import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class Test {

    private static final int FIRST_SIZE = 300;
    private static final int SECOND_SIZE = 150;

    public static void main(String[] args) throws Exception {

        final JFrame f = new JFrame();

        final JPanel p = new JPanel() {
            public void paintComponent(Graphics g) {
                super.paintComponent(g);
                g.fillRect(0, 0, SECOND_SIZE, SECOND_SIZE);
            }
        };

        p.setPreferredSize(new Dimension(FIRST_SIZE, FIRST_SIZE));
        f.add(p);

        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);

        Thread.sleep(2000);

        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                f.pack();
                f.setSize(0, 0);
                p.setPreferredSize(new Dimension(SECOND_SIZE, SECOND_SIZE));
                f.invalidate();
                f.validate();
                f.pack();
            }
        });
    }
}