Java Swing 用鼠标点击和拖动画线

Question

I want to bring back a question that was asked before: java draw line as the mouse is moved

"I would like to add a feature to my application which allows the user to draw a straight line by clicking the mouse at the start location and releasing it at the end location. The line should move as the mouse moves until it is finally released; similar to the way that a line can be drawn using the Microsoft Paint application.

How can implement this so that the line is repainted as it moves without repainting other things that may already be drawn in that rectangular area?"

Question is: How can I draw multiple lines with the old lines still there?

This is the code that works for me, but the previous line gets erased as soon as you draw a new one:

public static void main(String args[]) throws Exception {
    JFrame f = new JFrame("Draw a Red Line");
    f.setSize(300, 300);
    f.setLocation(300, 300);
    f.setResizable(false);
    JPanel p = new JPanel() {
        Point pointStart = null;
        Point pointEnd   = null;
        {
            addMouseListener(new MouseAdapter() {
                public void mousePressed(MouseEvent e) {
                    pointStart = e.getPoint();
                }

                public void mouseReleased(MouseEvent e) {
                    pointStart = null;
                }
            });
            addMouseMotionListener(new MouseMotionAdapter() {
                public void mouseMoved(MouseEvent e) {
                    pointEnd = e.getPoint();
                }

                public void mouseDragged(MouseEvent e) {
                    pointEnd = e.getPoint();
                    repaint();
                }
            });
        }
        public void paint(Graphics g) {
            super.paint(g);
            if (pointStart != null) {
                g.setColor(Color.RED);
                g.drawLine(pointStart.x, pointStart.y, pointEnd.x, pointEnd.y);
            }
        }
    };
    f.add(p);
    f.setVisible(true); 
}

Answer 1

This is the code that works for me, but the previous line gets erased as soon as you draw a new one:

There are two common approaches:

1. Keep an ArrayList of objects to paint. Then the paintComponent() method repaints all the objects each time the component needs to repaint itself

2. Paint onto a BufferImage and then just paint the BufferedImage.

Check out Custom Painting Approaches for a working example of both of these approaches.

Answer 2

Just set in mouseRelesed

pointStart = e.getPoint instead of pointStart = null.

there is no need for any lists or things like that when you set it to null it starts from the beginning and deletes the last one