Smalltalk and Object Orientation: an Introduction - Free

20.3.2.3 Design optimization
The analysis model was only intended to describe the application and its requirements and did not
attempt to take into account efficient information access or processing. These issues need to be
considered by the designer at this point. In particular they should consider:
• adding redundant associations to minimize access cost and maximize convenience,
• rearranging the computation for greater efficiency,
• saving derived attributes to avoid recomputation of complicated expressions.
20.3.2.4 Implementation of control
During the system design, an approach for handling the internal control of the system must have been
identified. That approach is fleshed out here. This includes determining how the selected approach can
be implemented and identifying any constraints this choice imposes on the design.
20.3.2.5 Adjust class structure
As the design progresses the class hierarchy is likely to change, evolve and become refined. It is quite
common to produce a design and then to rearrange it in light of commonalities which were hidden at an
earlier stage. The designer should:
• rearrange and adjust classes and operations to increase inheritance,
• abstract common behavior out of groups of classes,
• use delegation to share behavior when inheritance is semantically invalid.
20.3.2.6 Design associations
Associations are an important aspect of the analysis object model. However, they are conceptual
relationships and not implementation oriented relationships. The designer needs to consider how the
associations can be implemented in a given language. The choices made for representing associations
may be made globally for the whole system, locally to a package or on an association by association
basis. The criteria u sed for determining how associations should be represented in the design are based
on how are they traversed. If they are traversed only in one direction then a pointer representation may
be sufficient. However, if they are bi -directional then an intermedi ate object may best represent the
association.
20.3.2.7 Determine object representation
In most situations it is relatively straight forward to identify how to represent an object. However in
some cases consideration needs to be given to decide whether to use a system primitive or an object. For
example, in Java there are basic types int and char and there are also classes Integer and
Character.
20.3.2.8 Package classes
The system will have been decomposed into logical packages in the system design. However, dif ferent
languages provide different facilities for physically packaging a system. VisualWorks provides class
categories which can be used in a similar manner to packages but they do not provide any information
hiding etc.
20.4 Implementation phase
OMT states tha t implementation is “an extension of the design process” and that “writing code should
be straightforward, almost mechanical, because all the difficult decisions should already have been
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