基本上我想做的是，我有一个对象，叫做mainball。mainball有一个keydecater类作为其内部类，keydecater被添加到构造函数的mainball中。mainball是在游戏中创建的，但是mainball不响应击键。

public Mainball(){
    super(150,150,SIZE,SIZE);
    c=  Color.RED;
    addKeyListener(new KeyDetecter());
}
class KeyDetecter extends KeyAdapter{

    public KeyDetecter(){
    }

    double velocityfactor = 0.8;

    public void keyPressed(KeyEvent e){
        if(e.getKeyChar() == 'a'){
            x_velocity = -velocityfactor;
        }
        if(e.getKeyChar() == 'd'){
            x_velocity = velocityfactor;
        }
        if(e.getKeyChar() == 's'){
            ball.y_velocity = velocityfactor;
        }
        if(e.getKeyChar() == 'w'){
            y_velocity = -velocityfactor;
        }
        if(e.getKeyCode() == '1'){
            Shoot_Type = the_Game.SHOOT_ARROW;
        }
        if(e.getKeyCode() == '2'){
            Shoot_Type = the_Game.SHOOT_PARTICLE;
        }
    }

游戏也要求关注这里的窗口

if(button.getText().equals("Game")){
            try {
                game.walls = (ArrayList<wall>) SaveNLoad.load("wall_info.txt");
            } catch (Exception e1) {
                game.walls = new ArrayList<wall>();
            }
            frame.remove(current_panel);
            frame.add(game);
            game.ball.requestFocusInWindow(); /* ball is an mainball instance */
            current_panel = game;
            game.ball.x_center = 100;
            game.ball.y_center = 40;
            game.ball.y_velocity = 0;
        }