Object-oriented requirements analysis

Developing object models during requirements analysis usually simplifies the transition to object-oriented design and programming. However, I have found that end-users of a system sometimes find object models unnatural and difficult to understand. They may prefer to adopt a more functional, data-processing view. Therefore, it is sometimes helpful to supplement object models with activity charts or data-flow models that show the end-to-end data processing in the system.

In object-oriented requirements analysis, you should model real-world entities using object classes. You should not include details of the individual objects (instantiations of the class) in the system. You may create different types of object model, showing how object classes are related to each other, how objects are aggregated to form other objects, how objects interact with other objects and so on. These each present unique information about the system that is being specified.

The analysis process for identifying objects and object classes is recognised as one of the most difficult areas of object-oriented development.  There have been a range of different approaches proposed to object identification including:

• The use of a grammatical analysis of a natural language description of a system. Objects and attributes are nouns; operations or services are verbs (Abbott, 1983). This approach has been embodied in the HOOD method for object-oriented design (Robinson, 1992) that was widely used in the European aerospace industry.
• The use of tangible entities (things) in the application domain such as aircraft, roles such as manager, events such as request, interactions such as meetings, locations such as offices, organisational units such as companies and so on (Shlaer and Mellor, 1988, Coad and Yourdon, 1990, Wirfs-Brock et al., 1990). You support this by identifying storage structures (abstract data structures) in the solution domain that might be required to support these objects.
• The use of a behavioural approach where the designer first understands the overall behaviour of the system. The various behaviours are assigned to different parts of the system and an understanding is derived of who initiates and participates in these behaviours. Participants who play significant roles are recognised as objects (Rubin and Goldberg, 1992).
• The use of a scenario-based analysis where various scenarios of system use are identified and analysed in turn. As each scenario is analysed, the team responsible for the analysis must identify the required objects, attributes and operations. A method of analysis called CRC cards where analysts and designers take on the role of objects is effective in supporting this scenario-based approach (Beck and Cunningham, 1989).

In practice, you normally use a mixture of all of these approaches to identify the objects in a system being analyzed or in the system that you are designing.

Various methods of object-oriented analysis and design were proposed in the 1990s (Coad and Yourdon, 1990) (Rumbaugh et al., 1991) (Jacobsen et al., 1993) (Booch, 1994). These methods had a great deal in common and three of the key developers (Booch, Rumbaugh and Jacobsen) decided to integrate their approaches to produce a unified method (Rumbaugh et al., 1999). The Unified Modeling Language (UML) used in this unified method has become a standard for defining object-oriented models.

References

Abbott R. (1983). Program Design by Informal English Descriptions. Comm. ACM, 26(11), 882—94.

Beck K. and Cunningham, W. (1989). A Laboratory for Teaching Object-Oriented Thinking. Proc. OOPSLA’89, New Orleans, ACM Press.

Booch G. (1994). Object-oriented Analysis and Design with Applications. Redwood City, Ca.: Benjamin Cummings.

Booch G., Rumbaugh, J., et al. (1999). The Unified Modeling Language User Guide. Reading, Ma.: Addison-Wesley.

Coad P. and Yourdon, E. (1990). Object-oriented Analysis. Englewood Cliffs, NJ: Prentice-Hall.

Jacobsen I., Christerson, M., et al. (1993). Object-Oriented Software Engineering. Wokingham, UK: Addison-Wesley.

Robinson P. J. (1992). Hierarchical Object-Oriented Design. Englewood Cliffs, N.J.: Prentice-Hall.

Rubin K. and Goldberg, A. (1992). Object Behaviour Analysis. Comm. ACM, 35(9), 48—62.

Rumbaugh J., Blaha, M., et al. (1991). Object-oriented Modeling and Design. Englewood Cliffs, N.J.: Prentice-Hall.

Rumbaugh J., Jacobson, I., et al. (1999). The Unified Modeling Language Reference Manual. Reading, Ma.: Addison-Wesley.

Shlaer S. and Mellor, S. (1988). Object-Oriented Systems Analysis: Modeling the World in Data. Englewood Cliffs, N.J.: Yourdon Press.

Wirfs-Brock R., Wilkerson, B., et al. (1990). Designing Object-Oriented Software. Englewood Cliffs, N.J.: Prentice-Hall.