Circles disappear when maximizing, minimizing or resizing my JFrame.

This is my code:

    JFrame frame = new JFrame();
    frame.setSize(400, 400);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    final JPanel panel1 = new JPanel(){
        @Override
        protected void paintComponent(Graphics g) {
            int w = getWidth();
            int h = getHeight();
            for (int i = 0; i < w; i+=100) {
                g.drawLine(i, 0, i, h);
            }
        }
    };
    panel1.addMouseListener(new MouseListener() {
        @Override
        public void mouseReleased(MouseEvent arg0) {
        }
        @Override
        public void mousePressed(MouseEvent arg0) {
        }
        @Override
        public void mouseExited(MouseEvent arg0) {
        }
        @Overri开发者_如何转开发de
        public void mouseEntered(MouseEvent arg0) {
        }
        @Override
        public void mouseClicked(MouseEvent arg0) {
            int x = arg0.getX();
            int y = arg0.getY();
            Graphics g = (Graphics) panel1.getGraphics();
            g.setColor(Color.black);
            g.fillOval(x, y, 100, 100);
        }
    });
    frame.add(panel1);
    frame.setVisible(true);

What it does is when i click on frame a circle is drawn. after drawing some circles when i maximize or minimize or resize my frame the circles gets disappeared. how can I solve it?

You should override paintComponent() method of the panel. The circles should be added in a list. In the paintComponent() call super and then iterate through the list painting each circle.