一个Frame中有多个Panel，想在其中一个指定的Panel中画图，怎么实现呢

import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;

import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.BevelBorder;

public class Test extends JFrame
{
    private static final long serialVersionUID = 1L;

    Test()
    {
        setSize(400, 300);
        setLayout(new FlowLayout());
        int num = 10;
        int rand = (int) (Math.random() * num);
        for(int i = 0; i < num; i++)
        {
            JPanel panel;
            if(i == rand)
            {
                panel = new JPanel()
                {
                    private static final long serialVersionUID = 1L;

                    @Override
                    public void paint(Graphics g)
                    {
                        super.paint(g);
                        g.setColor(Color.orange);
                        g.fillOval(15, 15, 20, 20);
                        g.dispose();
                    }
                };
            }
            else
            {
                panel = new JPanel();
            }
            panel.setBorder(BorderFactory.createBevelBorder(BevelBorder.LOWERED, Color.red, Color.black));
            panel.setPreferredSize(new Dimension(40, 50));
            add(panel);
        }
        setLocationRelativeTo(null);
        setResizable(false);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    public static void main(String[] args)
    {
        new Test().setVisible(true);
    }
}