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Object-Oriented Concepts:

Object-Oriented Concepts Shehrevar Davierwala shehrevar@live.com 1

Object-Oriented Programming:

Object-Oriented Programming Objects can be used effectively to represent real-world entities For instance, an object might represent a particular employee in a company Each employee object handles the processing and data management related to that employee 2

Objects:

Objects An object has: state - descriptive characteristics behaviors - what it can do (or what can be done to it) The state of a bank account includes its account number and its current balance The behaviors associated with a bank account include the ability to make deposits and withdrawals Note that the behavior of an object might change its state 3

Classes:

Classes An object is defined by a class A class is the blueprint of an object Multiple objects can be created from the same class 4

Objects and Classes:

Objects and Classes 5 Bank Account A class (the concept) John’s Bank Account Balance: $5,257 An object (the realization) Bill’s Bank Account Balance: $1,245,069 Mary’s Bank Account Balance: $16,833 Multiple objects from the same class

Attributes:

Attributes Contain current state of an object. Attributes can be classified as simple or complex. Simple attribute can be a primitive type such as integer, string, etc., which takes on literal values. Complex attribute can contain collections and/or references. Reference attribute represents relationship. complex object: contains one or more complex attributes 6

Methods and messages:

Methods and messages Method : Defines behavior of an object, as a set of encapsulated functions. Message: Request from one object to another asking second object to execute one of its methods. 7 (a) (b) (a) Object showing attributes and methods (b) Example of a method

Classes:

Classes Class : Blueprint for defining a set of similar objects. Objects in a class are called instances . 8

Inheritance:

Inheritance One class can be used to derive another via inheritance Classes can be organized into hierarchies 9 Bank Account Account Charge Account Savings Account Checking Account

Inheritance (contd):

Inheritance (contd) Inheritance allows one class of objects to be defined as a special case of a more general class. Special cases are subclasses and more general cases are superclasses . 10 Generalization: process of forming a superclass Specialization: forming a subclass Subclass inherits all properties of its superclass and can define its own unique properties. Subclass can redefine inherited methods. All instances of subclass are instances of superclass. Principle of substitutability : instance of subclass can be used whenever method/construct expects instance of superclass. A KIND OF (AKO): Name for relationship between subclass and superclass

Types of inheritance:

Types of inheritance 11 (a) (b) (c) (a) Single (b) Multiple (c) Repeated (b)

Inheritance (contd):

Inheritance (contd) Define humanBeing to be a class A humanBeing has attributes , such as age, height, gender Assign values to attributes when describing object Define Parent to be a subclass of HumanBeing A Parent has all attributes of a HumanBeing, plus attributes of his/her own (name of oldest child, number of children) A Parent inherits all attributes of humanBeing The property of inheritance is an essential feature of object-oriented languages such as Smalltalk, C++, Ada 95, Java (but not C, FORTRAN) 12

Inheritance (contd):

Inheritance (contd) UML notation Inheritance is represented by a large open triangle 13

Java implementation:

Java implementation 14

Overriding and Overloading:

Overriding and Overloading Overriding: Process of redefining a property within a subclass. Overloading: Allows name of a method to be reused with a class or across classes. 15 Overriding Example: Might define method in Staff class to increment salary based on commission method void giveCommission(float branchProfit) { salary = salary + 0.02 * branchProfit; } May wish to perform different calculation for commission in Manager subclass: method void giveCommission(float branchProfit) { salary = salary + 0.05 * branchProfit; }

Aggregation :

Aggregation UML Notation 16

Association:

Association UML Notation 17

Equivalence of Data and Action:

Equivalence of Data and Action Classical paradigm record_1.field_2 Object-oriented paradigm thisObject.attributeB thisObject.methodC() 18

Example – :

Example – Design of an operating system for a large mainframe computer. It has been decided that batch jobs submitted to the computer will be classified as high priority, medium priority, or low priority. There must be three queues for incoming batch jobs, one for each job type. When a job is submitted by a user, the job is added to the appropriate queue, and when the operating system decides that a job is ready to be run, it is removed from its queue and memory is allocated to it Design 1 (Next slide) Low cohesion—operations on job queues are spread all over product 19

Data Encapsulation — Design 1:

Data Encapsulation — Design 1 20

Data Encapsulation — Design 2:

Data Encapsulation — Design 2 21

Data Encapsulation:

Data Encapsulation m_encapsulation has informational cohesion m_encapsulation is an implementation of data encapsulation Data structure ( job_queue ) together with operations performed on that data structure Advantages Development Maintenance 22

Data Encapsulation and Development:

Data Encapsulation and Development Data encapsulation is an example of abstraction Job queue example Data structure job_queue Three new functions i nitialize_job_queue add_job_to_queue delete_job_from_queue Abstraction Conceptualize problem at higher level job queues and operations on job queues not lower level records or arrays 23

Stepwise Refinement:

Stepwise Refinement Step 1: Design in terms of high level concepts It is irrelevant how job queues are implemented Step 2: Design low level components Totally ignore what use will be made of them In 1st step, assume existence of lower level Concern is the behavior of the data structure job_queue In 2nd step, ignore existence of high level Concern is the implementation of that behavior In a larger product, there will be many levels of abstraction 24

Data Encapsulation and Maintenance:

Data Encapsulation and Maintenance Identify aspects of product likely to change Design product so as to minimize the effects of change Data structures are unlikely to change Implementation may change Data encapsulation provides a way to cope with change 25

Implementation of Class JobQueue:

Implementation of Class JobQueue C++ Java 26

Implementation of queueHandler:

Implementation of queueHandler C++ Java 27

Data Encapsulation and Maintenance (contd):

Data Encapsulation and Maintenance (contd) What happens if queue is now implemented as a two-way linked list of JobRecord ? Module that uses JobRecord need not be changed at all, merely recompiled C++ Java 28

Abstract Data Types:

Abstract Data Types Problem with both implementations Only one queue Need We need: Data type + operations performed on instantiations of that data type Abstract data type 29

Abstract Data Type (contd):

Abstract Data Type (contd) (Problems caused by public attributes solved later) 30

Information Hiding:

Information Hiding Data abstraction Designer thinks at level of an ADT Procedural abstraction Define a procedure—extend the language Instances of a more general design concept, information hiding Design the modules in way that items likely to change are hidden Future change is localized Changes cannot affect other modules 31

Information Hiding (contd):

Information Hiding (contd) C++ abstract data type implementation with information hiding 32

Information Hiding (contd):

Information Hiding (contd) Effect of information hiding via private attributes 33

Polymorphism and Dynamic Binding:

Polymorphism and Dynamic Binding Classical paradigm Must explicitly invoke correct version 34

Polymorphism and Dynamic Binding (contd):

Polymorphism and Dynamic Binding (contd) Object-oriented paradigm 35

Polymorphism and Dynamic Binding (contd):

Polymorphism and Dynamic Binding (contd) All that is needed is myFile.open() Correct method invoked at run-time (dynamically) Method open can be applied to objects of different classes Polymorphic 36

Polymorphism and dynamic binding (contd):

Polymorphism and dynamic binding (contd) Method checkOrder (b : Base) can be applied to objects of any subclass of Base 37

Polymorphism and Dynamic Binding (contd):

Polymorphism and Dynamic Binding (contd) Can have a negative impact on maintenance Code is hard to understand if there are multiple possibilities for a specific method Polymorphism and dynamic binding Strength and weakness of the object-oriented paradigm 38

Polymorphism and dynamic binding (contd):

Polymorphism and dynamic binding (contd) Polymorphism: Means ‘ many forms ’. Dynamic Binding: Runtime process of selecting appropriate method based on an object’s type. 39 Example: With list consisting of an arbitrary no. of objects from the Staff hierarchy, we can write: list[i]. print and runtime system will determine which print() method to invoke depending on the object’s (sub)type.

Cohesion and Coupling of Objects:

Cohesion and Coupling of Objects No such thing! Object-oriented cohesion and coupling always reduces to classical cohesion The only feature unique to the object-oriented paradigm is inheritance Cohesion has nothing to do with inheritance Two objects with the same functionality have the same cohesion It does not matter if this functionality is inherited or not Similarly, so-called object-oriented coupling always reduces to classical coupling 40

Object-Oriented Metrics (contd):

Object-Oriented Metrics (contd) Two types of so-called object-oriented metric Not related to inheritance Reduces to a classical metric Inheritance-related May reduce to a classical metric No problem Classical metrics work just fine But don’t mislead others by calling them object-oriented 41

Advantages of Objects:

Advantages of Objects Same as as advantages of abstract data types Information hiding Data abstraction Procedural abstraction Inheritance provides further data abstraction Easier and less error-prone product development Easier maintenance Objects are more reusable than modules with functional cohesion 42

Summary:

Summary 43

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