How to make apples from oranges in UML

Unified Modeling Language (UML) provides various diagram types for describing a system from different perspectives or abstraction levels. Hence, various UML models of the same system are dependent and strongly overlapping. This paper discusses various general approaches and viewpoints of model transformations in UML. The possible source and target diagram types are analyzed and categories are given for different transformations. It is argued that such transformations should be defined in terms of the UML metamodel, rather than on the level of the actual diagrams. A detailed example of a transformation operation from sequence diagrams into class diagrams is presented to illustrate such operations. It is concluded that the transformation operations can automate a substantial part in both forward and reverse engineering. These operations can be used, for example, for model checking, merging, slicing and synthesis.