Trail: Creating a GUI with JFC/Swing
Lesson: Using Other Swing Features
How to Use Icons
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Solving Common Problems Using Other Swing Features
Many Swing components, such as labels, buttons, and tabbed panes, can be decorated with an icon — a fixed-sized picture. An icon is an object that adheres to the Icon interface. Swing provides a particularly useful implementation of the Icon interface: ImageIcon, which paints an icon from a GIF, JPEG, or (as of 1.3) PNG image.

Here's a snapshot of an application with three labels, two decorated with an icon:

An example of using image icons to decorate labels.

The program uses one image icon to contain and paint the yellow splats. One statement creates the image icon and two more statements include the image icon on each of the two labels: 
ImageIcon icon = createImageIcon("images/middle.gif",
                                 "a pretty but meaningless splat");
label1 = new JLabel("Image and Text", icon, JLabel.CENTER);
...
label3 = new JLabel(icon);
The createImageIcon method (used in the preceding snippet) is one we use in many of our code samples. It finds the specified file and return an ImageIcon for that file, or null if that file couldn't be found. Here is a typical implementation: 
/** Returns an ImageIcon, or null if the path was invalid. */
protected static ImageIcon createImageIcon(String path,
                                           String description) {
    java.net.URL imgURL = LabelDemo.class.getResource(path);
    if (imgURL != null) {
        return new ImageIcon(imgURL, description);
    } else {
        System.err.println("Couldn't find file: " + path);
        return null;
    }
}
If you copy createImageIcon, be sure to change the name of the class used for the getResource call; it should be the name of the class that contains the createAppletImageIcon method. For example, you might change LabelDemo.class.getResource to MyApp.class.getResource. In the preceding snippet, the first argument to the ImageIcon constructor is relative to the location of the class LabelDemo, and will be resolved to an absolute URL. The description argument is a string that allows assistive technologies to help a visually impaired user understand what information the icon conveys.

Generally, applications provide their own set of images used as part of the application, as is the case with the images used by many of our demos. You should use the Class getResource method to obtain the path to the image. This allows the application to verify that the image is available and to provide sensible error handling if it is not. When the image is not part of the application, getResource should not be used and the ImageIcon constructor is used directly. For example: 

ImageIcon icon = new ImageIcon("/home/sharonz/images/middle.gif",
                               "a pretty but meaningless splat");

When you specify a filename or URL to an ImageIcon constructor, processing is blocked until after the image data is completely loaded or the data location has proven to be invalid. If the data location is invalid (but non-null), an ImageIcon is still successfully created; it just has no size and, therefore, paints nothing. As we showed in the createImageIcon method, it's wise to first verify that the URL points to an existing file before passing it to the ImageIcon constructor. This allows graceful error handling when the file isn't present. If you want more information while the image is loading, you can register an observer on an image icon by calling its setImageObserver method.

Under the covers, each image icon uses an Image object to hold the image data.

The rest of this section covers the following topics:

A More Complex Image Icon Example

Here's an applet that uses eight image icons. In the snapshot, you can see three of them: one displays the photograph and two decorate buttons with small arrows.

Click this figure to run the applet.
This is a picture of the applet's GUI. To run the applet, click the picture. The applet will appear in a new browser window.

[PENDING: New screenshot forthcoming.]

Try this: 
  1. Run IconDemoApplet using JavaTM Web Start. Or, to compile and run the example yourself, consult the example index.
  2. Click the Previous Picture and Next Picture buttons to view the photographs.
  3. Hold the mouse over a photograph. A tool tip appears that indicates the filename of the current photograph and its width and height.

IconDemoApplet demonstrates icons used in the following ways:

The code that follows creates the four arrow image icons and attaches them to the two buttons. You can find the applet's code in IconDemoApplet.java. 

//Create the next and previous buttons.
ImageIcon nextIcon = createAppletImageIcon("images/right.gif",
                                           "a right arrow");
ImageIcon dimmedNextIcon = createAppletImageIcon("images/dimmedRight.gif",
                                                 "a dimmed right arrow");
ImageIcon previousIcon = createAppletImageIcon("images/left.gif",
                                               "a left arrow");
ImageIcon dimmedPreviousIcon = createAppletImageIcon("images/dimmedLeft.gif",
                                                     "a dimmed left arrow");

nextButton = new JButton("Next Picture", nextIcon);
nextButton.setDisabledIcon(dimmedNextIcon);
nextButton.setVerticalTextPosition(AbstractButton.CENTER);
nextButton.setHorizontalTextPosition(AbstractButton.LEFT);
...

previousButton = new JButton("Previous Picture", previousIcon);
previousButton.setDisabledIcon(dimmedPreviousIcon);
previousButton.setVerticalTextPosition(AbstractButton.CENTER);
previousButton.setHorizontalTextPosition(AbstractButton.RIGHT);

The action handler for the buttons initiates loading the photographs into the image icon: 

//User clicked either the next or the previous button.
public void actionPerformed(ActionEvent e) {
    //Show loading message.
    photographLabel.setIcon(null);
    photographLabel.setText("Loading image...");

    //Compute index of photograph to view.
    if (e.getActionCommand().equals("next")) {
        current += 1;
        if (!previousButton.isEnabled())
            previousButton.setEnabled(true);
        if (current == pictures.size() - 1)
            nextButton.setEnabled(false);
    } else {
        current -= 1;
        if (!nextButton.isEnabled())
            nextButton.setEnabled(true);
        if (current == 0)
            previousButton.setEnabled(false);
    }

    //Get the photo object.
    Photo pic = (Photo)pictures.elementAt(current);

    //Update the caption and number labels.
    captionLabel.setText(pic.caption);
    numberLabel.setText("Picture " + (current+1) +
                        " of " + pictures.size());

    //Update the photograph.
    ImageIcon icon = pic.getIcon();
    if (icon == null) {     //haven't viewed this photo before
        loadImage(imagedir + pic.filename, current);
    } else {
        updatePhotograph(current, pic);
    }
}

The photographs are loaded in a separate thread by the loadImage method — its code is shown a little later in this section.

The Photo class, in Photo.java, is a simple class that manages an image icon and its properties. 

public class Photo {
    public String filename;
    public String caption;
    public int width;
    public int height;
    public ImageIcon icon;

    public Photo(String filename, String caption, int w, int h) {
        this.filename = filename;
        if (caption == null)
            this.caption = filename;
        else
            this.caption = caption;
        width = w;
        height = h;
        icon = null;
    }

    public void setIcon(ImageIcon i) {
        icon = i;
    }

    public ImageIcon getIcon() {
        return icon;
    }
}

The Loading Images Into Applets section discusses how images are loaded into this applet and shows the createAppletImageIcon method — a version of createImageIcon customized for applets that are deployed using Java Plug-in.

Loading Images Using getResource

Most often, an image icon's data comes from an image file. There are a number of valid ways that your application's class and image files may be configured on your file server. You might have your class files in a JAR file, or your image files in a JAR file; they might be in the same JAR file, or they might be in different JAR files. The following figures illustrate a few of the ways these files can be configured:

[PENDING: New figures forthcoming. The new figures will show more family-tree style lines.]

Diagram showing MyDemo.class and images/myImage.gif under dot (parent directory) Diagram showing MyDemo.class and image.jar under dot (parent directory)
Class file next to an image directory containing the image file, in GIF format. Class file in same directory as JAR file. The JAR file was created with all the images in an images directory.
Diagram showing MyDemo.jar and image.jar under (parent directory) Diagram showing MyDemo.class and images/myImage.gif in the same JAR file.
Class file in one JAR file and the images in another JAR file. Class and image files in same JAR file.

If you are writing a real-world application, it is likely (and recommended) that you put your files into a package. For more information on packages, see Creating and Using Packages in the Learning the Java Language trail. Here are some possible configurations using a package named "omega":

Diagram showing omega package with MyDemo.class and image/myImage.gif Diagram showing omega package with MyDemo.class and image.jar
Class file in directory named omega. Image in omega/images directory. Class file in omega directory. Image in JAR file not inside of omega directory, but created with omega/images hierarchy.
Diagram showing omega.jar which contains omega/MyDemo.class and omega/images/myImage.gif
One big JAR file with class files under omega directory and image files under omega/images directory.

All seven configurations shown are valid, and the same code reads the image: 

java.net.URL imageURL = myDemo.class.getResource("images/myImage.gif");
...
if (imageURL != null) {
    ImageIcon icon = newImageIcon(imageURL);
}

The getResource method causes the class loader to look through the directories and JAR files in the program's class path, returning a URL as soon as it finds the desired file. In our example the MyDemo program attempts to load the image/myImage.gif file. The class loader looks through the directories and JAR files in the program's class path for image/myImage.gif. If the class loader finds the file, it returns the URL of the JAR file or directory that contained the file. If another JAR file or directory in the class path contains the images/myImage.gif file, the class loader returns the first instance that contains the file.

Version note: In versions of Java Plug-in before 1.4, getResource doesn't look in JAR files. See Loading Images Into Applets for details.

Here are three ways to specify the class path:

Most of our examples put the images in an images directory under the directory that contains the examples' class files. When we create JAR files for the examples, we keep the same relative locations, although often we put the class files in a different JAR file than the image JAR file. No matter where the class and image files are in the file system — in one JAR file, or in multiple JAR files, in a named package, or in the default package — the same code finds the image files using getResource. For more information, see Accessing Resources in a Location-Independent Manner and the Application Development Considerations.

Loading Images Into Applets

Applets generally load image data from the computer that served up the applet. There are two reasons for this. First, untrusted applets can't read from the file system on which they're running. Second, it just makes sense to put an applet's class and data files together on the server.

The IconDemoApplet program initializes each of its image icons from GIF files whose locations are specified with URLs. Because IconDemoApplet is designed to be an untrusted applet, we must place the image files under the applet's code base (the server directory containing the applet's class files). The following figure shows the locations of files for IconDemoApplet.

[PENDING: New figure forthcoming. The new figure will have box removed from around the text "code base" and "image directory (specified by IMAGEDIR parameter)". The following text replaces the previous comment: "The image files or image directory can be deleted if the applet loads image files from a JAR file."]

The APPLET tag is where you specify information about the images used in the applet. For example, here's part of the tag for IconDemoApplet: 

<applet code="IconDemoApplet.class"
        codebase="examples/"
        archive="iconAppletClasses.jar,
                 iconStartupImages.jar,
                 iconAppletImages.jar"
        width="400" height="360">
    <param NAME="IMAGEDIR" VALUE="images">

    <param NAME="IMAGE0" VALUE="stickerface.gif">
    <param NAME="CAPTION0" VALUE="Sticker Face">
    <param NAME="WIDTH0" VALUE="230">
    <param NAME="HEIGHT0" VALUE="238">
    ...
</applet>

The IMAGEDIR parameter indicates that the image files should be in a directory named images relative to the applet's code base. Applets generally use a URL that is constructed relative to the applet's code base.

As you can see from the archive attribute of the preceding APPLET tag, we have deployed IconDemoApplet using three JAR files. The classes are in one JAR file, the images required for starting up the UI (the arrow images) are in another JAR file, and the rest of the images are in a third JAR file. Separating the UI images from the other images means a quicker start-up time.

For more information on specifying JAR files with the APPLET tag, see Using the APPLET Tag and the JAR File Overview.

When using Java Web Start to deploy an applet, you can use the same approach for loading resources as you do for applications — the getResource method. However, for applets deployed using Java Plug-in, getResourceAsStream is more efficient for loading images.

Version note: Prior to release 1.4, when called from an applet deployed using Java Plug-in, getResource did not look in JAR files for resources, it looked only in the code base. In this situation, you must either put the images in the code base, or you must use getResourceAsStream.
Here is the code from IconDemoApplet that reads the images using getResourceAsStream: 
public class IconDemoApplet extends JApplet ... {
    protected String leftButtonFilename = "images/left.gif";
    ...
    public void init() {
        ...
        ImageIcon leftButtonIcon = createAppletImageIcon(leftButtonFilename,
                                                   "an arrow pointing left");
        ...
    }
    ...
    //Returns an ImageIcon, or null if the path was invalid.
    //When running an applet using Java Plug-in,
    //getResourceAsStream is more efficient than getResource.
    protected static ImageIcon createAppletImageIcon(String path,
                                              String description) {
        int MAX_IMAGE_SIZE = 75000; //Change this to the size of
                                    //your biggest image, in bytes.
        int count = 0;
        BufferedInputStream imgStream = new BufferedInputStream(
           IconDemoApplet.class.getResourceAsStream(path));
        if (imgStream != null) {
            byte buf[] = new byte[MAX_IMAGE_SIZE];
            try {
                count = imgStream.read(buf);
            } catch (IOException ieo) {
                System.err.println("Couldn't read stream from file: " + path);
            }

            try {
                imgStream.close();
            } catch (IOException ieo) {
                 System.err.println("Can't close file " + path);
            }

            if (count <= 0) {
                System.err.println("Empty file: " + path);
                return null;
            }
            return new ImageIcon(Toolkit.getDefaultToolkit().createImage(buf),
                                 description);
        } else {
            System.err.println("Couldn't find file: " + path);
            return null;
        }
    }
    ...
}
You might want to copy the createAppletImageIcon method for use in your applet. Be sure to change the IconDemoApplet string in the call to getResourceAsStream to the name of your applet.

Improving Perceived Performance When Loading Image Icons
Because the photograph images are large, IconDemoApplet uses several techniques to improve the performance of the program as perceived by the user. As with all performance-related issues, these techniques are applicable in some situations and not others. These are not general recommendations for all programs, but some techniques you can try to improve the user's experience. Furthermore, the techniques described here are designed to improve the program's perceived performance, but don't necessarily impact its real performance.

Creating a Custom Icon Implementation

If an image differs depending on the state of the component it's within, consider implementing a custom Icon class to paint the image. The really nice thing about a custom icon is that you can easily change the icon's appearance to reflect its host component's state.

Look-and-feel implementations often use custom icons. For example, the Java look and feel uses a single MetalCheckBoxIcon object to paint all of the check boxes in the GUI. The MetalCheckBoxIcon paints itself differently depending on whether its host component is enabled, pressed, or selected.

In this section, we'll convert a program called ButtonDemo so that it uses a custom icon to paint these two arrows:

A simple left arrow A simple right arrow
You can see a picture of ButtonDemo in How to Use the Common Button API. Its source code is in ButtonDemo.java. ButtonDemo uses the following code to load the arrows from GIF files and put the arrows into buttons: 
ImageIcon leftButtonIcon = createImageIcon("images/right.gif",
                                           "an arrow pointing right");

...
ImageIcon rightButtonIcon = createImageIcon("images/left.gif",
                                            "an arrow pointing left");

b1 = new JButton("Disable middle button", leftButtonIcon);
...
b3 = new JButton("Enable middle button", rightButtonIcon);
Here is the new code, which uses a custom icon class named ArrowIcon. Only the bold lines have changed. You can run CustomIconDemo using Java Web Start. Or, to compile and run the example yourself, consult the example index. 
Icon leftButtonIcon = new ArrowIcon(SwingConstants.TRAILING);
...
Icon rightButtonIcon = new ArrowIcon(SwingConstants.LEADING);

b1 = new JButton("Disable middle button", leftButtonIcon);
...
b3 = new JButton("Enable middle button", rightButtonIcon);
You can find the implementation of the custom icon class in ArrowIcon.java. Here are the interesting parts of its code: 
class ArrowIcon implements Icon, SwingConstants {
    public ArrowIcon(int direction) {
        if (direction == LEADING) {
            xPoints[0] = width;
            yPoints[0] = -1;
            xPoints[1] = width;
            yPoints[1] = height;
            xPoints[2] = 0;
            yPoints[2] = height/2;
            xPoints[3] = 0;
            yPoints[3] = height/2 - 1;
        } else /* direction == TRAILING */ {
            xPoints[0] = 0;
            yPoints[0] = -1;
            xPoints[1] = 0;
            yPoints[1] = height;
            xPoints[2] = width;
            yPoints[2] = height/2;
            xPoints[3] = width;
            yPoints[3] = height/2 - 1;
        }
    }
    ...
    public void paintIcon(Component c, Graphics g,
                          int x, int y) {
        if (c.isEnabled()) {
            g.setColor(c.setForeground());
        } else {
            g.setColor(Color.gray);
        }

        g.translate(x, y);
        g.fillPolygon(xPoints, yPoints, xPoints.length);
        g.translate(-x, -y);  //Restore Graphics object
    }
}
Note that the icon sets the current color. If you don't do this, then the icon's painting might not be visible. For more information about painting, see Performing Custom Painting.

Using a custom icon to paint the arrows has a few implications:

The Image Icon API

The following tables list the commonly used ImageIcon constructors and methods. Note that ImageIcon is not a descendent of JComponent or even of Component.

The API for using image icons falls into these categories:

Setting, Getting, and Painting the Image Icon's Image
Method or Constructor Purpose
ImageIcon()
ImageIcon(byte[])
ImageIcon(byte[], String)
ImageIcon(Image)
ImageIcon(Image, String)
ImageIcon(String)
ImageIcon(String, String)
ImageIcon(URL)
ImageIcon(URL, String)		 Create an ImageIcon instance, initializing it to contain the specified image. The first argument indicates the source — image, byte array, filename, or URL — from which the image icon's image should be loaded. The source must be in a format supported by the java.awt.Image class: namely GIF, JPEG, or (as of 1.3) PNG. The second argument, when present, provides a description for the image. The description may also be set via setDescription and provides useful textual information for assistive technologies.
void setImage(Image)
Image getImage()		 Set or get the image displayed by the image icon.
void paintIcon(Component, Graphics, int, int) Paint the image icon's image in the specified graphics context. You would override this only if you're implementing a custom icon that performs its own painting. The Component object is used as an image observer. You can rely on the default behavior provided by Component class, and pass in any component. The two int arguments specify the top-left corner where the icon is painted.
URL getResource(String)
in (java.lang.ClassLoader) 		Find the resource with the given name. For more information, see Loading Images Using getResource.
InputStream getResourceAsStream(String)
in (java.lang.ClassLoader) 		Find the resource with the given name and return an input stream for reading the resource. For more information, see the Loading Images Into Applets discussion.

Setting or Getting Information about the Image Icon
Method Purpose
void setDescription(String)
String getDescription()		 Set or get a description of the image. This description is intended for use by assistive technologies.
int getIconWidth()
int getIconHeight()		 Get the width or height of the image icon in pixels.

Watching the Image Icon's Image Load
Method Purpose
void setImageObserver(ImageObserver)
ImageObserver getImageObserver()		 Set or get an image observer for the image icon.
int getImageLoadStatus() Get the loading status of the image icon's image. The values returned by this method are defined by MediaTracker.

Examples that Use Icons

The following table lists just a few of the many examples that use ImageIcon.

Example Where Described Notes
LabelDemo This section and
How to Use Labels		 Demonstrates using icons in an application's label, with and without accompanying text.
IconDemoApplet This section An applet. Uses a label to show large images; uses buttons that have both images and text.
CustomIconDemo This section Uses a custom icon class implemented by ArrowIcon.java.
TumbleItem How to Make Applets Uses image icons in an animation. Shows how to call ImageIcon's paintIcon method.
ButtonDemo How to Use Buttons, Check Boxes, and Radio Buttons Shows how to use icons in an application's buttons.
CheckBoxDemo How to Use Check Boxes Uses multiple GIF images.
TabbedPaneDemo How to Use Tabbed Panes Demonstrates adding icons to tabs in a tabbed pane.
DialogDemo How to Make Dialogs Shows how to use standard icons in dialogs.
TreeIconDemo How to Use Trees Shows how to change the icons displayed by a tree's nodes.
ActionDemo How to Use Actions Shows how to specify the icon in a tool-bar button or menu item using an Action.
FileChooserDemo2 How to Use File Choosers Uses a PNG image. Shows how to implement an image previewer and an image filter in a file chooser.

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