• A detailed UML guide

• A UML tutorial

• Links to UML-related sites

• How to model the UNION (category) entity types in EER by using UML?

The hint is to use an abstract class with multiple "solid" sub-classes.  See the following Web pages for further information:

UML Abstract Classes and Interfaces:

• A detailed UML Guide at http://www.ifs.univie.ac.at/UML/ECOOP99/ , esp. slide 46 and slide 69.

• UML Tutorial for Java: Chapter 6

• UML Introduction , with some examples of abstract classes  

• Logical Model - The Class Model