Designing Classes in OOP: UH Lecture, Spring 2005, Exams of Computer Science

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D. Mirkovic, C++ Programming, Spring 2005

Lecture 5: Designing Classes

Dragan Mirkovic

Department of Computer Science

University of Houston

Announcements

• Today:
  • Designing classes in OOL
  • Ch 5. In Irvine.
  • Midterm exam
    • Thursday 3/10/

D. Mirkovic, C++ Programming, Spring 2005

Introduction

• So far we have discussed the syntax

and semantics of C++

• Here we discuss the basic ideas on the

design of classes and components

• Object model
• How to build components

Analysis, Design and Programming

• OO programming concentrates on the physical entities that make up an application
• OO design: method that uses oo decomposition and abstraction to implement a model of a system
• OO decomposition:
  • Using class and object abstractions to logically structure a system
• OO analysis:
  • Examination of application requirements in terms of classes and objects

D. Mirkovic, C++ Programming, Spring 2005

Abstraction types

• Two general types of abtractions:
  • Control (action) type:
    • Operations (insert, move, resize, find, …)
  • Entity type
    • Specific static elements in the problem domain
    • Easy to recognize (correspond to real objects)
    • Examples:
      • Window, display, cursor, screen, …
      • Employee, Student, Course, …

Encapsulation

• Enclosing of both variables and

functions inside a class with a well

defined interface

• Achieved though information hiding
  • Implementation details are hidden
  • User interface is public and well defined
• Leads to standardization of interfaces
  • Great improvement in software development

D. Mirkovic, C++ Programming, Spring 2005

Modularity

• One of the oldest and best concepts in programming - Compilation units (sharing and hiding of information) - Subroutines (common subtasks used in different parts of the application) - Modules
• Procedural languages achieve modularity by using procedural abstractions - Defining problem in terms of its operations

Hierarchy

• Hierarchy (inheritance)
  • Parent-child relationship between classes
  • Derived classes:
    • Inherit operations and data from their base classes
    • Only additional attributes need to be defined
• Example:
  • Point
  • Point3D

x y x y z

D. Mirkovic, C++ Programming, Spring 2005

Software design goals

• Simplicity
  • Well designed and meaningful objects
  • User friendly class interfaces
• Flexibility
  • Easy accommodation of changes
• Extensibility
  • Additions are easy to implement without destroying initial design
• Portability
  • Hardware independence
• Reusability
  • Well designed software can be used in large number of applications

Designing OO Programs

• OO program should be a faithful model of reality - It is important to identify key concepts in an application - Construct corresponding classes - Iterative process
• Traditional approach:
  • Top-down procedural design
  • Dependencies are hierarchical
  • Changes may be difficult to implement

D. Mirkovic, C++ Programming, Spring 2005

Components

• Component = group of related classes

that work together

• Well defined interfaces
• Design steps:
• Find the classes
• Specify class operations
• Specify class dependencies
• Specify class interfaces

Find the Classes

• This is the most obvious step in the

design

• If the class is not obvious it shouldn't be there!
• Discovery of inherent structure of the

application

• It is important to understand the

application well

D. Mirkovic, C++ Programming, Spring 2005

Class interfaces

• Function prototypes for member

functions define the class interface

• The interfaces can be shared with other

programmers

• Efficient design

Example

• Doctor’s Office Scheduling
• 1 st^ Step: Specifications
  • Automatically schedule appointments for patients
  • Multiple patients and doctors
  • 15 min time slots between 8:00 am and 6:00 pm.
  • Print out a separate daily schedule for each doctor
  • Interactive program with screen i/o and file output.

D. Mirkovic, C++ Programming, Spring 2005

2 nd^ Step: Analysis

• Find the classes:
  • What are the objects in the application
    • Doctor
    • Patient
    • DailySchedule
    • Appointment
    • Scheduler
• What would be a typical scenario for the appointment scheduling process: - A Patient enters her/his name into the Scheduler - Patient chooses a Doctor - Scheduler adds the appointment to the doctor’s schedule and to the patients records - Scheduler confirms the appointment

Class Dependencies

• Interactions between objects Scheduler

Doctor Patient

DailySchedule Appointment Notation: Composite Link

D. Mirkovic, C++ Programming, Spring 2005

Additional Classes

• We will need a time slot class
  • Translation and formatting of appointment times (enter/display time class in hh:mm format, store as integer) TimeSlot { public:TimeSlot( const unsigned n = 0 ); unsignedfriend istream AsInteger() & operator >>(istream const; // Returns & theinp, integer value TimeSlot & T); of the time. friend// ------ Static ostream & operator <<(ostreammember functions ------- & os, const TimeSlot & T); staticstatic unsigned GetStartHour();unsigned GetApptLen(); staticstatic voidvoid SetStartHour( unsigned nSetApptLen( unsigned n ); ); private:static unsigned StartHour; staticunsigned unsigned ApptLen; intValue; };void^ SetIntValue(^ unsigned^ n^ );

3 rd^ Step: Design

• Once all classes have been identified we can specify their interfaces
• Result of the design phase is the header file with all classes and their interfaces defined
• Classes:
  • TimeSlot
  • Doctor
  • Patient
  • DailySchedule
  • Appointment
  • Scheduler

D. Mirkovic, C++ Programming, Spring 2005

4 th^ Step: Main program

• The main program creates an instance of the Scheduler #include "doctors.h" #include "schedlr.h“ static Doctor doctorArray[NumDoctors]; int main() { cout << "Doctors Office Scheduling Program\n\n"; Scheduler officeSchedule( doctorArray ); officeSchedule.ScheduleAllAppointments(); officeSchedule.PrintAllAppointments("appts.txt"); return 0; }

5 th^ Step: Class Implementations

• See the code in the text.