在计时器上制作新图像

Question

嘿我试图每800毫秒创建一个新图标，并随机定位他们的x值，并将他们的y放在android屏幕下方。我想让这些图标在屏幕上垂直移动，但是开始工作我不断将多个图标叠加在一起。任何帮助表示赞赏。

public class GameView extends SurfaceView{
    Bitmap icon;
    LinkedList<Sprite> balloonList;
    Timer spawnTimer;
    TimerTask spawnTask;
    Random rand;
    SurfaceHolder holder;
    int spawnTime;
    final int SPAWN_DIFFERENCE = 10;

    public GameView(Context context){
        super(context);
        icon = BitmapFactory.decodeResource(getResources(), R.drawable.ic_launcher);

        holder = getHolder();

        balloonList = new LinkedList<Sprite>();

        rand = new Random();

        spawnTime = 1;

        spawnTimer = new Timer();
        spawnTask = new TimerTask() {

            @Override
            public void run() {
                spawnNewBalloon();
            }
        };

        spawnTimer.scheduleAtFixedRate(spawnTask, 800, 800);
    }

    private void spawnNewBalloon(){
        Canvas canvas = holder.lockCanvas();

        if(spawnTime == 0){
            balloonList.insertTail(new Sprite(icon, rand.nextInt(getWidth() - icon.getWidth() / 2), getHeight()));
            balloonList.getTailData().draw(canvas);
            spawnTime = SPAWN_DIFFERENCE;
        }
        else spawnTime--;

        if(balloonList.length() == 0){
            for(int i = 0; i < balloonList.length(); i++) {
                balloonList.getData(i).update(canvas);
            }
        }
        holder.unlockCanvasAndPost(canvas);
    }
}

class Sprite{
    public Bitmap image;
    public int x, y;

    public Sprite(Bitmap image, int x, int y){
        this.image = image;
        this.x = x;
        this.y = y;
    }

    public void draw(Canvas canvas){
        canvas.drawBitmap(image, x, y, null);
    }

    public void update(Canvas canvas){
        if(y > 0) y -= 15;
        draw(canvas);
    }
}