We use diagrams throughout the book to illustrate important ideas. Some diagrams are informal, like a screen shot of a dialog box or a schematic showing a tree of objects. But the design patterns in particular use more formal notations to denote relationships and interactions between classes and objects. This appendix describes these notations in detail.
We use three different diagrammatic notations:
A class diagram depicts classes, their structure, and the static relationships between them.
An object diagram depicts a particular object structure at run-time.
An interaction diagram shows the flow of requests between objects.
Each design pattern includes at least one class diagram. The other notations are used as needed ...