How to draw shapes continuously on a panel?

Question

I've been trying to draw shapes continuously without erasing the previous shape, but I'm always stuck on this.

I've also tried DrawOnImage based on this site https://tips4java.wordpress.com/2009/05/08/custom-painting-approaches/

That's why the code below is mostly similar to that example:

public class exo {
static class DrawingArea extends JPanel {

        private final static int AREA_SIZE = 600;
        private BufferedImage image = new BufferedImage(AREA_SIZE, AREA_SIZE, BufferedImage.TYPE_INT_ARGB);
        public CustomShape shape;
        public Shape s1;

    public DrawingArea() {
        setBackground(Color.WHITE);
        MyMouseListener ml = new MyMouseListener();
        addMouseListener(ml);
        addMouseMotionListener(ml);

    }

    class MyMouseListener extends MouseInputAdapter {

        private Point pointStart;
        private Point pointEnd;

        public void mousePressed(MouseEvent e) {
            pointStart = e.getPoint();
            s1 = null;
            shape = new CustomShape();
            //System.out.println(pointStart);
        }

        public void mouseDragged(MouseEvent e) {
            pointEnd = e.getPoint();
            shape.setX1(pointStart.x);
            shape.setX2(pointEnd.x);
            shape.setY1(pointStart.y);
            shape.setY2(pointEnd.y);

            s1 = drawInductor(shape.getX1(), shape.getX2(), shape.getY1(), shape.getY2());
            repaint();
        }

        public void mouseReleased(MouseEvent e) {
            //pointEnd = e.getPoint();
            if (shape.getX1() != shape.getX2()) {
                addShape(s1, e.getComponent().getForeground());
            }
            //s1 = null;
            //System.out.println(pointEnd);
        }
    }

    @Override
    public Dimension getPreferredSize() {
        return isPreferredSizeSet()
                ? super.getPreferredSize() : new Dimension(AREA_SIZE, AREA_SIZE);
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        //  Custom code to support painting from the BufferedImage
        if (image != null) {
            g.drawImage(image, 0, 0, null);
        }

        //  Paint the Rectangle as the mouse is being dragged
        if (shape != null) {
            Graphics2D g2d = (Graphics2D) g;

            g2d.draw(s1);
        }
    }

    public void addShape(Shape s, Color color) {
        //  Draw the Rectangle onto the BufferedImage

        Graphics2D g2d = (Graphics2D) image.getGraphics();
        g2d.draw(s);
        repaint();
    }

}
public static Shape drawInductor(double X1, double X2, double Y1, double Y2) {
    int count = 0;
    final int dx = 7;
    final int dy = 8;
    float x1, y1, x2, y2, x3, y3;
    final double alpha = Math.toRadians(75.0);
    double phi = Math.atan2((Y2 - Y1), (X2 - X1));
    double dist = Math.sqrt(Math.pow((X1 - X2), 2) + Math.pow((Y1 - Y2), 2));

    GeneralPath path = new GeneralPath();
    double x = X1;
    double y = Y1;

    path.moveTo(x, y);
    x += (float) (0.3 * dist);
    y = Y1;
    path.lineTo(x, y);

    do {
        path.curveTo(x - dx / 2, y + dy, x + dx + dx / 2, y + dy, x + dx, y);
        x += dx;
        y = Y1;
        count++;
    } while (x < X1 + 0.7 * dist && count < 35);

    x = X1 + (float) dist;
    y = Y1;
    path.lineTo(x, y);

    AffineTransform at = new AffineTransform();
    at.rotate(phi, X1, Y1);
    Shape resShape = at.createTransformedShape(path);
    return (resShape);
}
public static void main(String args[]) throws Exception {
    JFrame f = new JFrame("Run MTFK");
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setSize(
            600, 600);
    f.setLocation(
            300, 300);
    f.setResizable(
            false);
    DrawingArea drawingArea = new DrawingArea();
    f.getContentPane().add(drawingArea);

    f.setVisible(true);

}
}

I'd like to know where are my problems and how to deal with them. Thank you very much!

Answer 1

In order to continuously draw components on in the panel, you need to always draw the previous ones before you draw the most recent. So if you as you add them to say a List you would call repaint after each one assuming that is when you wanted to display them.

Assume each image is represented by a number.

Create image 1, add it to list and repaint
This draws 1.
Create image 2, add it to the list and repaint
This redraws 1 and now draws 2
Create image 3, add it to the list and repaint
This redraws 1,2 and now draws 3.

So if you have a List called shapes, your paintComponent method would have something like this in it.

for (Shape s : shapes) {
   g.draw(s); // notice the casting Graphics g to Graphics2D gives you access to draw()
}

And painting as well as event processing is done on the EDT so keep activity there do a minimum. Do as many of your calculations as possible outside of that thread.