调整大小或移动滚动条时，线条和椭圆形消失

Question

i have aj Panel in a jSrollbarPane in netbeans design mode. I want to paint permanently on it untill user presses the button "clear". My lines and ovals created in path() disappear when I resize or move scrollbars on UI.

My code segment is below, thank you in advance

private void jMenuItem1ActionPerformed(java.awt.event.ActionEvent evt) { 
//i create some public double arrays here like x[] and y[]
}

 private void jButton1MouseClicked(java.awt.event.MouseEvent evt) {                                      

        path(x, y, 0);
    }    
 public void path(double[] X, double[] Y, int type) {
        Graphics2D gfx = (Graphics2D) jPanel1.getGraphics();
        int xT, yT, xL, yL;
        getContentPane();
        scale = jSlider1.getValue();
        switch (type) {
            case 0:
                gfx.setStroke(new BasicStroke(3));
                break;
            case 1:
                gfx.setStroke(new BasicStroke(1));
                gfx.setPaint(Color.blue);
                break;
            case 2:
                gfx.setStroke(new BasicStroke(1));
                gfx.setPaint(Color.green);
                break;
            case 3:
                gfx.setStroke(new BasicStroke(1));
                gfx.setPaint(Color.red);
                break;
            default:
                gfx.setStroke(new BasicStroke(1));
                gfx.setPaint(Color.yellow);
                break;
        }

        for (int l = 1; l < size; l++) {
            xT = (int) (scale * X[l - 1]);
            yT = (int) (scale * Y[l - 1]);
            xL = (int) (scale * X[l]);
            yL = (int) (scale * Y[l]);

            gfx.drawOval(xT, yT, 5, 5);
            gfx.drawLine(xT, yT, xL, yL);

        }
    } 

Answer 1

As soon as I saw your title, I knew that you were drawing using a Graphics object obtained via Component#getGraphics() . Don't do this.

You should not draw with a Graphics object obtained by calling getGraphics() on a component. This will return a Graphics object that is short lived, risking disappearing graphics or worse, a NullPointerException. Instead, draw in the JPanel's paintComponent(...) method either directly, or indirectly by drawing on a BufferedImage (yes, you can get its Graphics object via getGraphics() ) and then drawing the BufferedImage to the GUI within the paintComponent method.