java jxl设置单元格格式为文本型？怎么设置的？

jxl.write.Number labelNF = new jxl.write.Number(j, i ,6677); NumberFormat nf = new NumberFormat("#00"); WritableCellFormat wcf = new WritableCellFormat(nf); labelNF.setCellFormat(wcf ); ws.addCell(labelNF);