auto generate barcode vb net Identifying Associations in Software

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Identifying Associations
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After the objects in a domain are identi ed, the next step is to identify the associations between the objects. An association denotes a relationship between two objects. The di erent kinds of associations were described in Section 2.4. An association between objects means that in the implementation of the system,
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Object-Oriented Development
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there will be a link between the objects. Thus, the importance of the associations is that the associations determine what access an object has to other objects. This access is essential to e cient implementation of functionality. There are di erent approaches for determining the associations between objects. One approach is to identify the associations that exist in the problem domain.
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EXAMPLE 11.9
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Develop associations for the family tree problem of Example 11.5. The problem statement mentions aunts, uncles, and cousins. These are all associations (relationships). They are not the primitive associations. The basic associations in genealogy are mother, father, and child. The inverse of each of these associations is marriage, marriage, and birthfamily, respectively. Additionally, there is an association (aggregation) from the top object, familytree, to marriage and to person. These can be called marriages and people, respectively. See Fig. 11-6.
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family tree marriages birthfamily family mother father children marriage marriage father mother child person birthfamily marriges people
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Fig. 11-6
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Another approach to identifying associations is to think about the functionality that is required. If one object is required to have functionality that requires access to other objects, then an association, or a sequence of associations, must exist between those objects.
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EXAMPLE 11.10
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A college wants a system that handles the courses, sections of courses, and students. Draw an object model and identify the associations between the objects. The college will need to access the students for printing out student information. To print out courses taken by students, there needs to be access from students to sections. To print out the line schedule that prints the sections that are available, there needs to be access to courses and then to sections for each course. See Fig. 11-7.
Object-Oriented Development
college students student taken by taking section sections courses course
Fig. 11-7
11.3.1 EXISTENCE DEPENDENCY
Another approach is to use the existence dependency (Section 2.4.1) relationship between objects to determine the required associations. Two objects have an existence dependency relationship if the existence of the child object is dependent on exactly one instance of the parent object. This means that the parent instance exists before the child instance is created and the child instance is deleted before the parent instance is deleted.
EXAMPLE 11.11
Use existence dependency to structure the associations in the library example, Example 2.6. Neither book nor person is existence dependent on library. However, their participation in the library in terms of patron and copy, respectively, does satisfy the existence dependency requirements. See Fig. 11-8.
person name address
library
book title author book
patron patron #
copy status
checked out
loan loan status
checked out
Fig. 11-8
EXAMPLE 11.12
Object-Oriented Development
Use existence dependency to determine the association in the student section problem of Example 11.10. The object model developed in Example 11.10 does not satisfy the existence dependency rules, since section cannot be existence dependent on student or vice versa. Thus, an additional object called enrollment must be used. See Fig. 11-9.
college students student
courses course sections section
taken by taking enrollment
Multiplicities are restrictions on the associations between instances of objects. The multiplicity is speci ed by an expression at the end of the association. The expression can be a single value, a range of values, or a list of ranges or single values. In the range, the two values are separated by two periods. The problem domain often has restrictions on how many relationships an instance of an object can have with instances of other objects.
EXAMPLE 11.13
Use multiplicities to restrict how many times a copy of a book can be borrowed at a given time. As shown in Fig. 11-10 the 0..1 at the loan end of the association restricts a copy to be participating in at most one loan relationship at a time. The 1 at the copy end of the association requires a loan to have exactly one association with a copy. That is, there cannot be a loan without exactly one copy associated with the loan. The check-out association restricts the loan instance to be associated with exactly one patron. The patron can have the association with zero or more loan instances.
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