10.8 Summary

One definition of an object is "a set of tightly coupled attributes and the operations that act on them." Detailed design takes this microscopic view to fully specify the characteristics of objects than have been hitherto abstracted away and ignored. These characteristics include the structuring of the attributes and identification of their representational invariants, resolution of abstract message passing into object operations, selection and definition of algorithms, including the handling of exceptional conditions.

Attributes are the data values subsumed within the objects. They must be represented in some fashion supported by the implementation language, but that is not enough. Most often, the underlying representation is larger than the valid ranges of the attribute, so the valid set of values of the attributes must be defined. Operations can then include checking the representational invariants to ensure that the object remains in a valid state.

Analysis models use the concept of message-passing to represent the myriad of ways that objects can communicate. Detailed design must decide the exact implementation of each message. Most often, messages are isomorphic with operations, but that is only true when the message source is always in the same thread of execution. When this is not true, other means, such as OS message queues, must be employed to provide inter-object communication.

Many objects are themselves algorithmically trivial and do not require a detailed specification of the interaction of the operations and attributes. However, in every system, a significant proportion of objects have "rich" behavior. Although this requires additional work, it also provides the designer with an opportunity to optimize the system performance along some set of criteria. Algorithms include the handling of exceptions and this is usually at least as complex as the primary algorithm itself. Algorithms can be expressed using state charts or activity diagrams. Other representations, such as mathematical equations, pseudocode, or text can be used as well.