[hardwired]

Code:

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import java.io.*;
import java.util.Hashtable;
import javax.imageio.ImageIO;
import javax.swing.*;
import javax.swing.event.*;

public class MapScale extends JPanel {
    BufferedImage image;
    double scale = 1.0;

    public MapScale(BufferedImage image) {
        this.image = image;
    }

    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
                            RenderingHints.VALUE_INTERPOLATION_BICUBIC);
        double x = (getWidth() - scale*image.getWidth())/2;
        double y = (getHeight() - scale*image.getHeight())/2;
        AffineTransform at = AffineTransform.getTranslateInstance(x,y);
        at.scale(scale, scale);
        g2.drawRenderedImage(image, at);
    }

    public Dimension getPreferredSize() {
        int w = (int)(scale*image.getWidth());
        int h = (int)(scale*image.getHeight());
        return new Dimension(w, h);
    }

    private JSlider getSlider() {
        int min = 1, max = 36, inc = 5;
        final JSlider slider = new JSlider(min, max, 16);
        slider.setMajorTickSpacing(5);
        slider.setMinorTickSpacing(1);
        slider.setPaintTicks(true);
        slider.setSnapToTicks(true);
        slider.setLabelTable(getLabelTable(min, max, inc));
        slider.setPaintLabels(true);
        slider.addChangeListener(new ChangeListener() {
            public void stateChanged(ChangeEvent e) {
                int value = slider.getValue();
                scale = (value+4)/20.0;
                revalidate();
                repaint();
            }
        });
        return slider;
    }

    private Hashtable getLabelTable(int min, int max, int inc) {
        Hashtable<Integer,JLabel> table = new Hashtable<Integer,JLabel>();
        for(int j = min; j <= max; j += inc) {
            String s = String.format("%.2f", (j+4)/20.0);
            table.put(Integer.valueOf(j), new JLabel(s));
        }
        return table;
    }

    public static void main(String[] args) throws IOException {
        String path = "images/world_map.gif";
        BufferedImage image = ImageIO.read(new File(path));
        MapScale test = new MapScale(image);
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(new JScrollPane(test));
        f.getContentPane().add(test.getSlider(), "Last");
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}