How can I rotate the text within an HSSFCell class of Apache POI?
Use HSSFCellStyle, that class has a method called setRotation(short rotation) which will rotate the text. All you do is apply the cell style to a cell:
HSSFCellStyle myStyle = workbook.createCellStyle();
myStyle.setRotation((short)90);
HSSFCell c = row.createCell(columnNumber);
c.setCellStyle(myStyle);