MY BOOK REVIEW

OBJECT ORIENTED ANALYSIS and DESIGN

Book

Edward Yourdon and Carl Argila

Author

QA 76.64 Y68 1996

Reference no.

CHAPTER 6

Some problem are so complex that you have to highly intelligent and well informed just to be undecided about them.      

                                                                                                                                  Laurence J. Peter

   Peter’s Almanac

The Structure Layer identifies relationship between object which establishes their composition, as well as inheritance relationship. Since inheritance allows us to identify something once, and then re-used it in different form, it is important to identify these structures in our OOA model. The structure Layer is one of the mechanisms we used to deal with complexity in the OOA model.

We identify two types of structure: Generalization-Specialization (Gen-Spec) and Whole-Part. Gen-Spec structures establish inheritance relationship and Whole-Part structure identifies relationship of composition.

Gen-Spec relationships are established when parent, or generalization object in the relationship, possesses attributes or characteristic which are shared by it’s all children, or specialization object. This sharing of attributes between parent and children is referred as inheritance.

Whole-Part relationship are established when the parent object is in some way composed of a number of child object. Typically these relationships are established on the basis of physical composition. Other types of composition are possible. Although Whole-Part relationship do not exhibit inheritance, as do Gen-Spec relationships, they posses the characteristics of multiplicity and participation. Multiplicity refers to the number of the child objects which may compose a parent. (Example, a car has four tires.) Participation refers to whether or not a parent or child object must participate in a Whole-Part relationship. (A car must have four tire, however a tire is not part of a car.)

Structures allow us to deal with application domain complexity.

Generalization-Specialization (Gen-Spec) structures involve inheritances, Inheritance allows us to define an attribute or service once and then to re-used it in other (specialization) objects.

Whole-Part structure involve some type of aggregation, they capture an application domain constrain or business rule.

In a Gen-Spec structure, it is the definition of attributes or service which are inherited, not values.

Whole-Part structures are similar to instance connection (chapter 9), we use  Whole-Part structures only when aggregation is involve.