花了2周时间做了一个俄罗斯方块的applet(完成度较高),现放在
http://www.cnbole.com/ggworld/test/eluosi/eluosi.html 
遇到的问题是:游戏当中按S键暂停,切换到别的应用程序等待一段时间后(等待的时间越长错误发生的概率越高),再返回俄罗斯方块的页面.此时再按S键无效,无法继续游戏,只有刷新页面重新开始,不知是何原因?
如须查看代码,请留mail,谢谢.

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

class SketchPanel extends JPanel
  implements KeyListener
{
InputMap mapaEntrada;
ActionMap mapaAction ;
HandlerKey hkLeft = new HandlerKey("LeftKey",KeyEvent.VK_LEFT);
HandlerKey hkRight = new HandlerKey("RightKey",KeyEvent.VK_RIGHT);
HandlerKey hkPageUp = new HandlerKey("PageUp",KeyEvent.VK_PAGE_UP);
HandlerKey hkPageDown = new HandlerKey("PageDown",KeyEvent.VK_PAGE_DOWN);
public SketchPanel()
  {  addKeyListener(this);
  mapaEntrada= getInputMap(WHEN_IN_FOCUSED_WINDOW);
  mapaAction = getActionMap();
  mapaEntrada.put(KeyStroke.getKeyStroke(KeyEvent.VK_LEFT,0),"LeftKey");
  mapaEntrada.put(KeyStroke.getKeyStroke(KeyEvent.VK_RIGHT,0),"RightKey");
  mapaEntrada.put(KeyStroke.getKeyStroke(KeyEvent.VK_PAGE_UP,0),"PageUp");
  mapaEntrada.put(KeyStroke.getKeyStroke(KeyEvent.VK_PAGE_DOWN,0),"PageDown");
  mapaAction.put("LeftKey",hkLeft);
  mapaAction.put("RightKey",hkRight);
  mapaAction.put("PageUp",hkPageUp);
  mapaAction.put("PageDown",hkPageDown);
  requestDefaultFocus();
  }

  public void keyPressed(KeyEvent evt)
  {  int keyCode = evt.getKeyCode();
      int d;
      if (evt.isShiftDown())
        d = 5;
      else
        d = 1;
      if (keyCode == KeyEvent.VK_LEFT) add(-d, 0);
      else if (keyCode == KeyEvent.VK_RIGHT) add(d, 0);
      else if (keyCode == KeyEvent.VK_UP) add(0, -d);
      else if (keyCode == KeyEvent.VK_DOWN) add(0, d);
  }

  public void keyReleased(KeyEvent evt)
  {}

  public void keyTyped(KeyEvent evt)
  {  char keyChar = evt.getKeyChar();
      int d;
      if (Character.isUpperCase(keyChar))
      {  d = 5;
        keyChar = Character.toLowerCase(keyChar);
      }
      else
        d = 1;
      if (keyChar == 'h') add(-d, 0);
      else if (keyChar == 'l') add(d, 0);
      else if (keyChar == 'k') add(0, -d);
      else if (keyChar == 'j') add(0, d);
  }

  public boolean isFocusTraversable() { return true; }

  public void add(int dx, int dy)
  {  end.x += dx;
      end.y += dy;
      Graphics g = getGraphics();
      g.drawLine(start.x, start.y, end.x, end.y);
      g.dispose();
      start.x = end.x;
      start.y = end.y;
  }

  private Point start = new Point(50, 50);
  private Point end = new Point(50, 50);
}

public class Sketch extends JApplet implements FocusListener, WindowListener
{
private Frame parentFrame;
public Sketch()
{
addFocusListener(this);
parentFrame = JOptionPane.getFrameForComponent(this);
parentFrame.addWindowListener(this);

}
public void jsForceFocus()
{

WindowEvent e = new WindowEvent(parentFrame,
WindowEvent.WINDOW_ACTIVATED);
parentFrame.dispatchEvent(e);

}


public void init()
{
Container contentPane = getContentPane();
      contentPane.add(new SketchPanel());
        FocusManager.getCurrentManager().focusNextComponent(contentPane);
    }
    public void focusLost(FocusEvent e)
    {
        FocusManager.getCurrentManager().focusNextComponent(getContentPane());

}
public void focusGained(FocusEvent e)
{
        FocusManager.getCurrentManager().focusNextComponent(getContentPane());

}
public void windowOpened(WindowEvent e) {
//System.out.println("windowOpened");
}

public void windowClosing(WindowEvent e) {
//System.out.println("windowClosing"); 
}

public void windowClosed(WindowEvent e) {
//System.out.println("windowClosed");
}

public void windowIconified(WindowEvent e) {
//System.out.println("windowIconified");
}

public void windowDeiconified(WindowEvent e) {
//System.out.println("windowDeiconified");
}

public void windowActivated(WindowEvent e) {
//System.out.println("windowActivated");

if (parentFrame.getFocusOwner() != null ) {
parentFrame.getFocusOwner().requestFocus();
}

}

public void windowDeactivated(WindowEvent e) {
//System.out.println("windowDeactivated");
}


}
class HandlerKey extends AbstractAction
{//LEFT,RIGHT,PAGEUP,PAGE DOWN will travel the images
int nKeyCode=0;
public HandlerKey(String s)
{
super(s);
}
public HandlerKey(String s,int keycode)
{
super(s);
nKeyCode=keycode;
}
public void actionPerformed(ActionEvent e)
{
System.out.println(nKeyCode);
}
}//end of HandlerKey

function focusToApplet ()
{
// This pulls focus away from the applet so the next stuffcan
// put it back. It seems without this every second try doesnot
// restore focus to the applet
hide.xx.focus();
document.TestApplet.jsForceFocus();
document.TestApplet.requestFocus();
}
window.onfocus = focusToApplet;

something here

要把IE窗口的大小固定为applet的大小，你用JS开一个新窗口，在里面指定不就行了……