- 16.1 Applying UML: Common Class Diagram Notation
- 16.2 Definition: Design Class Diagram
- 16.3 Definition: Classifier
- 16.4 Ways to Show UML Attributes: Attribute Text and Association Lines
- 16.5 Note Symbols: Notes, Comments, Constraints, and Method Bodies
- 16.6 Operations and Methods
- 16.7 Keywords
- 16.8 Stereotypes, Profiles, and Tags
- 16.9 UML Properties and Property Strings
- 16.10 Generalization, Abstract Classes, Abstract Operations
- 16.11 Dependency
- 16.12 Interfaces
- 16.13 Composition Over Aggregation
- 16.14 Constraints
- 16.15 Qualified Association
- 16.16 Association Class
- 16.17 Singleton Classes
- 16.18 Template Classes and Interfaces
- 16.19 User-Defined Compartments
- 16.20 Active Class
- 16.21 Whats the Relationship Between Interaction and Class Diagrams?
16.17 Singleton Classes
In the world of OO design patterns, there is one that is especially common, called the Singleton pattern. It is explained later, but an implication of the pattern is that there is only one instance of a class instantiated—never two. In other words, it is a “singleton” instance. In a UML diagram, such a class can be marked with a ‘1’ in the upper right corner of the name compartment. See Figure 16.17.
Figure 16.17 Showing a singleton.