验证码程序

package Yzm.awt;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.GridLayout;
import java.util.Random;

import javax.swing.*;
public class Yanzhengma extends JFrame {
 public Yanzhengma() {
  
  JPanel panel1 = new JPanel();
  panel1.setLayout(new GridLayout(2,1));
     panel1.add(new JButton("登陆"));
     panel1.add(new JButton("快速注册"));
     add(panel1, BorderLayout.SOUTH);
    
     JPanel panel2 = new JPanel(new BorderLayout());
     panel2.setLayout(new GridLayout(3,3));
     panel2.add(new JButton("登录名"));
     panel2.add(new JTextField("     "),BorderLayout.CENTER);
     JTextField jf = new JTextField(" ");
     jf.setEditable(false);
     panel2.add(jf,BorderLayout.CENTER);
     panel2.add(new JButton("密码"));
     panel2.add(new JTextField("     "),BorderLayout.CENTER);
     JTextField jf1 = new JTextField("忘记用户名/密码？");
     jf1.setEditable(false);
     panel2.add(jf1,BorderLayout.CENTER);
     panel2.add(new JButton("验证码"));
     int a = new Random().nextInt(9);
     int b = new Random().nextInt(9);
     int c = new Random().nextInt(9);
     int d = new Random().nextInt(9);
     panel2.add(new JTextField("     "),BorderLayout.CENTER);
     JTextField jf2 = new JTextField(""+a+b+c+d);
     jf2.setEditable(false);
     panel2.add(jf2,BorderLayout.CENTER);
     add(panel2,BorderLayout.NORTH);
}
   public static void main(String args[]) {
    Yanzhengma frame = new Yanzhengma();
    frame.setTitle("请登录");
    frame.setSize(380,175);
    frame.setLocationRelativeTo(null);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
   }
}

存在的问题：文本框的边框无法去除，影响美观。随机生成的0-9的数字来作为验证码，没有那种有划线来阻碍观察的验证码。