问GUI对象无法绘制

EN

import java.awt.*;
import java.awt.event.*;

public class Agar {
    //instance field
    public static final int DEFAULT_SIZE = 1000, DEFAULT_X = 
    ArenaPanel.PANEL_WIDTH/2, DEFAULT_Y = ArenaPanel.PANEL_HEIGHT/2;

    private int x, y, size;

    private Color clr;

    public Agar(Color c) {
        x = DEFAULT_X;
        y = DEFAULT_Y;

        clr = c;
    }

    public void move() {
        //blank for now, not used yet
    }

    //grows by 10% width
    public void grow() {
        size += 10;
    }

    public void draw(Graphics fred) {
        fred.setColor(clr);
        fred.fillOval(0,0,size,size);

        fred.setColor(Color.black);
        fred.drawOval(0,0,size,size);
    }
}

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

public class ArenaPanel extends JPanel {
    //instance field
    public static final int PANEL_WIDTH = 1000, PANEL_HEIGHT = 500;

    private ArrayList<Food> food;
    private Timer foodAdder;

    private Agar agar;

    public ArenaPanel() {
        //agar stuff
        agar = new Agar(getRandomColor());

        //food stuff
        food = new ArrayList<Food>();
        //add some initial food
        for (int i = 0; i < 50; i++)
            addRandomFood();

        //"this will last a while"
        foodAdder = new Timer(3000, new FoodAdder());
        foodAdder.start();

        //listeners
        addComponentListener(new ResizeListener());

        //more basic stuff
        setPreferredSize(new Dimension(PANEL_WIDTH, PANEL_HEIGHT));
        setBackground(Color.white);
    }

    public void paintComponent(Graphics fred) {
        super.paintComponent(fred);

        agar.draw(fred);

        for (int i = 0; i < food.size(); i++)
            food.get(i).draw(fred);
    }

    //put it here so I can use it for both agar and food
    private Color getRandomColor() {
        int rand = (int)(Math.random()*7);
        Color c;

        switch (rand) {
            case 0:
            c = Color.red;
            break;
            case 1:
            c = Color.orange;
            break;
            case 2:
            c = Color.yellow;
            break;
            case 3:
            c = Color.green;
            break;
            case 4:
            c = Color.cyan;
            break;
            case 5:
            c = Color.blue;
            break;
            default:
            c = Color.pink;
        }

        return c;
    }

    private void addRandomFood() {
        int x = (int)(Math.random()*PANEL_WIDTH);
        int y = (int)(Math.random()*PANEL_HEIGHT);

        food.add(new Food(x,y,getRandomColor()));
    }

    private class FoodAdder implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            addRandomFood();

            repaint();
        }
    }

    private class ResizeListener extends ComponentAdapter {
        public void componentResized(ComponentEvent e) {
            setPreferredSize(getSize());
        }
    }
}