I "know" Java because I had to do many undergrad projects in it, but I don't advertise the fact that I know Java at all because I actively do not want to work with it. It is a language designed for incompetent replaceable programmers, and I find working with it sort of insulting in a way, and just not enjoyable in general.

Depends on your (or your student's) approach to employment, really. If you want any programming job at any cost, of course you should know Java. There are lots of Java jobs, though the flip-side to that is that the market for Java programmers is extremely flooded.

On the other hand, if you're more worried about actually finding "interesting" work, pitching yourself as a Java programmer might help more than hurt. I would personally rather stack boxes or something along those lines than work on some kind of enterprise payroll nightmare in Java. Some would not. As always, it depends.

That said, Java is pretty simplistic if you're familiar with any even vaguely similar language (which you certainly should be, since the overwhelming majority of code out there is written in them). There's really very little to "learn" anyway...