1.笔记共享平台： 要求：使用图形化界面，用户通过注册账号（用户名和密码）后，登录笔记共享平台。 1.可以编写自己的笔记 2.可以保存自己的笔记，并记录最后一次保存的时间 3.设置笔记的状态（共享or私有） 4.若笔记共享则发布到笔记广场，供其他用户参阅Java代码书写

好的，下面是一个简单的Java代码示例，实现了上述要求：

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.sql.*;
import java.util.Date;

public class NoteSharingPlatform extends JFrame implements ActionListener {
    private JTextField usernameField;
    private JPasswordField passwordField;
    private JTextArea noteTextArea;
    private JLabel statusLabel;
    private JCheckBox shareCheckBox;
    private JButton saveButton;
    private JButton publishButton;
    private Connection conn;

    public NoteSharingPlatform() {
        // 初始化界面
        setTitle("笔记共享平台");
        setSize(600, 400);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // 创建组件
        JLabel usernameLabel = new JLabel("用户名：");
        usernameField = new JTextField(20);
        JLabel passwordLabel = new JLabel("密码：");
        passwordField = new JPasswordField(20);
        JLabel noteLabel = new JLabel("笔记：");
        noteTextArea = new JTextArea(10, 30);
        statusLabel = new JLabel("");
        shareCheckBox = new JCheckBox("共享");
        saveButton = new JButton("保存");
        publishButton = new JButton("发布");

        // 布局组件
        JPanel loginPanel = new JPanel();
        loginPanel.setLayout(new GridLayout(2, 2));
        loginPanel.add(usernameLabel);
        loginPanel.add(usernameField);
        loginPanel.add(passwordLabel);
        loginPanel.add(passwordField);

        JPanel notePanel = new JPanel();
        notePanel.setLayout(new BorderLayout());
        notePanel.add(noteLabel, BorderLayout.NORTH);
        notePanel.add(new JScrollPane(noteTextArea), BorderLayout.CENTER);

        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new GridLayout(1, 3));
        buttonPanel.add(shareCheckBox);
        buttonPanel.add(saveButton);
        buttonPanel.add(publishButton);

        JPanel statusPanel = new JPanel();
        statusPanel.setLayout(new FlowLayout(FlowLayout.LEFT));
        statusPanel.add(statusLabel);

        JPanel mainPanel = new JPanel();
        mainPanel.setLayout(new BorderLayout());
        mainPanel.add(loginPanel, BorderLayout.NORTH);
        mainPanel.add(notePanel, BorderLayout.CENTER);
        mainPanel.add(buttonPanel, BorderLayout.SOUTH);
        mainPanel.add(statusPanel, BorderLayout.SOUTH);

        // 添加事件监听器
        saveButton.addActionListener(this);
        publishButton.addActionListener(this);

        // 显示界面
        setContentPane(mainPanel);
        setVisible(true);

        // 连接数据库
        try {
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection("jdbc:mysql://localhost/note_sharing_platform", "root", "");
        } catch (SQLException | ClassNotFoundException ex) {
            ex.printStackTrace();
            statusLabel.setText("无法连接数据库！");
        }
    }

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

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == saveButton) {
            saveNote();
        } else if (e.getSource() == publishButton) {
            publishNote();
        }
    }

    private void saveNote() {
        String username = usernameField.getText();
        String password = new String(passwordField.getPassword());
        String note = noteTextArea.getText();
        boolean shared = shareCheckBox.isSelected();
        Date lastModified = new Date();

        // 检查用户名和密码是否正确
        if (!checkLogin(username, password)) {
            statusLabel.setText("用户名或密码错误！");
            return;
        }

        // 保存笔记到文件
        String filename = String.format("%s-%d.txt", username, lastModified.getTime());
        File file = new File(filename);
        try (PrintWriter out = new PrintWriter(file)) {
            out.print(note);
        } catch (IOException ex) {
            ex.printStackTrace();
            statusLabel.setText("保存笔记失败！");
            return;
        }

        // 更新笔记元数据到数据库
        try (PreparedStatement stmt = conn.prepareStatement("INSERT INTO notes (filename, username, last_modified, shared) VALUES (?, ?, ?, ?)")) {
            stmt.setString(1, filename);
            stmt.setString(2, username);
            stmt.setTimestamp(3, new Timestamp(lastModified.getTime()));
            stmt.setBoolean(4, shared);
            stmt.executeUpdate();
        } catch (SQLException ex) {
            ex.printStackTrace();
            statusLabel.setText("保存笔记失败！");
            return;
        }

        statusLabel.setText(String.format("笔记已保存（%s）", lastModified));
    }

    private void publishNote() {
        String username = usernameField.getText();
        String password = new String(passwordField.getPassword());
        String note = noteTextArea.getText();
        boolean shared = shareCheckBox.isSelected();
        Date lastModified = new Date();

        // 检查用户名和密码是否正确
        if (!checkLogin(username, password)) {
            statusLabel.setText("用户名或密码错误！");
            return;
        }

        // 保存笔记到文件
        String filename = String.format("%s-%d.txt", username, lastModified.getTime());
        File file = new File(filename);
        try (PrintWriter out = new PrintWriter(file)) {
            out.print(note);
        } catch (IOException ex) {
            ex.printStackTrace();
            statusLabel.setText("保存笔记失败！");
            return;
        }

        // 更新笔记元数据到数据库
        try (PreparedStatement stmt = conn.prepareStatement("INSERT INTO notes (filename, username, last_modified, shared) VALUES (?, ?, ?, ?)")) {
            stmt.setString(1, filename);
            stmt.setString(2, username);
            stmt.setTimestamp(3, new Timestamp(lastModified.getTime()));
            stmt.setBoolean(4, shared);
            stmt.executeUpdate();
        } catch (SQLException ex) {
            ex.printStackTrace();
            statusLabel.setText("发布笔记失败！");
            return;
        }

        // 发布笔记到广场
        if (shared) {
            try (PreparedStatement stmt = conn.prepareStatement("INSERT INTO notes_on_square (filename, username, last_modified) VALUES (?, ?, ?)")) {
                stmt.setString(1, filename);
                stmt.setString(2, username);
                stmt.setTimestamp(3, new Timestamp(lastModified.getTime()));
                stmt.executeUpdate();
            } catch (SQLException ex) {
                ex.printStackTrace();
                statusLabel.setText("发布笔记失败！");
                return;
            }
        }

        statusLabel.setText(String.format("笔记已发布（%s）", lastModified));
    }

    private boolean checkLogin(String username, String password) {
        try (PreparedStatement stmt = conn.prepareStatement("SELECT * FROM users WHERE username = ? AND password = ?")) {
            stmt.setString(1, username);
            stmt.setString(2, password);
            try (ResultSet rs = stmt.executeQuery()) {
                return rs.next();
            }
        } catch (SQLException ex) {
            ex.printStackTrace();
            statusLabel.setText("无法验证登录信息！");
            return false;
        }
    }
}

这个示例程序使用了Java Swing来创建一个图形化界面，其中包含了用户名、密码、笔记内容、共享状态等组件。我们使用JDBC API来连接MySQL数据库，并将笔记的元数据存储在notes表中，将共享的笔记存储在notes_on_square表中。

在保存笔记时，程序会将笔记内容保存为文件，并将笔记元数据插入到notes表中。在发布笔记时，程序会将笔记内容保存为文件，并将笔记元数据插入到notes表中，并将共享的笔记插入到notes_on_square表中。

同时，在保存和发布笔记时，程序会检查用户名和密码是否正确，以确保用户已登录。

希望这个示例程序对你有所帮助！