jPanel is not refreshing until I resize the app window

Question

I have one problem with my jPanel. I have a button which PNG image from String input (math formula) and then it will repaint the old image in jPanel. And there goes the problem. The image got changed, but the jPanel wont repaint until I manually resize the app window.

Looks like the Panel wont repaint until that resizing. But I have no idea how to do it in that button.

I tried this and this but no change.

btw. I am using GUI Builder in netbeans.

My code... first attempt:

public class ImagePanel extends JPanel {

   private String path;
   Image img;
   public ImagePanel() {
        try {
            //save path
            path = "Example5.png";
            //load image
            img = ImageIO.read(new File(path));
        } catch (IOException ex) {
        }
    }
   @Override
   public void paint(Graphics g) {
      //draw the image
      if (show) {
        try {
            if (img != null) {
                img = ImageIO.read(new File(path));
                g.drawImage(img, 0, 0, this);
            }
        } catch (IOException ex) {
        }
     } else {
        show = true;
     }
   }
}

and in Window class/button method:

   imagePanel = new ImagePanel();
   imagePanel.repaint();
   imagePanel.updateUI();

second attempt:

public class ImagePanel extends JPanel {

   private String path;
   Image img;
   ImagePanel(Image img) {
        this.img = img;
   }

   public void setImg(Image img) {
       this.img = img;
   }

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

       // Draw image centered in the middle of the panel
       g.drawImage(img, 0, 0, this);
   }

}

and Button:

imagePanel.setImg(new ImageIcon("2.png").getImage());
imagePanel.repaint();

Answer 1

You are overriding your paint(...) method, which is really not a good strategy in any way, since in Swing whenever possible try to override your paintComponent(...) method. Moreover I guess you might have missed to put your code inside SwingUtilities.invokeLater(...) method.Try your hands on this updated code of yours, do place the images alongside your .class files (inside image folder), so structure would be,

PanelTest.class      ImagePanel.class        image(Folder)
                                             |     |     |
                                         image1 image2  image3(and so on)

==================================================================================

import java.awt.*;
import java.awt.event.*;
import java.net.URL;
import javax.swing.*;

public class PanelTest
{
    private URL[] url = new URL[5]; 
    private int counter = 0;
    private ImageIcon image;
    private JButton updateButton;

    public PanelTest()
    {
        try
        {
            for (int i = 0; i < 5; i++)
            {
                url[i] = getClass().getResource("/image/geek" + i + ".gif");
            }
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }

    private void createAndDisplayGUI()
    {
        JFrame frame = new JFrame("Panel Test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        image = new ImageIcon(url[0]);
        final ImagePanel ip = new ImagePanel(image.getImage());
        updateButton = new JButton("UPDATE");
        updateButton.addActionListener(new ActionListener()
        {
            public void actionPerformed(ActionEvent ae)
            {
                counter++;
                if (counter < 5)
                {
                    image = new ImageIcon(url[counter]);
                    ip.setImg(image.getImage());
                }
                else
                    counter = -1;
            }
        });

        frame.getContentPane().add(ip, BorderLayout.CENTER);
        frame.getContentPane().add(updateButton, BorderLayout.PAGE_END);

        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String... args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                new PanelTest().createAndDisplayGUI();
            }
        });
    }
}
class ImagePanel extends JPanel {

   private String path;
   private Image img;
   ImagePanel(Image img) {
        this.img = img;
   }

   public void setImg(Image img) {
       this.img = img;
       // Added by me, so as to update the image
       // as a new Image is made available.
       repaint();
   }

   /*
    * Added by me, make this a customary
    * habbit to override this method too
    * as you override paintComponent(...)
    * method of the said JComponent.
    */
   @Override
   public Dimension getPreferredSize()
   {
       return (new Dimension(300, 300));    
   }

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

       // Clears the previously drawn image.     
       g.clearRect(0, 0, getWidth(), getHeight());  
       // Draw image centered in the middle of the panel
       g.drawImage(img, 0, 0, this);
   }

}

If you want the images too, here they are

GEEK0 GEEK1 GEEK2 GEEK3 GEEK4

Answer 2

You can take care of this with a backround repaint thread. You can place this in the constructor of your JPanel subclass.

Thread repainter = new Thread(new Runnable() {
    @Override
    public void run() {
        while (true) { // I recommend setting a condition for your panel being open/visible
            repaint();
            try {
                Thread.sleep(30);
            } catch (InterruptedException ignored) {
            }
        }
    }
});
repainter.setName("Panel repaint");
repainter.setPriority(Thread.MIN_PRIORITY);
repainter.start();