Re: Image doesn't display properly

From:

Knute Johnson <nospam@rabbitbrush.frazmtn.com>

Newsgroups:

comp.lang.java.programmer

Date:

Thu, 27 Apr 2006 16:25:21 -0700

Message-ID:

<Apc4g.159$oz1.0@newsfe06.phx>

rob.anteau@chartermi.net wrote:

Hello everyone,

I am very new to Java programming, or any programming for that matter.
I have been playing with getting images to display in an application
when an action is performed, but I have been experiencing problems.
When I click the button the image doesn't display. If I click the
button a second time the image displays. Could anyone tell me how I
could resolve this?

Here is a very simple version of my code.

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

public class SimpleImage extends JFrame implements ActionListener{

    Image exampleImage;
    JButton btnExample;

    public SimpleImage( ) {

        super("Image Example");
        // Get the image from the file
        btnExample = new JButton("Click Me");

        Container pane = getContentPane( );

        FlowLayout flow = new FlowLayout();

        pane.setLayout(flow);
        pane.add(btnExample);

        // Set up the frame features
        setSize(400, 300);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);

        //event handler
        btnExample.addActionListener(this);
     }
    public void actionPerformed(ActionEvent e)
    {
        if(e.getSource()== btnExample){
            exampleImage =
Toolkit.getDefaultToolkit().getImage("happyface.jpg");
            Graphics draw = getGraphics();
         draw.drawImage(exampleImage, 100, 175, this);
        }
    }

    public static void main(String[ ] args) {
        SimpleImage frame = new SimpleImage( );
    }
}

Rob:

You've got some general problems with your program design. First, all
drawing should take place in the paint() of AWT components or the
paintComponent() of Swing components. I know that this is
counter-intuitive but it has to be or your program will not be able to
repaint itself when another window is placed over it and removed or it
is minimized and then subsequently maximized. So draw your image in the
paintComponent(). To change images start a thread in your
actionListener that downloads a new image and then call repaint. The
reason to start a new thread is that you want your GUI to be functional
while the image is being retrieved. If you wait in your actionListener
you are blocking the EDT and none of your GUI will redraw or respond to
input. Also unless you need to use an out of date JDK, forget the
Toolkit.getImage(). Use the ImageIO.read() instead. It is synchronous
and is much simpler than getImage().

Here is a simple example of what you said you wanted to do. Replace the
image file name with something you want to see.

import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import java.io.*;
import javax.imageio.*;
import javax.swing.*;

public class ImageTest extends JPanel {
     BufferedImage image;

     public ImageTest(BufferedImage image) {
         this.image = image;
     }

     public void paintComponent(Graphics g) {
         // if there is no image yet draw the message instead
         if (image != null)
             g.drawImage(image,0,0,getWidth(),getHeight(),null);
         else
             g.drawString("No Image Loaded!",20,getHeight()/2);
     }

     // this method is used to change images
     public void setImage(BufferedImage image) {
         this.image = image;
     }

     public static void main(String[] args) {
         // all Swing apps should be created on the EDT
         // do your GUI creation in a static method (in this case main)
started
         // on the EDT with EventQueue.invokeLater()
         Runnable r = new Runnable() {
             public void run() {
                 JFrame f = new JFrame("ImageTest");
                 f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

                 // itest is final so that it can be seen in the
actionlistener
                 // of the button below
                 final ImageTest itest = new ImageTest(null);
                 // set the preferred size so the layout manager knows
how big
                 // to make your component
                 itest.setPreferredSize(new Dimension(400,300));
                 f.add(itest,BorderLayout.CENTER);

                 JButton b = new JButton("Load Image");
                 f.add(b,BorderLayout.NORTH);
                 b.addActionListener(new ActionListener() {
                     public void actionPerformed(ActionEvent ae) {
                         // start a new thread to get the image so that this
                         // method will return immediately
                         Runnable r = new Runnable() {
                             public void run() {
                                 try {
                                     // you can use a File, URL or
InputStream
                                     // as the source of an image with
ImageIO
                                     BufferedImage bi =
                                      ImageIO.read(new File("saturn.jpg"));
                                     // set the image you just read as
the image
                                     // to draw on your component
                                     itest.setImage(bi);
                                     // call repaint to tell the manager to
                                     // redraw the component
                                     itest.repaint();
                                 } catch (IOException ioe) {
                                     ioe.printStackTrace();
                                 }
                             }
                         };
                         new Thread(r).start();
                     }
                 });
                 f.pack();
                 f.setVisible(true);
             }
         };
         EventQueue.invokeLater(r);
     }
}

--

Knute Johnson
email s/nospam/knute/