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Relationships between Objects
Associations
Lecture 7
2
Overview
n Association
n Dependency
n Aggregation
n Composition
n Implementation of Relationships
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Associations between classes
n A system consists of coordinating objects n Inheritance describes the hierarchical relationship between classes/ objects n Association between classes describes the role the objects of these classes play in the system n For example: Lecturer teaches Student; If Lecturer and Student are classes then teaches is an association between them
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More Examples…
n BookBorrower borrows Copy of Book n Copy is copy of Book n Author writes Book n Student takes Module n Semester has Module
n Pattern: ClassName associationName ClassName
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Understanding Associations
n Conceptually
n Implementation point of view
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Conceptual Meaning
n Two classes are associated if the object
of one class sends a message to (uses)
the object of the other class
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Implementation of Point 3
n An instance of class A has one or more
attributes whose values are instances
of class B
class A { B b; // declare ‘b’ void somethingACanDo() { b = anotherInstanceOfClassB; } }
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Implementation of Point 4
n An instance of class A receives a
message with an instance of class B as
an argument
class A { ... void somethingACanDo(B b) { b.doSomething(); } }
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… knows about …
In all the previous cases:
class A knows about class B
n Indicates direction of message (i.e., A
sends message to B)
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UML Representation of
Association
n Name of association and direction of message
n Associations with roles
University^ τ^ studies in Student
University^ paysυ Lecturer
University Person
student
employer teacher
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Cardinality – Multiplicity of
Association
n Multiplicity information specifies the number of instances that participate in a relationship
n Example: n A particular university has several students n A given student belongs to a single university
University^ |^ studies in Student
1 *
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More Examples
Examiner^ examines Paper
1 *
WorkDay^ has Shift
1 4
Worker^ works Shift
10..20 1..
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UML Notation : Dependency
Person
House buys
Semester Date
starts on
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UML Notation : Aggregation
1..* 5..* Course Module
1..* 1..* EmployeeList Employee
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UML Notation : Composition
1 0..* Window Button
1 9 TicTacToeGame Square
In compositions, the whole has a multiplicity of zero or 0..1; that is.. A part cannot be part of more than one whole
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Aggregation vs. Composition
n In C++ we can refer to objects either
by value or by reference
n It is aggregation if the whole contains a
reference or pointer to the part
n It is composition if the whole contains
the part by value
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Summary
n Association between classes
n Links between objects
n Types of Associations
n UML Notations
n Multiplicity of associations
n Implementation considerations
