为什么keyPressed（）方法的简单KeyListenerDemo没有响应？ （使用Eclipse）

Question

我的Java经验有限，我正在使用下面的程序来教授高中水平的学生入门Java。在扩展它之前，该类中的大多数程序应该按原样运行，如下面的程序。我能够让程序在Eclipse中运行，并在代码中绘制一个蓝色球。但是，我不能让球向上，向下，向左或向右移动。是什么东西遗留在这个＆＃39;起动器＆＃39;绘制球的代码，但不响应箭头键？好像我的所有keyPressed（）代码都被忽略了。任何见解都非常感谢。

import java.awt.*;
import java.awt.event.*;                            // #1
import javax.swing.*;   

/******************************************************************************
 * 
 * KeyListenerDemo.java
 * Demonstrates getting keyboard input using the KeyListener interface.
 * 
 * Program 18: Extend this program by adding a few more keystroke commands:
 *      z     (VK_Z)    - Cause the ball to jump to a random new location.
 *      s     (VK_S)    - Make the ball smaller - multiply its diameter 1/2.
 *      b     (VK_B)    - Make the ball bigger - multiply its diameter by 2.
 *      c     (VK_C)    - Change the color (in any way you'd like).
 *
 *  In addition, modify the program to ensure the following:
 *  - The ball goes all the way to the edge of the screen but stays
 *          completely on the screen. 
 *  - If a doubled diameter doesn't fit, make it as large as possible.
 *  - Be sure the ball never completely disappears.
 * 
 *****************************************************************************/
public class KeyListenerDemo extends JFrame
                        implements KeyListener      // #2
{
// Class Scope Finals
private static final int SCREEN_WIDTH = 1000;
private static final int SCREEN_HEIGHT = 800;
private static final int START_RADIUS = 25;
private static final int START_X = 100;
private static final int START_Y = 100;
private static final int STEP_SIZE = 10;

// Class Scope Variables
private static int x = START_X;             // x at center of the ball
private static int y = START_Y;             // y at center of the ball
private static int radius = START_RADIUS;   // radius of the ball

// Methods
/**
 * Create the window and register this as a KeyListener
 * 
 * @param args
 */
public static void main (String[] args)
{
    // Set up the JFrame window.
    KeyListenerDemo gp = new KeyListenerDemo();
    gp.setSize(SCREEN_WIDTH, SCREEN_HEIGHT);
    gp.setVisible(true);

    gp.addKeyListener(gp);                          // #3
    // If this class had a constructor and you moved this line into
    //   that constructor it could not refer to gp since that variable
    //   is local to this method.  Instead you would write::
    // addKeyListener(this);
}

/**
 * Called when a key is first pressed
 * Required for any KeyListener
 * 
 * @param e     Contains info about the key pressed
 */
public void keyPressed(KeyEvent e)                  // #4A
{
    int keyCode = e.getKeyCode();
    if (keyCode == KeyEvent.VK_LEFT)
    {
        x = x - STEP_SIZE;
    }
    else if (keyCode == KeyEvent.VK_RIGHT)
    {
        x = x + STEP_SIZE;
    }
    else if (keyCode == KeyEvent.VK_UP)
    {
        y = y - STEP_SIZE;
    }
    else if (keyCode == KeyEvent.VK_DOWN)
    {
        y = y + STEP_SIZE;
    }
    repaint();
}

/**
 * Called when typing of a key is completed
 * Required for any KeyListener
 * 
 * @param e     Contains info about the key typed
 */
public void keyTyped(KeyEvent e)                    // #4B
{
}

/**
 * Called when a key is released
 * Required for any KeyListener
 * 
 * @param e     Contains info about the key released
 */
public void keyReleased(KeyEvent e)                 // #4C
{
}

/**
 * paint - draw the figure
 * 
 * @param g     Graphics object to draw in
 */


   public void paint(Graphics g)
    {
        g.setColor(Color.white);
        g.fillRect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT);

        g.setColor(Color.blue);
        g.fillOval(x - radius, y - radius, 2 * radius, 2 * radius);
    }
}