I love it!

And that is the reason I still write in Python or Lisp when I don't need something (Hadoop, Lucene, etc) from the JVM ;-)

I seem to remember this same feature causing lots of headache in C in the form of subtle bugs mostly the result of this being used in awkward ways.

Personally I'd rather have a distinct boolean value to test for, the java version is much more explicit as to exactly what its doing.

I have no idea what the annotations mean, but if we use exceptions for transmitting error conditions, then there is no point having a true | exception function.

    public void deleteStudent(int uid) {
      if (STUDENTS.remove(uid) == null) {
        throw new WebApplicationException(Response.Status.NOT_FOUND);
      }
    }

In that case it is preferable to use "or" over "||" since then you do not have to worry so much about operator precedence because "or" has about the weakest of all operators. This means in this case that you can skip the explicit ".new" for the exception.

  def delete_student(uid)
    STUDENTS.remove(uid) or raise WebApplicationException, Response.Status.NOT_FOUND
  end

In Ruby, you don't write code, code writes you! ;-)