Battlecode 2016
battlecode.orgAre there any ongoing tournaments for this or similar? I'd love to get involved, but I feel like I wouldn't get the most benefit from this by starting from scratch so late in the game.
For Battlecode it's not too late to start. Plenty of people are still getting started. The competition lasts until January 30th and it's only the 4th day.
Why Java only?
Since computation is measured by bytecode cost of instructions, supporting other languages would require a great deal of effort to do similar bytecode counting and to make sure that the counting across languages is fair and balanced. They've considered doing other forms of computation measurement, but none of the alternatives are very good or deterministic (time, lower-level instruction counting, etc).
Also you can use Scala.