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Object Orientation Yaodong Bi, Ph.D. Department of Computer Sciences University of Scranton May 24, 2017 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 5/24/2017 2 Classes and objects for modeling Classes 5/24/2017 “Customers John and Susan entered the MakeMoney bank and were served by teller Andy.” Objects: 3 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 5/24/2017 List, stack, tree, queue – containers Exception classes 4 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 5/24/2017 Attributes (private and protected) are not accessible directly by client Operations are used to read/write the values of attributes 5 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 5/24/2017 6 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 5/24/2017 7 Member access modes in Java Specifier Class Package Subclass World Private Y N N N No Specifier Y Y N N Protected Y Y Y N Public Y Y Y Y 5/24/2017 8 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 5/24/2017 9 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 – A class may implement multiple interfaces 5/24/2017 Its objects have more than one set of services 10 Inheritance Specifies is-a or a-kind-of relationship Generalization and specialization Superclasses and subclasses Single and multiple inheritance Class and interface inheritance 5/24/2017 11 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 5/24/2017 12 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 5/24/2017 Through interface implementation 13 Class inheritance lattice Person SSN Name getSSN() getName() Faculty office getOffice() Student major getMajor() Staff dept getDept() FullTime salary getSalary() Graduate PartTime PayRate getPayRate() PayRate getPayRate() Underg PayRate getPayRate() TA 5/24/2017 labs getLabs() 14 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 5/24/2017 15 Pure class inheritance with C++ Class BinaryTree { getRoot() {;} setRoot() {;} leftTree() {;} rightTree() {;} } The operations of BinaryTree are not accessible/visible to the clients of BinSearchTree because BinaryTree is private 5/24/2017 Class BinSearchTree: private BinaryTree { insert() {;} remove() {;} find() {;} } They use the operations of BinaryTree 16 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. 5/24/2017 17 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 5/24/2017 18 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() {;} } 5/24/2017 Class YourStack: public Stack { push() {;} pop() {;} isEmpty() {;} isFull() {;} } 19 Interface inheritance with Java Interface Stack { push(); pop(); isEmpty(); isFull() 0; } A Java Interface Class MyStack implements Stack { push() {;} pop() {;} isEmpty() {;} isFull() {;} } 5/24/2017 Class YourStack implements Stack { push() {;} pop() {;} isEmpty() {;} isFull() {;} } 20 Polymorphism When a client sends a request to a reference, the method executed depends on the object behind the reference Polymorphism and inheritance are very powerful tool in software design So powerful that if they are used properly, they can hurt really bad – If something goes wrong, it is hard to tell which class/object caused the problem 5/24/2017 21 Dynamic (late) binding Dynamic binding is a way of implementing polymorphism It means that a function call is not linked to the actual function until execution time Since a reference may be used to point to different types (subtypes or subclasses) of objects at execution time, so the actual function to be executed may not be known when the program is compiled. The opposite of dynamic binding is static binding which links a function call to the actual function at compilation time. 5/24/2017 22 Method Overriding A subclass overrides the operation(s) defined in the parent/grandparent classes. This is for specialized subclasses to override the behavior defined in the super class(es). For example: – Superclass Employee’s print method prints employee’s SSN, Name, Address. – Its subclass Engineer’s print method prints engineer’s type and level in addition to employee’s normal information. 5/24/2017 23 Function overloading The same name is used for more than function For example, a function of adding two integers may be named the same as the one of adding two floats: – Add(int x, int y); – Add(float x, float y); – Two functions use the same name for the same purpose with different data parameters 5/24/2017 24 Class Dependencies 5/24/2017 25