设计一个留言板，使用Application

好的，以下是一个基本的留言板设计，使用Java Swing和Application进行实现：

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.time.LocalDateTime;
import java.util.Scanner;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.SwingConstants;

public class MessageBoard extends JFrame implements ActionListener {

    private static final long serialVersionUID = 1L;
    private static final String FILE_NAME = "messages.txt"; // 存储留言的文件名

    private JPanel mainPanel;
    private JTextArea messageArea;
    private JLabel nameLabel, emailLabel, messageLabel;
    private JTextField nameField, emailField;
    private JTextArea messageField;
    private JButton submitButton, clearButton;

    public MessageBoard() {
        super("留言板");

        // 创建主面板
        mainPanel = new JPanel(new BorderLayout());
        mainPanel.setBorder(BorderFactory.createEmptyBorder(10, 10, 10, 10));

        // 创建留言面板
        JPanel messagePanel = new JPanel(new GridLayout(3, 2, 10, 5));
        messagePanel.setBorder(BorderFactory.createTitledBorder("留言信息"));

        nameLabel = new JLabel("姓名：", SwingConstants.RIGHT);
        nameLabel.setFont(new Font("宋体", Font.PLAIN, 16));
        emailLabel = new JLabel("邮箱：", SwingConstants.RIGHT);
        emailLabel.setFont(new Font("宋体", Font.PLAIN, 16));
        messageLabel = new JLabel("留言：", SwingConstants.RIGHT);
        messageLabel.setFont(new Font("宋体", Font.PLAIN, 16));

        nameField = new JTextField();
        emailField = new JTextField();
        messageField = new JTextArea();
        messageField.setLineWrap(true);
        JScrollPane messageScrollPane = new JScrollPane(messageField);

        messagePanel.add(nameLabel);
        messagePanel.add(nameField);
        messagePanel.add(emailLabel);
        messagePanel.add(emailField);
        messagePanel.add(messageLabel);
        messagePanel.add(messageScrollPane);

        // 创建按钮面板
        JPanel buttonPanel = new JPanel(new GridLayout(1, 2, 10, 5));

        submitButton = new JButton("提交");
        submitButton.addActionListener(this);
        clearButton = new JButton("清空");
        clearButton.addActionListener(this);

        buttonPanel.add(submitButton);
        buttonPanel.add(clearButton);

        // 创建留言显示面板
        JPanel messageDisplayPanel = new JPanel(new BorderLayout());
        messageDisplayPanel.setBorder(BorderFactory.createTitledBorder("留言信息"));

        messageArea = new JTextArea();
        messageArea.setEditable(false);
        messageArea.setBackground(Color.WHITE);
        messageArea.setLineWrap(true);
        messageArea.setWrapStyleWord(true);
        JScrollPane messageScroll = new JScrollPane(messageArea);

        messageDisplayPanel.add(messageScroll, BorderLayout.CENTER);

        // 添加组件到主面板
        mainPanel.add(messagePanel, BorderLayout.NORTH);
        mainPanel.add(buttonPanel, BorderLayout.CENTER);
        mainPanel.add(messageDisplayPanel, BorderLayout.SOUTH);

        // 加载之前存储的留言
        loadMessages();

        // 设置窗口属性
        setContentPane(mainPanel);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(600, 500);
        setLocationRelativeTo(null);
        setVisible(true);
    }

    // 加载之前存储的留言
    private void loadMessages() {
        try {
            Scanner scanner = new Scanner(new File(FILE_NAME));
            while (scanner.hasNextLine()) {
                String name = scanner.nextLine();
                String email = scanner.nextLine();
                String message = scanner.nextLine();
                LocalDateTime dateTime = LocalDateTime.parse(scanner.nextLine());
                addMessage(name, email, message, dateTime);
            }
            scanner.close();
        } catch (IOException e) {
            System.out.println("无法加载之前的留言！");
        }
    }

    // 添加新留言
    private void addMessage(String name, String email, String message, LocalDateTime dateTime) {
        String text = String.format("[%tF %tT] %s (%s) 说：\n%s\n\n", dateTime, dateTime, name, email, message);
        messageArea.append(text);
    }

    // 保存留言到文件中
    private void saveMessage(String name, String email, String message, LocalDateTime dateTime) {
        try {
            BufferedWriter writer = new BufferedWriter(new FileWriter(FILE_NAME, true));
            writer.write(name + "\n");
            writer.write(email + "\n");
            writer.write(message + "\n");
            writer.write(dateTime.toString() + "\n");
            writer.flush();
            writer.close();
        } catch (IOException e) {
            System.out.println("无法保存留言！");
        }
    }

    // 清空留言输入框
    private void clearFields() {
        nameField.setText("");
        emailField.setText("");
        messageField.setText("");
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource() == submitButton) {
            String name = nameField.getText().trim();
            String email = emailField.getText().trim();
            String message = messageField.getText().trim();

            // 检查输入是否为空
            if (name.isEmpty() || email.isEmpty() || message.isEmpty()) {
                JOptionPane.showMessageDialog(this, "请填写完整的留言信息！", "错误", JOptionPane.ERROR_MESSAGE);
                return;
            }

            // 添加新留言
            addMessage(name, email, message, LocalDateTime.now());

            // 保存留言并清空输入框
            saveMessage(name, email, message, LocalDateTime.now());
            clearFields();

            JOptionPane.showMessageDialog(this, "留言已提交，谢谢您的参与！", "提示", JOptionPane.INFORMATION_MESSAGE);
        } else if (e.getSource() == clearButton) {
            clearFields();
        }
    }

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

}

这个留言板的功能比较简单，用户可以输入姓名、邮箱和留言内容，并点击“提交”按钮将留言添加到留言板中。同时，程序会将留言存储到本地文件中，并在程序启动时加载之前的留言。用户还可以点击“清空”按钮清空输入框的内容。