As people have already listed a flavor of Lisp/Scheme would probably give you the biggest bang for your buck on changing how you think when programming. And tha

As people have already listed a flavor of Lisp/Scheme would probably give you the biggest bang for your buck on changing how you think when programming. And that perspective will translate back to any language you use. Since you're already familiar with Java, Clojure is likely the best bet for you as it builds on top if it. Otherwise the slightly easier to learn but a bit less real world practical is Racket Scheme.