In Java Servlets, hidden form fields are used to store data that is not displayed to the user but is submitted with the form when it is submitted. Hidden form fields are often used to store data that is used to identify the form or to store data that is required for processing the form.

Here's an example of how to create a hidden form field in a Servlet:

import java.io.IOException;
import javax.servlet.ServletException;
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 {
        response.setContentType("text/html");
        
        // Create a form with a hidden field
        String form = "<form action='processForm' method='post'>";
        form += "<input type='hidden' name='hiddenField' value='hiddenValue'>";
        form += "<input type='submit' value='Submit'>";
        form += "</form>";
        
        // Send the form to the client
        response.getWriter().println(form);
    }
}

In this example, we create a form with a hidden field using the <input> tag with a type of "hidden". The name of the field is "hiddenField" and the value of the field is "hiddenValue". We then send the form to the client using the getWriter() method.

Here's an example of how to retrieve the value of a hidden form field in a Servlet:

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

public class MyServlet extends HttpServlet {
    
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // Get the value of the hidden field
        String hiddenValue = request.getParameter("hiddenField");
        
        // Send the value to the client
        response.getWriter().println("Hidden field value: " + hiddenValue);
    }
}

In this example, we retrieve the value of the hidden field using the getParameter() method with the name of the field as the parameter. We then send the value to the client using the getWriter() method.

This is an example of how to use hidden form fields in a Servlet to store data that is required for processing the form.