Model transformations constitute the key technology for model-driven software development, a software engineering discipline which became more and more important during the last decade. While tool support for unidirectional batch transformations is rather mature, bidirectional and incremental transformations are only weakly investigated. Nevertheless, several usage scenarios demand for incremental and bidirectional transformations, like round-trip engineering between UML class models and Java source code. This paper presents a bidirectional transformation between UML class models and a Java model which is obtained from Java source code. The transformation is written in QVT Relations, a declarative model transformation language provided by the OMG. While the case study demonstrates that it is possible to specify bidirectional transformations between heterogeneous metamodels in a single relational specification, it also reveals some inherent limitations of the language and the corresponding tool support.