Copy-on-Write Objects in Java

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The Flyweight pattern uses just a few object instances to represent many different objects in a program All of them normally have the same base properties as intrinsic data, as well as a few properties that represent extrinsic datathatvary with each manifestation of the class instance However, these instanceseventually could take on new intrinsic properties (such as shape or folder tab position) and require a new specific instance of the class to representthem Rather than creating these in advance as special subclasses, you cancopy the class instance and change its intrinsic properties when the programflow indicates that a new, separate instance is required The class thuscopies itself when the change becomes inevitable, changing those intrinsic properties inthe new class This process is called copy-on-write It can be builtinto Flyweights as well as a number of other classes, such as the Proxy pattern discussed in the next chapter
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1 Consider a JTable with a cell renderer to highlight certain cells Write a Flyweight pattern for this system 2 Suppose that JButtons can appear on several different tabs of a JTabbedPane, but each controls the same one or two tasks How could you use a Flyweight pattern here
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Description Complete code for drawing folders that are selected by a mouse\Flyweight\FlyFolders\ over event FlyCanvasjava Similar code in which the folders are generated in advance and \Flyweight\FlyVectors\ kept in a Vector FlyCanvasjava Tests Integers and Strings for Flyweight properties \Flyweight\Flytestjava
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15 The Proxy Pattern
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The Proxy pattern is used to represent with a simpler object an object that is complex or timeconsuming to create If creating an object is expensive in time or computer resources, a Proxy allows you to postpone this creation until you need the actual object A Proxy usually has the same methods as the full object that it represents Once that full object is loaded, the Proxy passes on the method calls to the full object There are several cases where a Proxy can be useful: 1 If an object, such as a large image, takes a long time to load 2 If the object is on a remote machine and loading it over the network might be slow, especially during peak network load periods 3 If the object has limited access rights The proxy can then validate the access permissions for that user Proxies can also be used to distinguish between requesting an instance of an object and the actual need to access it For example, program initialization might set up a number of objects, all of which might not be used right away In that case, the Proxy can load the real object only when it is needed Consider a program that needs to load and display a large image When the program starts, some indication that an image is to be displayed is needed so that the screen is set out correctly; however, the actual image display can be postponed until the image is completely loaded This is particularly important in programs such as word processors and Web browsers that lay out text around the images before the images are available An image proxy can note the image and begin loading it in the background, while drawing a simple rectangle or other symbol to represent the image's extent on the screen before it appears The proxy can even delay loading the image at all until it receives a paint request and only then begin the loading process
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In this example program, we create a simple program to display an image on a JPanel when it is loaded Rather than loading the image directly, we use a class we call ImageProxy to defer loading and draw a rectangle around the image area until loading is completed
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public class ProxyDisplay extends JxFrame { public ProxyDisplay() { super("Display proxied image"); JPanel p = new JPanel(); getContentPane()add(p); psetLayout(new BorderLayout()); ImageProxy image = new ImageProxy("elliottjpg", 321, 271); padd("Center", image); padd("North", new Label(" padd("West", new Label(" setSize(370, 350); setVisible(true); }
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Note that we create the instance of the Image Proxy just as we would have for an Image and that we add it to the enclosing JPanel as we would an actual image The ImageProxy class sets up the image loading and creates a MediaTracker object to follow the loading process within the constructor
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public class ImageProxy extends JPanel implements Runnable { private int height, width; private MediaTracker tracker; private Image img; private JFrame frame; private Thread imageCheck; //to monitor loading //----------------public ImageProxy(String filename, int w, int h) { height = h; width = w; tracker = new MediaTracker(this); img = ToolkitgetDefaultToolkit()getImage(filename); trackeraddImage(img, 0); //watch for image loading imageCheck = new Thread(this); imageCheckstart(); //start 2nd thread monitor //this begins actual image loading try { trackerwaitForID(0,1); } catch (InterruptedException e) { } }
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The waitForID method of the MediaTracker actually initiates loading In this case, we put in a minimum wait time of 1 millisecond so that we can minimize apparent program delays The constructor also creates a separate thread imageCheck that checks the loading status every few milliseconds and then starts that thread running
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public void run() { /**this thread monitors image loading and repaints when done the 1000 millisecond is artificially long to allow the demo to display with a delay*/ try { Threadsleep(1000); while (! trackercheckID(0)) Threadsleep(1000); } catch (Exception e) { } repaint(); }
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For the purposes of this illustration program, we slowed the polling time down to 1 second so that we can see the program draw the rectangle and then refresh the final image
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Finally, the Proxy is derived from the JPanel component and therefore has a paint method In this method, we draw a rectangle if the image is not loaded If the image has been loaded, we erase the rectangle and draw the image
public void paint(Graphics g) { if (trackercheckID(0)) { height = imggetHeight(frame);//get height width = imggetWidth(frame); //and width getsetColor(ColorlightGray); //erase box gfillRect(0,0, width, height); gdrawImage(img, 0, 0, this); //draw image } else { //draw box outlining image if not loaded yet gsetColor(Colorblack); gdrawRect(1, 1, width-2, height-2); } }
The program's two states are illustrated in Figure 151 Figure 151 The proxy image display on the left is shown until the image on the right loads