Object Orientation

1. Object Orientation Yaodong Bi, Ph.D. Department of Computer Sciences University of Scranton August 19, 2005

2. Classes and objects for modeling • Classes • Real world concepts • Student, course, bicycle, etc • Objects • Instances of real word concepts • John Smith, Operating Systems, Mongoose Mountain S100, etc

3. Classes and objects for modeling “Customers John and Susan entered the MakeMoney bank and were served by teller Andy.” Classes Objects:

4. Classes and objects in software • Objects • Packaging both data and the procedures that operate on the data into one • Operations are the only way to change the data items – encapsulation • Classes • Template of objects of the same type • Write once and use many times • Implementation classes/objects • Some classes/objects do not match to concepts in the real world • List, stack, tree, queue – containers • Exception classes

5. Attributes and operations • Instance attributes (simply attributes) • Properties (internal state) of an object • Student: name, major, class, GPA • Course: title, # of credits, description • Each object has its own value for an attribute • Instance operations (simply operations) • Services of an object • Stack: push, pop, isEmpty, isFull • Encapsulation • Attributes (private and protected) are not accessible directly by client • Operations are used to read/write the values of attributes

6. Attributes and Operations • Class attributes (static attributes) • Represent properties that can be applied to all objects of the same class • Only one value for each class attribute shared by all objects of the class • All accounts of the CheckingAccount class have the same interest rate • Can be accessed in both instance and class operations • Class operations (static operations) • Used to access class attributes • Cannot access instance attributes and operations

7. Class member access modes • Public • Public attributes and public operations • Accessible to all clients or the world • Protected • Protected attributes and operations • Accessible to the class and its subclasses • Private • Private attributes and operations • Only accessible by the operations of the class

8. Member access modes in Java

9. Abstract and concrete classes • Abstract classes • Define a common interface for its subclasses • Defer some or all of its implementation of operations to its subclasses • Cannot be instantiated • Concrete classes • That are not abstract classes • Can be instantiated • Concrete subclasses implement all the operations

10. Interfaces • Signature of an operation: • Operation name • Objects/classes it takes as parameters • Return value and type • Interface: • The set of signatures of related operations • Representing a capability or a set of services • An object may implement multiple interfaces

11. Inheritance • Specifies is-a or a-kind-of relationship • Generalization and specialization • Superclasses and subclasses • Single and multiple inheritance • Class and implementation inheritance

12. Superclasses and subclasses • Superclass • Also called base class or parent class • All of its members are inherited by its subclasses • Subclasses • Also called child classes • Inherit all the properties of its superclasses

13. Single and multiple inheritance • Single inheritance • A subclass cannot have more than one parent • Multiple inheritance • A subclass may have more than one parent • Java does not allow multiple inheritance • Java allows multiple interface inheritance • Through interface implementation

14. Person SSN Name getSSN() getName() Class inheritance lattice Faculty office getOffice() Student major getMajor() Staff dept getDept() Graduate PayRate getPayRate() Underg PayRate getPayRate() FullTime salary getSalary() PartTime PayRate getPayRate() TA labs getLabs()

15. Implementation inheritance • Also called class inheritance • Define an object’s implementation in terms of another object’s implementation • Pure class inheritance in C++ • Pure class inheritance in Java

16. Pure class inheritance with C++ Class BinaryTree { getRoot() {;} setRoot() {;} leftTree() {;} rightTree() {;} } Class BinSearchTree: private BinaryTree { insert() {;} remove() {;} find() {;} } The operations of BinaryTree are not accessible/visible to the clients of BinSearchTree because BinaryTree is private They use the operations ofBinaryTree

17. Pure class inheritance with Java • Java does not allow private parent • The parent and grandparents are public and accessible through the subclass • Java cannot implement pure implementation inheritance.

18. Interface inheritance • Describe when an object can be used in place of another • Pure interface inheritance in C++ • Superclasses are pure abstract classes • Pure interface inheritance in Java • Superclasses are Java interfaces

19. Interface inheritance with C++ Class Stack { virtual push() = 0; virtual pop() = 0; virtual isEmpty() = 0; virtual isFull() = 0; } A C++ Abstract class Class MyStack: public Stack { push() {;} pop() {;} isEmpty() {;} isFull() {;} } Class YourStack: public Stack { push() {;} pop() {;} isEmpty() {;} isFull() {;} }

20. Interface inheritance with Java Interface Stack { push(); pop(); isEmpty(); isFull() 0; } A Java Interface Class MyStack implements Stack { push() {;} pop() {;} isEmpty() {;} isFull() {;} } Class YourStack implements Stack { push() {;} pop() {;} isEmpty() {;} isFull() {;} }

21. Polymorphism • When a client sends a request to a reference, the method executed depends on the object behind the reference.

22. Dynamic (late) binding

23. Method overriding

24. Function overloading