重新绘制方法后无法在JPanel上绘制

Question

I'm experimenting on a GUI that I programmed and I don't understand how I can fix my problem: My GUI contains a jPanel that on receiving a mouseclick, paints a point with filloval command.

private void myPnlMousePressed(java.awt.event.MouseEvent evt) {                                   
    changed = true;
    p.x = evt.getX();
    p.y = evt.getY();
    drewPoints(p.x, p.y);
} 

private void drewPoints (int x, int y) {
    if (gf == null) {
        gf = (Graphics)myPnl.getGraphics();
    }
    myPointsList.add(new Point(x, y));
    gf.fillOval(x, y, 5, 5);
    xVal.setText("X = " + x);
    yVal.setText("Y = " + y);
}

everything works fine but when I want to open an XML file that I created to save all the points it doesn't work. The problem is that when I use the repaint method on the jPanel after choosing a file, all the points loads fine but the panel can't draw the points. If I put the repaint method in the open button listener (before the choosing file) it works, but then if the user cancels the open option so the panel stays blank and I don't want to draw the points again. I think it happens because the repaint process is not finished. All the points added to a private List.

private void OpenFile() {
    try {
        File thisFile;
        JFileChooser of = new JFileChooser();
        int option = of.showOpenDialog(of);

        if (option == JFileChooser.APPROVE_OPTION){
            thisFileName = of.getSelectedFile().getPath();
            thisFile =  new File(thisFileName);                
            if (!of.getSelectedFile().getName().endsWith(".xml")) {
                String error = "Error, You didn't select XML file";
                JOptionPane.showMessageDialog(this, error, "Wrong type of file", JOptionPane.INFORMATION_MESSAGE);
                return;
            }            
            myPnl.repaint();
            myPointsList.clear();
                        ....
                        ....
                        ....
            for (int i = 0; i < pointsList.getLength(); i++) {
                Element point = (Element) pointsList.item(i);
                p.x = Integer.parseInt(point.getElementsByTagName("X").item(0).getTextContent());
                p.y = Integer.parseInt(point.getElementsByTagName("Y").item(0).getTextContent());
                drewPoints(p.x, p.y);
            }
                        ....

how can I make it work??

Answer 1

Don't use gf = (Graphics)myPnl.getGraphics(); , this is not how painting in Swing works. The getGraphics method can return null and is nothing more then a snap shot of the last paint cycle, any thing you paint to it will be erased on the next paint cycle (repaint).

Instead, override the JPanel s paintComponent and put all you painting logic there. There is an expectation that when called, you are expected to fully re-paint the current state of the component.

See Painting in AWT and Swing and Performing Custom Painting for more details about how painting works in Swing

Answer 2

You have to use the repaint() and override the paint() method:

class MyPanel extends JPanel implements MouseListener
{
    private int x;
    private int y;

    public MyPanel() {
        super();
        addMouseListener(this);
    }

    @Override public void mouseEntered(MouseEvent e) { }

    @Override public void mouseExited(MouseEvent e) { }

    @Override public void mouseClicked(MouseEvent e) { }

    @Override public void mousePressed(MouseEvent e) { }

    @Override public void mouseReleased(MouseEvent e) {
        x = e.getX();
        y = e.getY();
        repaint();
    }

    @Override public void paint(Graphics g) {
        super.paint(g);
        g.fillOval(x, y, 10, 10);
    }
}

If you want to draw all points, don't use x and y but a list of points :

class MyPanel extends JPanel implements MouseListener
{
    private ArrayList<Point> points = new ArrayList<>();

    public MyPanel() {
        super();
        addMouseListener(this);
    }

    @Override public void mouseEntered(MouseEvent e) { }

    @Override public void mouseExited(MouseEvent e) { }

    @Override public void mouseClicked(MouseEvent e) { }

    @Override public void mousePressed(MouseEvent e) { }

    @Override public void mouseReleased(MouseEvent e) {
        points.add(new Point(e.getX(), e.getY()));
        repaint();
    }

    @Override public void paint(Graphics g) {
        super.paint(g);
        for (Point p : points)
            g.fillOval(p.getX(), p.getY(), 10, 10);
    }
}

where:

class Point
{
    private int x;
    private int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public int getX() {
        return x;
    }

    public int getY() {
        return y;
    }
}

Then use it:

public static void main(String[] args) {
    JFrame frame = new JFrame("Test");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(400, 400);
    frame.setLocationRelativeTo(null);

    MyPanel myPanel = new MyPanel();

    frame.add(myPanel);
    frame.setVisible(true);
}