public void setImg(int movingDir) {
        if (isVisible()) {
            return;
        }
        switch (movingDir) {
            case POS_UP:
                img = IMG_BULLET_UP;
                break;
            case POS_DOWN:
                img = IMG_BULLET_DOWN;
                break;
            case POS_LEFT:
                img = IMG_BULLET_LEFT;
                break;
            case POS_RIGHT:
                img = IMG_BULLET_RIGHT;
                break;
        }
    }