The UML is a visual language (meaning a drawing notation with semantics) used to create models of programs. By models of programs, I mean a diagrammatic representation of the programs in which one can see the relationships between the objects in the code.
The UML has several different diagrams—some for analysis, others for design, and still others for implementation (or more accurately, for the dissemination, that is, the distribution of the code) (see Table 2-1). Each diagram shows the relationships among the different sets of entities, depending upon the purpose of the diagram.
Table 2-1. UML Diagrams and Their Purposes
When You Are…
Use the UML Diagram…
In the analysis phase
Use Case Diagrams, which involve entities interacting with the system (say, users and other systems) and the function points that I need to implement.
Activity Diagrams, which focus on workflow of the problem domain (the actual space where people and other agents are working, the subject area of the program) rather than the logic flow of the program.
Note:Since this book is principally focused on design, I will not cover Use Case Diagrams or Activity Diagrams here.
Looking at object interactions
Interaction Diagrams, which show how specific objects interact with each other. Since they deal with specific cases rather than general situations, they are helpful both when checking requirements and when checking designs. The most popular kind of Interaction Diagram is the Sequence Diagram.
In the design phase
Class Diagrams, which detail the relationships between the classes.
Looking at an object's behaviors that differ based upon the state that the object is in
State Diagrams, which detail the different states an object may be in as well as the transitions between these states.
In the deployment phase
Deployment Diagrams, which show how different modules will be deployed. I will not talk about these diagrams here.
