Question

I am making a program where one can click on the window, and a point will be placed there. If the user clicks again, the point will be removed. Programmatically, each click will create a new instance of another class called “Element”, which contains the (X, Y) positions for a single point.

To achieve this, I am extending JPanel and implementing a MouseListener. To draw the points, I’m overriding the paint() method. Every time a user clicks, code the MouseListener’s mouseReleased() either adds to the ArrayList, or removes from it, then calls paint(), where the screen is cleared and the ArrayList is redrawn.

The problem I am having is that the points aren’t going away when clicked on. I don’t know if it’s my lack of understanding of paint(), or something to do with the ArrayList.

Here is my paint():

public void paint(Graphics g)
{
    // Clear screen
    g.setColor(Color.WHITE);
    g.fillRect(0, 0, getWidth(), getHeight());

    // Display what elements will be drawn (for debugging)
    System.out.println("About to draw the following elements:");
    for (Element e : elements)
    {
        System.out.println("\t" + e);
    }

    // Draw Elements
    g.setColor(Color.BLACK);
    for(int i=0; i < elements.size(); i++)
    {
        g.fillOval(elements.get(i).x, elements.get(i).y, 10, 10);
    }

}

And here is the mouse click method:

public void mouseReleased(MouseEvent e)
{
    // Rounds to the nearest grid space (spacing is currently 20px)
    int roundX = (int) ((float)(Math.round(e.getX() / GRID_SPACING)) * GRID_SPACING);
    int roundY = (int) ((float)(Math.round(e.getY() / GRID_SPACING)) * GRID_SPACING);

    System.out.println("Clicked (" + roundX + ", " + roundY + ")");

    // Go through each element...
    for (int i=0; i < elements.size(); i++)
    {
        // if an element exists at the coordinates clicked,
        if (elements.get(i).getX() == roundX && elements.get(i).getY() == roundY)
        {
            // remove it from the elements list
            elements.remove(i);
            i--;
            System.out.println("\tElement exists at (" + roundX + ", " + roundY + "). Removing it.");
        }
    }

    elements.add(new Element(roundX, roundY));

    repaint();
}

The output of this is as follows:

  About to draw the following elements: (None)

Clicked (140, 100)
About to draw the following elements:
    This element's coordinates are (140, 100)

Clicked (160, 100)
About to draw the following elements:
    This element's coordinates are (140, 100)
    This element's coordinates are (160, 100)

Clicked (140, 100)
    Element exists at (140, 100). Removing it.
About to draw the following elements:
    This element's coordinates are (160, 100)
    This element's coordinates are (140, 100)

Answer 1

You should not add(new Element(roundX, roundY)) if the element has been removed in the previous loop