即使扩展了JPanel并覆盖PaintComponent，面板也不会显示PaintComponent

Question

我试图将用paintComponent（）方法构造的图像放在JFrame的其他对象下面。我无法使paintComponent（）方法显示在我的框架/面板上。

我扩展了JPanel，在带有图形参数的paintComponent（）中使用了super，并且我还尝试重写该方法，这些方法都是解决此问题的常用方法，但是它们都无济于事，所以我假设它是另一个错误。

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

public class LayoutPractice extends JPanel{

    Color currentColor;
    private int numClicks = 0;

    public void changeColor(Color currentColor) {
        if (currentColor.equals(Color.RED)) {
            currentColor = Color.GREEN;
        } else {
            currentColor = Color.RED;
        }
    }

    public LayoutPractice() {
        this.currentColor = Color.RED;
        JFrame frame = new JFrame();
        frame.setTitle("Mario GUI");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(600, 400);

        JPanel panel = new JPanel();
        JLabel counterText = new JLabel("Button Clicked 0 times");
        JButton dialogButton = new JButton("Click Me");
        JButton characterButton = new JButton("Change Character");


        panel.addMouseListener(new MouseAdapter() {
            public void mouseClicked(MouseEvent mv) {
                numClicks++;
                counterText.setText("Button Clicked " + numClicks + " times");
            }
        });

        dialogButton.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent e) {
                JOptionPane.showMessageDialog(panel,
                        "You have clicked " + numClicks + " times",
                        "Clicker Counter",
                        JOptionPane.INFORMATION_MESSAGE);
            }
        });
        dialogButton.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent e) {
                changeColor(currentColor);
            }
        });

        panel.add(counterText);
        panel.add(dialogButton);
        panel.add(characterButton);
        frame.getContentPane().add(panel);
        frame.setVisible(true);
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        int width = 600;
        int height = 400;
        int pxW = width/13;
        int pxH = height/7;
        //hat, red color
        g.setColor(currentColor);
        g.fillRect((3 * pxW), 0, (6 * pxW), pxH);
        g.fillRect((2 * pxW), pxH, (10 * pxW), pxH);

        //skin, tan color
        g.setColor(new Color(255,186,170));
        g.fillRect((2 *pxW), (3 * pxH), pxW, (2*pxH));
        g.fillRect((5 *pxW), (2 *pxH), (5 *pxW), pxH);
        g.fillRect((4 *pxW), (3*pxH), (8 *pxW), pxH);
        g.fillRect((5 *pxW), (4*pxH), (8 *pxW), pxH);
        g.fillRect((3 *pxW), (5*pxH), (5*pxW), pxH);
        g.fillRect((3 *pxW), (6*pxH), (8*pxW), pxH);

        //hair, brown color
        g.setColor(new Color(153, 102, 17));
        g.fillRect((2 *pxW), (2 * pxH), (3 * pxW), pxH);
        g.fillRect(pxW, (3 * pxH), pxW , (3 * pxH));
        g.fillRect((3*pxW), (3* pxH), pxW, (2 * pxH));
        g.fillRect((2*pxW), (5 *pxH), pxW, pxH);
        g.fillRect((4 *pxW), (4 * pxH), pxW, pxH);

        //eyes, black color
        g.setColor(Color.BLACK);
        g.fillRect((8*pxW), (2 * pxH), pxW, (2 *pxH));
        g.fillRect((9*pxW), (4 *pxH), pxW, pxH);
        g.fillRect((8*pxW), (5 *pxH), (4 *pxW), pxH);

    }

    public static void main(String args[]) {
        SwingUtilities.invokeLater(new Runnable(){
            public void run() {
                new LayoutPractice();
            }
        });
    }
}