I think Gavin Bierman is an unsung hero as far as steering the Java language is concerned. I had the privilege to sit next to him in the office when I was working on TruffleRuby, and our conversations on language design were always elucidating, and his ability to gently steer others away from pitfalls was something I wish I could do as well.

Hearing the work he and others did to gradually introduce pattern matching without painting themselves into a corner was fascinating and inspiring.