Java image viewer

MyCanvas.java

package PictureViewer;
  
import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
public class MyCanvas extends Canvas implements ComponentListener{
    private BufferedImage bi;
    private Image im;
    private int image_width;
    private int image_height;
    public void setImage(BufferedImage bi){
        this.bi = bi;
        this.zoom();
    }
    public void paint(Graphics g){
        g.drawImage(im,(this.getWidth()-image_width)/2,(this.getHeight()-image_height)/2,this);
    }
    public void componentResized(ComponentEvent e){
        if(bi != null){
            System.out.println("resize!!");
              
            this.zoom();
            this.repaint();
        }
    }   
    public void componentMoved(ComponentEvent e){}
    public void componentShown(ComponentEvent e){}
    public void componentHidden(ComponentEvent e){}
    public void zoom(){
        if(bi == null)
            return;
        int screen_width = this.getWidth();
        int screen_height = this.getHeight();
        double screen_proportion = 1.0 * screen_height / screen_width;  
        System.out.println("screen: w "+screen_width+" ,h "+screen_height+" ,p0 "+screen_proportion);
          
        image_width = bi.getWidth(this);
        image_height = bi.getHeight(this);
        double image_proportion = 1.0 * image_height / image_width;
        System.out.println("image: w "+image_width+" ,h "+image_height+" ,p1 "+image_proportion);
          
        if(image_proportion > screen_proportion){
            image_height = screen_height;
            image_width = (int)(image_height / image_proportion);   
            System.out.println("  p1>p0  w= "+image_width);
        }else{
            image_width = screen_width;
            image_height = (int)(image_width * image_proportion);   
            System.out.println("  p0>p1  h= "+image_height);
        }
        im = bi.getScaledInstance(image_width,image_height,Image.SCALE_SMOOTH);
    }
      
}

MyFilter.java

package PictureViewer;
  
import java.io.File;
import java.io.FilenameFilter;
  
public class MyFilter implements FilenameFilter{
    private String[] extension;  
    public MyFilter(){
        extension = new String[]{".jpg", ".JPG", ".gif", ".GIF", ".png", ".PNG", ".jpeg", ".JPEG"};
    }
    public MyFilter(String[] extension){
        this.extension = extension;
    }
    public boolean accept(File dir,String name){
        for(String s : extension){
            if(name.endsWith(s)){
                return true;   
            }
        }  
        return  false;
    }  
}

PictureViewer.java

package PictureViewer;
  
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);
    }
      
    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 validateButton(){
        previous.setEnabled((files!=null) && (findex > 0));
        next.setEnabled((files!=null) && (findex<(files.length-1))); 
    }
}