A Simple Servlet

The following class completely defines servlet:
    public class SimpleServlet extends HttpServlet
    { 
        /**
         * Handle the HTTP GET method by building a simple web page.
         */
        public void doGet (HttpServletRequest request,
                           HttpServletResponse response)
        throws ServletException, IOException
        {
            PrintWriter         out;
            String              title = "Simple Servlet Output";

            // set content type and other response header fields first
            response.setContentType("text/html");

            // then write the data of the response
            out = response.getWriter();

            out.println("<HTML><HEAD><TITLE>");
            out.println(title);
            out.println("</TITLE></HEAD><BODY>");
            out.println("<H1>" + title + "</H1>");
            out.println("<P>This is output from SimpleServlet.");
            out.println("</BODY></HTML>");
            out.close();
        }
    }
That's it!
The classes mentioned in the Architecture of the Servlet Package section are shown in the example in bold:

SimpleServlet extends the HttpServlet class, which implements the Servlet interface.

SimpleServlet overrides the doGet method in the HttpServlet class. The doGet method is called when a client makes a GET request (the default HTTP request method), and results in the simple HTML page being returned to the client.

Within the doGet method,

The user's request is represented by an HttpServletRequest object.

The response to the user is represented by an HttpServletResponse object.

Because text data is returned to the client, the reply is sent using the Writer object obtained from the HttpServletResponse object.