The following example is extracted from a larger
project.  I have a subclass of JPanel added to the
center of my window, and a BufferedImage which is
displayed on the JPanel.  The panel sits inside a
JScrollPane, and if I size the window smaller than
the image I get scrollbars.  If the window is sized
large enough to see the entire image, I don't get
scrollbars.  This is what I want -- so far so good.

I also added a JSlider to control zoom (it's crude
right now, but I'm just trying to get it functional).
If I zoom the image, I lose the part of the image
that isn't visible after I zoom.  I want to be able
to use the scrollbars to scroll to the remainder of
the image, but I can't.  I tried setting the
horizontalScrollBarPolicy and
verticalScrollBarPolicy to always show the scroll
bars, but that didn't help -- I stilt can't ever get
to the rest of the image without zooming out.  Any
ideas?

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

public class ZoomPanel extends JFrame {
 private ImagePanel   imagePanel;
 private JSlider      zoomSlider;

 public ZoomPanel() {
   super("Image Processing Demo");

   Container container = getContentPane();
   container.setLayout(new BorderLayout(5, 5));

   imagePanel = new ImagePanel();
   container.add(new JScrollPane(imagePanel), BorderLayout.CENTER);
       
   String fileName = "c:\\larry\\test1.jpg";
   File file = new File(fileName);
   try {
     imagePanel.setImage(ImageIO.read(file));
   } catch (IOException x) {}
       
   zoomSlider = new JSlider(SwingConstants.VERTICAL, 1, 10, 1);
   zoomSlider.setMajorTickSpacing(1);
   zoomSlider.setPaintTicks(true);
   zoomSlider.addChangeListener(new ChangeListener() {
     public void stateChanged(ChangeEvent e) {
       repaint();
     }
   });
   container.add(zoomSlider, BorderLayout.EAST);

   pack();
   setVisible(true);
 }

 private class ImagePanel extends JPanel {
   private BufferedImage image;

   public void paintComponent(Graphics g) {
     super.paintComponent(g);
     if (image == null) return;
     g.drawImage(image, 0, 0, getWidth() * zoomSlider.getValue(),
                 getHeight() * zoomSlider.getValue(), this);
   }

   public void setImage(BufferedImage image) {
     this.image = image;
   }

   public Dimension getPreferredSize() {
     return getMinimumSize();
   }

   public Dimension getMinimumSize() {
     return new Dimension(300, 300);
   }
 }

 public static void main(String args[]) {
   ZoomPanel app = new ZoomPanel();
   app.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 }
}