; }
target:  This method registers an IdentityHandler to the list of possible IdentityHandlers. An IdentityHandler will not be registered more than once.