Question

I’m new to GUI programming and I tried out some code and wanted to constantly change the background of a JPanel called “HeaderPanel”.

Why isnt this working as I wished? (Color stays the same…)

private void changeColors() {
    int r = 0;
    int g = 155;
    int b = 12;

    while(true) {
        r = (r+1)%255;
        g = (g+1)%255;
        b = (b+1)%255;

        Color color = new Color(r,g,b);
        HeaderPanel.setBackground(color);
    }
}

Answer 1

Your while loop is stopping the repaint manager from ever getting around to changing the colors.

You need to, some how, execute the request in the background, something like this

public class TestLabel extends JLabel {

    private Timer timer;

    private int r = 0;
    private int g = 155;
    private int b = 12;

    public TestLabel() {

        setText("Hello world");
        setOpaque(true);

        timer = new Timer(1000, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {

                System.out.println("tick");

                r = (r + 1) % 255;
                g = (g + 1) % 255;
                b = (b + 1) % 255;

                Color color = new Color(r, g, b);
                setBackground(color);

                System.out.println(color);

                if (r == 0 && b == 0 && g == 0) {

                    r = 0;
                    g = 155;
                    b = 12;

                }

                invalidate();
                revalidate();
                repaint();

            }
        });

        timer.setRepeats(true);
        timer.setCoalesce(true);
        timer.start();

    }
}

You might like to read up on

• The Event Dispatching Thread
• Concurrency in Swing

UPDATED with Extended Example

public class TestLabel extends JLabel {

    private Timer timer;

    private Object[][] colors = {{"Black", Color.BLACK},
        {"Blue", Color.BLUE},
        {"Cyan", Color.CYAN},
        {"Dark Gray", Color.DARK_GRAY},
        {"Gray", Color.GRAY},
        {"Green", Color.GREEN},
        {"Light Gary", Color.LIGHT_GRAY},
        {"Mangenta", Color.MAGENTA},
        {"Orange", Color.ORANGE},
        {"Pink", Color.PINK},
        {"Red", Color.RED},
        {"White", Color.WHITE},
        {"Yellow", Color.YELLOW}};

    public TestLabel() {

        setText("Hello world");
        setOpaque(true);

        timer = new Timer(1000, new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {

                System.out.println("tick");

                int index = (int) Math.round((colors.length - 1) * Math.random());

                Object[] group = colors[index];

                setBackground((Color)group[1]);
                setText((String)group[0]);

            }
        });

        timer.setInitialDelay(0);
        timer.setRepeats(true);
        timer.setCoalesce(true);
        timer.start();

    }
}