初学图形化界面的一个小程序出了点有关问题

import java.awt.*;import java.awt.event.*;import java.io.*;class  MenuDemo{    private Frame f;    private MenuBar mb;    private Menu m;    private MenuItem dm,gm,bm;    private FileDialog open,save;    private TextArea ta;    private File file;    MenuDemo()    {        init();    }    public void init()    {        f=new Frame("魔兽世界");        mb=new MenuBar();        m=new Menu("文件");        dm=new MenuItem("打开");        gm=new MenuItem("退出");        bm=new MenuItem("保存");        open=new FileDialog(f,"打开WoW",FileDialog.LOAD);        save=new FileDialog(f,"关闭WoW",FileDialog.SAVE);        ta=new TextArea();        f.setBounds(300,100,600,600);        //f.setLayout(new FlowLayout());使用默认布局，没设置东南西北，就填充了        f.setMenuBar(mb);        f.add(ta);        mb.add(m);        m.add(dm);        m.add(bm);        m.add(gm);        myEvent();        f.setVisible(true);    }     private void myEvent()       {           bm.addActionListener(new ActionListener()         {            public void actionPerformed(ActionEvent e)            {                if(file==null)                {                    save.setVisible(true);                    String s1=save.getFile();                    String s2=save.getDirectory();                    if (s1==null||s2==null)                    {                        return;                    }                    file=new File(s2,s1);                }                else                {                    BufferedWriter bw=null;                    try                    {                        bw=new BufferedWriter(new FileWriter(file));                        String s=ta.getText();                        bw.write(s);                        bw.flush();                        bw.close();                    }                    catch (IOException e1)                    {                        throw new RuntimeException("写入失败");                    }                }                            }         });        dm.addActionListener(new ActionListener()        {            public void actionPerformed(ActionEvent e)            {                open.setVisible(true);                String filePath=open.getDirectory();                String name=open.getFile();                if (filePath==null||name==null)                {                    return;                }                ta.setText("");                file=new File(filePath,name);                BufferedReader br=null;                try                {                    br=new BufferedReader(new FileReader(file));                    String line=null;                    while ((line=br.readLine())!=null)                    {                        ta.append(line+"\r\n");                    }                }                catch (IOException e1)                {                    throw new RuntimeException("读取失败");                }                finally                {                    try                    {                        if (br!=null)                        {                            br.close();                        }                    }                    catch (IOException e1)                    {                        System.out.println("读取流关闭失败");                    }                }            }        });        f.addWindowListener(new WindowAdapter()          {              public void windowClosing(WindowEvent e)              {                  System.exit(0);              }          });          gm.addActionListener(new ActionListener()        {            public void actionPerformed(ActionEvent e)            {                System.exit(0);            }        });            }    public static void main(String[] args)     {        new MenuDemo();    }}