im trying to fix the part of my program that allows the user to edit some classes. but i keep getting this error error java.lang.NumberFormatException: For input string: "" for this part of my code Integer id = Integer.valueOf( request.getParameter( "id" ) ) and i have no idea how to fix it. i get that error in my controller class at the bottom

heres my jsp that does the display
<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
<%@ taglib prefix="c" uri=""%>
<jsp:useBean id="gb" class="cs320Homework1.servlet.CourseDisplay"
	scope="application" />
<!-- param takes the value of what is typed in the edit -->
<c:if test="${not empty param.edit}">
	<c:set target="${}" property="name" value="${param.code}" />
	<c:set target="${}" property="message" value="${param.title}" />
	<c:redirect url="CourseDisplay.jsp" />
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "">
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Edit Course</title>
		<c:when test="${not empty user}">
			<form action="EditCourse" method="post">
				Code: <input type="text" name="code" value="${entry.code}" /><br />
			    Message: <input type="text" name="code" value="${entry.message}" /><br />
				<input type="submit" name="edit" value="Edit" /> <input type='hidden' name='id' value="${}" />
			<a href='Logout'>Logout</a>
			<c:redirect url="Login"></c:redirect>

and here my controller
package cs320Homework1.servlet;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import cs320Homework1.servlet.CourseEntry;
public class EditCourse extends HttpServlet {
	private static final long serialVersionUID = 1L;
	public EditCourse() {
	 @SuppressWarnings({ "unchecked", "unused" })
	private CourseEntry getEntry( Integer id )
	        List<CourseEntry> entries = (List<CourseEntry>) getServletContext().getAttribute(
	            "entries" );
	        for( CourseEntry entry : entries )
	            if( entry.getId()==id ) return entry;
	        return null;
	protected void doGet(HttpServletRequest request,
			HttpServletResponse response) throws ServletException, IOException {
		 Integer id = Integer.valueOf( request.getParameter( "id" ) );
		 CourseEntry entry = getEntry( id );
		 request.setAttribute( "entry", entry );
		 request.getRequestDispatcher( "/WEB-INF/EditCourse.jsp" ).forward(
		            request, response );
	protected void doPost(HttpServletRequest request,
			HttpServletResponse response) throws ServletException, IOException {
		String code = request.getParameter("code");
		String title = request.getParameter("title");
		String[] prerequisites = request.getParameterValues("prerequisites");
		Integer index = Integer.valueOf(request.getParameter("id"));
		List<CourseEntry> entries = (List<CourseEntry>) getServletContext()
		if (prerequisites == null) {
			entries.get(index).setPrerequisites(new String[] {});
		} else {