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: