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();
    }
}