Null pointer exception while using cookie to remember the previous color

**oss_2008** · June 26th, 2009, 07:53 AM

I'm trying to write a simple program to have the background on a html page changed by selecting the colour on radio buttons, this I managed to do successfully. As soon as I add a cookie to remember the colour and load the same background the next time I get null pointer exceptions all over the place.

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
public class ColourSelect extends HttpServlet {
	public void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException{
			String pageColour = "Black";
			String colour = req.getParameter("colour");
			Cookie colourCookie = new Cookie("colour", colour);
			if (colourCookie != null) {	
				res.addCookie(colourCookie);
				colourCookie.setMaxAge(100000);
				Cookie[] cookies = req.getCookies();
				pageColour = getLastColour(cookies);
			} else {
					pageColour = colour;
			}
			res.setContentType("text/html");	
			ServletOutputStream out = res.getOutputStream();
			out.println("<html>");
			out.println("<body bgcolor=\"" + pageColour + "\">");
			out.println("<font color=\"White\">");
			out.println("<head><title>Change Colour</title></head>");
			out.println("<h1><p>Background Colour</p></h1>");
			out.println("<p>What colour do you want the background?</p>");
			out.println("<form>");
			out.println("Yellow</br><input type=\"radio\" name=\"colour\" value=\"Yellow\" /></br>");
			out.println("Green</br><input type=\"radio\" name=\"colour\" value=\"Green\" /></br>");
			out.println("Blue</br><input type=\"radio\" name=\"colour\" value=\"Blue\" /></br>");
			out.println("Black</br><input type=\"radio\" name=\"colour\" value=\"Black\" /></br>");
			out.println("<input type=submit value=\"Change Background\"/>");
			out.println("</font>");
			out.println("</form>");
			out.println("</body>");
			out.println("</html>");
		}
 
	public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException, ServletException {
			doPost(req, res);
		}
 
		String getLastColour(Cookie[] cookies){
				String inColour="white";
				for (int i = 0; i < cookies.length; i++) {
						String cookieName = cookies[i].getName();
						if (cookieName.equals("colour"))
							inColour = cookies[i].getValue();
					}
					return inColour;
				}
}

Can anyone help, I'm really stuck with this, no matter what I try it just doesn't seem to work.

**oss_2008** · June 26th, 2009, 10:47 AM

OK I've solved the null pointer problem, if there are no cookies sent to the server it returns null so I added an if statement in the right place. Now however when I first set the colour it turns the background white no matter which colour I choose, then I select the next colour and it turns the background the colour of the first one I chose etc etc. Once that's done it's not remembering the colours when I reload it. Here's the code test it out and you'll see what I mean. I'll continue working away on this myself but would appreciate some help.

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
public class ColourSelect extends HttpServlet {
	public void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException{
			String pageColour;
			String colour = req.getParameter("colour");
			Cookie colourCookie = new Cookie("colour", colour);	
			res.addCookie(colourCookie);
			colourCookie.setMaxAge(100000);
			Cookie[] cookies = req.getCookies();
			if (cookies != null) {	
				pageColour = getLastColour(cookies);
			} else { pageColour = colour; }
			res.setContentType("text/html");	
			ServletOutputStream out = res.getOutputStream();
			out.println("<html>");
			out.println("<body bgcolor=\"" + pageColour + "\">");
			out.println("<font color=\"White\">");
			out.println("<head><title>Change Colour</title></head>");
			out.println("<h1><p>Background Colour</p></h1>");
			out.println("<p>What colour do you want the background?</p>");
			out.println("<form>");
			out.println("Yellow</br><input type=\"radio\" name=\"colour\" value=\"Yellow\" /></br>");
			out.println("Green</br><input type=\"radio\" name=\"colour\" value=\"Green\" /></br>");
			out.println("Blue</br><input type=\"radio\" name=\"colour\" value=\"Blue\" /></br>");
			out.println("Black</br><input type=\"radio\" name=\"colour\" value=\"Black\" /></br>");
			out.println("<input type=submit value=\"Change Background\"/>");
			out.println("</font>");
			out.println("</form>");
			out.println("</body>");
			out.println("</html>");
		}
 
	public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException, ServletException {
			doPost(req, res);
		}
 
		String getLastColour(Cookie[] cookies){
				String inColour="Black";
				for (int i = 0; i < cookies.length; i++) {
						String cookieName = cookies[i].getName();
						if (cookieName.equals("colour"))
							inColour = cookies[i].getValue();
					}
				return inColour;
			}
}

**oss_2008** · June 26th, 2009, 10:51 AM

Ah I see the problem now. Is there anyway of reloading a cookie only when the page is loaded and not everytime when the submit button is clicked?

**oss_2008** · June 26th, 2009, 11:06 AM

OK it displays ok and changes colour ok it just doesn't do what it's supposed to do and remember the colour for the next time the page is loaded.

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
public class ColourSelect extends HttpServlet {
	public void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException{
			String pageColour;
			String colour = req.getParameter("colour");
			Cookie colourCookie = new Cookie("colour", colour);	
			colourCookie.setMaxAge(100000);
			Cookie[] cookies = req.getCookies();
			if (cookies != null) {	
				res.addCookie(colourCookie);
				pageColour = getLastColour(cookies);
			} else { pageColour = colour; }
			res.setContentType("text/html");	
			ServletOutputStream out = res.getOutputStream();
			out.println("<html>");
			out.println("<body bgcolor=\"" + pageColour + "\">");
			out.println("<font color=\"White\">");
			out.println("<head><title>Change Colour</title></head>");
			out.println("<h1><p>Background Colour</p></h1>");
			out.println("<p>What colour do you want the background?</p>");
			out.println("<form>");
			out.println("Yellow</br><input type=\"radio\" name=\"colour\" value=\"Yellow\" /></br>");
			out.println("Green</br><input type=\"radio\" name=\"colour\" value=\"Green\" /></br>");
			out.println("Blue</br><input type=\"radio\" name=\"colour\" value=\"Blue\" /></br>");
			out.println("Black</br><input type=\"radio\" name=\"colour\" value=\"Black\" /></br>");
			out.println("<input type=submit value=\"Change Background\"/>");
			out.println("</font>");
			out.println("</form>");
			out.println("</body>");
			out.println("</html>");
		}
 
	public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException, ServletException {
			doPost(req, res);
		}
 
		String getLastColour(Cookie[] cookies){
				String inColour="Black";
				for (int i = 0; i < cookies.length; i++) {
						String cookieName = cookies[i].getName();
						if (cookieName.equals("colour"))
							inColour = cookies[i].getValue();
					}
				return inColour;
			}
}

Thread: Null pointer exception while using cookie to remember the previous color

LinkBack

Thread Tools

Display

Null pointer exception while using cookie to remember the previous color

Related threads:

Re: Struggling with cookies

Re: Struggling with cookies

Re: Struggling with cookies

Similar Threads

Details about 'CopyTo' of Arrays in Java