Question

I have a swing gui application. I want to type into a image path and then click button to load the image into a jpanel. The problem is it won’t be loaded, but if I add the extended jpanel which is able to load image when I instiate the jframe, image can be loaded normally. why is that?

code related:

package com.xdg.graphic;

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;
import javax.swing.JPanel;

public class ImageLoader extends JPanel {
    private String imgPath;
    private BufferedImage image;


    public ImageLoader(String imgPath) {
        this.imgPath = imgPath;
        try {
            this.image=ImageIO.read(new File(imgPath));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void paint(Graphics g) {
        g.drawImage(image, 0, 0, null);
    }

    @Override
    protected void paintComponent(Graphics g) {
        g.drawImage(image, 0, 0, null);
    }

    public Dimension getPreferredSize() {
        if (image == null) {
             return new Dimension(100,100);
        } else {
           return new Dimension(image.getWidth(null), image.getHeight(null));
       }
    }

    public String getImgPath() {
        return imgPath;
    }

    public void setImgPath(String imgPath) {
        this.imgPath = imgPath;
    }



}

invoker class:

package com.xdg.image;

import com.xdg.graphic.ImageLoader;
import sun.awt.windows.ThemeReader;

import java.awt.*;

public class FrmImgCropper extends javax.swing.JFrame {
    private ImageLoader imageLoader;

    /** Creates new form FrmImgCropper */
    public FrmImgCropper() {
        initComponents();
    }

    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
     */
    //GEN-BEGIN:initComponents
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        jPanel2 = new javax.swing.JPanel();
        tfImagePath = new javax.swing.JTextField();
        btnPreview = new javax.swing.JButton();
        tfRatioW = new javax.swing.JTextField();
        tfRatioH = new javax.swing.JTextField();
        jLabel1 = new javax.swing.JLabel();
        btnLoad = new javax.swing.JButton();
        jLabel2 = new javax.swing.JLabel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        btnPreview.setText("Preview");

        jLabel1.setText(":");

        btnLoad.setText("Load");
        btnLoad.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnLoadActionPerformed(evt);
            }
        });

        jLabel2.setText("Image Path:");

        javax.swing.GroupLayout jPanel2Layout = new javax.swing.GroupLayout(jPanel2);
        jPanel2.setLayout(jPanel2Layout);
        jPanel2Layout.setHorizontalGroup(jPanel2Layout
                .createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(
                        jPanel2Layout
                                .createSequentialGroup()
                                .addGap(34, 34, 34)
                                .addComponent(tfRatioW, javax.swing.GroupLayout.PREFERRED_SIZE, 55,
                                        javax.swing.GroupLayout.PREFERRED_SIZE)
                                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                                .addComponent(jLabel1)
                                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                                .addComponent(tfRatioH, javax.swing.GroupLayout.PREFERRED_SIZE, 53,
                                        javax.swing.GroupLayout.PREFERRED_SIZE)
                                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 123,
                                        Short.MAX_VALUE)
                                .addComponent(btnLoad, javax.swing.GroupLayout.PREFERRED_SIZE, 82,
                                        javax.swing.GroupLayout.PREFERRED_SIZE).addGap(46, 46, 46)
                                .addComponent(btnPreview).addGap(276, 276, 276))
                .addGroup(
                        jPanel2Layout.createSequentialGroup().addGap(66, 66, 66).addComponent(jLabel2)
                                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                                .addComponent(tfImagePath, javax.swing.GroupLayout.DEFAULT_SIZE, 593, Short.MAX_VALUE)
                                .addGap(29, 29, 29)));
        jPanel2Layout.setVerticalGroup(jPanel2Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                .addGroup(
                        javax.swing.GroupLayout.Alignment.TRAILING,
                        jPanel2Layout
                                .createSequentialGroup()
                                .addContainerGap(20, Short.MAX_VALUE)
                                .addGroup(
                                        jPanel2Layout
                                                .createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                                                .addComponent(jLabel2)
                                                .addComponent(tfImagePath, javax.swing.GroupLayout.PREFERRED_SIZE,
                                                        javax.swing.GroupLayout.DEFAULT_SIZE,
                                                        javax.swing.GroupLayout.PREFERRED_SIZE))
                                .addGap(18, 18, 18)
                                .addGroup(
                                        jPanel2Layout
                                                .createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                                                .addComponent(jLabel1)
                                                .addComponent(tfRatioW, javax.swing.GroupLayout.PREFERRED_SIZE,
                                                        javax.swing.GroupLayout.DEFAULT_SIZE,
                                                        javax.swing.GroupLayout.PREFERRED_SIZE)
                                                .addComponent(tfRatioH, javax.swing.GroupLayout.PREFERRED_SIZE,
                                                        javax.swing.GroupLayout.DEFAULT_SIZE,
                                                        javax.swing.GroupLayout.PREFERRED_SIZE)
                                                .addComponent(btnPreview).addComponent(btnLoad)).addGap(20, 20, 20)));

        getContentPane().add(jPanel2, java.awt.BorderLayout.SOUTH);

        pack();
    }// </editor-fold>
    //GEN-END:initComponents

    private void btnLoadActionPerformed(java.awt.event.ActionEvent evt) {
        if (imageLoader == null) {
            imageLoader = new ImageLoader(tfImagePath.getText());
            imageLoader.setBackground(Color.green);

            getContentPane().add(imageLoader, BorderLayout.CENTER);
            getContentPane().repaint();
            this.repaint();//image does not show up this way dnamically
        } else {
            imageLoader.setImgPath(tfImagePath.getText());
            imageLoader.repaint();
        }

    }

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {

                FrmImgCropper cropper = new FrmImgCropper();
                cropper.getContentPane().add(new ImageLoader("i:\\temp4\\car.jpg"));  //if I add the image loader here directly,image shows up
                cropper.setSize(800, 900);
                cropper.setVisible(true);
            }
        });
    }

    //GEN-BEGIN:variables
    // Variables declaration - do not modify
    private javax.swing.JButton btnLoad;
    private javax.swing.JButton btnPreview;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JPanel jPanel2;
    private javax.swing.JTextField tfImagePath;
    private javax.swing.JTextField tfRatioH;
    private javax.swing.JTextField tfRatioW;
    // End of variables declaration//GEN-END:variables

}

any ideas? I’ve been working on it two hours.

Answer 1

To update your panel you should call revalidate() method on panel’s parent, not repaint() on the new panel since you change the content itself (the panel) but the image. In your case its content pane, which content should be validated.

But its still not the best way to update the image inside the panel if you ask me…
You can try this simple example (its much more simple and works perfectly):

private static BufferedImage image = null;

public static void main ( String[] args )
{
    final JFrame imageFrame = new JFrame ();
    imageFrame.setLayout ( new BorderLayout () );

    final JPanel panel = new JPanel ()
    {
        protected void paintComponent ( Graphics g )
        {
            super.paintComponent ( g );

            if ( image != null )
            {
                g.drawImage ( image, getWidth () / 2 - image.getWidth () / 2,
                        getHeight () / 2 - image.getHeight () / 2, this );
            }
        }
    };
    imageFrame.add ( panel, BorderLayout.CENTER );

    imageFrame.add ( new JButton ( "Load image" )
    {
        {
            addActionListener ( new ActionListener ()
            {
                public void actionPerformed ( ActionEvent e )
                {
                    JFileChooser fc = new JFileChooser ();
                    fc.setDialogType ( JFileChooser.OPEN_DIALOG );

                    if ( fc.showOpenDialog ( imageFrame ) == JFileChooser.APPROVE_OPTION )
                    {
                        try
                        {
                            image = ImageIO.read ( fc.getSelectedFile () );
                            panel.repaint ();
                        }
                        catch ( IOException e1 )
                        {
                            //
                        }
                    }
                }
            } );
        }
    }, BorderLayout.SOUTH );

    imageFrame.setSize ( 500, 500 );
    imageFrame.setLocationRelativeTo ( null );
    imageFrame.setDefaultCloseOperation ( JFrame.EXIT_ON_CLOSE );
    imageFrame.setVisible ( true );
}

The only actual thing you need to do if you change the image inside panel – repaint the affected panel rect (or the whole panel if you don’t like to get deep into the way graphics in Swing works).

As you can see i didn’t even touch the panel itself – just changed the image source.