当图片判断为空时，想要强行赋给它一张图片，应该怎么写？

各位高手请帮我看看下面的程序，
当图片判断为空时，想要强行赋给它一张图片，应该怎么写？
在线等待private UserService userService;
public ActionForward execute(ActionMapping mapping, ActionForm form,
HttpServletRequest request, HttpServletResponse response) {
UserForm userForm = (UserForm) form;// TODO Auto-generated method stub
try {
User user=new User();
userService.fillUser(userForm,user);
if(userForm.getPicture()!=null){
String fileName=userService.upload(userForm);
user.setPicture(fileName);
System.out.println(fileName);
}
//就在这个区域写一个判断图片为空时强行赋给它一张图片


   userService.addUser(user);
// modify by zhongbj 将成功保存的数据 能够在主界面查询出来
request.setAttribute("departmentList", userService.getDepartmentOptionList(user.getUserId()));
request.setAttribute("allDepartmentList", userService.getAllDepartmentOptions(user.getUserId()));
request.setAttribute("userGroupList", userService.getUserGroupOptionList());
request.setAttribute("stateList", userService.getStateOptionList());
request.setAttribute("parentList", userService.getParentUserOptions());
request.setAttribute("positionGradeList", userService.getPositonGradeOptionList());
request.setAttribute("titleList", userService.getTitleOptionList());
request.setAttribute("positionList", userService.getPositonOptionList());
request.setAttribute("educationList",userService.getEducationOptionList());

request.setAttribute("auth", AuthorizationObject.getAuthObject(user,"/user.do"));


List<User> users=new ArrayList<User>(0);
users.add(user) ;
List<UserForm> forms=userService.convertUserToForm(users);
request.setAttribute("users",forms);
request.setAttribute("DIVShow","");

} catch (StringIndexOutOfBoundsException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return mapping.findForward("userPage");
}