Program freezes during Thread.sleep() and with Timer

Question

Original question:

This method is supposed to change the image being displayed on a JFrame gradually into another image. However, without some way to slow it down, it just seems to change from one image to the new image. In order to slow it down, I put in a Thread.sleep(1000) so the changes wouldn't happen instantly. However, with this line in there, my program freezes completely. No error message, no nothing. Can anyone please help me out? Suggest a better method to slow it down, or how this can be fixed.

For clarification: int k is the number of gradual steps in the change. k = 1 would be an instant change. Anything greater would be gradual changes. int l meanwhile controls the ratio of how much of each image is displayed.

public void morphImg(int width, int height, BufferedImage morphImage, int k) {
    //creates new image from two images of same size
    BufferedImage image2 = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
    for (int i = 0; i < width; i++) {
        for (int j = 0; j < height; j++) {
            //get color from original image
            Color c = new Color(image.getRGB(i, j));

            //get colors from morph image
            Color c2 = new Color(morphImage.getRGB(i, j));

            for (int l = 1; l <= k; l++) {
                //gets colors at different stages
                int r = ((k-l)*c.getRed()/k) + (l*c2.getRed()/k);
                int g = ((k-l)*c.getGreen()/k) + (l*c2.getGreen()/k);
                int b = ((k-l)*c.getBlue()/k) + (l*c2.getBlue()/k);   
                Color newColor = new Color(r, g, b);
                //set colors of new image to average of the two images
                image2.setRGB(i, j, newColor.getRGB());

                //display new image
                try {
                    imageLabel.setIcon(new ImageIcon(image2));
                    Thread.sleep(1000);
                }
                catch (InterruptedException e){
                    System.out.println("Exception caught.");
                }
            }
        }
    }

    //sets modified image as "original" for further manipulation
    setImage(image2);
}

UPDATED CODE: Using a Timer also causes the program to freeze...Am I not using it right?

public void morphImg(int width, int height, BufferedImage morphImage, int k) {
    //creates new image from two images of same size
    final BufferedImage image2 = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
    for (int l = 1; l <= k; l++) {
        for (int i = 0; i < width; i++) {
            for (int j = 0; j < height; j++) {
                //get color from original image
                Color c = new Color(image.getRGB(i, j));

                //get colors from morph image
                Color c2 = new Color(morphImage.getRGB(i, j));

                //gets colors at different stages
                int r = ((k-l)*c.getRed()/k) + (l*c2.getRed()/k);
                int g = ((k-l)*c.getGreen()/k) + (l*c2.getGreen()/k);
                int b = ((k-l)*c.getBlue()/k) + (l*c2.getBlue()/k);   
                Color newColor = new Color(r, g, b);

                //set colors of new image to average of the two images
                image2.setRGB(i, j, newColor.getRGB());
                //display new image

                imageLabel.setIcon(new ImageIcon(image2));
                final Timer t = new Timer(500,null);
                t.setInitialDelay(500);
                t.start();
            }
        }
    }

    //sets modified image as "original" for further manipulation
    setImage(image2);
}

Answer 1

Never use Thread.sleep() when code is executing on the Event Dispatch Thread.

Instead you should use a Swing Timer to schedule your animation.

See the sections from the Swing tutorial on:

1. Concurrency in Swing
2. How to Use Timers

Or if you don't want to use a Timer, then you can use a SwingWorker (as described in the tutorial on concurrency) and then just publish() the image after you change it. Then you can use a Thread.sleep() since the SwingWorker doesn't execute on the EDT.

Simple Timer example:

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import javax.swing.*;
import javax.swing.Timer;

public class TimerTime extends JPanel implements ActionListener
{
    private JLabel timeLabel;
    private int count = 0;

    public TimerTime()
    {
        timeLabel = new JLabel( new Date().toString() );
        add( timeLabel );

        Timer timer = new Timer(1000, this);
        timer.setInitialDelay(1);
        timer.start();
    }

    @Override
    public void actionPerformed(ActionEvent e)
    {
        //  Update the time

        timeLabel.setText( new Date().toString() );
        count++;

        //  Stop after 10 events have been generated

        if (count == 10)
        {
            Timer timer = (Timer)e.getSource();
            timer.stop();
            System.out.println( "Timer stopped" );
        }
    }

    private static void createAndShowGUI()
    {
        JFrame frame = new JFrame("TimerTime");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add( new TimerTime() );
        frame.setLocationByPlatform( true );
        frame.pack();
        frame.setVisible( true );
    }

    public static void main(String[] args)
    {
        EventQueue.invokeLater( () -> createAndShowGUI() );
    }
}