9.5 Design Representations

Designing an object-oriented system is aided by a high-level, graphical representation of classes, objects, and their relationships. The graphical nature of the representation allows easy visualization of the structural aspects of the system. The design is represented at a high level in that only the essential elements of the design are present independent of lexical details or the specifics of the implementation.

There are three specific uses for a high-level, graphical design representation. First, the representation provides a language that quickly describes an idea being considered by a single designer or a design team. Because it can be quickly drawn and easily modified, the representation aids in thinking about the design and exploring design alternatives. Second, the representation is a means of documenting a design. The design document serves as a guideline for the implementor and, later, for those who will modify and maintain the system. The graphical and high-level nature of the representation make it useful as a documentation device because through them it describes only the essential structural elements, leaving the code and other documentation to add more detailed information. Third, once represented, the design may itself be the object of reuse. Commonly occuring design solutions may be identified, documented, and collected for use in situations other than the ones in which they were initially encountered. The recent development of design patterns is a realization of this idea. One of these patterns is shown and discussed in section 9.6.

The design representation presented here employs three kinds of diagrams:

The first two diagrams depict relationships that are static in the sense that they do not change over time. Object-interaction diagrams are used to illustrate sequences of interactions (method invocations) among objects. The sequence of interactions are usually those in a typical or significant scenario.

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