public final void checkOccurrenceNumbers ( ) { if ( LOGGER_ . isTraceEnabled ( ) ) LOGGER_ . trace ( toString ( ) + " occurences: " + printOccurrenceNumbers ( ) ) ; if ( positiveOccurrenceNo < 0 || negativeOccurrenceNo < 0 ) throw new ElkUnexpectedIndexingException ( toString ( ) + " has a negative occurrence: " + printOccurrenceNumbers ( ) ) ; }
verifies that occurrence numbers are not negative