import java.awt.Frame;
import java.awt.event.*;

public class $2121_Key {
    class $2121_Window extends Frame{
        $2121_Window(){
            this.setBounds(100, 100, 100, 100);
            this.addKeyListener(new KeyAdapter(){
                public void keyPressed(KeyEvent e){
                    if(e.getKeyCode()==e.VK_KP_DOWN){
                        //我想在这里实现窗口的关闭，怎么写？
                        //因为是内部类
                        //不能用System.exit(0);
                    }
                }
            });
            this.setVisible(true);
        }
    }    
}