int Svolt = 0;
double Amperes = 0.0;
double LEDdrop = 0.0;

if (request.getParameter("Svolt") != null) {
try {
    Svolt = Integer.parseInt(request.getParameter("Svolt"));
} catch ( NumberFormatException e ) {
    %>
    <script type ="text/javascript">
        alert("The source voltage must be an integer. Please fix this.");
    </script>
    <%
}

final double MA_TO_A = 0.001;
int Svolt = 0;
double Amperes = 0.0;
double LEDdrop = 0.0;

try {
    if (request.getParameter("Svolt") != null) {
        try {
            Svolt = Integer.parseInt(request.getParameter("Svolt"));
        } catch (Exception e)
        {
            %>
            <script type ="text/javascript">
                alert("The source voltage must be an integer. Please fix this.");
            </script>
            <%
        }
        Amperes = Double.parseDouble(request.getParameter("Amperes"));
        LEDdrop = Double.parseDouble(request.getParameter("LEDdrop"));

        out.println("Ideal resistance is: " + ((Svolt - LEDdrop) / (Amperes * MA_TO_A)) + " Ohms.");
    }
} catch (Exception e) {
        out.println("Please make sure your inputs are correct.");
}

org.apache.jasper.JasperException: java.lang.NumberFormatException: For input string: "a"

final double MA_TO_A = 0.001;
int Svolt = 0;
double Amperes = 20.0;
double LEDdrop = 1.8;

try {
    if (request.getParameter("Svolt") != null) {
        try {
            Svolt = Integer.parseInt(request.getParameter("Svolt"));
        } catch (Exception e)
        {
            %>
            <script type ="text/javascript">
                alert("The source voltage must be an integer. Please fix this.");
            </script>
        <%
        }
        out.println("Ideal resistance is: " + ((Svolt - LEDdrop) / (Amperes * MA_TO_A)) + " Ohms.");
    }
} catch (Exception e) {
    out.println("Please make sure your inputs are correct.");
}

final double MA_TO_A = 0.001;
                    int Svolt;
                    double Amperes;
                    double LEDdrop;

                    if (request.getParameter("Svolt") != null)
                    {
                        try
                        {

                            try
                            {
                                Svolt = Integer.parseInt(request.getParameter("Svolt"));
                            } catch (Exception e)
                            {
                            %>
                            <script type ="text/javascript">
                                alert("The source voltage must be an integer. Please fix this.");
                            </script>
                            <%
                            Svolt = 0;
                            }
                            try
                            {
                                Amperes = Double.parseDouble(request.getParameter("Amperes"));
                            } catch (Exception e)
                            {
                                %>
                                <script  type = "text/javascript">
                                    alert("The source voltage must be an integer. Please fix this.");
                                </script>
                                <%
                                Amperes = 0.0;
                            }
                            try
                            {
                                LEDdrop = Double.parseDouble(request.getParameter("LEDdrop"));
                            } catch (Exception e)
                            {
                            %>
                                <script  type = "text/javascript">
                                    alert("The source voltage must be an integer. Please fix this.");
                                </script>
                            <%
                            LEDdrop = 0.0;
                            }
                            out.println("Ideal resistance is: " + ((Svolt - LEDdrop) / (Amperes * MA_TO_A)) + " Ohms.");
                        } catch (Exception e)
                        {
                            out.println("Please make sure your inputs are correct.");
                        }
                    }

final double MA_TO_A = 0.001;
    int Svolt = 0;
    double Amperes = 0.0;
    double LEDdrop = 0.0;
    try {
        if (request.getParameter("Svolt") != null) {
            try {
                Svolt = Integer.parseInt(request.getParameter("Svolt"));
             } catch (Exception e)
            {
                //Changed to avoid the message if no exception, or for some other reason it fails to execute the conditions
                out.println("<script type ="text/javascript">")
                out.println("alert("The source voltage must be an integer. Please fix this.");") ;
                out.println("</script>")
            }
            Amperes = Double.parseDouble(request.getParameter("Amperes"));
            LEDdrop = Double.parseDouble(request.getParameter("LEDdrop"));

            out.println("Ideal resistance is: " + ((Svolt - LEDdrop) / (Amperes * MA_TO_A)) + " Ohms.");
        }
    } catch (Exception e) {
            out.println("Please make sure your inputs are correct.");
    }

int Svolt = 0;
double Amperes = 0.0;
double LEDdrop = 0.0;
if (request.getParameter("Svolt") != null) {
      try {
          Svolt = Integer.parseInt(request.getParameter("Svolt"));
      } catch ( NumberFormatException e ) {
          %>
          <script type ="text/javascript">
              alert("The source voltage must be an integer. Please fix this.");
          </script>
          <%
      }
}