方法一:
(利用imageio类的读写函数)
import javax.imageio.ImageIO;
import java.io.File;
import java.io.IOException;
import java.awt.image.BufferedImage;

public class ChangeImageStyle {

  public static void main(String args[]) throws IOException{
 
    File inputFile = new File("test.bmp");
    BufferedImage input = ImageIO.read(inputFile);

    //转换为gif格式的图片
    File outputFile = new File("test.gif");
    ImageIO.write(input, "GIF", outputFile);

    //转换为jpg格式的图片  
    outputFile = new File("test.jpg");
    ImageIO.write(input, "JPG", outputFile);

    //转换为png格式的图片
    outputFile = new File("test.png");
    ImageIO.write(input, "PNG", outputFile);
  }
}

方法二:
如果你已经把某个文件读入,并存为了Image类的一个实例.
转变方法如下:
其中:image为Image类的一个实例
try{
BufferedImage bi = new BufferedImage(image.getWidth(null), image.getHeight(null),BufferedImage.TYPE_INT_RGB);
Graphics2D biContext = bi.createGraphics();
biContext.drawImage(image, 0, 0, null);
FileOutputStream out=new FileOutputStream("./a1.jpeg");                    
JPEGImageEncoder encoder=JPEGCodec.createJPEGEncoder(out);                    
JPEGEncodeParam param=encoder.getDefaultJPEGEncodeParam(bi);                     
param.setQuality(1.0f,false);
encoder.setJPEGEncodeParam(param);
encoder.encode(bi);
out.flush();
 out.close();
 }catch(Exception EE)
   {
  System.out.println(EE);
   }

import java.awt.*;
import java.awt.geom.Ellipse2D;
import java.awt.geom.GeneralPath;
import java.awt.geom.RoundRectangle2D;

import javax.swing.*;

public class ShapedWindow extends JFrame {

 public ShapedWindow() {
  super("Test oval-shaped window");
  this.setLayout(new FlowLayout());
  this.add(new JButton("test"));
  this.add(new JCheckBox("test"));
  this.add(new JRadioButton("test"));
  this.add(new JProgressBar(0, 100));

  this.setSize(new Dimension(400, 300));
  this.setLocationRelativeTo(null);
  this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 }

 public static void main(String[] args) {
  JFrame.setDefaultLookAndFeelDecorated(true);
  SwingUtilities.invokeLater(new Runnable() {
   @Override
   public void run() {
    Window w = new ShapedWindow();
    w.setVisible(true);
    
    Rectangle bounds = w.getBounds();
               
                Rectangle newrec = new Rectangle(0, 0, bounds.width-100, bounds.height);
                GeneralPath gp;
                gp = new GeneralPath(newrec);
                gp.moveTo(bounds.width-100, 0);
                gp.lineTo(bounds.width, bounds.height/2);
                gp.lineTo(bounds.width-100, bounds.height);
                gp.lineTo(bounds.width-100, 0);
 
    /*com.sun.awt.AWTUtilities
      .setWindowShape(w, new Ellipse2D.Double(0, 0, w
        .getWidth(), w.getHeight()));
        */
                com.sun.awt.AWTUtilities.setWindowShape(w, gp); 
  }
  });
 }

}

   /* Convert RGBA -> ARGB */
   for (int i = 0; i < width * height; i++)
   {
    icon_image_ARGB[i + 2] =
     icon_image_RGBA[i * 4 + 3] << 24 |
     ((icon_image_RGBA[i * 4 + 0] << 16) & 0x00FF0000) |
     ((icon_image_RGBA[i * 4 + 1] << 8) & 0x0000FF00) |
     ((icon_image_RGBA[i * 4 + 2] << 0) & 0x000000FF);
   }
   
   BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);
   image.setRGB(0, 0, width, height, icon_image_ARGB, 0, width);
   try {
    ImageIO.write(image, "jpeg", imgage_file);
   } catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   }
  }