Page 178 - DCAP308_OBJECT_ORIENTED_ANALYSIS_AND_DESIGN

Object Oriented Analysis and Design

Notes 3. Saving Derived Attributes to Avoid Recomputation: Data that is redundant because it can
be derived from other data can be “cached” or store in its computed form to avoid the
overhead of recomputing it. The class that contains the cached data must be updated if any
of the objects that it depends on are changed.
Derived attributes must be updated when base values change. There are three ways to
recognise when an update is needed:
Explicit update: Each attribute is defined in terms of one or more fundamental base
objects. The designer determines which derived attributes are affected by each change
to a fundamental attribute and inserts code into the update operation on the base
object to explicitly update the derived attributes that depend on it.
Periodic Recomputation: Base values are updated in bunches. Recompute all derived
attributes periodically without recomputing derived attributes after each base value
is changed. Recomputation of all derived attributes can be more efficient than
incremental update because some derived attributes may depend on several base
attributes and might be updated more than once by incremental approach. Periodic
recomputation is simpler than explicit update and less prone to bugs. On the other
hand, if the data set changes incrementally a few objects at a time, periodic
recomputation is not practical because too many derived attributes must be
recomputed when only a few are affected.
Active values: An active value is a value that has dependent values. Each dependent
value registers itself with the active value, which contains a set of dependent values
and update operations. An operation to update the base value triggers updates all
dependent values, but the calling code need not explicitly invoke the updates. It
provides modularity.

14.1.5 Implementation of Control

The designer must refine the strategy for implementing the state – event models present in the
dynamic model. As part of system design, you will have chosen a basic strategy for realizing
dynamic model, during object design flesh out this strategy. There are three basic approaches to
implementing the dynamic model:
1. State as Location within a Program: This is the traditional approach to representing
control within a program. The location of control within a program implicitly defines the
program state. Any finite state machine can be implemented as a program. Each state
transition corresponds to an input statement. After input is read, the program branches
depending on the input event received. Each input statement need to handle any input
value that could be received at that point. In highly nested procedural code, low-level
procedures must accept inputs that they may know nothing about and pass them up
through many levels of procedure calls until some procedure is prepared to handle them.
One technique of converting state diagram to code is as follows:
(a) Identify the main control path. Beginning with the initial state, identify a path
through the diagram that corresponds to the normally expected sequence of events.
Write the name of states along this path as a linear sequence of events. Write the
names of states along this path as a linear sequence. This becomes a sequence of
statements in the program.
(b) Identify alternate paths that branch off the main path and rejoin it later. These
become conditional statements in the program.

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