[Java] สอบถามเรื่องการ Login เงื่อนไขคือไม่ให้ ค่าใน textbox มันว่างครับ

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.*;

import javax.swing.*;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class addManager
 */
@WebServlet("/Logincheck")
public class Logincheck extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public Logincheck() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
    
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		response.setContentType("text/html;charset=utf-8");
		PrintWriter out = response.getWriter();
		String un = request.getParameter("user");
		String ps = request.getParameter("pass");
		
		Connection cnn = null;
	    PreparedStatement pst = null;
	    ResultSet rs = null;
	    cnn = MysqlConnect.connectDB();
	    String sql = "select * from logintable where Username='"+un+"' and Pssword='"+ps+"' ";
		try{
			pst = cnn.prepareStatement(sql);
			rs = pst.executeQuery();
			if(rs.next()){
				if(rs.getString(3).equals("admin")){
					JOptionPane.showMessageDialog(null, "ยินดีต้อนรับผู้ดูแลระบบ");
					response.sendRedirect("AdminanagerForm.html");
				}else if(rs.getString(3).equals("user")){
					JOptionPane.showMessageDialog(null, "ยินดีต้อนรับสมาชิก");
					response.sendRedirect("UsermanagerForm.html");
				}
				}else{
					JOptionPane.showMessageDialog(null, "กรุณากรอก Username หรือ Password ให้ถูกต้อง");
					response.sendRedirect("LoginForm.html");	
				}
				cnn.close();
				pst.close();
				rs.close();
		}catch(SQLException sqle){
			JOptionPane.showMessageDialog(null, sqle);
		}
	}
	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
	}

}