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)));       }  }