No, a few weeks (or even days) should be enough. If you don't use all that fancy object oriented stuff and the huge library (most of which will never be of use in the ICPC), then Java code looks pretty much like C++ code.
You should start learning the library with the classes in the package java.io (Input/Output), particularly BufferedReader, StreamTokenizer and Printstream.
Then, I'd suggest you look at the package java.math, with its classes BigInteger and BigDecimal.
Also very useful is stuff in java.util, especially the classes Arrays, Collections, Dictionary, Hashtable, Treemap, Treeset, Vector and StringTokenizer.
Oh, I forgot. Of course the very first package you should look at is java.lang, which contains all the basic types.
Look at the API specification at http://java.sun.com/j2se/1.4/docs/api/index.html
It really shouldn't take too long to learn this most useful stuff, but as I said, the BigInteger is probably the only reason you might want to learn Java if it's only for the contest. On the other hand, if you ever want to build a web applet or work for a software company, then knowing Java could be an advantage anyway.