Regardless of whether it's necessary or not, it's kind of expected knowledge to have as a programmer. Around here you'll find a lot more anti-Java sentiment than in most places (and most of it is probably justified), but I still doubt you could find many expert hackers here that don't know how to code Java. Everyone hates it because everyone has to use it from time to time, kind of like C.

At the moment, the ten most popular languages are (according to http://www.tiobe.com/index.php/content/paperinfo/tpci/index.... and whatever metric they use):

1) Java

2) C

3) PHP

4) C++

5) (Visual) Basic

6) C#

7) Python

8) Perl

9) JavaScript

10) Ruby

Given that Java is #1 on that list, I'd find it hard to call some time spent learning it wasted, you're just going to run across it too much, at conferences, in papers, in code that you inherit, in libraries you have to interact with or debug, etc.

Besides, if you know C or C++ it won't take very long to learn the syntax, and the libraries are worth learning because you'll use them in any JVM language anyways. If you don't know C/C++...well, um, add another one to your list, every programmer should have at least a basic understanding of some flavor of C for the same reasons they should know some Java.

If you do start with Java, make sure you actually use Java tools for the job, though, at least once you have the basics down; if you're coding Java in a text editor, even a good one, you're making things far more difficult than they need to be, because it's a language where tools are absolutely crucial if you don't want to drive yourself crazy. Whether it's Eclipse, Netbeans, or IntelliJ, you haven't really learned Java if you haven't learned how to use one of them well.