Page 59 - DCAP308_OBJECT_ORIENTED_ANALYSIS_AND_DESIGN

Unit 5: Advanced Class Modelling – II

Notes
Figure 5.2: A Subset Constraint between Associations

Task Why do we draw a dotted arrow between classes or associations? Discuss.

Self Assessment

Fill in the blanks:
4. A ....................... is a functional relationship between modeling constructs such as classes,
attributes, and associations.

5. A constraint restricts the values of .......................
6. The ....................... of a constraint should clearly indicate the affected data.
7. We often use ....................... to document the rationale for subtle modeling decisions and
convey important enumerations.

5.3 Derived Data

Derived data is data that can be completely determined from other data. Classes, attributes, and
associations can all be derived. The underlying data can, in turn, be base data or further derived.

Notes Do not confuse our use of the term “derived” with the C++ derived class. A C++
derived class refers to the subclass of a generalization; it has nothing to do with OMT’s
meaning of derived data.

As a rule, you should not show derived data during analysis unless the data appears in the
problem description. During design you can add derived data to improve efficiency and ease
implementation. During implementation you can compute derived data on demand from
constituent data (lazy evaluation) or precompute and cache it (eager evaluation).

!
Caution Derived data that is precomputed must be marked as invalid or recomputed if
constituent data is changed.
The notation for derived data is a slash preceding the name of the attribute, class, association, or
role.

Example: Figure 5.3 shows an example of a derived attribute for airline flightdescriptions.

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