无法显示g.fillRect

Question

以下是我从YouTube视频中复制的代码。我不知道为什么g.fillRect（x，10,20,20）;没有显示。我检查了几次，仍然可以在代码中找到错误。请帮我找出问题所在：（

package sanke.playground;

import java.awt.Color;  
import java.awt.Graphics; 
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;


import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;


public class SankePlayground extends JPanel implements ActionListener {

    Timer tm = new Timer (5,this);
    int x = 0, velX = 2;
    //int y = 0, velY = 0;

    public SankePlayground(){

    }
public void paintComponent(Graphics g){

    super.paintComponent(g);

    g.setColor(Color.RED);
    g.fillRect(x, 10, 20, 20);

    tm.start();
}

public void actionPerformed(ActionEvent e){

    x = x+ velX;
    repaint();
}

public static void main(String args[]){

    SankePlayground t = new SankePlayground();
    JFrame jf = new JFrame();
    jf.setSize(600,400);
    jf.setVisible(true);
    jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    jf.add(t);
}
}