用箭头键移动图形与java

Question

我正在尝试使用箭头键（对于游戏）进行矩形移动，但我收到错误消息。它说这个，但是当我改变它时它仍然不起作用。我已经多次尝试但仍然收到相同的错误。有谁知道为什么会出现这个错误？ 这是错误：

java.lang.Error: Do not use javax.swing.JFrame.add() use javax.swing.JFrame.getContentPane().add() instead
        at javax.swing.JFrame.createRootPaneException(Unknown Source)
        at javax.swing.JFrame.addImpl(Unknown Source)
        at java.awt.Container.add(Unknown Source)
        at Tutorial.main(Tutorial.java:109)

这是我的代码：

// The "Test" class.
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyListener;
import java.awt.event.KeyEvent;

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


public class Tutorial extends JPanel implements ActionListener, KeyListener
{
    Timer tm = new Timer (5,this);
    int x = 0, y = 0, velX = 0, velY = 0;

    public Tutorial ()
    {
        tm.start();
        addKeyListener(this);
        setFocusable (true);
        setFocusTraversalKeysEnabled(false);
    }   

    public void paintComponent (Graphics g)
    {
        super.paintComponent (g);
        g.setColor (Color.RED);
        g.fillRect (x, y, 50, 30);
    }

    public void actionPerformed (ActionEvent e)
    {
        if (x < 0)
        {
            velX = 0;
            x = 0;
        }
        else if (x > 530)
        {
            velX = 0;
            x = 530;
        }

        if (y < 0)
        {
            velY = 0;
            y = 0;
        }

        else if (y > 330)
        {
            velY = 0;
            y = 330;
        }

        x = x + velX;
        y = y + velY;
        repaint ();
    }

    public void keyPressed (KeyEvent e)
    {
        int c = e.getKeyCode ();
        if (c == KeyEvent.VK_LEFT)
        {
            velX = -1;
            velY = 0;
        }

        if (c == KeyEvent.VK_UP)
        {
            velX = 0;
            velY = -1;
        }

        if (c == KeyEvent.VK_RIGHT)
        {
            velX = 1;
            velY = 0;
        }

        if (c == KeyEvent.VK_DOWN)
        {
            velX = 0;
            velY = 1;
        }
    }   

    public void keyTyped (KeyEvent e) {}
    public void keyReleased (KeyEvent e) 
    {
        velX = 0;
        velY = 0;
    }

    public static void main (String arge [])
    {
        Tutorial t = new Tutorial ();
        JFrame jf = new JFrame ();
        jf.setTitle ("Tutorial");
        jf.setSize (600, 400);
        jf.setVisible (true);
        jf.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
        jf.add(t); 
    }


} // Test class