Read/Write Jar files?

This is really a newbie question, but since there's a forum specifically for jar questions I figured it was better to start here and move it to the newbie section if y'all deem it appropriate.
I have written a desktop application using Java 1.4.1 class libraries and intend the application to run on multiple platforms including Mac OS X, Windows XP and Linux. The application works standalone (not yet jarred) on the machine it was developed on, but now it's time to begin figuring out the distribution method.
Right now, the application reads several files from a flat text-file database, allows the user to peruse and display the information in a variety of ways, and gives the user a method to add to the flat-file database as needed. The file is about a megabyte and is excerpted into memory at initialization, then not referred to again unless the user writes additional data. When the user generates new data, write traffic to the file is fairly light, maybe 2-3kbytes per session.
I searched the forums for the best way to handle read/write data files for distribution and so far I haven't found anything that seems relevant, but surely the question must have been asked and answered before - maybe I'm using the wrong keywords?
Anyway, I have three basic questions:
1) Can I both read and write a file that's enclosed in a jar file? Or are files read-only once "jarred"?
2) Assuming I can both read and write a file within my jar file, is reading and rewriting within the jar so inefficient as to make that a non-preferred approach?
3) How do other folks who have a local read/write datafile in a desktop application deal with distribution? Keep the read/write datafile within the jar? Make a copy of it outside the first time the application is run and always read/write the copy outside the jar? Or some other strategy?
Thanks for any suggestions you can give a newbie at the Java game.
Jon

Thanks, that was pretty much what I suspected.
I have several data files and configuration files for this project, so I was trying to make the distribution as clean as possible. At least some of the config information can be hidden in Preferences, but I was struggling with the data files. I'll include the data files in my jar, then unpack them to the user's directory when launched the first time and work with them thereafter in the user's directory.
Enjoy the Dukes!
Jon

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