实现java swing的鼠标聚焦缩放图层功能。代码如下：

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);          }      }  }