真正无人能解的问题，哈哈
源码如下（corejava例8-3 卷一page289稍作改动），其效果应该是移动方向键绘制直线，可是程序没有发现绘制效果，请高手指教（已通过编译，可运行）
import java.awt.*;
import javax.swing.*;
import java.awt.geom.*;
import java.awt.event.*;
import java.util.*;

public class SktStu
{
public static void main(String[] args)
{
SktStu.start();
}
public static void start()
{
SktJFrame frame=new SktJFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
}
class SktJFrame extends JFrame
{
public SktJFrame()
{
setTitle( "SktStu ");
setExtendedState(JFrame.MAXIMIZED_BOTH);
setSize(600,400);
SktPanel panel=new SktPanel();
add(panel);
}
}
class SktPanel extends JPanel
{
public SktPanel()
{
last=new Point2D.Double(100,100);
lines=new ArrayList <Line2D> ();
KeyHandler listener=new KeyHandler();
addKeyListener(listener);
setFocusable(true);
}
private Point2D last;
private ArrayList <Line2D> lines;
private int SMALL_INCREMENT=1;
private int LARGE_INCREMENT=5;
private static int d;

public void paintComponent(Graphics g)
{
super.paintComponent(g);
Graphics2D g2=(Graphics2D)g;
for(Line2D l:lines)
{
g2.draw(l);
}
}
private class KeyHandler implements KeyListener
{
public void keyPressed(KeyEvent event)
{
int keyCode=event.getKeyCode();
if(event.isShiftDown())
d=LARGE_INCREMENT;
else
d=SMALL_INCREMENT;
if(keyCode==KeyEvent.VK_LEFT) moveLeft();
else if(keyCode==KeyEvent.VK_RIGHT) moveRight();
else if(keyCode==KeyEvent.VK_UP) moveUp();
else if(keyCode==KeyEvent.VK_DOWN) moveDown();
}

public void keyReleased(KeyEvent event){}

public void keyTyped(KeyEvent event)
{
}

private void moveLeft(){move(-d,0);}
private void moveRight(){move(d,0);}
private void moveUp(){move(0,-d);}
private void moveDown(){move(0,d);}
private void move(int dX,int dY)
{
Point2D end=new Point2D.Double(last.getX()+dX,last.getY()+dY);
Line2D line=new Line2D.Double(last,end);
lines.add(line);
repaint();
last=end;
}

}
}


[解决办法]
都说了无人能解了,大家还浪费什么力气呢.