Error Jsp Adding Subtracting Numbers

this web app is a simple calculator that adds and subtracts. my problem is that the add and the subtract buttons don't work

i get the following errors : "18 in the jsp file: /adder.jsp opchar cannot be resolved " and "An error occurred at line: 52 in the jsp file: /adder.jsp opchar cannot be resolved" i know i need to initialize opchar but dont know how

<%
        String name = (String) session.getAttribute("name");
        if (name==null) {
                name = request.getParameter("name");
                if (name!=null) session.setAttribute("name",name);
        }
    
        String sum = (String) session.getAttribute("sum");
        if (sum==null) {
                sum = "0";
                session.setAttribute("sum",sum);
        }
        int isum = Integer.parseInt(sum);
   
        String number = request.getParameter("number");
        if (number==null) number = "0";
        int inumber = Integer.parseInt(number);
         switch(opchar){
       case "add": 
        isum += inumber;
        session.setAttribute("sum",""+isum);
        break;
       case "subtract": 
        isum += inumber;
        session.setAttribute("sum",""+isum);
        break;}
        %>
        <html>
        <head>
   <title>Adding Machine</title>
        </head>
        <body>
                <form method='get' action='adder.jsp'>
        <%
        if (name==null) {
        %>
                        <p>
                                Name: <input type='text' name='name' id='name'>
                        </p>
                        <p>
                                <input type='submit' name='submit' id='submit' value='signin'>
                        </p>
        <%
        } else {
        %>
                        <p>Welcome, <%=name%>!</p>
                        <p>Current sum is: <%=isum%></p>
                        <p>
                                Number to add: <input type='text' name='number' id='number' value='add'>
                        </p>
                        <p>
                               <% if (opchar == "add") %> <input type='submit' name='submit' id='submit' value='add'>
                        </p>
                        <p>
                               <% if (opchar == "subtract") %> <input type='submit' name='submit' id='submit' value='subtract'>
                        </p>
        <%
        }
        %>
                </form>
        </body>
        </html>

#1 don't put java code in a jsp. Java code belongs in a java class.
#2 Unless you're using java 7, you can't switch on a String.
#3 You can't compare strings with ==. Use the equals() method.
#4 The subtraction operator is '-', not '+'

To initialize opchar you need to declare the variable, and give it a value.

something like:

Depending upon which button you clicked in your form, the value of opchar would then be either 'add' or 'subtract'

thanks for your help