Proper way to use JLabels to update an image

Question

I am creating a GUI, and am fairly new to swing and awt. I am trying to create a gui that, upon launch, sets the background to an image, then uses a method to create a slide

Answer 1

You're calling Thread.sleep(...) and likely on the EDT or Swing event thread (full name is the Event Dispatch Thread). This thread is responsible for all Swing painting/drawing and user interactions, and so sleeping it will only serve to freeze your entire GUI. Instead you should use a Swing Timer to allow you to swap a JLabel's ImageIcon.

So, briefly:

• Don't call Thread.sleep(...) on the Swing event thread (Event Dispatch Thread or EDT).
• Do use a Swing Timer to do your repeating delayed actions.
• Don't make and add many JLabels. Just make and add one.
• Do Swap the ImageIcon that the JLabel displays by calling setIcon(...) on the label.
• Better (cleaner) to write if (busy == false) { as if (!busy) {

e.g.,

ImageIcon[] icons = {...}; // filled up with your ImageIcons

if (!busy) {
  int timerDelay = 2000;
  new Timer(timerDelay, new ActionListener() {
    private int i = 0;
    public void actionPerfomed(ActionEvent e) {
      myLabel.setIcon(icons(i));
      i++;
      if (i == icons.length) {
        ((Timer)e.getSource).stop();
      } 
    };
  }).start();
}

Answer 2

See ImageViewer for a working example of displaying images using a Swing based Timer.

See also How to use Swing Timers.

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And while I'm here, another (prettier) example of animating an image. It uses this Mercator map of land masses. The image can be tiled horizontally, and therefore be scrolled left/right as needed.

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.Point2D;
import java.awt.image.BufferedImage;
import javax.swing.*;

import java.net.URL;
import javax.imageio.ImageIO;

public class WorldView {

    public static void main(String[] args) throws Exception {
        URL url = new URL("http://i.stack.imgur.com/P59NF.png");
        final BufferedImage bi = ImageIO.read(url);
        Runnable r = new Runnable() {

            @Override
            public void run() {
                int width = 640;
                int height = 316;
                Graphics2D g = bi.createGraphics();

                float[] floats = new float[]{0f, .4f, .55f, 1f};
                Color[] colors = new Color[]{
                    new Color(20, 20, 20, 0),
                    new Color(0, 10, 20, 41),
                    new Color(0, 10, 20, 207),
                    new Color(0, 10, 20, 230),};
                final LinearGradientPaint gp2 = new LinearGradientPaint(
                        new Point2D.Double(320f, 0f),
                        new Point2D.Double(0f, 0f),
                        floats,
                        colors,
                        MultipleGradientPaint.CycleMethod.REFLECT);

                final BufferedImage canvas = new BufferedImage(
                        bi.getWidth(), bi.getHeight() + 60,
                        BufferedImage.TYPE_INT_RGB);

                final JLabel animationLabel = new JLabel(new ImageIcon(canvas));
                ActionListener animator = new ActionListener() {

                    int x = 0;
                    int y = 30;

                    @Override
                    public void actionPerformed(ActionEvent e) {
                        Graphics2D g = canvas.createGraphics();
                        g.setColor(new Color(55, 75, 125));

                        g.fillRect(0, 0, canvas.getWidth(), canvas.getHeight());

                        int offset = (x % bi.getWidth());
                        g.drawImage(bi, offset, y, null);
                        g.drawImage(bi, offset - bi.getWidth(), y, null);

                        g.setPaint(gp2);
                        g.fillRect(0, 0, canvas.getWidth(), canvas.getHeight());

                        g.dispose();

                        animationLabel.repaint();

                        x++;
                    }
                };
                Timer timer = new Timer(40, animator);
                timer.start();
                JOptionPane.showMessageDialog(null, animationLabel);
                timer.stop();
            }
        };
        // Swing GUIs should be created and updated on the EDT
        // http://docs.oracle.com/javase/tutorial/uiswing/concurrency
        SwingUtilities.invokeLater(r);
    }
}

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Here is a version of that image with the equator added (it is 44 pixels 'south' of the center of the image).