Concepts in Object-Oriented Design CSCI 240 Dale Roberts, Lecturer Computer Science, IUPUI

CSCI 240Dale Roberts 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 GeorgeWendle Lee

CSCI 240Dale Roberts 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

CSCI 240Dale Roberts Objects, Instances of a Class Classes define what properties will exist in each instance Objects provide distinct instances that exhibit those properties George : Student · Name=“George” · Attendence=100% · Current Grade=A Lee : Student · Name=“Lee” · Attendence=85% · Current Grade=B Class Objects 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.

CSCI 240Dale Roberts 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.

CSCI 240Dale Roberts 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.

CSCI 240Dale Roberts 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

CSCI 240Dale Roberts 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 Person · Name Student · Name · Attendence · Current Grade

CSCI 240Dale Roberts 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

CSCI 240Dale Roberts Comparison of Functional vs. OO Views Register Student Print Transcript Submit Grade Students Grades Student/Grades

CSCI 240Dale Roberts Addition of a New Student Type Changes in data types cause significant impact to functional approaches OO approaches allow new object types to re-define functionality Register Student Print Transcript Submit Grade Students Students/ Pass Fail Students Grades/PF Student/Grades/PF Impact Areas function override

CSCI 240Dale Roberts Addition of New Report Type Changes in functionality based on stable data causes significant impact across objects Functional approaches allow new functions to augment functionality Print Transcript Submit Grade Students Grades Student/Grades/PF Register Student Print Report Card Student/Grades/PF Impact Areas

CSCI 240Dale Roberts Re-organization of OO Abstractions Data dependent behavior handled by derived classes New functionality handled by new associated classes ("wrappers", "adapters", "views")

CSCI 240Dale Roberts UML Sample Package View

CSCI 240Dale Roberts UML Sample Class View Notice that this sample class view does not include visibility indicators. Diamond implies composition: All MediaTopics together comprise a CardCatalog.

CSCI 240Dale Roberts Acknowledgements This presentation is an adaptation of materials developed by Jim Stafford, John Hopkins University. Used by permission.