CS 631 FAQ

Joseph Bergin
Pace University


1. What kind of software is to be used to draw sequence and class diagram?

You can use any drawing program that you like. There are UML tools that you can get if you want to go fancy. You can go to http://rational.com to get a trial version of rose.Look at http://argouml.tigris.org for a different tool. Some of the diagrams done in the past with Rose were too fancy and hid the information in a flood of detail. You need to do use cases, CRC cards (not part of uml), class diagrams, and sequence diagrams. The class diagrams can be simple. They don't need to show all class detail (especially the instance variables). This is a small part of UML.

2. What needs to be done after the project scope document is approved?

If you have been approved to continue then your next task is to analyze and design your system. Start with use cases and develop them for all of the interactions with the system. I will create a page on which you can see each other's work as it is developed, but to make this work you need to post your work on your project pages and tell me when you have made changes. I will comment as we go along on things. After use cases you can start on CRC cards. You can do something like what we did in class (coffee machine). Once you have CRC cards you should walk through your use cases to see that the classes you have will support your use cases and that the communication is simple and clean. From the CRC cards you can develop class and then sequence diagrams.

Put up the use cases as you build them. Same with CRC, etc. If you give it all to me for the first time at midterm I will take too long giving you feedback and ultimate approval to continue.You can learn a bit more about use cases on the web at http://c2.com/cgi/wiki?UseCase. You can also look for http://c2.com/cgi/wiki?UserStory which is the XP (see below) equivalent. There is a lot of stuff about use cases at http://www.crim.ca/hci/indiv/hayne_uc/

3. When should we learn Java?

The midterm will require that you can write Java code so you need to be sure you learn it in the first half of the course. You should also explore internet programming in Java since the midterm may be an internet program (probably with tcp/ip sockets). See Core Java (Horstman) and http://csis.pace.edu/~bergin/InternetProgramming.html

4. When should we begin programming our project?

You need to develop the analysis (Use Case) and Design (Class diagrams, CRC cards, and Sequence diagrams) first. Programming comes after midterm.

5. What is a good place to learn about Object-Oriented Design.

Two good sources are the CRC Card Book (Bellin...) and Designing Object Oriented Software (Wirfs-Brock...). Another book is Hunt's Java for Practitioners, though this focuses more on programming than design. There are many more resources on the course "Student Task Page" (STP). Note that although the techniques are different the quality criteria are the same in OO as in structured design: cohesion, encapsulation, loose binding, thin interfaces...

6. What are design patterns?

Design patterns are a tool for capturing expertise about design and for improving design. For an introduction to patterns look at http://www.enteract.com/~bradapp/docs/patterns-intro.html You can also use the Wiki Web: http://c2.com/cgi/wiki Look especially for their find page. The wiki is the virtual meeting place of the patterns community, actually. It will have information about many aspects of this course including Extreme Programming.

7. What is Extreme Programming (XP)?

Extreme Programming (or XP) is a simple methodology for teams of up to about 10 members on projects of up to about 100 person years. It differs radically from most others. Another similar one is called Scrum. Look on wiki. There is a good intro at http://c2.com/cgi/wiki?ExtremeProgramming. See the book Extreme Programming Explained by Kent Beck. There is also a site devoted to XP http://www.xprogramming.com/ It has a lot of information including an introduction to XP. Note that XP focuses on the implementation part though, not the design part so it is for later in the course. Scrum Methodology is discussed at http://c2.com/cgi/wiki?ScrumMethodology and on their home site: http://www.controlchaos.com/

8. There is a lot to do in this course. How should we use our time wisely?

You have a lot to learn and a lot to do. Take the books I've suggested and parcel them out to your team so you can each develop an outline of the important ideas in one book and share them with each other--in your group and outside. Use each other and your teams to lessen the load. Again, if you have a team you can have different people investigate the things I've suggested you read and report back to the team. You could even organize across teams to do this. Note that I'm treating you like professionals here, not like students. You can/should help each other learn. You could post summaries of what you learn in reading to one of your web sites or send it to me to post. That would help you learn more since you will be writing as well as reading and it would help the others too. What they do will help you. Share out the work.

You can also have a team member serve as a consultant on some aspect of your project, investigating it in depth and reporting to the others. So, one member might focus on GUI stuff and another on RMI or TCP sockets, for example.

9. How important is reuse in designing for OO? Do we need to consider this for the project?

OO programmers/designers are learning that reuse is overrated. A much more powerful benefit of OO is the the fact that it enables piecemeal growth. Build a small system that works and then add to it to get a big system. This is the XP way, actually. It has been said that no LARGE project has ever been succesful if built in one go. All successful LARGE projects grew out of well designed small projects. There is a LOT of evidence for this in the telecom industry in which they depend on hundreds of millions of lines of code. This is why Star Wars and its replacements cannot be successful. They are huge and there is no way to build them incrementally. The wiki has this page: http://c2.com/cgi/wiki?PiecemealGrowth

I will suggest that after midterm you build the implementation this way. First build a small part of your project that is still end to end. (client-server-back end) and then add features to it. You will then always have something to show even if you come to the end of the course before you come to the end of the project.

I will not be judging your project on how well you design for reuse, but I will require that you do reuse the available resources, like the Java libraries.

10. How does the midterm exam work?

I will distribute the exam by email. You will send me your solution by email after one week. It will be a Java program to write, probably a simple distributed program using GUI elmements and TCP sockets. The midterm is strictly individual work. You may obtain no help from any source other than books and the instructor. Any evidence of collaboration will be dealt with harshly. Sorry.

11. Why do we each have to participate in the team presentations at Midterm and at the End?

You need to learn to to presentations in front of people to be successful in this field. If you don't the marketplace will penalize you horribly. The university provides a safer place to get the practice and get your feet wet as we say. One way to get really good practice is to rehearse your presentation before you give it. Capture it on video tape if you can and then review it.

You can use powerpoint if you like in your presentation or use html presentations for graphics. Be sure that things are nicely visible, however. Everyone must speak at the presentation and everyone must give some significant part of their project if they want a decent number of points. I will be grading your oral presentation primarily on your presentation skills and less on content. There will be a content grade on the written work. However, if you are boring as you speak and only give boring information, this will affect your grade here.

Each person should cover only a part of the project, not repeat over things done by their teammates. The purpose is so that everyone in the class can understand everyone else's design. This will make mutual help possible later. All your life you will be making presentations to people who need to understand what you are doing. Mostly so that you can continue to get paid.

12. What is a good place to learn about Client/Server and Distributed Computing?

The STP has many resources including books and web resources. You can refer to: Java Distributed Programming," Jim Farley, O'Reilly & Associates, 1998. The wiki has this: http://c2.com/cgi/wiki?DistributedComputing

13. Does my communication topology need to be a star (centered at the server) or can it be more general?

It can be more general. In RMI, for example it is relatively easy to use a generalized interconnection topology. In effect, each remote object is both a client and a server. An RMI application looks like one large application and hides the fact that it is distributed. If you have a reference to an object local or remote you can send it messages in the same way. It doesn't matter where it is. If each "client" has a list of references to all the other "clients" It could talk to each of them directly and wouldn't have to relay communication thru a server (in RMI)

However, if your application needs a database to store information persistently, a star topology may be the most useful since all communication goes through the server which can capture it for the database. Otherwise you may need too much redundancy to be sure information isn't lost.

14. Can we use "found code" in our project?

If you find code on the net or in a book that will help you may use it with permission of the instructor provided that the owner of the code also grants permission and you cite all uses explicitly.

15. How do I get a Button inside a Panel in my GUI? Do I need inner classes for this?

Actually you don't need inner classes for this. Distinguish between inner classes which is a programming concept and containment of GUI elements which is a human factors design concept. A panel is a Java Component that can contain other Components. The design pattern is called Composite. You send the add message to the Panel giving the button as param. Where it shows up depends on the layout manager of the panel and the form of the add message.

Inner classes on the other hand both control name spaces and provide a handy way to interlink tightly related objects. An object of a (non-static) inner class has an automatic hidden link (reference) to a specific outer class object. This is what makes inner classes so nice for listeners. The listener has a link to the GUI it was created within as well as to the button or whatever it is listening to. The link to the button is done explicitly (addActionListener...). The other is implicit.

16. Should we use the Event pattern "Component Listens to Itself" primarily?

No. You can use it, but it has a down side. One Component per Listener is much more flexible. Component Listens to Itself is implemented by subclassing the component and also implementing one of the listener interfaces. This binds two kinds of code together that should usually not be bound.

One of the objections made to Component Listens to Itself is that the gui (display) and the actions performed on the "model" should be separate. Read the stuff on model-view-controller for a look at what the separation should be. Your application minus any gui is the model. The look and feel of the screen (gui) is one or more models. And the listeners are the controllers (MVC). All separate.

If two components need to behave in exactly the same way (as a "business rule") then they can/should share a listener. The pattern is "Say it Once". If Listeners can be parametrized so that a set of components can be created to work with the same Class of listeners then you don't need as many listener classes as you need active components, but you need as many listener objects, generally. An example is in a calculator where a NumberButtonListener class can be used to listen to the 10 digit buttons. But you create 10 Listener objects from this class so that you don't have one object doing testing to determine which button it is handling an event for.

17. How do I handle "messages" coming from the client to the server that need to be distributed to all the clients?

The term "message" can mean two things. First is the message protocol of any OO system (send a message to an object ). Second is a distributed/telecom concept( send some information from one place (usually the client) to another (usually the server). Here we are talking about the second kind.

I have a chat room on my site that does this distribution. A chatroom works something like the following, and requires that the communication be thread safe (synchronized). A client sends a message to the server which catches the resulting "message" using a separate thread. One thread per client. This thread is responsible only for receiving information from the client.

When the message is received, it is passed using sync'd communication to a broadcaster object in the server, which queues the incoming messages from the many threads. All these threads are on the server. The broadcaster is in its own thread also. The broadcaster then sends the incoming messages from the queue to all clients.

The code is available at http://csis.pace.edu/~bergin/sol/java/coursefiles/ . Get the chat applet and the server. The interesting stuff is in the server.Make sure you get the 1.1 version of the applet. The other is very primitive java (jdk1.0).

18. How do we implement the message Queue in the above scheme?

You can use a LinkedList from java.util for this. It needs to be sync'd to work in a multi-threaded environment, however. There is a class in java.util called Collections that has a syncing "wrapper" for linked lists and other collections. Don't confuse Collection (root of the collection hierarchy) with Collections (lots of utility code for working with Collection objects.

In my chat code, the Queue is much simpler. It is just a single cell with sync'd access. If it is full and some thread wants to insert into it, the thread blocks until it becomes empty. This is probably sufficient for communication (slow) coming from humans unless there are thousands of them (like at Amazon.com).

19. What level of detail do we need to show in our Class diagrams? Inner classes? AWT code?

This depends on the level of detail needed to define your project. I will assume that you have used inner classes for each button, etc, so a higl level diagram is probably enough without inner classes. However, some projects may need some of that to show how it works. There is no reason why you can't have both a high level diagram and several lower level ones. They are supposed to help you and others understand what is going on, after all. The level of detail must be appropriate for that. A low level class diagram would be used for just some portion of your application, not the whole thing.

You may or may not need to show classes from the jdk in your class diagram. Again this depends on the level of the diagram and what is needed to make the design understandable. A low level supplementary diagram can be helpful to show details fo a part of the design.

20. How fancy do the "screen layouts" need to be in the initial design?

They don't need to be screen shots. They can be made with any draw program. They are just mockups in the initial design documents. It is "initial" design, after all. In particular, you don't build the GUI to then snap shot it to show in your design. The design better preceed the build.

21. Can we use Visual Cafe (etc) to build the GUI?

You can use these things as IDE's only, not as GUI graphic builders. Otherwise the code will be too ugly. Don't use any tool to draw your gui on the screen. Write it in Java. You can prototype it on the screen, of course.

You will suffer in grading if you give me code as ugly as produced by GUI tools.

Another problem with GUI tools is that if you start with them and then decide you want to modify their code and then later decide you want to go back to the GUI tool to change the look, it may wipe out your changes at the next save. They only permit certain edits and wipe the rest. Very frustrating.

22. In Java is there any advantage of declaring and initialising objects at the same time?

Yes, you can't forget to initialize if you do it as part of declaration. Much safer and definitely preferred. Do it the other way only when necessary.

23. Do we need to use Swing or can we use AWT?

Use what works. AWT is easier, but Swing has a nice design. Use AWT if you need to deploy Applets, however as Swing isn't well supported yet by browsers. For deployed applications, either will do. However, use one or the other. Don't try to mix them. That may not work correctly.

Last Updated: October 12, 2000