如何在 Java 框架內將矩形居中，考慮框架裝飾？

import javax.swing.*;
import java.awt.*;

public class CenteredRectangle extends JFrame {

    public static void main(String[] args) {
        new CenteredRectangle();
    }

    public CenteredRectangle() {
        setDefaultCloseOperation(EXIT_ON_CLOSE);

        // Set the content pane's preferred size
        setContentPane(new PaintablePanel());
        pack();

        // Center the frame on the screen
        setLocationRelativeTo(null);

        // Make the frame visible
        setVisible(true);
    }

    public class PaintablePanel extends JPanel {

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(800, 400);
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);

            // Draw the rectangle centered within the content area
            paintRectangle(g);
        }

        private void paintRectangle(Graphics g) {
            g.setColor(Color.RED);
            int x = (getWidth() - 700) / 2;
            int y = (getHeight() - 300) / 2;
            g.drawRect(x, y, 700, 300);
        }
    }
}