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

public class Undo extends HttpServlet {

    public void doGet(HttpServletRequest request,
                      HttpServletResponse response)
        throws IOException, ServletException {

        try {
            HttpSession session = request.getSession(true) ;
            DBSession dbs   = (DBSession) session.getAttribute("dbs") ;
            Vector undoList = (Vector) session.getAttribute("undoList") ;

            if(dbs == null || undoList == null) {
                response.sendError(HttpServletResponse.SC_OK,
                             "Session timed out.  Cannot undo change.") ;
                return ;
            }

            // Undo most recent change

            int last = undoList.size() - 1 ;
            dbs.stat.executeUpdate((String) undoList.remove(last)) ;

            // Redirect browser back to selection page

            response.sendRedirect(
                  response.encodeRedirectURL("/students/servlet/Select"));

        } catch (SQLException e) {
            response.sendError(HttpServletResponse.SC_OK,
                               "SQL error: " + e.getMessage()) ;
        }
    }
}