What's the matter with Java, exactly?

I've copied a jre directory on my stick, and configured OpenOffice to use it (in tools/options/java).

All OOo features relying on Java now seem to work properly.

I understand that if the stick later mounts as another drive letter, I might have to change the settings, but is that the only reason that makes java "non-portable", at least from OOo point of view?