完成简易记事本。能打开txt文件，保存文件。

package GUI;

import javax.swing.;
import javax.swing.filechooser.FileFilter;
import javax.swing.filechooser.FileNameExtensionFilter;
import java.awt.;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.\*;
import java.net.URI;


public class NotepadDemo {


    public JPanel jp;
    public JFrame jf;
    public JMenuBar jMenuBar;
    public JMenu menu_File, menu_Edit, menu_View, menu_About;
    public JMenuItem menuItem_Open, menuItem_Exit1, menuItem_Exit2,
            menuItem_Exit3, menuItem_Exit4, menuItem_Save, menuItem_SaveAs,
            menuItem_Cut, menuItem_Copy, menuItem_MyBlog;
    public JTextArea jTextArea;
    public JScrollPane jScrollPane;
    public File oneFile;
    public FileReader fr;
    public FileWriter fw;
    public BufferedReader br;
    public String line;

    public NotepadDemo(){

        jf = new JFrame("菜单");
        jf.setSize(800, 600);
        jf.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        jf.setLocationRelativeTo(null);
        //jf.setResizable(false);

        jp = new JPanel(new GridLayout(1, 1));

        jMenuBar = new JMenuBar();

        menu_File = new JMenu("文件");
        menu_Edit = new JMenu("编辑");
        menu_View = new JMenu("视图");
        menu_About = new JMenu("关于");

        jMenuBar.add(menu_File);
        jMenuBar.add(menu_Edit);
        jMenuBar.add(menu_View);
        jMenuBar.add(menu_About);

        //-=-=-=-=-=-=-文件-=-=-=-=-=-=-
        menuItem_Open = new JMenuItem("打开");
        menuItem_Save = new JMenuItem("保存");
        menuItem_SaveAs = new JMenuItem("另存为");
        menuItem_Exit1 = new JMenuItem("退出");

        menu_File.add(menuItem_Open);
        menu_File.add(menuItem_Save);
        menu_File.add(menuItem_SaveAs);
        menu_File.addSeparator();
        menu_File.add(menuItem_Exit1);

        //-=-=-=-=-=-=-编辑-=-=-=-=-=-=-
        menuItem_Copy = new JMenuItem("复制");
        menuItem_Cut = new JMenuItem("裁剪");
        menuItem_Exit2 = new JMenuItem("退出");

        menu_Edit.add(menuItem_Copy);
        menu_Edit.add(menuItem_Cut);
        menu_Edit.addSeparator();
        menu_Edit.add(menuItem_Exit2);

        //-=-=-=-=-=-=-视图-=-=-=-=-=-=-
        menuItem_Exit3 = new JMenuItem("退出");
        menu_View.add(menuItem_Exit3);

        //-=-=-=-=-=-=-关于-=-=-=-=-=-=-
        menuItem_MyBlog = new JMenuItem("银月のBlog");
        menuItem_Exit4 = new JMenuItem("退出");

        menu_About.add(menuItem_MyBlog);
        menu_About.addSeparator();
        menu_About.add(menuItem_Exit4);

        ActL actL = new ActL();

        menuItem_Open.addActionListener(actL);
        menuItem_Save.addActionListener(actL);
        menuItem_SaveAs.addActionListener(actL);
        menuItem_Exit1.addActionListener(actL);
        menuItem_Exit2.addActionListener(actL);
        menuItem_Exit3.addActionListener(actL);
        menuItem_Exit4.addActionListener(actL);
        menuItem_MyBlog.addActionListener(actL);

        jTextArea = new JTextArea();
        jTextArea.setLineWrap(true);
        jScrollPane = new JScrollPane(jTextArea);

        jp.add(jScrollPane);

        jf.setJMenuBar(jMenuBar);
        jf.setContentPane(jp);
        jf.setVisible(true);

    }

    class ActL implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent actionEvent) {
            String m = actionEvent.getActionCommand();
            if(m.equals("退出")) {
                System.exit(0);
            } else if(m.equals("打开")) {
                //↓判断用户单击的是否为"打开"按钮
                if (fileChooser.showOpenDialog(jf.getContentPane()) == JFileChooser.APPROVE_OPTION) {

                    File selectedFile = fileChooser.getSelectedFile();
                    oneFile = new File(selectedFile.getAbsolutePath());

                    try {
                        fr = new FileReader(oneFile);
                        br = new BufferedReader(fr);
                        line = "";
                        while((line = br.readLine()) != null) {
                            jTextArea.append(line + "\r\n");
                        }
                    } catch (IOException e){
                        e.printStackTrace();
                    } finally {
                        try {
                            if(br != null) {
                                br.close();
                            }
                        } catch(IOException e) {
                            e.printStackTrace();
                        }
                    }
                }
            } else if(m.equals("保存")) {

                try {
                    if (oneFile != null) {
                        fw = new FileWriter(oneFile, false);
                        fw.write(jTextArea.getText());
                        fw.flush();
                    } else {
                        System.out.println("未打开文件");
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                } finally {
                    try{
                        if(fw != null){
                            fw.close();
                        }
                    } catch (IOException e){
                        e.printStackTrace();
                    }
                }

            } else if(m.equals("另存为")) {
                //↓判断用户单击的是否为"打开"按钮
                if (fileChooser.showSaveDialog(jf.getContentPane()) == JFileChooser.APPROVE_OPTION) {

                    File selectedFile = fileChooser.getSelectedFile();
                    oneFile = new File(selectedFile.getAbsolutePath());

                    try {
                        if (oneFile.createNewFile()) {
                            fw = new FileWriter(oneFile, false);
                            fw.write(jTextArea.getText());
                            fw.flush();
                            fw.close();
                        }
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            } else if(m.equals("银月のBlog")) {
                URI uri=java.net.URI.create("https://i.loli.link");
                Desktop dp=java.awt.Desktop.getDesktop();
                //↓获取系统默认浏览器打开链接
                if(dp.isSupported(java.awt.Desktop.Action.BROWSE)){
                    try {
                        dp.browse(uri);
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            } else {
                System.out.println(m);
            }
        }

        JFileChooser fileChooser;
        {
            fileChooser = new JFileChooser();
            FileFilter filter = new FileNameExtensionFilter
                    ("文本文件(txt/html/htm/xml/php/js/java/c/cpp/py/vb/vba/vbs)",
                            "txt", "html", "htm", "xml", "php", "js",
                            "java", "c", "cpp", "py", "vb", "vba", "vbs");
            fileChooser.setFileFilter(filter);
        }
    }

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



`}
`

大部分代码都是从12.2中搬过来的,
也就只改了文件读写部分的代码