java中的简单的照片阅读器疑问？我应当如何修改呢？能否给出具体修改方法呢，谢谢

import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import java.io.*;
import javax.imageio.*;

public class PictureViewer implements ActionListener{
private Frame frame;
private MyCanvas mc;
private String fpath;
private String fname;
private File[] files;
private int findex;
private FileDialog fd_load;
private MyFilter filter;
private Button previous;
private Button next;
public static void main(String args[]) throws Exception{
new PictureViewer().init();
}
public void init(){
frame = new Frame("PictureViewer");
Panel pb = new Panel();
Button select = new Button("选择图片");
previous =new Button("上一张");
next = new Button("下一张");
select.addActionListener(this );
previous.addActionListener(this );
next.addActionListener(this);
pb.add(select);
pb.add(previous);
pb.add(next);
mc = new MyCanvas();
mc.setBackground(new Color(200,210,230));
mc.addComponentListener(mc);
frame.add(pb,"North");
frame.add(mc,"Center");
frame.setSize(360,360);
frame.setLocation(400,200);
frame.addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent e){
System.exit(0);
}
});
frame.setVisible(true);
this.validateButton();
filter = new MyFilter();
fd_load = new FileDialog(frame,"打开文件",FileDialog.LOAD);
fd_load.setFilenameFilter(filter);
}
@Override
public void actionPerformed(ActionEvent e){
String command = e.getActionCommand();
if (command.equals("选择图片")){
fd_load.setVisible(true);
fpath = fd_load.getDirectory();
fname = fd_load.getFile();
if ((fpath != null)&&(fname != null))
{
this.display(new File(fpath + fname));
files = new File(fpath).listFiles(filter);
this.setIndex();
}
}else if ( command.equals("上一张"))
{
findex--;
if (findex<0)

                findex = 0;
                this.display(files(findex));
            }else if (command.equals("下一张"))
            {
                findex++;
                if(findex >= files.length)
                    findex = files.length-1;
                this.display(files[findex]);
            }
            this.validateButton();
        }
        public void display(File f){
            try
            {
                BufferedImage bi = ImageIO.read(f);
                mc.setImage(bi);
                frame.setTitle("PictureViewer - [" + f.getName() +"]");
            }
            catch (Exception e)
            {
                e.printStackTrace();
            }
            mc.repaint();
        }
        public void  setIndex(){
            File current = new File(fpath + fname);
            if (files  != null)
            {
                for (int i = 0;i<files.length ;i++ )
                {
                    if (current.equals(files[i]))
                    {
                        findex = i;
                    }
                }
            }
        }
        public void valideteButton(){
            previous.setEnabled((files !=null)&&(findex >0));
            next.setEnabled((files != null)&&(findex<(files.length-1)));
        }
}