用java语言编写一个程序，模仿实现Windows自带记事本功能：新建文件、保存、编辑、格式、帮助等功能

下面是一个基于Java Swing框架的记事本程序，实现了新建文件、保存、编辑、格式、帮助等功能：

import javax.swing.*;
import javax.swing.filechooser.FileNameExtensionFilter;
import java.awt.*;
import java.awt.event.*;
import java.io.*;

public class Notepad extends JFrame implements ActionListener {
    private JTextArea textArea;
    private JScrollPane scrollPane;
    private String fileName;
    private boolean changed = false;

    public Notepad() {
        setTitle("记事本");
        setSize(500, 500);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

        textArea = new JTextArea();
        scrollPane = new JScrollPane(textArea);
        add(scrollPane, BorderLayout.CENTER);

        JMenuBar menuBar = new JMenuBar();
        JMenu fileMenu = new JMenu("文件");
        JMenu editMenu = new JMenu("编辑");
        JMenu formatMenu = new JMenu("格式");
        JMenu helpMenu = new JMenu("帮助");

        JMenuItem newMenuItem = new JMenuItem("新建");
        JMenuItem openMenuItem = new JMenuItem("打开");
        JMenuItem saveMenuItem = new JMenuItem("保存");
        JMenuItem saveAsMenuItem = new JMenuItem("另存为");
        JMenuItem exitMenuItem = new JMenuItem("退出");

        newMenuItem.addActionListener(this);
        openMenuItem.addActionListener(this);
        saveMenuItem.addActionListener(this);
        saveAsMenuItem.addActionListener(this);
        exitMenuItem.addActionListener(this);

        fileMenu.add(newMenuItem);
        fileMenu.add(openMenuItem);
        fileMenu.add(saveMenuItem);
        fileMenu.add(saveAsMenuItem);
        fileMenu.addSeparator();
        fileMenu.add(exitMenuItem);

        menuBar.add(fileMenu);
        menuBar.add(editMenu);
        menuBar.add(formatMenu);
        menuBar.add(helpMenu);

        setJMenuBar(menuBar);

        textArea.addKeyListener(new KeyAdapter() {
            @Override
            public void keyPressed(KeyEvent e) {
                changed = true;
            }
        });

        addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e) {
                exit();
            }
        });
    }

    private void newFile() {
        if (changed) {
            int result = JOptionPane.showConfirmDialog(this, "是否保存当前文件？", "提示", JOptionPane.YES_NO_CANCEL_OPTION);
            if (result == JOptionPane.YES_OPTION) {
                save();
            } else if (result == JOptionPane.CANCEL_OPTION) {
                return;
            }
        }
        fileName = null;
        setTitle("记事本");
        textArea.setText("");
        changed = false;
    }

    private void open() {
        if (changed) {
            int result = JOptionPane.showConfirmDialog(this, "是否保存当前文件？", "提示", JOptionPane.YES_NO_CANCEL_OPTION);
            if (result == JOptionPane.YES_OPTION) {
                save();
            } else if (result == JOptionPane.CANCEL_OPTION) {
                return;
            }
        }
        JFileChooser fileChooser = new JFileChooser();
        fileChooser.setFileFilter(new FileNameExtensionFilter("文本文件(*.txt)", "txt"));
        int option = fileChooser.showOpenDialog(this);
        if (option == JFileChooser.APPROVE_OPTION) {
            File file = fileChooser.getSelectedFile();
            try {
                BufferedReader reader = new BufferedReader(new FileReader(file));
                textArea.read(reader, null);
                reader.close();
                fileName = file.getAbsolutePath();
                setTitle("记事本 - " + fileName);
                changed = false;
            } catch (IOException e) {
                JOptionPane.showMessageDialog(this, "打开文件失败！", "错误", JOptionPane.ERROR_MESSAGE);
            }
        }
    }

    private void save() {
        if (fileName == null) {
            saveAs();
            return;
        }
        try {
            BufferedWriter writer = new BufferedWriter(new FileWriter(fileName));
            textArea.write(writer);
            writer.close();
            setTitle("记事本 - " + fileName);
            changed = false;
        } catch (IOException e) {
            JOptionPane.showMessageDialog(this, "保存文件失败！", "错误", JOptionPane.ERROR_MESSAGE);
        }
    }

    private void saveAs() {
        JFileChooser fileChooser = new JFileChooser();
        fileChooser.setFileFilter(new FileNameExtensionFilter("文本文件(*.txt)", "txt"));
        int option = fileChooser.showSaveDialog(this);
        if (option == JFileChooser.APPROVE_OPTION) {
            File file = fileChooser.getSelectedFile();
            if (file.exists()) {
                int result = JOptionPane.showConfirmDialog(this, "文件已存在，是否覆盖？", "提示", JOptionPane.YES_NO_OPTION);
                if (result == JOptionPane.NO_OPTION) {
                    return;
                }
            }
            try {
                BufferedWriter writer = new BufferedWriter(new FileWriter(file));
                textArea.write(writer);
                writer.close();
                fileName = file.getAbsolutePath();
                setTitle("记事本 - " + fileName);
                changed = false;
            } catch (IOException e) {
                JOptionPane.showMessageDialog(this, "保存文件失败！", "错误", JOptionPane.ERROR_MESSAGE);
            }
        }
    }

    private void exit() {
        if (changed) {
            int result = JOptionPane.showConfirmDialog(this, "是否保存当前文件？", "提示", JOptionPane.YES_NO_CANCEL_OPTION);
            if (result == JOptionPane.YES_OPTION) {
                save();
            } else if (result == JOptionPane.CANCEL_OPTION) {
                return;
            }
        }
        System.exit(0);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        JMenuItem source = (JMenuItem) e.getSource();
        if (source.getText().equals("新建")) {
            newFile();
        } else if (source.getText().equals("打开")) {
            open();
        } else if (source.getText().equals("保存")) {
            save();
        } else if (source.getText().equals("另存为")) {
            saveAs();
        } else if (source.getText().equals("退出")) {
            exit();
        }
    }

    public static void main(String[] args) {
        Notepad notepad = new Notepad();
        notepad.setVisible(true);
    }
}

这个程序实现了基本的新建、打开、保存、另存为、退出功能，并且在关闭窗口时会自动进行保存提示。用户也可以通过菜单栏进行相应操作。