private void addPoint(int x ,int y) {
        String key = x+","+y;
        if (pointMap.containsKey(key)) {
            return;
        }
        pointMap.put(key, true);
        Point p = new Point(x, y);
        points.add(p);
        pointCanvas.repaint();
        label.setText("一共 " + points.size() + " 个点");
    }