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Goals for this Lab

Today we learn about applets. You will also learn more about concurrency, exceptions, and event handling

Deliverable

By the deadline you should have at least one original Java applet on a HTML web page with link to the source code. I hope this will be a public web page that is linked to your CS320 web page. I expect the code to include some thing original... and your page to have notes saying what is new about it.

You page should also have a paragraph or picture that defines the relationship between these classes: Throwable, Error, Exception, RuntimeException, and IOException.

Process

Start from with my existing Java and HTML samples. Modify them. You may have to take classes and extend them to do what you'd like them to do. You can(probably will) implement existing interfaces and abstract classes. Add comments indicating what you changed.

Study the examples and experiments below. Pick one. Download the page and source code into your working directory and make changes. When done put the test page, the source code, and the compiled applet in your public web site(below). Find the index page and add a link to it to your Java page.

Review Java

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1. Have a look at this page, with four Applets, code, and documentation: [ java.test.html ]

2. It is easy to extend TextDemo to do new things by over-riding the stub something with a new something in the new class. I decided to write a something that downloads the result of requesting a URL from the WWW: [ testURLDemo.html ] (this is a Java2 version...and works better on some browsers than others because of security issues with accessing remote computers).

3. Here is an updated variation of the Hello, Goodbye series that is more object-oriented. It imports a class of Colored Text Objects: [ ColoredText.html ] [ ColoredText.java ] and so the "main" Applet is made simpler: [ test.Goodbye2.html ] Perhaps the ColoredText class may have other uses?
   Net
   The

    		import ColoredText;

   does not seem to be compatible with all versions of Java. Very old and newer ones automatically import local classes so I had to make it into a comment:

    		//import ColoredText;

   You may hit a Java where it has to be uncommented.

   
   (End of Net)
   
   

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Hints

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1. Look in your Previous Lab notes and my handout.
2. Avoid using Netscape/MS IE to debug applets. Include a simple main in each Applet to let you debug it as an application before publishing it.
3. Open the Java Console before running a new Applet in a browser for the first time.
4. Put an applet called AaA in file AaA.java.
5. More examples [ java.html ] and [ http://ftp.csci.csusb.edu/public/faculty/dick/ ]

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Exceptions

Explore the Sun documentation of the Throwable hierarchy starting from [ Throwable.html ] and taking note of what is special about Error, Exception, and RuntimeException. Where does IOException fit in this hierarchy? Which class do you not catch? Which subclass of Exception do you not need catch or declare in a throws clause?

By drawing a UML diagram or in text describe the hierarchy of Throwable classes.

Concurrency

Threads in Java

In Netscape/IE make sure you open the Java Console so you can see the "System.out" before looking at the following two pages:
Here is a sample of a Java program that uses an extra thread to display changing data: [ test.Spinner2.html ]

Here is the same applet running only one thread: [ test.Spinner.html ]

Warning: close down and reopen your browser after running the second one above.

For more on Thread see [ Thread.html ]

CPU Eaters

Java has a nasty little flaw: it is inherently multithreaded. On the other hand if you avoid Threads you tend to get blank images:

Worse several browsers do not allow you control of these thread. If you look at a page with an Applet that has a thread that doesn't stop itself it will continue running after you leave the page. Slowly and steadily as you view badly behaved applets your CPU gets full of things you can not see or control. The cursor flickers, and so on.

It would be unethical for me to show you a demonstration page! I have perpetrated this kind of problem in some of my code. The following shows a program that shows a blank when run as an application and grabs the CPU if run in a browser: [ Surat2.java ]

For more see, [ mbox ]

. . . . . . . . . ( end of section Concurrency) <<Contents | End>>

More Java

More Layouts

You may need or want to do some sophisticated graphic interfaces if so this experiment may help you.

A layout manager automagically places some graphic parts inside a larger graphic Container. Here is an example of some of the standard layouts.

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1. (FlowLayout): Parts follow each other Left-right and "word-wrap"
2. (GridLayout): A two dimensional array
3. (BorderedLayout): One part in the center and one to four parts at edge
4. (Stack of CardsLayout): Only one part is visible at a time.
5. (GridBagLayout): This is not a simple layout. Read The Famous Sun Documentation.

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The example below came from the Java developers. I have modified it to make it a little more obvious what of going on. Warning, you will need to read, study and think about both the code and Sun's documentation: [ CardTest.java ] and [ test.CardTest.html ]

By the way take note of the relation

 		instanceof

in the CardTest code it allows Java to find out the type of an object at run time.

A Graphic Applet

Java is one of the few languages to provide the functionality needed to graphics as part of its standard library. Previous languages have left it up to others to create libraries for doing graphics. Dr. Zemoudeh has a nice example of this in the Scribble applets and applications. The source code for an application is in [ StandaloneScribble.java ] and the compiled byte-code file [ Scribble.class ] Try this out at [ Scribble.html ]

Graphic Recursion

Here is a page+applet that uses the Le Riche algorithm to plot a curve. I've used the Henrici spiral as a test case. [ test.RecursiveHenrici.html ] Try it out and then modify the curve (leave Le Riche alone!).

Entropy and Randomness

Here is a page that demonstrates how to get Random Numbers [ test.HowRandom.html ] by painting 4000 random points. Here are a some more [ test.Surat.html ] that produce more or less random pointilist paintings. George Seurat's real paintings are much better than these. And his name is better when spelled write.

I'm still developing the following one. On some Browsers you have to reload to get the display to update:-) [ test.Surat4.html ]

The Crispy Program in Color

1. Turn the Crispy application into an applet... with lots of Snap/Crackle/Pop in random places and different colors on the screen. [ Entropy and Randomness ] and the ColoredText objects [ ColoredText.html ] and [ ColoredText.java ]

   Chaos

   Here is a variation of the [ test.Henrici.html ] technique that shows the iteration of a 2 or 3 dimensional map that produces some chaotic type shapes, one dot at a time: [ test.IterMap.html ] (this may not work on all browsers.....). Download and test. If it works try out some changes to the code to see what happens. Look at the comments for ideas.

. . . . . . . . . ( end of section More Java) <<Contents | End>>

. . . . . . . . . ( end of section CS320: Java Applets) <<Contents | End>>

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