Fast Answers to Frequently-Asked Questions

Answer : If you are running netscape on a PC, then you can simply move the
         cursor to the background, and click on the right button. A small
         window will appear asking where you want the background image to
         be saved.

         Otherwise, you can retrieve a background color pattern directly
         from the Netscape cache (the cache stores images and patterns
         locally on your computer, thereby enhancing performance).

         The cache can be accessed by typing

         about:cache

         in the "Location" window of Netscape.

Question 13 : How can I put a counter on my home page?

Answer : Free counters are available at:

         http://counters.qpt.com/

Question 14 : I can't send E-mail from my Netscape Browser. What's the problem?

Answer : First, check your html code to make sure you haven't made a
         silly typo. The html should look something like:

         E-mail : <a href="mailto:your-login-name@glue.umd.edu"><b>
                                  your-login-name@glue.umd.edu </b></a>

         If your E-mail still doesn't work then perhaps
         your Netscape browser settings are incorrect.

         To check your mail preferences go to the Netscape Options
         menu and button on "Mail and News Preferences". Then select
         the services page. The SMTP (simple main transfer protocol)
         setting for outgoing mail should be

             smtp.glue.umd.edu

         for glue users, and 
         
             smtp.isr.umd.edu

         for users at ISR.

Question 15 : How do I get a domain name for my new company?

Answer : You can register a domain name for your company by contacting
         Network Solutions Inc. See

              www.internic.net.

         I believe the fee is $100/yr.

         Nothing happens physically? All this means is that some nameserver
         in the Internet world will contain an entry for your domain and
         point it to some IP address. If there is a machine on the Internet
         with that IP address, then all packets will be forwarded there.
         Otherwise, nothing will happen.

         Do I need to purchase a server and have it on for 24 hours/day?

         No. There are several companies out there that can house your
         domain for you for a fee. However, you are free to do so if you
         wish. 

         For more information, see the internic web site

         www.internic.net

         and the FAQ at

         http://rs.internic.net/domain-info/registration-FAQ.html

Question 16 : What is a script file? How do I create one?

Answer : A script file contains a copy of all of the commands
         given at the keyboard and all of the output sent to 
         the computer screen.

         The syntax for starting a script file is:

             prompt >> script name-of-script-file

             ..... now go ahead and run your programs. A copy
                   of the commands and program output will be
                   echoed to file "name-of-script-file"
         
         The script file is terminated by typing

             prompt >> exit

         Now send the "name-of-script-file" file to the printer.

Question 17 : What colors can I use English names for in HTML?

Question 18 : From where can I download NetTerm?

Answer : Try ......

         http://www.netterm.com/html/downloads.html

Question 19 : How do I make acrobat pdf files?

Answer: The isr has a daemon running that automtically makes pdf
        files. To use it, you log into an isr machine, and ftp the
        postscript file that you wish to convert to your account.
        Then copy the file to the directory

         /software/isrwww/html/misc/tools/acrobat/IN

         Wait for a few minutes, and the postscript file that you
         copied should appear along with a pdf file in the directory

         /software/isrwww/html/misc/tools/acrobat/OUT

         Move the pdf file and the postscript file out of the OUT
         directory to the place in your own homespace where you want
         to put them.

Question 20 : What info do I need to access WebCT?

    The Web Courses Tool (WebCT) at Maryland operates as a secure
    web server. The web address is:

        https://www.courses.umd.edu/

    Students use their SID number and their MARS logon password
    to get onto the page.

Question 1 : What are some of the key differences between C and Java?

    No preprocessor  -- Java does not include a preprocessor
    and does not define any analog of C's #define, #include, and so
    forth.  Program constants are replaced with

       static final data_type = data_value;

    for example,

       static final double dPi = 3.1415926;

    No global variables  -- Java defines a very clean namespace.
    Packages contain classes, classes contain fields and methods, and
    methods contain local variables.

    Well-defined primitive type sizes  -- Unlike C, all of
    the primitive datatypes in Java have well-defined sizes.

    No pointers  -- Java classes and arrays are reference types
    and references to objects and arrays are akin to pointers in C.
    However, in Java there is no address operator or mechanism to
    increment or decrement a pointer (i.e., do pointer arithmetic).

    Garbage collection  -- The Java virtual machine performs
    garbage collmection so that Java programmers do not have to
    explicitly manage (e.g., release memory to the operating systems)
    memory used by all objects and arrays.

    Method overloading  -- Java programmers can define multiple
    methods with the same name, so long as the methods have different
    parameter lists.

    Variable declarations anywhere  -- C requires local variable
    declarations to be made at the beginning of a method or block.
    Java allows them anywhere in the method or block.

    For more information, see the Nutshell book (2000).

Question 2 : How has Java changed from versions 1.0 through 1.3?

Answer : During the past five years (1996-2001) four versions of
         Java have been released. The key features in each release
         are as follows:

         Java 1.0  -- The first public release of Java contained 212
         classes organized into 8 packages.

         Java 1.1  -- The second release doubled the size of the Java
         platform to 504 classes organized into 23 packages.

         This release introduced the delegation event handling model
         and inner classes, leading to significant improvements in
         performance of the Java virtual machine.

         Java 1.2  -- The third release tripled the size of Java to
         1520 classes organized into 59 packages.

         This release is also called the Java 2 Platform.

         Java 1.3  -- Minor updates to Java 1.2 release (e.g., includes
         support for Java Sound as part of the Java Platform).

         This release is also called the Java 2 Platform, version 1.3.

Question 3 : What editions of Java exist?

Answer : SUN is working to create different versions of the Java platform
         for different uses. In this class we will use:

         -- The Standard Edition (details shown in previous question).

         Several versions are in development:

         -- Java 2 Enterprise Edition (J2EE) for Enterprise developers of
            the Java Platform.

         -- Micro Edition for consumer electronics, handheld PDAs and 
            cellular telephones.

Question 4 : How do I access the Java 1.1 and Java 1.2 compilers?

Answer : To access the Java 1.1 compiler on the Glue system type

         >> tap oldjava

         To access the Java 1.2 compiler type

         >> tap java

         Access to Java 1.0 is no longer provided. 

         Java 1.3 has been installed in the PCs in the Engineering
         labs. We hope that version 1.3 will also be installed on
         the glue system, Spring Semester 2001.

Question 5 : How do I use the just-in-time compiler?

Answer : One problem with Java bytecodes is their slow execution
         by Java virtual machines. A promising advance is just-in-
         time compilation, where Java bytecodes are converted 
         on-the-fly into native-platform machine code (see, for
         example, SUN's hotspot technology).

         Add details on compilation ....

Question 6 : What is the Java Runtime Environment (JRE)?

Answer : The Java Runtime Environment contains everything needed to run a
         Java program, but does not software needed to compile Java source
         code.

         One version of the JRE is implemented as a Java-plug in, designed
         to be connected to Netscape and Microsoft IE browsers.

Question 7 : What are the basic data types supported in Java?

The Java language has eight basic data types:

Question 8 : How do the keyword modifiers work in Java?

Answer : Java uses keyword modifiers to adjust the scope and storage
         of program elements in much the same way as C.

         However, the details of implementation are quite different...
         as shown the the table.

Question 9 : What is a Java package?

Answer : A Java package is a named collection of classes (and possibly
         sub-packages.

         Packages allow for the grouping of related classes and the
         definition of a namespace for the classes they contain.

Question 10 : What are the Java packages I will most likely use in this course?

We will use packages associated with Java language, utilities and networking (e.g., downloading images/data over the web), the abstract windowing toolkit (AWT), and Swing.

Java Language, Utilities and Networking

Abstract Window Toolkit (AWT)

Java Swing

To import all of the classes within a package into a Java program, simply append ".*" to the package name. For example, the applet classes can be imported with the statement:

    import java.applet.*;

If you don't want to import all of the classes within a package, then simply name the classes you want in the import statement. For example, the statement:

    import java.net.URL;

imports a set of classes (contained within java.net) that can represent a uniform resource locator (URL).

Question 11 : What are the Java Foundation Classes (JFC)?

Answer : The production version of the Java Development Kit 1.2 (also
         called Java 2) was released in December 1998. The JDK 1.2
         contains a sophisticated set of graphics and user interface
         application interfaces called the Java Foundation Classes (JFCs).

Question 12 : What is the relationship between AWT, Swing and the JFCs?

Answer : The Java Foundation Classes contain five technologies:

         Accessibility API  -- Assistive technologies provide disabled
         people with additional assistance in using the user interface.

         Examples of assistive technologies include: speech recognition
         systems and screen magnifiers.

         Abstract Windowing Toolkit and Swing  -- The Abstract
         Windowing Toolkit (AWT) was launched with Java 1.0 and remains
         a key part of the JFC.

         When you write a user interface using Swing components, you still
         need to use the layout managers and event models supported by AWT.

         Java 2D Graphics API  -- The 2D Graphics API supports advanced
         2D Graphics and imaging.

         Drag and Drop  -- On native platforms supporting drag-and-drop
         (e.g., Windows 98), this technology provides drag-and-drop services
         between Java programs and native applications.

         Two of the key features of Swing are:

         Lightweight Components  -- Starting with the JDK 1.1, the AWT
         supports lightweight classes, meaning that the AWT classes do not depend
         on the classes of the underlying native system. Indeed, in Swing most
         of components have their own view supported by the Java look-and-feel
         classes.
 
         Pluggable Look-and-Feel  -- This feature enables a user to switch
         the look-and-feel of Swing components without restarting the application.

        For more information, see Pantham (1999).

Question 13 : Why should I upgrade my Java graphics from AWT to Swing?

Answer : The biggest difference between the AWT components and the Swing
         components is that the later are implemented with absolutely no
         native code. As a result, the Swing components are not restricted
         to the least common denominator (i.e., features common to every
         platform on which Java runs), and they can have more functionality
         than the AWT components.

         Examples of expanded functionality for the Swing components are
         as follows:

         1. New components for tables, trees, sliders, progress bars,
            internal frames and text components.

         2. Swing buttons and labels can display images instead of or in
            addition to text.

         3. Swing components don't have to be rectangular (e.g., Buttons
            can be round).

         4. You can easily add or change the borders drawn around most
            Swing components.

         5. The behavior or appearance of a Swing component can be easily
            changed by invoking methods on it or creating a subclass of it.

            For example, Swing components can have tooltips placed over
            them (a tooltip is a textual popup that momentarily appears
            when the mouse cursor rests inside the component's painting
            region).

         For more information on this topic, see Walrath (1999).

Question 14 : Are the AWT and Swing components designed to work together?

Answer : No.

Question 15 : What is the model-view-controller (MVC) architecture?

Answer : The swing components are based on the model-view-controller (MVC)
         design. Components of the MVC models are as follows:

         Model  -- The model encompasses the state data for each
         component.

         View  -- The view refers to how each component will appear
         on the screen.

         Controller  -- The controller is the portion of the user
         interface that dictates how the components interact with events
         (e.g., keyboard events, mouse clicks). The controller in each 
         component knows what events require an action and which events
         can be ignored.

         Each element in the MVC model requires information from the other
         elements to function properly. For details, see the adjacent
         figure.

Question 16 : How does the MVC architecture work in Swing?

Answer : The Swing components actually make use of a simplified variant
         of the MVC model called model-delegate. This design combines
         the view and controller objects into a single element that 
         draws the component to the screen and handles GUI events know
         as the user interface (UI) delegate.

         Bundling of the graphics capabilities and event handling in
         Swing components makes sense in Java because most of the event
         handling is taken care of by the AWT.

         A second purpose of the UI delegate is to react to various events
         that propagate through the component.

Question 17 : What is a JAR file and how do I use them?

Answer : A Java archive (JAR) file is a ZIP file that contains Java class
         files, auxiliary resource files required by those classes, and
         optional meta-information (e.g., a list of files contained within
         the JAR file).

         JAR files are created with jar, the Java archive tool.

Question 18 : What is a JavaBean?

Answer : The JavaBeans API provides a framework for defining reusable, embeddable,
         modular software components that can be manipulated visually in a 
         builder tool. Beans are characterized by the properties, events and
         methods they export.

         The most common use of beans is for graphical user interface components,
         such as those in the AWT and Swing packages. There are, however, no 
         limitations on bean complexity -- for example, embeddable spreadsheet
         applicaion programs have also been implemented as JavaBeans.

         For more information, see Flanagan (1999).

Question 19 : How can I run an applet?

Answer : The simplest way of running an applet is with the appletviewer.

         An appletviewer reads or downloads the one or more HTML documents
         specified by the filename or URL on the command line. Next, it
         downloads any applets specified in any of those files and runs
         each applet in a separate window. If the specified document or
         documents does not contain an applet, the appletviewer does nothing.

         In Java 1.1, the appletviewer recognizes applets specified with
         the <APPLET> tag. A simple implementation might look like:

            <html>
            <body>

            <applet code = "SimpleExample.class"
                    width = 200 height = 200
            </applet>

            </body>
            </html>

         Suppose that these markup tags are contained within a file
         called simple.html. To run the appletviwer, simply type:

         appletviewer simple.html

         This example assumes that SimpleExample.class file is located on
         the same machine as the appletviewer. The applet is displayed in
         a window having dimensions 200 by 200 pixels. More complicated
         implementations download the class file over the net and initialize
         the Java applet with parameter values...

         In Java 1.2 and 1.3, the appletviewer also recognizes applets specified
         with the <OBJECT> and <EMBED> tags. The java code page
         for this class contains numerous examples of these tags.

1. Campione M., Walrath K. and Humi A., The Java Tutorial, Third Edition , A Short Course on the Basics, Addison-Wesley, 2001.
2. Flanagan D., Java in a Nutshell 3rd Edition, O'Reilly, 1999.
3. Gutz S., Up to Speed with Swing , Second Edition, Manning, Grenwich, 2000.
4. Pantham S., Pure JFC Swing , SAMS Publishing, 1999.
5. Walrath K. and Campione M., The JFC Swing Tutorial : A Guide to Constructing GUIs, Addison-Wesley, 1999.

Developed in January 1997 by Mark Austin
Copyright © 1997-2002, Department of Civil Engineering, University of Maryland