Fundamentals of OO

1. Fundamentals of OO

2. 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

3. 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, attendance) • "Attributes" • simple access • behavior (completes assignments, attends class) • "Operations/Methods" • complex properties • relationships (student in course) • external implementations

4. 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 · Name=“George” · Attendence=100% · Current Grade=A Lee : Student · Name=“Lee” · Attendence=85% · 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. Grade=B

5. · GetName() Student Encapsulation and Objects • Classes form the interface for interaction, Objects provide the implementation • they are instances of a class (student) • they have unique behavior • they have state Class Objects

6. 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.

7. 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.

8. 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

9. 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

10. 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

11. Summary of Object-Oriented Concepts • Abstraction • Class • Attribute • Behavior • Relations • Encapsulation • Object • Message • Polymorphism • Hierarchies • Inheritance • Aggregation “Is-A” relationships are implemented through inheritance. “Has-A” relationships are implemented through data members. Data structures are used to implement the various “Has-A” relationships. For example, multiple children are maintained through container data structures such as arrays and linked lists.

12. Comparison of Functional vs. OO Views Register Student Submit Grade Students Grades Students Student/Grades Print Transcript

13. 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 Submit Grade Students/ Pass Fail Students Grades/PF Impact Areas Students Student/Grades/PF Print Transcript function override

14. 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 Register Student Submit Grade Impact Areas Students Grades Students Student/Grades/PF Student/Grades/PF Print Report Card Print Transcript

15. Re-organization of OO Abstractions • Data dependent behavior handled by derived classes • New functionality handled by new associated classes ("wrappers", "adapters", "views")

16. Exercise: Identifying OO Properties • Provide examples of the following concepts using an Answering Machine. • Class: • Attribute: • Behavior: • Object: • Association: • Inheritance: • Aggregation:

17. Assignment: Identifying OO Properties • Provide examples of the following concepts using a domain you are familiar with. Prepare your results on a transparency for presentation to the class. • Class: • Attribute: • Behavior: • Object: • Association: • Inheritance: • Aggregation:

18. Sample Package View

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

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