显示首页：<input type="hidden" name="a" value="first">
<input type=submit value="first">
</form>
<form action="<%=str%>" method="post">
显示下一页：<input type="hidden" name="a" value="next">
<input type=submit value="next">
</form>
<form action="<%=str%>" method="post">
显示上一页：<input type="hidden" name="a" value="previous">
<input type=submit value="previous">
</form>
<form action="<%=str%>" method="post">
输入要显示的页数：<input type="text" name="a" value="1" style="text-align:right">
<input type=submit value="submit">
</form>
<%
string s=request.getparameter("a");
if(s==null){
