Drawing lines in java (Graphics 2D)

问题:

I am trying to do a little program on Eclipse. The program goes like this: when I click for the 1st time on thr Panel on the frame, a line has to be drawn regarding the Y position of my mouse listener.The line takes all the width of the panel. On the 2nd click, another line has to be drawn, again regarding the Y position of where I clicked. After, I'll put a little circle between the 2 lines and make a little animation with it. But now, I have a problem. When I click on the panel, a line is drawn, but if i click another time, the first line disappears and the 2nd line takes it place... This is the code of the painComponent and my mousr listener. What is wrong with it ?

public Lines() {
    addMouseListener(new MouseAdapter() {
        @Override
        public void mouseClicked(MouseEvent e) {

            posY=e.getY();
            posX=e.getX();
            nbClic=e.getClickCount();
            repaint();
        }

    });
    setBackground(Color.black);
}

@Override
public void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2d = (Graphics2D) g;
    g2d.setColor(Color.blue);

    if(nbClic>=1){
        line1=new Line2D.Double(0, posY, getWidth(), posY);
        g2d.draw(line1);
        repaint();
    }
    if(nbClic>=2){
        g2d.setColor(Color.YELLOW);
        line2=new Line2D.Double(0, posY, getWidth(), posY);
        g2d.draw(line2);
    }
    repaint();
} 

回答1:

Painting is an event that draws the entire component. You can't depend on past events because they are erased each time a repaint happens.

You would need to keep something like a List and each time you create a new line, you add it to the List.

List<Integer> yClicks = new ArrayList<>();

... {
    addMouseListener(new MouseAdapter() {
       @Override
        public void mouseClicked(MouseEvent e) {
            yClicks.add(e.getY());
            repaint();
        }
    });
}

@Override
protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2d = (Graphics2D)g.create();

    for(int y : yClicks) {
        g2d.draw(new Line2D.Double(0, y, getWidth(), y));
    }

    g2d.dispose();
}

Also:

• Never call repaint inside paintComponent! This will cause an endless cycle of repaints.
• paintComponent is a protected method and should remain so unless there is a compelling reason to make it public.
• Be careful changing the state of the Graphics object passed in to paintComponent because it is used elsewhere. Usually we create a local copy which is disposed when we are done.