Java The Complete Reference, Seventh Edition

762 Part II: The Java Library

Notice that it is okay to pass in anullas the fourth parameter todrawImage( ). This is

the parameter used to pass anImageObserverobject that receives notification of image

events. Since this is an image that is not being produced from a network stream, we have no

need for notification. The left snapshot in Figure 25-2 is what the applet looks like with the

mouse button not pressed. As you can see, the image was in the middle of repainting when

this snapshot was taken. The right snapshot shows how, when a mouse button is pressed,

the image is always complete and clean due to double buffering.

MediaTracker

Many early Java developers found theImageObserverinterface far too difficult to understand

and manage when there were multiple images to be loaded. The developer community asked

for a simpler solution that would allow programmers to load all of their images synchronously,

without having to worry aboutimageUpdate( ). In response to this, Sun Microsystems added a

class tojava.awtcalledMediaTrackerin a subsequent release of the JDK. AMediaTrackeris an

object that will check the status of an arbitrary number of images in parallel.

To useMediaTracker, you create a new instance and use itsaddImage( )method to track

the loading status of an image.addImage( )has the following general forms:

void addImage(ImageimgObj, intimgID)

void addImage(ImageimgObj, intimgID, intwidth, intheight)

Here,imgObjis the image being tracked. Its identification number is passed inimgID. ID

numbers do not need to be unique. You can use the same number with several images as a

means of identifying them as part of a group. In the second form,widthandheightspecify

the dimensions of the object when it is displayed.

Once you’ve registered an image, you can check whether it’s loaded, or you can wait for

it to completely load. To check the status of an image, callcheckID( ). The version used in

this chapter is shown here:

boolean checkID(intimgID)

Here,imgIDspecifies the ID of the image you want to check. The method returnstrueif all

images that have the specified ID have been loaded (or if an error or user-abort has terminated

FIGURE 25-2

Output from

DoubleBuffer

without (left) and

with (right) double

buffering