Add the following code, properly
RegionUtil.setBorderBottom(1, region, hssfSheet, wb);
RegionUtil.setBorderTop(1, region, hssfSheet, wb);
RegionUtil.setBorderLeft(1, region, hssfSheet, wb);
RegionUtil.setBorderRight(1, region, hssfSheet, wb);
As for what is in the parameters, you can click to see the official API