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写在下面的 BasicLinePix 类中，我尝试创建一个程序，该程序允许用户通过按住 shift 键同时按住鼠标按钮并将其拖动到端点来绘制线条。问题是当鼠标被释放时，这条线就消失了。我试图让线条显示在面板中，并显示以相同方式绘制的多条线条。

我的代码如下所示：

    // this method overrides the paint method defined in JFrame
    public void paint(Graphics g) {
        super.paint(g);


    }


    // Inner class - instances of this class handle action events
    private class EventHandler implements ActionListener, MouseListener,
            MouseMotionListener {

        private Point startPoint = null; // line's start point
        private Point endPoint = null; // line's most recent end point

        public void actionPerformed(ActionEvent arg0) {
            if (arg0.getActionCommand().equals("Exit")) {
                statusLabel.setText("Exiting program...");
                System.exit(0);
            }

        }

        @Override
        public void mouseClicked(MouseEvent arg0) {
            // TODO Auto-generated method stub

        }

        @Override
        public void mouseEntered(MouseEvent arg0) {
            // TODO Auto-generated method stub

        }

        @Override
        public void mouseExited(MouseEvent arg0) {
            // TODO Auto-generated method stub

        }

        @Override
        public void mousePressed(MouseEvent e) {

            if (e.isShiftDown()) {

                // record starting point for line
                startPoint = new Point(e.getX(), e.getY());

                // initialize endPoint
                endPoint = startPoint;
            }

            if (e.isControlDown()) {
                Graphics g = drawingPanel.getGraphics();
                g.drawString("Hello", e.getX(), e.getY());
            }

        }

        @Override
        public void mouseReleased(MouseEvent arg0) {
            //repaint the frame and its contents
            //this executes the paint method defined above
            repaint();
        }

        @Override
        public void mouseDragged(MouseEvent e) {

            if (e.isShiftDown()) {
                // Implement rubber-band cursor
                Graphics g = drawingPanel.getGraphics();
                g.setColor(Color.black);

                g.setXORMode(drawingPanel.getBackground());

                // REDRAW the line that was drawn
                // most recently during this drag
                // XOR mode means that yellow pixels turn black
                // essentially erasing the existing line
                drawLine(g, startPoint, endPoint);

                // Update the end point of the line to current mouse position
                endPoint = new Point(e.getX(), e.getY());

                // Draw line to current mouse position
                // XOR mode: yellow pixels become black
                // black pixels, like those from existing lines, temporarily
                // become
                // yellow
                drawLine(g, startPoint, endPoint);

            }

        }

        @Override
        public void mouseMoved(MouseEvent arg0) {
            // TODO Auto-generated method stub

        }

        private void drawLine(Graphics g, Point start, Point end) {
            if (startPoint != null && endPoint != null) {
                int startX = ((Double) start.getX()).intValue();
                int startY = ((Double) start.getY()).intValue();

                int endX = ((Double) end.getX()).intValue();
                int endY = ((Double) end.getY()).intValue();

                g.drawLine(startX, startY, endX, endY);
            }
        }

    }

}

任何帮助将不胜感激！

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1 回答 1

6

不要使用getGraphics()对组件的调用来获取组件的 Graphics。画在paintComponent(...)方法里。您会在此站点和图形教程中多次找到此建议。

问题是通过调用getGraphics()组件获得的 Graphics 对象不是持久的 Graphics 对象，并且会随着下一次重绘而消失。

请注意，您发布的 80% 的代码与您的问题无关，完全没有必要和分散注意力。

编辑：请注意，您的程序看起来可能会受益于使用 BufferedImage，绘制到它，然后在 JPanel 的 paintComponent 方法中显示 BufferedImage。如果你确实走这条路，那么是的，只要你在完成后处理它，就可以通过它来获取BufferedImage 的Graphics 对象。getGraphics()

编辑2：虽然删除不相关的代码是个好主意，但不要删除太多，以免使您的代码无法编译且无法运行。而是研究创建一个sscce。

于 2013-03-29T04:00:12.843 回答