java怎样判断图片格式

我用java的GifEncoder和JPEGImageEncoder来实现java图片的显示和保存 但是在保存的时候总是只能保存其中一种格式 如过是Jpg格式的话保存后 仍是Gif格式 反过来相同……  private void saveImage()
    { 
      if(flag==1)
      {
      
        try
        {
        int saved=fileChooser.showSaveDialog(this);
        if(saved==fileChooser.APPROVE_OPTION)
         {
            File filenameSave=fileChooser.getSelectedFile();
            String filename=new String(filenameSave.getPath()+".jpg");
            FileOutputStream out=new FileOutputStream(filename);
            FileInputStream in=new FileInputStream(this.openFile.toString());
            tempImage=ImageIO.read(in);
            JPEGImageEncoder encoder=JPEGCodec.createJPEGEncoder(out);
            encoder.encode((BufferedImage) tempImage);
            out.flush();
            out.close();
          }
         }
        catch(Exception e)
        {
            e.printStackTrace();
        }   
      }       
       else if(flag==0)
       {
        try
          {       
        int saved=fileChooser.showSaveDialog(this);
        if(saved==fileChooser.APPROVE_OPTION)
           {
        File filenameSave=fileChooser.getSelectedFile();
        String filename=new String(filenameSave.getPath()+".gif");
        FileOutputStream out=new FileOutputStream(filename);
        FileInputStream in=new FileInputStream(this.openFile.toString());
        BufferedImage tempImage=ImageIO.read(in);
        encoder1=new GifEncoder(tempImage,out);
         encoder1.encode();
        out.flush();
        out.close();
        in.close();       
            } 
           }
        catch(Exception e)
        {
            e.printStackTrace();
        }    
      }   
    }  保存按钮的响应事件代码：
     switch(flag)
       {
           case 0:
           {
               flag=0;
               this.saveImage();
           }break;       
           case 1:
           {
               flag=1;
               this.saveImage();
           }break;
       }