This may be a problem if your truck breaks down in the desert, your 3g is out of range and you never thought to install an ide beforehand but apart from that when would you find yourself writing java without an ide?
Edge case. I develop on my own machine (That said, I absolutely hate the verbosity of Java and love Nods.js exactly for this reason).
When was the last time any professional software developer was forced to do any real development on someone elses computer? In that case it is likely that one will miss his keyboard, desktop shortcuts, shell aliases, browser bookmarks much more than tour ide.
So you have never been asked for help from a junior developer whose development setup is different than yours?
Help in this context means discussion, advice, him showing me what he had done, me suggesting quick solution, or talking to him what to type, similar to pair programming. If _I_ had to actually write more than few lines of code, we'd just switch to my computer.
We have a standard IDE set-up for our team and require more junior devs to use it -- a least when starting out. It makes pairing with them easier.
The junior developer who insists on not using an IDE working with Java is precisely the one who needs the most help.