java poi excel导出怎么制作标题

一个导出的excel，内容包括大标题，列标题和数据，现在这个大标题不会弄。用的是HSSFWorkbook。我是新手，希望能说的详细一点，多谢！我这样写的
HSSFWorkbook wb = new HSSFWorkbook();

HSSFSheet sheet = wb.createSheet("report");

//---
sheet.addMergedRegion(new CellRangeAddress(0,0,0,7));
HSSFRow rowheader = sheet.createRow(0);
HSSFCell cellheader = rowheader.createCell(0);
cellheader.setCellValue("标题内容");
//--
HSSFRow row = sheet.createRow(1);
HSSFCell cell = row.createCell(0);
String[] headNames=new String[]{"工号","姓名","迟到次数","早退次数","夜班","周末班","节假日班","总工时"};
for (int k=0;k cell=row.createCell(k);
cell.setCellValue(headNames[k]);
cell.setCellStyle(headStyle);
}
List list = workhourBizImpl.findByParamGruopForExcel(workhour);
for (int i = 0; i < list.size(); i++){
--数据
}

但是我一加//里面的代码，列标题就没了。。。