Rectangles strips after scrolling in Java

Question

I use Netbeans8 and Java7. Inside a JPanel embedded in a JScrollPane I draw a lot of coloured rectangles. When I scroll down via the scroll bar some of the rectangles are shown in strange strips, as illustrated in the following image (specifically, see the blue rectangle).

I paint rectangles as follows in the class that extends JPanel.

List<Rectangle> rectagles = ...
List<Color> colours = ...

@Override
public void paintComponent(Graphics g) {
    int index = 0;
    int maxX = 0;
    int maxY = 0;
    for (Rectangle r : rectangles) {
        g.setColor(colours.get(index));
        int x = r.x;
        int y = r.y;
        int width = r.width;
        int height = r.height;

        maxX = Math.max(maxX, x + width);
        maxY = Math.max(maxY, y + height);

        g.fillRect(x, y, width, height);
        g.setColor(Color.BLACK);
        g.drawRect(x, y, width, height);
        index++;
    }
    this.setMinimumSize(new Dimension(maxX, maxY));
    this.setMaximumSize(new Dimension(maxX, maxY));
    this.setPreferredSize(new Dimension(maxX, maxY));
}

How can I prevent this annoying situation?

Answer 1

You need to add:

super.paintComponent(g);

at the top of your paintComponent() method to clear the background before you do your custom painting.