Java Swing - 如何在CardLayout中显示不同的面板大小

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.Window;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import java.awt.image.BufferedImage;

import javax.swing.ImageIcon;
import javax.swing.JComboBox;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;

public class Main {
  public static void swapComponentsAndResizeUI(JComponent ui,
      JComponent current, JComponent next) {
    ui.remove(current);
    ui.add(next);
    current = next;
    Component c = ui.getTopLevelAncestor();
    if (c instanceof Window) {
      Window w = (Window) c;
      w.pack();
    }
  }

  public static void main(String[] args) {
    JPanel ui = new JPanel(new BorderLayout(4, 4));
    ui.setBorder(new EmptyBorder(6, 6, 6, 6));

    JPanel controls = new JPanel(new FlowLayout(FlowLayout.LEADING));
    ui.add(controls, BorderLayout.PAGE_START);
    int s = 100;
    Dimension[] sizes = { new Dimension(s * 4, s * 2),
        new Dimension(s * 6, s * 3), new Dimension(s * 8, s * 4) };
    final JComboBox cb = new JComboBox(sizes);
    controls.add(cb);
    final JPanel[] panels = new JPanel[sizes.length];
    for (int ii = 0; ii < sizes.length; ii++) {
      Dimension d = sizes[ii];
      BufferedImage bi = new BufferedImage(d.width, d.height,
          BufferedImage.TYPE_INT_RGB);
      JPanel p = new JPanel(new GridLayout());
      JLabel l = new JLabel(new ImageIcon(bi));
      p.add(l);
      panels[ii] = p;
    }
    ItemListener sizeListener = new ItemListener() {

      JPanel current = panels[0];

      @Override
      public void itemStateChanged(ItemEvent e) {
        JPanel next = panels[cb.getSelectedIndex()];
        swapComponentsAndResizeUI(ui, current, next);
        current = next;
      }
    };
    cb.addItemListener(sizeListener);

    ui.add(panels[0], BorderLayout.CENTER);

    JFrame f = new JFrame("Three Sized Panels");
    f.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    f.setContentPane(ui);
    f.pack();
    f.setLocationByPlatform(true);
    f.setVisible(true);
  }
}