if (now != null && prevValue != null) return now.subtract(prevValue); };
if (now != null && prevValue != null) { return now.subtract(prevValue); } else { return null; } }
        public Object getValueAt(int rowIndex, int columnIndex) {            if (_key == null) return null;            SessionID id = _model.getSessionIDAt(_key, rowIndex);            switch(columnIndex) {                case 0: return id.getDate();                case 1: return id.getValue();                case 2: return _model.getSessionIDValue(_key, id);                case 3:                     if (rowIndex == 0) {                        return null;                    } else {                        SessionID prev = _model.getSessionIDAt(_key, rowIndex - 1);                        BigInteger prevValue = _model.getSessionIDValue(_key, prev);                        BigInteger now = _model.getSessionIDValue(_key,  id);                        if (now != null && prevValue != null) return now.subtract(prevValue);                    };                default: return null;            }        }