relationships between classes

8/3/2019 Relationships Between Classes

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Lecture 8 -Relationships

between Classesdelivered by John Stockwell


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Objectives

At the end of this lecture you will be able to: Interpret the association relationship on a class

diagram Correctly apply multiplicity and understand its

significance Correctly configure generalization/specialization

relationships Distinguish between association and

compositional aggregation Show all key class relationships on a class

diagram.


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Links and Association


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Association


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Multiplicity

Notation Meaning

* or 0..* From 0 up to any number

5..8 From 5 to 8 inclusive

4..* From 4 up to any number

1 Exactly 1.

When there is no number shown 1 is assumed.


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Association format for interpretation

on class diagrams

A link description multiplicity

at Second Class end (s)

A link description reversed

multiplicity at Class end (s)

A Lecturer teaches on zero or more ModuleOfferings

A Module Offering is taught by one Lecturer


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Multiplicity

The multiplicity then helps us to formulate

some of the business rules for the

information system under consideration.


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Possible links according tomultiplicity specification on class

diagram


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Examples of Association

A Cup of Tea is drunk by one Person. A Person drinks zero or more Cups of Tea.

A Tea Bag makes one or two Cups of Tea. A Cup of Tea is made by one Tea Bag.

A Person buys forty or more Tea Bags. A Tea Bag is bought by zero or one Persons.


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Forming associations


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Choosing Associations

Take classes in pairs and form simple

phrases between them

Verbs may be chosen from case study

Consider SFU


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Student Module Lecturer

Semester Module Offering Invoice


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Student studies Modules ?

Student registers on Module Offering ?

Student learns from Lecturer ?

Student ----------- Semester ????


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Student is sent Invoice

Module results in Module Offering

Lecturer teaches Module Offering

Module Offering occurs in Semester


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Association relationships onSFU


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Inheritance

(Generalization/Specialization)

Two or more classes appear to share commoncharacteristics

Object-oriented programming languagesenable us to use the same code to generatethe common features of these classes

If a class is in existence it may be possible tore-use the class and inherit characteristicsfrom it that can then be refined to give us amore specialized version of it


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Inheritance The class from which the characteristics are

inherited is called the super-class

The class or classes that inherit from it are

known as the sub-classes The characteristics that are inherited are the

attributes and the operations of the super-

class


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Inheritance In addition the relationships that the super-

class has to other classes is also inherited bythe subclasses.

If class A is the super-class and class B is asub-class then we should be able to sayclass B is-a-kind-of class A . It should alsobe clear that there are a number ofcharacteristics in the super-class that thesub-class can sensibly inherit.


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Inheritance

(Generalization/Specialization)


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Inheritance

Inheritance may be deduced from two

perspectives.

A super-class or generalization might

suggest sub-classes or specializations. Similar sub-classes or specializations

suggest a super-class or generalization.


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Generalization/Specialization onSFU


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Aggregation and composition

Aggregation is less important than the

association and inheritance

It exists in two forms ordinary aggregation

a stronger form known as compositional

aggregation or sometimes simply a composition


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Composition

Composition is-a-part-of. The test to see if

composition exists is twofold

Does it make sense to say that one class is a

part of another class? Is there a life dependendency ?

If the answer to both questions is yes then the

smaller of the two classes considered has itslife dependent on the larger class.


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Composition


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Class diagram for SFU so far


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Can you understand theserelationships ?


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References

Bennett, S., McRobb, S. and Farmer,R.

Object-Oriented Systems Analysis and

Design using UML, London: McGraw-Hill,

2002 Britton, C. and Daoke, J. Object-Oriented

Systems Development a gentle

introduction, London: McGraw-Hill, 2000

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