Concepts in Object-Oriented Design

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Concepts in Object-Oriented Design

• CSCI 240

Dale Roberts, Lecturer Computer Science,
IUPUI E-mail droberts_at_cs.iupui.edu
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Example Classroom

• Attending the lecture we have several individuals
• Wade - loves Chinese food
• George - an outdoorsman
• Wendle - is a sports nut
• Lee - doesn't eat anything with a vowel in its
  name
• These are the students of our lecture
• Note that the individuals are not identical.

Wade George
Wendle
Lee
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Abstraction and Classes

• From our perspective, we see them as an
  instantiation of a "Class", Student
• has a name
• attends class
• has a grade
• completes assignments
• Note the each student looks the same
• even though the are different individuals
• A Class defines an "Abstraction"
• OO Abstractions are based on
• data (name, grade, attendance record)
• "Attributes"
• simple access
• behavior (completes assignments, attends class)
• "Operations/Methods"
• complex properties
• relationships (student in course)
• external implementations

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Objects, Instances of a Class

• Classes define what properties will exist in each
  instance
• Objects provide distinct instances that exhibit
  those properties

Class
Objects
George Student


NameGeorge


Attendence100


Current
GradeA
Lee Student


NameLee


Attendence85


Current
Notice that the data members are replicated in
each object. Each object is an instance of
Student. It does not make sense to replicate
member functions.
GradeB
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Messaging Supports Encapsulation

• We interact with objects through "messages"
• Messages allow object to determine implementation
  rather than the sender determining the
  implementation for each instance.

Messages are passed and handled, rather than
invoked like functions.
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Polymorphism Supports Encapsulation

• Allows an object of any implementation type,
  which satisfies the interface defined by the
  abstraction, to be used by a generic reference to
  an abstraction.
• Promotes a separation of interface and
  implementation

class GraduateStudent Student float
calculateScore(Student s) GraduateStudent
George calculateScore(George)
Wade is an undergraduate Student
George is an graduate Student
Polymorphism says that you can treat both Wade
and George as Students if the distinction between
undergraduate and graduate is not important.
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Hierarchies

• Organize lower level abstractions into more
  complex abstractions
• Hierarchies allow more complex abstractions to be
  understood
• Two basic forms
• Inheritance
• Generalization - Specialization
• Base Class - Derived Class
• Parent - Child
• Abstract Class - Concrete Class
• Aggregation (many-to-one relationships)
• Whole - Part
• Containership
• Collection
• Group

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Inheritance Hierarchy

• an "is-A" relationship
• Inheritance
• for type - a re-use of common interface
• for class - a re-use of common interface and
  implementation

Student
Person


Name


Name


Attendence


Current
Grade
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Aggregation Hierarchy

• a "has-A" relationship
• assembly-part - where the aggregation of parts
  makes up the whole
• ex airplane is an aggregation of wings, wheels,
  motor, prop, etc.
• container-contents - where the container exists
  with or without contents
• ex classroom is an aggregation of students,
  instructor, tables, chairs, etc.
• group-member - where members are logically
  associated with whole
• ex course has students and teachers as members
• "weak has-A" - where there is more of a
  peer-to-peer relationship between abstractions
  (association)
• ex Instructor has students, Students have
  instructor

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Comparison of Functional vs. OO Views
Register Student
Submit Grade
Students
Grades
Students
Student/Grades
Print Transcript
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Addition of a New Student Type
Register Student
Submit Grade
Students/ Pass Fail Students
Grades/PF
Impact Areas
Students
Student/Grades/PF
Print Transcript
function override

• Changes in data types cause significant impact to
  functional approaches
• OO approaches allow new object types to re-define
  functionality

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Addition of New Report Type
Register Student
Submit Grade
Impact Areas
Students
Grades
Students
Student/Grades/PF
Student/Grades/PF
Print Report Card
Print Transcript

• Changes in functionality based on stable data
  causes significant impact across objects
• Functional approaches allow new functions to
  augment functionality

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Re-organization of OO Abstractions

• Data dependent behavior handled by derived
  classes
• New functionality handled by new associated
  classes ("wrappers", "adapters", "views")

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UML Sample Package View
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UML Sample Class View
Diamond implies composition All MediaTopics
together comprise a CardCatalog.
Notice that this sample class view does not
include visibility indicators.
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Acknowledgements

• This presentation is an adaptation of materials
  developed by Jim Stafford, John Hopkins
  University. Used by permission.