The wording of this question (as summarized in the headline) suggests professional negligence on my part for not having spent a year, five, or ten learning Java. I'd like to respectfully disagree.

Ruby is my primary programming language, but I've also built apps and scripts in Python, C, C++, Objective-C, PHP, and Scala.

I'm a firm believer that exposure to a wide variety of programming languages is a great way to experience new constructs, paradigms, and approaches to computer programming, which is why I've spent the past few years branching out. During this process, I've worked with a few languages built atop the JVM (Scala and JRuby), and thus learned a bit of Java, basic language structure / primitives, and some underpinnings of the JVM. I believe that making an effort to understand and employ these various platforms has helped me to become a better programmer.

It sounds as if you're coming from the perspective of an academic advisor. This surprises me, as your question is primarily career-focused. Had Java been forced on me in University, I would have left disappointed by my education and mentally prepared for five years as a junior developer in a cube farm working on obscure, broken internal HR software. Instead, exposure to this variety of languages, architectures, and concomitant development methodologies has taught me a lot about programming / software engineering, and opened my eyes to careers I might not have imagined otherwise.

Circling back to the original question, I can tell you that I live a happy, productive, fulfilling life as a software developer at a great company doing what I love without "knowing Java."