HelloWorld

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class HelloWorld extends HttpServlet {

  public void doGet(HttpServletRequest req, HttpServletResponse res)
                               throws ServletException, IOException {

    res.setContentType("text/html");
    PrintWriter out = res.getWriter();

    out.println("<!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Strict//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd\">");
    out.println("<html xmlns=\"http://www.w3.org/1999/xhtml\" xml:lang=\"en\" lang=\"en\">");
    out.println("<head><title>Hello World</title></head>");
    out.println("<body>");
    out.println("<h1>Hello World</h1>");
    out.println("</body></html>");
  }
}

HttpServletRequest has information about the client, like parameter.
HttpServletResponse is sent back to the client.