jsp页面简单的提交表单数据并转为double类型，是下面代码有错吗？
<form action="" method="get">
<input type="text" name="a" value="a"><input><br>
<input type="submit" name="submit" value="submit"></input>
</form>
<%
String a1=request.getParameter("a");
double o=Double.parseDouble(a1);
out.println(o); %>
运行显示：
An exception occurred processing JSP page /b.jsp at line 16
13: </form>
14: <%
15: String a1=request.getParameter("a");
16: double o=Double.parseDouble(a1);
17: out.println(o);
14: <%
15: String a1=request.getParameter("a");
16: double o=Double.parseDouble(a1);
17: out.println(o);
18:
19:
