要求输入两个数，只输入一个，返回一个提示
public class AddAction extends DispatchAction {
    public ActionForward doAdd(ActionMapping mapping,
            ActionForm form,
            HttpServletRequest request,
            HttpServletResponse response) {
        Calculator calc = new Calculator();
        //获得ActionForm的引用
        AddForm addForm = (AddForm) form;
        double model = calc.Add(
            Double.parseDouble(addForm.getFirstNumber()),Double.parseDouble(addForm.getSecondNumber()) );
        //将计算结果存储到request中
        if(addForm.getFirstNumber()== null){
            String a ="you should input something";
            request.setAttribute("sum",a);
            return mapping.findForward("result");
        }
        else{
        String a = String.valueOf(model);
        request.setAttribute("sum", a);
        //将请求转发到"result"出口指向的页面
        return mapping.findForward("result");
        }
    }

谁能告诉我怎么改？没有报错  就是实现不了结果