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EOS Data Gateway Java FAQ

FREQUENTLY ASKED QUESTIONS


General Topics

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.

Problems running Java

Q: Why doesn't the applet run for me?
A: There are several possibilities:

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?
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.

Bottom line:
Netscape support for Java seems superior to Internet Explorer's for platforms other than the Mac and is recommended.
Mac users will find IE 4.01 more dependable than Netscape for Java.

General Applet Interface and Behavior

Q: Why are the applets in separate windows?
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.

Q: Why didn't you just make it an application instead of an 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: It seems to take a long time to update the map. Why?
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.

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?
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

Q: Why can't I enter the points for the latitude and longitude?
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.

Q: Why is there a small globe (which you can't do anything to) and a large globe as well?
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.

Q: Why does the system slow down when I use the applet a lot?
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.

Q: Hey, the applet just disappeared all of a sudden! Why'd it do that?
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.

Q: Why is the applet so spread-out looking, with lots of space between the buttons and fields and things?
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

Q: (Netscape Macintosh users) Hey, this applet just goes into a loop! Now what do I do?
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.

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: 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.