用Java.swing写一个生成从“0123456789abcdefghijkmnopqrstuvwxyzABCDEFG”中每次产生四个随机数作为验证码的程序

好的，下面是Java.swing生成验证码的程序代码：

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Random;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class VerifyCode extends JFrame implements ActionListener {

    private static final long serialVersionUID = 1L;

    private final String VERIFY_CODES = "0123456789abcdefghijkmnopqrstuvwxyzABCDEFG";// 验证码字符集

    private final int VERIFY_SIZE = 4;// 验证码长度

    private JPanel jp;

    private JButton jb;

    private String verifyCode;

    public VerifyCode() {
        setTitle("验证码");
        setSize(250, 150);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        jp = new JPanel();
        jp.setBackground(Color.WHITE);
        add(jp, BorderLayout.CENTER);

        jb = new JButton("生成验证码");
        jb.addActionListener(this);
        add(jb, BorderLayout.SOUTH);

        setVisible(true);
    }

    private String generateVerifyCode() {
        Random rand = new Random();
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < VERIFY_SIZE; i++) {
            sb.append(VERIFY_CODES.charAt(rand.nextInt(VERIFY_CODES.length())));
        }
        return sb.toString();
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == jb) {
            verifyCode = generateVerifyCode();
            jp.repaint();
        }
    }

    public static void main(String[] args) {
        new VerifyCode();
    }

    class VerifyPanel extends JPanel {

        private static final long serialVersionUID = 1L;

        public void paint(Graphics g) {
            super.paint(g);
            g.setColor(Color.BLACK);
            g.setFont(new Font("Times New Roman", Font.BOLD, 20));
            g.drawString(verifyCode, 60, 60);
        }

    }

}

这个程序会在窗口中生成一个“生成验证码”的按钮，点击它可以生成一个由指定字符集中的字符组成的长度为4的随机字符串作为验证码，并在窗口中显示出来。