Why repaint() not always work?

Question

In my first program repaint work corectly, but in the second program i have a problem with it.

    import java.awt.Graphics;
    import java.awt.image.BufferedImage;
    import java.io.File;
    import java.io.IOException;
    import javax.imageio.ImageIO;
    import javax.swing.JFrame;
    import javax.swing.JPanel;

    public class repaint1 {

        public static void main(String[] args) {
            JFrame win = new JFrame("");
            test1 content = new test1();
            win.setContentPane(content);
            win.setSize(600, 400);
            win.setLocation(100, 100);
            win.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            win.setResizable(false);
            win.setVisible(true);
            content.function();

        }

    }

    class test1 extends JPanel {

        private BufferedImage img;

        public int x = 50;
        public int y = 50;

        public test1() {

        }

        public void function() {

            try {

                Thread.sleep(1000);
            } catch (InterruptedException ex) {
                Thread.currentThread().interrupt();
            }

            for (int i = 50; i < 150; i++) {
                x = i;
                y = i;

                try {

                    Thread.sleep(10);
                } catch (InterruptedException ex) {
                    Thread.currentThread().interrupt();
                }

                repaint();
            }
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);

            try {
                img = ImageIO.read(new File("images.jpg"));
                g.drawImage(img, x, y, null); 

            } catch (IOException ex) {
                // handle exception...
            }

        }

    }



import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.FocusEvent;
import java.awt.event.FocusListener;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

public class repaint2 {

    public static void main(String[] args) {

        JFrame f = new JFrame("JFrame");
        test2 content = new test2();
        f.setContentPane(content);
        f.setSize(600, 400);
        f.setLocation(100, 100);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setResizable(false);
        f.setVisible(true);

    }

}

class test2 extends JPanel {

    private BufferedImage img;

    public int x = 50;
    public int y = 50;

    public test2() {

        JButton button = new JButton("Start !");
        button.setBounds(458, 24, 122, 23);
        button.setVisible(true);
        add(button);

        button.addActionListener(new ActionListener() {

            public void actionPerformed(ActionEvent e) {
                // Execute when button is pressed

                function();

            }

        });

    }

    public void function() {

        for (int i = 50; i < 150; i++) {

            x = i;
            y = i;

            repaint();

            try {

                Thread.sleep(10);
            } catch (InterruptedException ex) {
                Thread.currentThread().interrupt();
            }

        }
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);

        try {
            img = ImageIO.read(new File("images.jpg"));
            g.drawImage(img, x, y, null);

        } catch (IOException ex) {
            // handle exception...
        }

    }

}

Answer 1

In the first case the function() method is invoked from the main Thread, so when you invoke sleep() this Thread sleeps, which does not affect painting of the GUI.

In the second case your function() method is invoked from the ActionListener. All code executed from a listener is invoked on the Event Dispatch Thread (EDT), which is responsible for repainting the GUI. When you tell the Thread to sleep() the EDT sleeps, so it can't repaint the GUI until the loop finishes executing.

In your ActionListener code you need to start a separate Thread. One way to do this is to use a SwingWorker.

Read the section from the Swing tutorial on Concurrency for more information.

Answer 2

To move your image every time you need to use implements Runnable in your JPanel class. Here this class (MyJPanleClass) will behave like both as JPanel and Thread.

public class MyJPanleClass extends JPanel implements Runnable{

   public MyJPanleClass(){
      //constractor
   }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        //do your staff with g
    }

    @Override
    public void run(){
       repaint();
    }

}

To execute it from main class just write this code:

MyJPnaleClass jp=new MyJPanelClass();
Thread t1=new Thread(jp);
t1.start();//this will call run class
try{
t1.join();//running the thread until it finish. 
}catch(Exception ex){}

If you don't have any idea about thread just follow the link to learn it: Thread in java tutorial