java初学者请教在JFrame中显示JPanel对象问题，为什么只能显示BUTTON,不能显 ...

代码如下：
ChangeBallColorTest.java代码:
import java.awt.Color;
import javax.swing.*;

public class ChangeBallColorTest {
    public static void main(String[] args) {
        ChangeBallColorFrame cbcf = new ChangeBallColorFrame();
        cbcf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        cbcf.setLocation(300,200);
        cbcf.setTitle("ChangeBallColor");
        cbcf.setVisible(true);
    }
}

ChangeBallColorFrame.java代码：
import java.awt.*;
import javax.swing.*;
class ChangeBallColorFrame extends JFrame {
        public ChangeBallColorFrame() {
            JButton cbcb1 = new JButton("red");
            JButton cbcb2 = new JButton("green");
            ChangeBallColorPanel cbcp = new ChangeBallColorPanel();
            setLayout(null);
            setSize(300,400);
            setBackground(Color.white);
            cbcb1.setBackground(Color.red);
            cbcb1.setBounds(160, 20, 75, 30);
            add(cbcb1);
            cbcb2.setBackground(Color.green);
            cbcb2.setBounds(70, 20, 75, 30);
            add(cbcb2);
            this.getContentPane().add(cbcp);
            
        }
    }

ChangeBallColorPanel.java代码：
import java.awt.*;
import javax.swing.*;
class ChangeBallColorPanel extends JPanel{
    public void paintComponents(Graphics g) {
        super.paintComponents(g);
        Graphics g1 = (Graphics) g;
        Color c = g1.getColor();
        g1.setColor(Color.red);
        g1.fillOval(120, 150, 20, 20);
        g1.setColor(c);
    }
}

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各位大侠给点回复啊！新手分比较少，请见谅！
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