5.4 Looking Ahead

One of the first things done on any project is to determine the detailed requirements of the system. These requirements may either be capabilities of the system or how well those capabilities are to be achieved. The UML provides use cases, scenarios, and statecharts to capture these requirements. Use cases identify the primary capabilities of a system. The details of how those capabilities are achieved at least from an outside, black-box view is captured in the scenarios or specifications associated with the use cases. A use case is normally detailed by a few up to several dozen scenarios, shown on sequence or timing diagrams. Alternatively, or in combination, a single statechart or activity diagram can provide the behavioral details of the entire use case.

This is crucial in order to understanding what system is to be built its capabilities and required characteristics. None of this, however, has anything (directly) to do with how it's built. Use cases are realized by collaborations of objects working together for a common purpose (to implement the use case). It is, at best, less than obvious, to go from a set of requirements regardless of how they are modeled to a set of objects. Many objects may work together to realize a use case, but the same object may participate in multiple use cases as well.

The identification of objects and their classes is done in two phases. Object analysis identifies the key, essential set of objects necessary to realize the set of identified use cases. That is the subject of the next two chapters. Chapter 6 deals with how to open the box and identify the key concepts required to construct the essential logical object model of the system. Chapter 7 focuses on how to define the behavior of those logical elements, and map them back to the use case behavioral details.

Chapters 8 through 10 go on to discuss design. While object analysis identifies the set of objects required for any reasonably resolution of the requirements, design is all about optimization. Design selects one particular solution that optimizes total system quality with respect to all of its quality criteria.

Armed with that understanding, let's go open up the box and peer inside.