Struts2 tag <s:select /> application

1.后台数据:
     /** List of days. */
    private Collection<Date> examDays;
         /** The map list. */
    private List<Map> examDayList = new ArrayList<Map>();

      SimpleDateFormat df = new SimpleDateFormat("dd");
        for (Date examDay : examDays) {
            Map examDayMap = new HashMap();
            examDayMap .put("examDay", examDay);
            //如果examDay = '2012-12-06' ,则 df.format(examDay) = 06
            examDayMap .put("day", df.format(examDay));
            examDayList .add(examDayMap);
        }
2.前台展示:
       <s:select
           name="examDay"
           requiredLabel="true"
           list="examDayList "
           listValue="%{day}" listKey
           ="examDay"
           headerKey="" headerValue="--" />
3. If you want the default value displayed in the drop-down box, you can set <s:select / > the value of the attribute value in