Java on the desktop: Which paths to use?

  Applications (like a word-processor) sometimes need to store
  or access files. To help users with little computer
  experience, there should be safe defaults for the paths,
  because not all users can find the best paths themselve.

  One can not assume that a Java application is running under a
  type of UNIX/Linux or Windows, so one only is allowed to rely
  on the standard methods and classes of Java SE, not to use
  code for specific host operating systems (at least, assume
  this for this question).

  How is the path to store the following type of files determined
  within such a Java applicataion?

    - User documents (the user can choose any path, but the
      application should suggest a reasonable default)

    - read-only data (The word-processor might use a file with data
      for a spell-checker that is not user specific)

    - User specific program configuration (like the background
      color of program's main window or user-specific extensions
      for the spell-checker)

    - log files (The word-processor keeps records of who edited which
      document at which time), this might be inspected by the user

    - debug log files (The word processor writes reports about
      abnormal program endings to a log file that is not intended
      to be read by users, but can be send to the manufacturer of
      the program as a bug report)

    - backup files (The word-processor keeps some old versions of a
      document, so the user can restore them later)

    - databases (The words processor supports sending form letters
      and can be used to maintain a database of customers, which
      eventually is stored in a file)

    - (meta) user specific settings for all of the above:
      The user can overwrite the defaults for all of the
      above paths, and these overwrite settings needs to be stored
      somewhere, too.

                               ~~

  A footnote to anticipate answers involving ?user.home?:

  The documentation says this:

      ?This set of system properties always includes values for
      the following key: [...]

      user.home User's home directory
      user.dir User's current working directory?

http://download.java.net/jdk7/docs/api/java/lang/System.html

  ?java.lang.System.getProperty( "user.home" )? might sound
  good, but gives ?C:\WINDOWS? on Windows 9x, which might not be
  the best place for user documents.

  It is not only broken on Windows 9x, but also on Windows NT:

      ?We made a conscious decision to change the definition of
      the "user.home" property on Win32; see 4100238 for more
      info. This is not a bug.?

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4162112

  (If Sun Microsystems, Inc. is free to make such a decision,
  but why don't they document it at the appropriate place, that is, in
  http://download.java.net/jdk7/docs/api/java/lang/System.html?)

  It also is broken on Vista:

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6519127

  And also Microsoft?'s VM does it wrong:

http://support.microsoft.com/kb/221206

  So we know now that there is a conspiracy among JVM producers
  to make ?java.lang.System.getProperty( "user.home" )? unusable.
  (When it can not be used on Windows, it can not be used in a
  portable desktop Java program generally, because this should run
  under /all/ supported platforms.)

  The question still is, what to use instead.

  I already had posted my idea for an answer recently myself,
  but would not like to mention this now, so that I can get
  your unbiased opinion about this.