How to optimize Image loading?

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Image;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.text.DecimalFormat;
 
import javax.imageio.ImageIO;
import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
 
/**
 * This test class creates an online catalog.
 * 
 * @author CB, crazy-bytes.com (C) 2007
 * @version 1.4
 * @since JDK / JRE 1.5
 */
@SuppressWarnings("serial")
public class TestCatalog 
	extends JPanel 
	implements ActionListener {
	// Catalog root directory
	private static final String CAT_ROOT_DIR = "C:\\Temp\\TestCatalog\\";
	// Catalog image directory
	private static final String CAT_IMG_DIR = CAT_ROOT_DIR + "images\\";
	private static final DecimalFormat THREE_DIGITS = new DecimalFormat("000");
	private static final int MAX_WIDTH = 379;
	private static final int MAX_HEIGHT = 539;
	private static final int MAX_PAGE = getMaximumPage();
 
	private JLabel lbl_left_page = new JLabel();
	private JLabel lbl_right_page = new JLabel();
	private JLabel lbl_left_index = new JLabel();
	private JLabel lbl_right_index = new JLabel();
	private JLabel lbl_goto = new JLabel("Go to page");
	private JTextField txt_goto = new JTextField(3);
	
	private JButton btn_prev = new JButton("previous");
	private JButton btn_next = new JButton("next");
	private JButton btn_goto = new JButton("\u00bb"); // ">>" character
	private int current_page = 0;
	
	public TestCatalog() {
		init();
	}
	
	private void init() {
		setLayout(new BorderLayout(0, 0));
		
		btn_next.addActionListener(this);
		btn_prev.addActionListener(this);
		
		// Create top navi bar
		JPanel pnl_top_nav = new JPanel(new BorderLayout(5, 5));
		pnl_top_nav.add(btn_prev, BorderLayout.WEST);
		pnl_top_nav.add(btn_next, BorderLayout.EAST);
		
		JPanel pnl_pages = new JPanel(new FlowLayout(FlowLayout.CENTER, 0, 0));
		pnl_pages.add(lbl_left_page);
		pnl_pages.add(lbl_right_page);
		
		// Create bottom navi bar
		JPanel pnl_goto = new JPanel(new FlowLayout(FlowLayout.CENTER));
		pnl_goto.add(lbl_goto);
		pnl_goto.add(txt_goto);
		pnl_goto.add(btn_goto);
		
		JPanel pnl_bottom_nav = new JPanel(new BorderLayout());
		pnl_bottom_nav.add(lbl_left_index, BorderLayout.WEST);
		pnl_bottom_nav.add(pnl_goto, BorderLayout.CENTER);
		pnl_bottom_nav.add(lbl_right_index, BorderLayout.EAST);
		
		// Create display
		Dimension dim = new Dimension(MAX_WIDTH, MAX_HEIGHT);
		lbl_left_page.setPreferredSize(dim);
		lbl_right_page.setPreferredSize(dim);
		
		add(pnl_top_nav, BorderLayout.NORTH);
		add(pnl_pages, BorderLayout.CENTER);
		add(pnl_bottom_nav, BorderLayout.SOUTH);
		
		loadPage(current_page);
	}
	
	/**
	 * Loads the desired page and its opposite page. If the desired
	 * page number is invalid nothing happens. Even pages are
	 * displayed left and odd pages right.
	 * 
	 * @param page the page that shall be displayed
	 */
	private void loadPage(int page) {
		// Ignore imvalid pages.
		if(page < 0 || page > MAX_PAGE) {
			return;
		}
		
		if(page % 2 == 0) {
			loadPage(lbl_left_page, lbl_left_index, page);
			loadPage(lbl_right_page, lbl_right_index, page + 1);
		}
		else {
			loadPage(lbl_left_page, lbl_left_index, page - 1);
			loadPage(lbl_right_page, lbl_right_index, page);
		}
		
		current_page = page;
	}
	
	/**
	 * Loads a desired page into the desired label. The index of the page is
	 * display in the index label.
	 * 
	 * @param page_display the label that shall display the desired page
	 * @param index_display the label that displays the page number
	 * @param page the desired page that shall be displayed
	 */
	private void loadPage(JLabel page_display, JLabel index_display, int page) {
		try {
			page_display.setText("");
			if(page > MAX_PAGE) {
				page_display.setIcon(null);
				index_display.setText("");
			}
			else {
				page_display.setIcon(loadImage(page));
				index_display.setText("Page " + page + " of " + MAX_PAGE);
			}
		}
		catch(Exception e) {
			page_display.setText("An error occured whil loading page " + page + ".");
			e.printStackTrace();
		}
	}
	
	private Icon loadImage(int page) throws IOException {
		File file = new File(CAT_IMG_DIR + THREE_DIGITS.format(page) + ".jpg");
		BufferedImage bi = ImageIO.read(file);
		int width = bi.getWidth(null);
		int height = bi.getHeight(null);
		int new_width;
		int new_height;
		// Default ratio is 1 (equals 100% of the image size) which results in no
		// resizing.
		double ratio = 1;
		// If the image is larger than the maximum bounds resize it to fit, but 
		// keep ratio between width and height to avoid distortion.
		if (width > MAX_WIDTH &&
			height > MAX_HEIGHT) {
			// If width > height use width to resize to fit MAX_WIDTH.
			if (width > height) {
				ratio = MAX_WIDTH * 100 / width;
			}
			// Else use height to resize to fit MAX_HEIGHT.
			else {
				ratio = MAX_HEIGHT * 100 / height;
			}
		}
		else if (width > MAX_WIDTH) {
			ratio = MAX_WIDTH * 100 / width;
		}
		else if (height > MAX_HEIGHT) {
			ratio = MAX_HEIGHT * 100 / height;
		}
		
		Icon ico = null;
		
		// If resizing is needes, create a scaled instance.
		if(ratio != 1) {
			// Calculate new image size
			new_width = (int) Math.round(width * ratio);
			new_height = (int) Math.round(height * ratio);
 
			// WORK AROUND
			// Set new width and height to 1 since sometimes the BufferedImage filter
			// is not able to read the width and height right and returns 0, which can
			// cause a division by zero error when scaling to the new width and height.
			new_width = new_width > 0 ? new_width : 1;
			new_height = new_height > 0 ? new_height : 1;
			ico = new ImageIcon(bi.getScaledInstance(new_width, new_height, Image.SCALE_DEFAULT));
		}
		// Else just return the image.
		else {
			ico = new ImageIcon(bi);
		}
		
		// Free resources and return the image.
		bi.flush();
		bi = null;
		System.gc();
		return ico;
	}
 
	@Override
	public void actionPerformed(ActionEvent e) {
		if(e.getSource() == btn_prev) {
			loadPage(current_page - 2);
		}
		if(e.getSource() == btn_next) {
			loadPage(current_page + 2);
		}
	}
 
	/**
	 * Every page is indexed from 000 to nnn. Since the page amount can vary
	 * from catalog to catalog, this methode always determines the highest
	 * page number when this class is invoked.
	 * 
	 * @return the highest page number of the catalog
	 */
	private static int getMaximumPage() {
		int max_page = 0;
		File img_dir = new File(CAT_IMG_DIR);
		if (img_dir.exists() &&
			img_dir.isDirectory()) {
			File[] files = img_dir.listFiles();
			for(File file : files) {
				if (file.isFile() &&
				    file.getName().matches("\\d\\d\\d.jpg")) {
					int num = Integer.parseInt(file.getName().substring(0, 3));
					if(num > max_page) max_page = num;
				}
			}
		}
		System.out.println(max_page);
		return max_page;
	}
 
	public static void main(String[] args) {
		final JFrame f = new JFrame("TestCatalog");
		f.getContentPane().add(new TestCatalog());
		f.addWindowListener(new WindowAdapter() {
		    public void windowClosing(WindowEvent windowEvent) {
			f.dispose();
			System.exit(0);
		    }
		});
		f.pack();
		f.setResizable(false);
	
		// Go!!!
		f.setVisible(true);
	}
}

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	private Icon loadImage(int page) throws IOException {
		File file = new File(CAT_IMG_DIR + THREE_DIGITS.format(page) + ".jpg");
		BufferedImage bi = ImageIO.read(file);
		int width = bi.getWidth(null);
		int height = bi.getHeight(null);
		int new_width;
		int new_height;
		// Default ratio is 1 (equals 100% of the image size) which results in no
		// resizing.
		double ratio = 1;
		// If the image is larger than the maximum bounds resize it to fit, but 
		// keep ratio between width and height to avoid distortion.
		if (width > MAX_WIDTH &&
			height > MAX_HEIGHT) {
			// If width > height use width to resize to fit MAX_WIDTH.
			if (width > height) {
				ratio = MAX_WIDTH / (double) width;
			}
			// Else use height to resize to fit MAX_HEIGHT.
			else {
				ratio = MAX_HEIGHT / (double) height;
			}
		}
		else if (width > MAX_WIDTH) {
			ratio = MAX_WIDTH / (double) width;
		}
		else if (height > MAX_HEIGHT) {
			ratio = MAX_HEIGHT / (double) height;
		}
		
		Icon ico = null;
		
		// If resizing is needes, create a scaled instance.
		if(ratio != 1) {
			// Calculate new image size
			new_width = (int) Math.round(width * ratio);
			new_height = (int) Math.round(height * ratio);
 
			// WORK AROUND
			// Set new width and height to 1 since sometimes the BufferedImage filter
			// is not able to read the width and height right and returns 0, which can
			// cause a division by zero error when scaling to the new width and height.
			new_width = new_width > 0 ? new_width : 1;
			new_height = new_height > 0 ? new_height : 1;
			ico = new ImageIcon(bi.getScaledInstance(new_width, new_height, Image.SCALE_DEFAULT));
		}
		// Else just return the image.
		else {
			ico = new ImageIcon(bi);
		}
		
		// Free resources and return the image.
		bi.flush();
		bi = null;
		System.gc();
		return ico;
	}
 
	@Override
	public void actionPerformed(ActionEvent e) {
		if(e.getSource() == btn_prev) {
			loadPage(current_page - 2);
		}
		if(e.getSource() == btn_next) {
			loadPage(current_page + 2);
		}
	}
 
	/**
	 * Every page is indexed from 000 to nnn. Since the page amount can vary
	 * from catalog to catalog, this methode always determines the highest
	 * page number when this class is invoked.
	 * 
	 * @return the highest page number of the catalog
	 */
	private static int getMaximumPage() {
		int max_page = 0;
		File img_dir = new File(CAT_IMG_DIR);
		if (img_dir.exists() &&
			img_dir.isDirectory()) {
			File[] files = img_dir.listFiles();
			for(File file : files) {
				if (file.isFile() &&
				    file.getName().matches("\\d\\d\\d.jpg")) {
					int num = Integer.parseInt(file.getName().substring(0, 3));
					if(num > max_page) max_page = num;
				}
			}
		}
		System.out.println(max_page);
		return max_page;
	}
 
	public static void main(String[] args) {
		final JFrame f = new JFrame("TestCatalog");
		f.getContentPane().add(new TestCatalog());
		f.addWindowListener(new WindowAdapter() {
		    public void windowClosing(WindowEvent windowEvent) {
			f.dispose();
			System.exit(0);
		    }
		});
		f.pack();
		f.setResizable(false);
	
		// Go!!!
		f.setVisible(true);
	}
}

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	private Icon loadImage(int page) throws IOException {
		File file = new File(CAT_IMG_DIR + THREE_DIGITS.format(page) + ".jpg");
		BufferedImage img = ImageIO.read(file);
		int width = img.getWidth(null);
		int height = img.getHeight(null);
		// Default ratio is 1 (equals 100% of the image size) which results in no
		// resizing.
		double ratio = 1;
		// If the image is larger than the maximum bounds resize it to fit, but 
		// keep ratio between width and height to avoid distortion.
		if (width > MAX_WIDTH &&
			height > MAX_HEIGHT) {
			// If width > height use width to resize to fit MAX_WIDTH.
			if (width > height) {
				ratio = MAX_WIDTH / (double) width;
			}
			// Else use height to resize to fit MAX_HEIGHT.
			else {
				ratio = MAX_HEIGHT / (double) height;
			}
		}
		else if (width > MAX_WIDTH) {
			ratio = MAX_WIDTH / (double) width;
		}
		else if (height > MAX_HEIGHT) {
			ratio = MAX_HEIGHT / (double) height;
		}
		
		Icon ico = null;
		
		// If resizing is needes, create a scaled instance.
		if(ratio != 1) {
			// Define tile size for each tile of the original image, so
			// each tile has a size of tile_size x tile_size pixels 
			int tile_size = 50;
			// Calculate new image size
			int new_width = (int) Math.round(width * ratio);
			int new_height = (int) Math.round(height * ratio);
			// WORK AROUND
			// Set new width and height to 1 since sometimes the BufferedImage filter
			// is not able to read the width and height right and returns 0, which can
			// cause a division by zero error when scaling to the new width and height.
			new_width = new_width > 0 ? new_width : 1;
			new_height = new_height > 0 ? new_height : 1;
 
			BufferedImage new_img = new BufferedImage(new_width, new_height, BufferedImage.TYPE_INT_RGB);
			Graphics2D g = new_img.createGraphics();
			// Count tiles along x and y axis
			int y_tile_count = (int) Math.ceil(img.getHeight(null) / tile_size);
			int x_tile_count = (int) Math.ceil(img.getWidth(null) / tile_size);
			int new_tile_size = (int) Math.round(tile_size * ratio);
			// Read each tile and resize it accordingly.
			for(int y = 0; y < y_tile_count; y++) {
				for(int x = 0; x < x_tile_count; x++) {
					int w = x < x_tile_count - 1 ? tile_size : width - (x * tile_size);
					int h = y < y_tile_count - 1 ? tile_size : height - (y * tile_size);
					// Read tile.
					BufferedImage tile = img.getSubimage(x * tile_size, y * tile_size, w, h);
					// Resize it.
					w = (int) Math.round(w * ratio);
					h = (int) Math.round(h * ratio);
					Image scaled_tile = tile.getScaledInstance(w, h, BufferedImage.SCALE_SMOOTH);
					// Free resources.
					tile.flush();
					// write tile into new output image.
					g.drawImage(scaled_tile, x * new_tile_size, y * new_tile_size, null);
				}
			}
			ico = new ImageIcon(new_img);
			new_img.flush();
		}
		// Else just return the image.
		else {
			ico = new ImageIcon(img);
		}
		
		// Free resources and return the image.
		img.flush();
		img = null;
		return ico;
	}

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