Ruby, Perl and Python packages usually come with a README that says:

  This depends on these external packages: ...

Java programs usually come with a bunch of .jar files which were once independent packages, but have been dropped into the release itself. No dependency problems!

Then, if someone wants to package a Java application for Debian, the process is:

1. Look through the collection of .jar files in the release

2. Do you recognize one of these as already being packaged for Debian?

3. Work with upstream to delete that .jar from Debian's copy and depend on the system's version instead

4. Repeat for every other .jar in the release, until you hit a wall

5. Upload the package to Debian with an acceptably small number of bundled .jars

6. Time permitting, get someone to package the other .jars that aren't available in Debian yet

That said, you can cause a similar amount of trouble in other languages, it's just not the convention (thanks to the success of gems, CPAN, PyPI). For example, Ubuntu appears to have deleted the sagemath package because upstream keeps their own patched copies of dozens of libraries they depend on:

http://packages.ubuntu.com/search?keywords=sagemath&sear...