移动两个矩形 - 框架显示零矩形，没有给出错误

Question

我正在尝试解决Java教科书中的问题。用户要求两个矩形在单个帧中以相反方向移动的问题请求。到目前为止，我附上了我的代码。但是，当我运行代码时，框架不显示任何内容，甚至不显示静止的矩形。此外，我没有收到任何有助于深入了解手头问题的错误。谢谢！

import java.awt.Graphics;
import javax.swing.JComponent;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.Timer;
import javax.swing.JPanel;

public class RectangleComponent extends JComponent
{
    private static final int RECTANGLE_WIDTH = 20;
    private static final int RECTANGLE_HEIGHT = 30;

    private int xLeft;
    private int yTop;

    public RectangleComponent(int x, int y)
    {
        xLeft = x;
        yTop = y;
    }

    public void paintComponent(Graphics g)
    {
        g.fillRect(xLeft, yTop, RECTANGLE_WIDTH, RECTANGLE_HEIGHT);
    }

    /**
     * Moves the rectangle by a given amount
     * @return dx the amount to move in the x-direction
     * @return dy the amount to move in the y-direction
     */
    public void moveRectangleBy(int dx, int dy)
    {
        xLeft = xLeft + dx;
        yTop = yTop + dy;
        repaint();
    }
}

public class RectangleFrame extends JFrame
{
    private static final int FRAME_WIDTH = 300;
    private static final int FRAME_HEIGHT = 400;

    private RectangleComponent rec1;
    private RectangleComponent rec2;

    class TimerListener implements ActionListener
    {
        public void actionPerformed(ActionEvent event)
        {
            rec1.moveRectangleBy(1, 1);
            rec2.moveRectangleBy(-1, -1);
        }
    }

    public RectangleFrame()
    {        
        createPanel();

        setSize(FRAME_WIDTH, FRAME_HEIGHT);

        ActionListener listener = new TimerListener();

        final int DELAY = 100; //Milliseconds between timer ticks
        Timer t = new Timer (DELAY, listener);
        t.start();
    }

    public void createPanel()
    {
        JPanel panel = new JPanel();

        rec1 = new RectangleComponent(0,0);          
        rec2 = new RectangleComponent(280,470);

        panel.add(rec1);
        panel.add(rec2);

        add(panel);
    }
}

public class RectangleFrameViewer
{
    public static void main (String[] args)
    {
        JFrame rectangleFrame = new RectangleFrame();
        rectangleFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        rectangleFrame.setVisible(true);
    }
}

在回顾了MadProgrammer的建议后，我将RectangleComponent类更改为以下内容：

public class RectangleComponent extends JComponent
{
    private static final int RECTANGLE_WIDTH = 20;
    private static final int RECTANGLE_HEIGHT = 30;

    private int xLeft1;
    private int yTop1;
    private int xLeft2;
    private int yTop2;

    public RectangleComponent()
    {
        xLeft1 = 0;
        yTop1 = 0;

        xLeft2 = 250;
        yTop2 = 350;
    }

    public void paintComponent(Graphics g)
    {
        g.fillRect(xLeft1, yTop1, RECTANGLE_WIDTH, RECTANGLE_HEIGHT);
        g.fillRect(xLeft2, yTop2, RECTANGLE_WIDTH, RECTANGLE_HEIGHT);
    }

/**
 * Moves the rectangle by a given amount
 * @return dx the amount to move in the x-direction
 * @return dy the amount to move in the y-direction
 */
    public void moveRectangleBy(int dx, int dy)
    {
    xLeft1 = xLeft1 + dx;
    yTop1 = yTop1 + dy;
    xLeft2 = xLeft2 - dx;
    yTop2 = yTop2 - dy;
    repaint();
    }
}