private void ensureColumnBounds(Sheet sheet) {
        if (gotBounds) {
            return;
        }

        Iterator<Row> iter = sheet.rowIterator();
        firstColumn = (iter.hasNext() ? Integer.MAX_VALUE : 0);
        endColumn = 0;
        while (iter.hasNext()) {
            Row row = iter.next();
            short firstCell = row.getFirstCellNum();
            if (firstCell >= 0) {
                firstColumn = Math.min(firstColumn, firstCell);
                endColumn = Math.max(endColumn, row.getLastCellNum());
            }
        }
        
        if (maxColumns > 0 && endColumn > maxColumns){
            endColumn = maxColumns;
        }
        gotBounds = true;
    }