Lecture01

Object Oriented Software Modeling and Design CE 350 Abdel-Karim Al-Tamimi, Ph.D. [email_address] http://faculty.yu.edu.jo/altamimi Al-Tamimi 2010 ©

Overview
Object Oriented Concepts
Object Oriented Programming (OOP) vs. Structured/Procedural Programming
Al-Tamimi 2010 ©

Introduction to OOP
Things in real life are objects , and an object has both: attributes and behaviors associated with it
Adam is a member of class Person , as a member of this class it takes on Person attributes and behaviors
Walking, Eating, Sleeping, … Eye-Color, Age, Height, … Attributes Behaviors

OOP Concepts
An object may have only attributes
Multiple attributes: struct
A single attribute (e.g. integer)
Attributes = data
Behavior = functions
Restricting the access to object’s data or functions is called data hiding
Combining data and behavior in the same object is called encapsulation

OOP vs. Procedural Programming Inputs Outputs object OOP Global Data Procedural Function 1 Function 2 Function 4 Function 3

Moving from Procedural to OOP
Procedural separates data from the data-manipulations done on it
OOP encapsulates both data and its manipulation inside one object

Procedural/Structured Programming
Carry out the required tasks using functions, if the function is too big, break it down to smaller functions
Functional decomposition
Functions are correlated with each other to form a data flow diagram (DFD)
Input Process Output

Structured Programming Students Exams Courses Tutors Add Student Enter for Exams Check Exam Marks Issue Certificate Expel Student

Structured Programming
Harder to maintain as the system grows
The impact of one change can affect many functions, which are hard to trace and to re-test
To overcome these difficulties in maintaining the code and to perform expected adjustments, object oriented approach was introduced

How to Think in Terms of Objects
Know the difference between interface and implementation
Think more abstractly
Give the user the minimal interface possible

Object to Object Communications MyObject AnotherObject Data Data MyFunction1 MyFunction2 Function_1 Function_2

Object to Object Communications
Each object provide a specific interface to allow other objects to communicate with it
The class interface includes the following:
The method/function name
The inputs passed to the method/function
The return type of the method/function
Any public data is considered also as a part of the class interface(better use get/set methods)
By designing the class interfaces , the developer hide extra or sensitive data

Object to Object Communications

Interface vs. Implementation

Interface vs. Implementation
Only the services the end user needs are represented
Data hiding with the use of encapsulation
Change in the class implementation should not require change in the class user’s code
Interface is still the same
Always provide the minimal interface
Use abstract thinking in designing interfaces
No unnecessary steps
Implement the steps in the class implementation

Interface vs. Implementation Student Age, GPA, … +getAge():integer +admit():void +expel():void +getFullName():string attributes methods

How to determine the minimum interface possible
Only what the user absolutely needs
Fewer interfaces as possible
Use polymorphism
Starts with hiding everything (private)
Only use public interfaces (try not to use public attributes, instead get/set)
Design your class from the user’s perspective and what they need (meet the requirements)

Determining the Class’s Environment
Determine the class user
What type of technical programmer
Determine the behavior of the object
Environmental constraints on objects
Hardware or/and software constraints
The next step is the implementation, anything that is not a public interface is considered an implementation

Concept of Scope
Local attributes
Local to a specific method
Object attributes
Attributes shared between object methods
Class attributes
Attributes share between objects
Starts with static identifier
public class Employee{ … static int Count; … }

Object Attributes vs. Class Attributes

Inheritance
Inheritance allows reusing code, with a better organization and overall management
Inheritance allows classes to inherit attributes from other classes, and you can still overwrite common behaviors
Inheritance categorizes classes into superclasses (parent class) and subclasses (child class)

Inheritance

Multiple Inheritance
Class can inherit multiple classes
In real life: kids inherit both father and mother attributes and behaviors
Multiple inheritance is a complex solution that can solve complex problems elegantly
Problem to the designer
Problem to the programming language compiler writer
Java and C# decided not to allow it in the form C++ allows it
C# uses interfaces to carry on the architecture of methods that a child class should have

Polymorphism
Polymorphism means many shapes
Polymorphism allows a single behavior to have different interfaces, and thus different inner-object behavior
Polymorphism means that similar objects can respond to the same message in different manners
Polymorphism gives the developer more options and different levels of customization to achieve a certain data manipulation or output

Polymorphism Al-Tamimi 2010 © +ReturnArea():double Shape +ReturnArea():double Circle +ReturnArea():double Square Area = Length^2 Area =   Radius^2 Returns a double value

Polymorphism Al-Tamimi 2010 © +GetArea(Shape) MainClass Will call Shape.ReturnArea() Area = Length^2 Area =   Radius^2 If shape is Circle If shape is Square

Constructors
Only with objects
No return value (even void)
Method name is identical to class name
public Student(int StudentNumber) { /* Implementation Code */ } ... Student BestStudent = new Student(1234); ...

Constructors
Default constructor (no arguments)
Any object will have a constructor even if you do not define it, since it inherits its superclass constructor
Every object is a subclass of object class
Superclass constructor is called by default
Always provide a constructor
Overloading methods/constructors
Same method name, different parameters
Polymorphism : using different objects that have the same superclass (child override parent)

Constructors
Initialize all attributes
Make sure that the initialization values leaves the object in a stable and safe state
Do not include implementation in the constructor
Call a method if needed
Student Age, GPA, … +Student: +getAge:integer +admit:void +expel:void +getFullName:string

Error Handling
What can be done as a software developer when there is a potential risk of causing an error:
Ignore the problem that might cause the error
Very dangerous, even if it has a very low probability
Check the problem then abort the program
Handle the error by aborting the program, and show the error message
Check the problem, and try to fix it
You already know how to fix it, may not be the best solution
Check the problem and throw an exception
Throw the exception to the calling object to handle it

Exception Handling
Exception is unexpected event that happens in the system
We use keywords : try and catch and throw to manage exceptions
try { // potential risky code } catch( Exception object) { // how to solve the exception // can not solve it, throw an exception }

Composition
Composition is to combine multiple objects to achieve a better or more complex objects
Example : A computer is composed of different objects : memory, hard drive, motherboard, and each of these objects are composed of smaller objects … etc

Resources
Matt Weisfeld, The Object Oriented Thought Process , Sams Publishing, ISBN:0-672-32611-6

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