Perform simple arithmetic operation and show the result on the same jsp page

Question

I'm trying to wrap my head around servlets and JSP and I got stuck when implementing a simple calculator.

Basically, I have two input fields, operator selection field and the submit button.

When I click on the submit button I need to perform the chosen arithmetic operation on two values from input elements and show the result on the same page.

Here's what I have:

<!-- hello.jsp page -->

<form action="hello.jsp" id="calc-form">
    <input type="number" name="num1" required>
    <select id="opers" name="oper">
        <option>+</option>
        <option>-</option>
        <option>*</option>
        <option>/</option>
    </select>
    <input type="number" name="num2" required>
    <input type="submit" value="Calculate">
</form>
<h2>The result is: ${result}</h2>

my doGet method in hello servlet:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    response.setContentType("text/html");

    System.out.println("Hello#doGet");

    String strNum1 = request.getParameter("num1");
    String strNum2 = request.getParameter("num2");
    String oper = request.getParameter("oper");

    double a, b, result = 0;

    if(validateNum(strNum1) && validateNum(strNum2) && validateOper(oper)) {

        try {           

            a = Double.parseDouble(request.getParameter("num1"));
            b = Double.parseDouble(request.getParameter("num2"));

            switch(oper) {
            case "+":
                result = a + b;
                break;
            case "-":
                result = a - b;
                break;
            case "*":
                result = a * b;
                break;
            case "/":
                if (b == 0) {
                    throw new ArithmeticException("Division by zero is not allowed");
                } else {
                    result = a / b;
                }
            }
        } catch(NumberFormatException | ArithmeticException e) {
            // handle the exception somehow
        }
        request.setAttribute("result", result);

    }

    RequestDispatcher dispatcher = request.getRequestDispatcher("/hello.jsp");
    dispatcher.forward(request, response);
}

So, when I go to http://localhost:8080/test2/hello, enter the numbers in input elements and press submit, I get redirected to address that looks pretty much like this: http://localhost:8080/test2/hello.jsp?num1=4&oper=*&num2=4

however, I don't get the result.

Could you please tell me what I'm doing wrong here?

Answer 1

Look at your action

   <form action="hello.jsp" id="calc-form">

You need to point your action to the servlet. not JSP.