This article discusses the issues that have arisen as we have updated a course for undergraduates on computers and physics developed at the University of Chicago by Leo Kadanoff, Michael Vinson, Amy Kolan, and Marcelo Magnasco. This course uses interactive computation to teach concepts in nonlinear dynamics, and, more generally, scientific hypothesis testing. Changing the computer language used from THINK Pascal to the object-oriented programming language Java has led not only to "cosmetic" changes (an increased emphasis on animations), but also to conceptual changes in the course (expansion of the discussion of the concept of universality).