java swing 實現鼠標聚焦縮放圖層

實現java swing的鼠標聚焦縮放圖層功能。代碼如下：

package test;

import java.awt.;
import java.awt.event.;
import java.awt.geom.;
import javax.swing.;
public class FPanel extends javax.swing.JPanel {

private Dimension preferredSize = new Dimension(400, 400);
private Rectangle2D[] rects = new Rectangle2D[50];

public static void main(String[] args) {
    JFrame jf = new JFrame("test");
    jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    jf.setSize(400, 400);
    jf.add(new JScrollPane(new FPanel()));
    jf.setVisible(true);
}

public FPanel() {
    // generate rectangles with pseudo-random coords
    for (int i = 0; i < rects.length; i++) {
        rects[i] = new Rectangle2D.Double(Math.random() * .8,
                Math.random() * .8, Math.random() * .2, Math.random() * .2);
    }
    // mouse listener to detect scrollwheel events
    addMouseWheelListener(new MouseWheelListener() {
        public void mouseWheelMoved(MouseWheelEvent e) {
            updatePreferredSize(e.getWheelRotation(), e.getPoint());
        }
    });
}

private void updatePreferredSize(int wheelRotation, Point stablePoint) {
    double scaleFactor = findScaleFactor(wheelRotation);
    scaleBy(scaleFactor);
    Point offset = findOffset(stablePoint, scaleFactor);
    offsetBy(offset);
    getParent().doLayout();
}

private double findScaleFactor(int wheelRotation) {
    double d = wheelRotation * 1.08;
    return (d > 0) ? 1 / d : -d;
}

private void scaleBy(double scaleFactor) {
    int w = (int) (getWidth() * scaleFactor);
    int h = (int) (getHeight() * scaleFactor);
    preferredSize.setSize(w, h);
}

private Point findOffset(Point stablePoint, double scaleFactor) {
    int x = (int) (stablePoint.x * scaleFactor) - stablePoint.x;
    int y = (int) (stablePoint.y * scaleFactor) - stablePoint.y;
    return new Point(x, y);
}

private void offsetBy(Point offset) {
    Point location = getLocation();
    setLocation(location.x - offset.x, location.y - offset.y);
}

public Dimension getPreferredSize() {
    return preferredSize;
}

private Rectangle2D r = new Rectangle2D.Float();

public void paint(Graphics g) {
    super.paint(g);
    g.setColor(Color.red);
    int w = getWidth();
    int h = getHeight();
    for (Rectangle2D rect : rects) {
        r.setRect(rect.getX() * w, rect.getY() * h, rect.getWidth() * w,
                rect.getHeight() * h);
        ((Graphics2D) g).draw(r);
    }
}

}</pre>