Having gone from Ruby to JRuby, I think it's worth learning enough Java so you can truly benefit from the JVM platform. You may not write much Java, but you'll

Having gone from Ruby to JRuby, I think it's worth learning enough Java so you can truly benefit from the JVM platform. You may not write much Java, but you'll want to use all the wonderful libraries available to you. I do, however, know C++ well so Java isn't much of a problem. But it's not a difficult language, especially as it's designed to stop people making mistakes. (!)