Beginning Java 2- JDK 1.3 Version
Images and Animation
Creating Image Objects
The Image class is at the heart of all images in Java.
An image will always be an object of a class that has Image as a base. You can't
directly create an Image object that contains an image because the class is
abstract. While a reference of type Image is used to refer to an object that
encapsulates the data for an image, the Image
class does not provide you with the means of obtaining the image data. There has
to be some other mechanism for creating an Image
object corresponding to a particular image.
The Applet
class defines two overloaded versions of a method, getImage(),
that return a reference to an Image
object. One version accepts an object of type URL
as an argument that specifies the source of the image data. The other accepts
two arguments, a URL
object plus a String
object, where the String
object defines the specification of the source of the image data in the context
of the URL
object. In the previous example, we could have created an Image
object directly with the following statement:
Image image = getImage(new URL(getCodeBase(),"Images/wrox_logo.gif"));
We still need to put this in a
try
block because of the possibility of a MalformedURLException
(or others) being thrown by the URL
class constructor. There is a significant difference between this statement
obtaining a reference to an Image
object for an image from a given source, and the statement in the example that
created an ImageIcon
object. When the ImageIcon
class constructor returned to the init()
method, the object was fully constructed and the image was available. In the
statement above we get a reference to an Image
object returned that may at some time
in the future contain the data for the image. This gets over the problem
implicit in using an ImageIcon
object . it hangs up your applet until all the image data for the icon has been
retrieved from the source. When you call getImage(),
your applet code can continue to execute and get on with other things while the
image data is being downloaded. This still leaves the question of determining
when the image is actually here which is where the ImageObserver
interface comes in.
Image Observers
The Component
class implements the ImageObserver
interface so all components will inherit this implementation. The ImageObserver
interface declares one method, imageUpdate(),
that is called automatically when information about an image that was previously
requested becomes available. The imageUpdate()
method will draw the image on the component. So how do you request information
about an image?
One way is to call the drawImage()
method for a Graphics
object. By calling this method you implicitly request all the information
necessary to draw the image in the graphics context . the width, the height, the
pixel data and so on. As you saw, the last argument to the drawImage()
method was a reference to an ImageObserver
object that in our case was the panel on which the image was to be drawn. If any
of the image data was not available at the time of the call, the
imageUpdate() method for the ImageObserver
object would be called as more information became available.
Another way you can request information about an image is to
call methods for the Image
object itself. The Image
class defines methods getWidth()
and getHeight()
to access the width and height of the image. Both of these methods require a
reference of type ImageObserver
to be passed . normally a reference to the component on which the image is to be
drawn . and that object's imageUpdate()
method is called when the information becomes available.
The last part we need to understand in all this is exactly
what the imageUpdate()
method does when it is called. It simply draws the image incrementally with the
information available. This has the effect on a slow Internet link or with a
large image of making the image appear piecemeal, as and when the data becomes
available. This usually gives the user a visual cue that, even though the entire
image has not been retrieved yet, something is still happening.
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