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EOS Data Gateway Java FAQFREQUENTLY ASKED QUESTIONS
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Q: Why Java?
A:
We've chosen to add Java applets to the EOS Data Gateway to enhance
its usability. Java provides better interactivity and a nicer user
interface than straight Web pages can.
Q: What if I don't have or don't want to use Java, or I can't get it
to work on my machine?
A:
We've retained the older CGI-only interface as well, to make sure that
users with older browsers, or with machines that aren't up to handling
Java, still have a way to access the features that Java handles as well.
You'll be able to choose a non-Java version of every page that uses Java.
Q: Where is Java being used in the EOS Data Gateway?
A:
At present (in version 2.1), as an option for making a
geographic selection, accessed through the "Geographic Region"
section of the Main Search Form, and as a replacement for the
previous multi-screen search criteria selection process in the
advanced search form.
Q: Gee, Java 1.1 has all these other neat interface things that I
don't see in the applet. Why aren't they there?
A:
Because the browsers that support Java only support 1.0.2. Sorry.
Q: Why doesn't the applet run for me?
A:
There are several possibilities:
Windows | Macintosh | Unix | |
Netscape | 2.0 | 3.0 | 2.0 |
Internet Explorer | 3.0 | 3.0 | n/a |
Q: Okay, so how do I find out what version my browser is?
A:
Choose one of the following, depending on which browser you're using:
Q: This applet doesn't seem to work right. What's wrong?
Q: Why are the applets in separate windows?
Q: Why didn't you just make it an application instead of an applet?
Q: It seems to take a long time to update the map. Why?
Also, embedding the entirety of the map database in the applet made it
very large and slow to load. We decided to trade off a slight decrease
in overall speed against a long initial delay in running.
Q: Why does the applet look different in different browsers and on
different systems?
Q: Why can't I enter the points for the latitude and longitude?
Q: Why is there a small globe (which you can't do anything to) and a
large globe as well?
Q: Why does the system slow down when I use the applet a lot?
Q: Hey, the applet just disappeared all of a sudden!
Why'd it do that?
Q: Why is the applet so spread-out looking, with lots of space between
the buttons and fields and things?
Q: (Netscape Macintosh users) Hey, this applet just goes into a loop! Now
what do I do?
Q: (Internet Explorer users) Hey, this applet doesn't seem to respond when I
try to hit a button or make a selection! What is wrong?
A:
Even though Java is supposed to be cross-platform compatible,
it is dependent on the browser manufacturers' implementing it
properly. There are a number of known bugs in the Java implementations
on various platforms; check the tables below to see whether or
not the problem you're seeing is the applet's fault or the browser's.
Windows NT (all browsers)
X-Windows Netscape
Windows 95 Netscape
Macintosh Netscape (all releases)
Macintosh Internet Explorer
Windows 95 Internet Explorer
Mac users will find IE 4.01 more
dependable than Netscape for Java. General Applet Interface and Behavior
A:
We considered a number of different implementations for the applets
when we were first designing them. We have found that using a separate
window for the applets allows us to minimize the memory and
resource impact of multiple uses of the applet.
A:
We wanted to provide the maximum integration possible of the existing
EOS Data Gateway software with new features. Making the geoselection applet
a separate application would have entailed more work for you as a user
(installing Java on your machine, installing the application properly,
etc.) and would have made it much harder to integrate the new function
into the existing gateway.
Q: Why does the map disappear and then reappear after a delay?
A:
We chose to have the maps generated "on the fly" and sent up from the
EOS Data Gateway machine. This allows us to leverage existing code and
to ensure that the map presentation is consistent across the application.
A:
Because the different browsers and systems chose different ways to
implement the Java Virtual Machine (the program that actually runs
the Java programs) and its interface to the underlying system. Each one
has its own way of displaying text, drawing controls, etc.
Geoselection Applet Interface and Behavior
A:
Because we would have to validate the shape that you entered and
display it effectively on the maps. We decided to not implement this
feature in the initial version of the applet because it is a much
more complex problem.
A:
The small globe acts as a reference point. If you zoom way in (4x or
more), a lot of the large globe isn't visible anymore, and it becomes
kind of difficult to see where you are. The reference globe pans in
unison with the big globe, so that whatever is centered in the large
globe's window is always centered in the small one.
A:
Each time you invoke the applet, it gets reloaded into the Java VM. When
you leave the page, the applet code that's already loaded hangs around.
When you access the page again, it gets reloaded. This causes the Java VM
to ask your browser for more memory. We've engineered the applet to
reuse and recycle as much memory as possible, but the browsers do seem to
keep part of it around each time it's accessed.
A:
Anytime you leave the applet geoselection page, the applet automatically
terminates itself, since you've told the EOS Data Gateway that you want to do
something else now. If you go back to the applet geoselection page, it
will reappear; anything that you did before leaving the page will be lost,
however.
A:
Because Java gives us (as programmers) relatively little control over
how we position these things inside the window.
Dependent Valids Applet Interface and Behavior
A:
Unfortunately, there seems to be a bug in the way that the Java virtual
machine - the part of Netscape that actually runs the Java code -
handles the selection list in the middle of the applet. We have not, so
far, been able to correct this problem. If you really want to switch to
using the Java dependent valids selection, you probably should switch to
Internet Explorer 4.01; it runs the applet quite reliably.
To get out of the loop, you can click on the browser window to bring it
to the top, and then click on the "Turn Java Off" link at the top of the
page. This will force the applet to quit running.
A:
The applet changes the cursor to a wait cursor and disables the frame when
it is posting to the server, but you can't see this and it looks like nothing is
happening. You need to download a newer version of the Microsoft Virtual
Machine for Java from http://www.microsoft.com/java.
Generated by EOS Data Gateway / Send us your comments
NASA/GSFC Task Representative: Robin Pfister, Robin.Pfister@gsfc.nasa.gov
Full contact information is available.