java.lang.NumberFormatException: For input string: "" - When I try to parse empty string as integer

Question

I wanna make a servlet where you write your name, surname, email address and you have to enter the correct sum of two numbers.

I've got one problem. When I don't enter anything in this "captcha", it gives me HTTP Status 500, Message: HTTP Status 500

Exception:

java.lang.NumberFormatException: For input string: ""
    java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:68)
    java.base/java.lang.Integer.parseInt(Integer.java:662)
    java.base/java.lang.Integer.parseInt(Integer.java:770)
    pl.coderslab.Sess05.doPost(Sess05.java:16)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:652)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

I'd like it to return to the form so you have to type the captcha (I'm not checking the other fields, don't pay attention to it).

Here's my code:

@WebServlet("/Sess05")
public class Sess05 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession sess = request.getSession();
        int userSum = Integer.parseInt(request.getParameter("captchaInput"));   //Here's 16 line
        int correctSum = (int) sess.getAttribute("captcha");
        if (userSum != correctSum || request.getParameter("captchaInput").equals("")) {
            Random randGenerator = new Random();
            int firstRand = randGenerator.nextInt(101);
            int secRand = randGenerator.nextInt(101);
            int sum = firstRand + secRand;
            sess.setAttribute("captcha", sum);

            response.getWriter().append("<form action='#' method='post'>");
            response.getWriter().append("<label>Name: <input type='text' name='name'></label><br>");
            response.getWriter().append("<label>Surname: <input type='text' name='surname'></label><br>");
            response.getWriter().append("<label>Email: <input type='email' name='email'></label><br>");
            response.getWriter().append("<label>Enter the sum " + firstRand + " + " + secRand + "<input type='number' name='captchaInput'></label><br>");
            response.getWriter().append("<input type='submit' value='Submit'>");
            response.getWriter().append("</form>");
            response.getWriter().append("Enter the correct sum!");
        } else {
            response.getWriter().append("Sum is correct!");
        }
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Random randGenerator = new Random();
        int firstRand = randGenerator.nextInt(101);
        int secRand = randGenerator.nextInt(101);
        int sum = firstRand + secRand;
        HttpSession sess = request.getSession();
        sess.setAttribute("captcha", sum);
        
        response.getWriter().append("<form action='#' method='post'>");
        response.getWriter().append("<label>Name: <input type='text' name='name'></label><br>");
        response.getWriter().append("<label>Surname: <input type='text' name='surname'></label><br>");
        response.getWriter().append("<label>Email: <input type='email' name='email'></label><br>");
        response.getWriter().append("<label>Enter the sum " + firstRand + " + " + secRand + "<input type='number' name='captchaInput'></label><br>");
        response.getWriter().append("<input type='submit' value='Submit'>");
        response.getWriter().append("</form>");
    }
}

Just check that request.getParameter("captchaInput") is not empty before parsing it — Federico klez Culloca
– Federico klez Culloca, Commented Nov 2, 2020 at 11:04

Mohamed Atef · Accepted Answer · 2020-11-02 11:23:17Z

The main problem is caused because you are trying to get integer value from empty variable which is not number so to fix the problem you should check if that parameter contains value and it is a numeric value by creating method like below in your code :

public static boolean isNumeric(String strNum) {
    if (strNum == null) {
        return false;
    }
    try {
        int d = Integer.parseInt(strNum);
    } catch (NumberFormatException nfe) {
        return false;
    }
    return true;
}

and then call this method in your code to check parameter value before doing anything and do whatever you want based on this boolean:

 @WebServlet("/Sess05")
    public class Sess05 extends HttpServlet {
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            HttpSession sess = request.getSession();
//  check if it is numeric or not           
 boolean isNumeric = Sess05.isNumeric(request.getParameter("captchaInput"));
if(isNumeric){
  // do your servlet logic 
}else {
  // do whatever you will do if you request hasn't a numeric value 
}

Anish B. · Accepted Answer · 2020-11-03 09:09:23Z

0

Update line number 16 :

  String value = request.getParameter("captchaInput");
  int userSum = value == null || value.isEmpty() ? userSum : Integer.parseInt(value);

edited Nov 3, 2020 at 9:09

answered Nov 2, 2020 at 11:16

Anish B.

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1 Comment

Maurice Perry Over a year ago

value == null || value.isEmpty() perhaps?

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