仿照Windows中的记事本，编写一个Java程序记事本。代码

以下是一个简单的Java程序记事本的代码示例：

import java.awt.BorderLayout;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;

import javax.swing.JFileChooser;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JOptionPane;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.KeyStroke;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.filechooser.FileNameExtensionFilter;

public class JavaNotepad extends JFrame implements ActionListener {
    private static final long serialVersionUID = 1L;
    private JTextArea textArea;
    private JFileChooser fileChooser;
    private String fileName;
    private boolean hasSaved;
    private String appName = "Java Notepad";

    public JavaNotepad() {
        super();
        setTitle(appName);
        setSize(800, 600);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setLocationRelativeTo(null);

        // create menu bar
        JMenuBar menuBar = new JMenuBar();
        JMenu fileMenu = new JMenu("File");
        JMenuItem newFile = new JMenuItem("New");
        newFile.addActionListener(this);
        newFile.setAccelerator(KeyStroke.getKeyStroke('N', java.awt.Toolkit.getDefaultToolkit().getMenuShortcutKeyMask()));
        fileMenu.add(newFile);
        JMenuItem openFile = new JMenuItem("Open");
        openFile.addActionListener(this);
        openFile.setAccelerator(KeyStroke.getKeyStroke('O', java.awt.Toolkit.getDefaultToolkit().getMenuShortcutKeyMask()));
        fileMenu.add(openFile);
        JMenuItem saveFile = new JMenuItem("Save");
        saveFile.addActionListener(this);
        saveFile.setAccelerator(KeyStroke.getKeyStroke('S', java.awt.Toolkit.getDefaultToolkit().getMenuShortcutKeyMask()));
        fileMenu.add(saveFile);
        JMenuItem saveAsFile = new JMenuItem("Save As");
        saveAsFile.addActionListener(this);
        fileMenu.add(saveAsFile);
        JMenuItem exit = new JMenuItem("Exit");
        exit.addActionListener(this);
        fileMenu.add(exit);
        menuBar.add(fileMenu);

        JMenu editMenu = new JMenu("Edit");
        JMenuItem cut = new JMenuItem("Cut");
        cut.addActionListener(this);
        cut.setAccelerator(KeyStroke.getKeyStroke('X', java.awt.Toolkit.getDefaultToolkit().getMenuShortcutKeyMask()));
        editMenu.add(cut);
        JMenuItem copy = new JMenuItem("Copy");
        copy.addActionListener(this);
        copy.setAccelerator(KeyStroke.getKeyStroke('C', java.awt.Toolkit.getDefaultToolkit().getMenuShortcutKeyMask()));
        editMenu.add(copy);
        JMenuItem paste = new JMenuItem("Paste");
        paste.addActionListener(this);
        paste.setAccelerator(KeyStroke.getKeyStroke('V', java.awt.Toolkit.getDefaultToolkit().getMenuShortcutKeyMask()));
        editMenu.add(paste);
        JMenuItem selectAll = new JMenuItem("Select All");
        selectAll.addActionListener(this);
        selectAll.setAccelerator(KeyStroke.getKeyStroke('A', java.awt.Toolkit.getDefaultToolkit().getMenuShortcutKeyMask()));
        editMenu.add(selectAll);
        menuBar.add(editMenu);

        setJMenuBar(menuBar);

        // create text area
        textArea = new JTextArea();
        textArea.setFont(new Font("Arial", Font.PLAIN, 16));
        textArea.setLineWrap(true);
        textArea.setWrapStyleWord(true);
        JScrollPane scrollPane = new JScrollPane(textArea);
        getContentPane().add(scrollPane, BorderLayout.CENTER);

        // create file chooser dialog
        fileChooser = new JFileChooser();
        fileChooser.setFileFilter(new FileNameExtensionFilter("Text Files (*.txt)", "txt"));
        fileChooser.setAcceptAllFileFilterUsed(false);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        String command = e.getActionCommand();
        switch (command) {
            case "New":
                newFile();
                break;
            case "Open":
                openFile();
                break;
            case "Save":
                saveFile();
                break;
            case "Save As":
                saveAsFile();
                break;
            case "Exit":
                exit();
                break;
            case "Cut":
                cut();
                break;
            case "Copy":
                copy();
                break;
            case "Paste":
                paste();
                break;
            case "Select All":
                selectAll();
                break;
            default:
                break;
        }
    }

    private void newFile() {
        if (!hasSaved && textArea.getText().length() > 0) {
            int result = JOptionPane.showConfirmDialog(this, "Do you want to save the changes?");
            if (result == JOptionPane.YES_OPTION) {
                saveFile();
            } else if (result == JOptionPane.CANCEL_OPTION) {
                return;
            }
        }
        textArea.setText("");
        fileName = null;
        setTitle(appName);
        hasSaved = false;
    }

    private void openFile() {
        if (!hasSaved && textArea.getText().length() > 0) {
            int result = JOptionPane.showConfirmDialog(this, "Do you want to save the changes?");
            if (result == JOptionPane.YES_OPTION) {
                saveFile();
            } else if (result == JOptionPane.CANCEL_OPTION) {
                return;
            }
        }
        int result = fileChooser.showOpenDialog(this);
        if (result == JFileChooser.APPROVE_OPTION) {
            fileName = fileChooser.getSelectedFile().getPath();
            setTitle(fileName + " - " + appName);
            try (BufferedReader reader = new BufferedReader(new FileReader(fileName))) {
                String line;
                StringBuilder sb = new StringBuilder();
                while ((line = reader.readLine()) != null) {
                    sb.append(line).append("\n");
                }
                textArea.setText(sb.toString());
                hasSaved = true;
            } catch (IOException e) {
                JOptionPane.showMessageDialog(this, "Error: " + e.getMessage(), "Error", JOptionPane.ERROR_MESSAGE);
            }
        }
    }

    private void saveFile() {
        if (fileName == null) {
            saveAsFile();
            return;
        }
        try (PrintWriter writer = new PrintWriter(new BufferedWriter(new FileWriter(fileName)))) {
            writer.print(textArea.getText());
            hasSaved = true;
        } catch (IOException e) {
            JOptionPane.showMessageDialog(this, "Error: " + e.getMessage(), "Error", JOptionPane.ERROR_MESSAGE);
        }
    }

    private void saveAsFile() {
        int result = fileChooser.showSaveDialog(this);
        if (result == JFileChooser.APPROVE_OPTION) {
            fileName = fileChooser.getSelectedFile().getPath();
            if (!fileName.endsWith(".txt")) {
                fileName += ".txt";
            }
            setTitle(fileName + " - " + appName);
            saveFile();
        }
    }

    private void exit() {
        if (!hasSaved && textArea.getText().length() > 0) {
            int result = JOptionPane.showConfirmDialog(this, "Do you want to save the changes?");
            if (result == JOptionPane.YES_OPTION) {
                saveFile();
            } else if (result == JOptionPane.CANCEL_OPTION) {
                return;
            }
        }
        dispose();
    }

    private void cut() {
        textArea.cut();
    }

    private void copy() {
        textArea.copy();
    }

    private void paste() {
        textArea.paste();
    }

    private void selectAll() {
        textArea.selectAll();
    }

    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (ClassNotFoundException | InstantiationException | IllegalAccessException
                | UnsupportedLookAndFeelException e) {
            e.printStackTrace();
        }

        JavaNotepad notepad = new JavaNotepad();
        notepad.setVisible(true);
    }
}

该程序具有基本的文件操作、编辑操作和界面元素，并且可以在Windows、Mac OS X和Linux等操作系统上运行。您可以根据自己的需求进行调整和扩展。