4.7 The ImageIcon Class

Swing provides a concrete implementation of the Icon interface that is considerably more useful than our OvalIcon class. ImageIcon uses a java.awt.Image object to store and display any graphic and provides synchronous image loading (i.e., the Image is loaded completely before returning), making ImageIcons very powerful and easy to use. You can even use an ImageIcon to display an animated GIF89a, making the ubiquitous "animation applet" as simple as this:

// AnimationApplet.java // import javax.swing.*; // A simple animation applet public class AnimationApplet extends JApplet {   public void init( ) {     ImageIcon icon = new ImageIcon("images/rolling.gif");  // Animated gif     getContentPane( ).add(new JLabel(icon));   } }

All we did here was load an animated GIF in the init( ) method and then add it to the applet. For more information on JApplet, see Chapter 8.

ImageIcon currently supports the JPEG, GIF (including animation and transparency), PNG, and XBM image formats. TIFF support should be coming soon. SVG might be supported eventually.

4.7.1 Properties

The ImageIcon class defines the properties listed in Table 4-4. The description property allows an arbitrary description of the image to be specified. One possible use of this property might be to give a blind user an audio description of the image.

Table 4-4. ImageIcon properties

Property	Data type	get	is	set	Default value
description	String	·		·	null
iconHeighto	int	·			-1
iconWidtho	int	·			-1
image	Image	·		·	null
imageLoadStatus	int	·			0
imageObserver	ImageObserver	·		·	null
ooverridden

The iconHeight and iconWidth properties default to -1 if no image is loaded by the constructor, while the image property simply contains the Image object rendered by the icon. ImageLoadStatus indicates the success or failure of the image load process using the constants defined in java.awt.MediaTracker (ABORTED, ERRORED, or COMPLETE). The default for this property is 0, which does not map to any of these constants.

The imageObserver property contains the ImageObserver that should receive notifications of changes to the image. If this property is null (as it is by default), the component containing the icon will be treated as the image observer when the image is painted.

Figure 4-9 shows a class diagram for ImageIcon and the classes related to it.

Figure 4-9. ImageIcon class diagram

4.7.2 Serialization

Like most Swing classes, ImageIcon implements Serializable. The keen observer may see a problem with this: the java.awt.Image class used by ImageIcon is not serializable. By default, this would keep ImageIcon objects from serializing properly. The good news is that ImageIcon implements its own readObject( ) and writeObject( ) methods so that the pixel representation of the image is stored and retrieved correctly.

4.7.3 Constructors

ImageIcon( )

Create an uninitialized ImageIcon.

ImageIcon(Image image)

ImageIcon(Image image, String description)

Create ImageIcon objects from an existing image. A textual description of the image may be provided. If no description is provided, an attempt is made to retrieve the "comment" property from the input Image. If this is a non-null string, it is used as the description.

ImageIcon(String filename)

ImageIcon(String filename, String description)

Create ImageIcon objects from the contents of the specified JPEG, PNG, GIF, or XBM file. The image is guaranteed to be completely loaded (unless an error occurs) when the constructor returns.

ImageIcon(URL location)

ImageIcon(URL location, String description)

Create ImageIcon objects from the contents of the specified java.net.URL. The image is guaranteed to be completely loaded (unless an error occurs) when the constructor returns.

public ImageIcon(byte imageData[])

public ImageIcon(byte imageData[], String description)

Create ImageIcon objects from an array of bytes containing image data in a supported format, such as JPEG, PNG, GIF, or XBM.

4.7.4 User Interface Method

public void paintIcon(Component c, Graphics g, int x, int y)

Paint the Image at the specified location on the supplied Graphics. The given Component is passed to the Graphics's drawImage( ) method as the ImageObserver (recall that java.awt.Component implements ImageObserver) if no image observer has been explicitly set.