Smalltalk and Object Orientation: an Introduction - Free

20.3.1.8 Setting trade offs between competing resources
In any design there are various trade offs to be made. For example, the trade off between speed of
access and data storage is a common one i n database systems. The larger the number of indexes used,
the faster data retrieval can be made (however the indexes must now be stored along with the data).
Such design trade offs must be made with regard to the system as a whole (including non -software
issues) as sub -optimal decision will be made if they are left for designers concentrating on a single
package.
20.3.2 Object design phase
The object design essentially takes the models produced by the analysis phase and fleshes them out
ready for the implementation. Thus the objects identified during the analysis act as the skeletons for the
design. However, the designer must now consider how the analysis objects (partitioned into packages)
should be implemented. They must express the operations identified earlier in terms of algorithms
which can be implemented and associations as appropriate references from one object to another (taking
into account the type of facilities provided by the target language). New classes may need to be
introduced to deal with aspects which are important for the design (and ultimately the implementation)
but are not significant for the analysis.
The following steps are performed by the designer during the object design:
1. Combine the three models to obtain operations on the classes.
2. Design algorithms to implement operations.
3. Optimize access paths to data.
4. Implement control for external interactions.
5. Adjust class structure to increase inheritance.
6. Design associations.
7. Determine object representation.
8. Package classes and associations into modules.
As stated before, these steps are iterative.
20.3.2.1 Combining the three models
If the implied operations from the dynamic and functional models have not been added to the object
model, then they should be added now.
20.3.2.2 Designing algorithms
Each of the DFD s in the functional model needs to be expressed as an algorithm to indicate how they
perform the function. The DFD says what it does, but the algorithm says how it does it. The algorithm
designer must:
• Choose algorithms that minimize the cost of implementing operations.
• Select data structures appropriate to the algorithms.
• Define new internal classes and operations as necessary.
• Assign responsibility for operations to appropriate classes.
Note that any algorithms defined in pseudo code during the analysis phase were intended to explain
the required functionality. It is therefore necessary at this stage to consider what algorithms are required
in the implementation. For example, although a bubble sort algorithm may have been used in the
analysis phase, an insertion sort may well be a more efficient algorithm to chose for the design.
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