In Java Servlets, cookies are small text files that are used to store information on the client-side. They are often used to implement session tracking and to pass data between a server and a client.

Here's an example of how to create a cookie and send it to the client in a Servlet:

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class MyServlet extends HttpServlet {
    
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // Create a cookie
        Cookie cookie = new Cookie("myCookie", "cookieValue");
        
        // Set the cookie's age to 24 hours
        cookie.setMaxAge(24 * 60 * 60);
        
        // Add the cookie to the response
        response.addCookie(cookie);
        
        // Send a message to the client
        response.getWriter().println("Cookie has been set!");
    }
}

In this example, we create a Cookie object with a name of "myCookie" and a value of "cookieValue". We then set the cookie's age to 24 hours using the setMaxAge() method and add it to the response using the addCookie() method. Finally, we send a message to the client using the getWriter() method.

Here's an example of how to retrieve a cookie from the client in a Servlet:

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class MyServlet extends HttpServlet {
    
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // Get all cookies from the request
        Cookie[] cookies = request.getCookies();
        
        // Loop through the cookies
        for (Cookie cookie : cookies) {
            // Check if the cookie has the name "myCookie"
            if (cookie.getName().equals("myCookie")) {
                // Get the cookie's value
                String cookieValue = cookie.getValue();
                
                // Send the cookie's value to the client
                response.getWriter().println("Cookie value: " + cookieValue);
            }
        }
    }
}

In this example, we retrieve all cookies from the request using the getCookies() method. We then loop through the cookies and check if any of them have a name of "myCookie" using the getName() method. If we find the cookie, we retrieve its value using the getValue() method and send it to the client using the getWriter() method.

This is an example of how to use cookies in a Servlet to pass data between a server and a client.