I use a lot of my (sometimes ancient) Java code with both Clojure and Scala. So far I have always written wrappers that copy Java data into Clojure or Scala 'native' data types. If you do this then there is little chance of having problems like those you are concerned about. It might be more efficient to use Java types, but not worth the hassle in most cases. (I also use (J)Ruby a lot, and I have found the secret to happy use of (J)Ruby is to give up the desire for good run time performance :-)