Page 165 - DCAP308_OBJECT_ORIENTED_ANALYSIS_AND_DESIGN

Unit 13: Class Design

Most organisations provide no reward for reusability; sometimes productivity is measured Notes
in terms of new lines of code written plus a discounted credit

Corollary 4: Strong Mapping

A strong mapping links classes identified during analysis and classes designed during the
design phase e.g., view and access classes.
The analyst identifies objects’ types and inheritance, and thinks about events that change
the state of objects.

The designer adds detail to this model perhaps designing screens, user interaction, and
client-server interaction.

Corollary 5: Standardisation

To reuse classes, we must have a good understanding of the classes.

Most object-oriented systems come with several built-in class libraries.
But these class libraries are not always well documented.
Sometimes they are documented, but not updated.
They must be easily searched, based on users’ criteria.

Corollary 6: Designing with Inheritance

When we implement a class, we have to determine its ancestor, what attributes it will
have, and what messages it will understand.
Then we have to construct its methods and protocols.
Ideally, one has to choose inheritance to minimise the amount of program instructions.

The primitive form of reuse is cut-and-paste reusability.
Now let us see the process of achieving multiple inheritance in a singe inheritance system:
Single inheritance means that each class has only a single super class.
The result of using a single inheritance hierarchy is the absence of ambiguity as to how an
object will respond in a given method.
We simply trace up the class tree beginning with the object’s class, looking for a method
of the same name.

But languages like LISP or C++ have a multiple inheritance scheme whereby objects can
inherit behavior from unrelated areas of the class tree.

The complication here is how to determine which behavior to get from which class,
particularly when several ancestors define the same method.
One way of resolving this is to inherit from the most appropriate class and add an object
of mother class as an attribute or aggregation. The other is to use the instance of the class
(object) as an attribute.

Task Make distinction between stamp coupling and data coupling.

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