用java写一个系统登陆界面的验证码验证

非常好的问题！以下是一个使用 Java Swing 编写的系统登录界面的验证码验证示例：

import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import java.util.*;
import javax.swing.*;
import javax.swing.border.*;
import java.security.*;
import java.math.*;

public class Login extends JFrame implements ActionListener {
    private JLabel userLabel, passLabel, verifyLabel;
    private JTextField userField;
    private JPasswordField passField;
    private JTextField verifyField;
    private JButton loginButton;
    private JPanel mainPanel, buttonPanel;
    private BufferedImage verifyImage;
    private int verifyCode;
    
    public Login() {
        setTitle("系统登录");
        setSize(400, 200);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        userLabel = new JLabel("用户名:");
        passLabel = new JLabel("密码:");
        verifyLabel = new JLabel("验证码:");
        
        userField = new JTextField(20);
        passField = new JPasswordField(20);
        verifyField = new JTextField(10);
        
        loginButton = new JButton("登录");
        loginButton.addActionListener(this);
        buttonPanel = new JPanel();
        buttonPanel.add(loginButton);
        
        mainPanel = new JPanel();
        mainPanel.setLayout(new GridLayout(3, 2));
        mainPanel.add(userLabel);
        mainPanel.add(userField);
        mainPanel.add(passLabel);
        mainPanel.add(passField);
        mainPanel.add(verifyLabel);
        mainPanel.add(verifyField);
        mainPanel.setBorder(new EmptyBorder(10, 10, 10, 10));
        
        add(mainPanel, BorderLayout.CENTER);
        add(buttonPanel, BorderLayout.SOUTH);
        
        Random rand = new Random();
        verifyCode = rand.nextInt(9000) + 1000;
        try {
            verifyImage = createVerifyImage(Integer.toString(verifyCode));
        } catch (NoSuchAlgorithmException ex) {
            System.err.println(ex.getMessage());
        }
        
        setVisible(true);
    }
    
    public void actionPerformed(ActionEvent evt) {
        String username = userField.getText();
        String password = new String(passField.getPassword());
        String verify = verifyField.getText();
        
        if (verify.equals(Integer.toString(verifyCode))) {
            JOptionPane.showMessageDialog(this, "欢迎 " + username);
        } else {
            JOptionPane.showMessageDialog(this, "验证码错误");
        }
    }
    
    private BufferedImage createVerifyImage(String code) throws NoSuchAlgorithmException {
        MessageDigest md = MessageDigest.getInstance("MD5");
        md.update(code.getBytes());
        byte[] hash = md.digest();
        String hex = new BigInteger(1, hash).toString(16);
        int width = 120, height = 50;
        BufferedImage img = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
        Graphics2D g = img.createGraphics();
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, width, height);
        for (int i = 0; i < 100; i++) {
            int x1 = (int)(Math.random() * width);
            int y1 = (int)(Math.random() * height);
            int x2 = (int)(Math.random() * width);
            int y2 = (int)(Math.random() * height);
            g.setColor(new Color((int)(Math.random() * 0x1000000)));
            g.drawLine(x1, y1, x2, y2);
        }
        g.setColor(Color.BLACK);
        g.setFont(new Font(Font.SANS_SERIF, Font.BOLD, 24));
        g.drawString(hex.substring(0, 4), 30, 30);
        g.dispose();
        return img;
    }
    
    public void paint(Graphics g) {
        super.paint(g);
        g.drawImage(verifyImage, 250, 25, this);
    }
    
    public static void main(String[] args) {
        new Login();
    }
}

该示例生成一个带有用户名、密码和验证码的登录窗体。在此示例中，验证码是使用 MD5 哈希对四位数的随机整数生成的。每次刷新验证码时，都会生成一个不同的随机数字。当用户单击“登录”按钮时，将检查输入的验证码是否与生成的验证码匹配。

这是一个简单的示例，您可以根据自己的需要进行自定义和扩展。