下面是一个用Java和MySQL编写的停车场管理系统的示例代码：
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.sql.*;public class ParkingManagementSystem extends JFrame {private JLabel carNumberLabel;private JTextField carNumberTextField;private JButton enterButton;private JButton exitButton;private Connection connection;public ParkingManagementSystem() {setTitle("停车场管理系统");setSize(300, 200);setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);setLayout(new FlowLayout());carNumberLabel = new JLabel("车牌号：");carNumberTextField = new JTextField(10);enterButton = new JButton("进入");exitButton = new JButton("离开");enterButton.addActionListener(new ActionListener() {public void actionPerformed(ActionEvent event) {String carNumber = carNumberTextField.getText();enter(carNumber);}});exitButton.addActionListener(new ActionListener() {public void actionPerformed(ActionEvent event) {String carNumber = carNumberTextField.getText();exit(carNumber);}});add(carNumberLabel);add(carNumberTextField);add(enterButton);add(exitButton);try {Class.forName("com.mysql.cj.jdbc.Driver");connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/parking", "username", "password");} catch (Exception ex) {ex.printStackTrace();}}private void enter(String carNumber) {try {PreparedStatement statement = connection.prepareStatement("INSERT INTO parking_cars (car_number, enter_time) VALUES (?, NOW())");statement.setString(1, carNumber);statement.executeUpdate();JOptionPane.showMessageDialog(null, "车辆进入停车场");} catch (SQLException ex) {ex.printStackTrace();}}private void exit(String carNumber) {try {PreparedStatement statement = connection.prepareStatement("UPDATE parking_cars SET exit_time = NOW() WHERE car_number = ? AND exit_time IS NULL");statement.setString(1, carNumber);int rowsAffected = statement.executeUpdate();if (rowsAffected > 0) {JOptionPane.showMessageDialog(null, "车辆离开停车场");} else {JOptionPane.showMessageDialog(null, "未找到该车辆或该车辆已离开停车场");}} catch (SQLException ex) {ex.printStackTrace();}}public static void main(String[] args) {SwingUtilities.invokeLater(new Runnable() {public void run() {new ParkingManagementSystem().setVisible(true);}});}
}
上述代码使用了Java的Swing库创建图形用户界面。它包含一个文本框用于输入车牌号码，一个“进入”按钮和一个“离开”按钮。当用户点击“进入”按钮时，程序将将车牌号码和当前时间插入到MySQL数据库的parking_cars表中。当用户点击“离开”按钮时，程序将更新parking_cars表中与输入车牌号码匹配且exit_time为空的记录的exit_time字段，并给出相应的提示。
请注意，上述代码中的数据库连接URL、用户名和密码需要根据实际情况进行修改。