![Procedural (open) version
enum ShapeType {circle, square};
struct Shape
{enum ShapeType itsType;};
struct Circle
{
enum ShapeType itsType;
double itsRadius;
Point itsCenter;
};
void DrawCircle(struct Circle*)
struct Square
{
enum ShapeType itsType;
double itsSide;
Point itsTopLeft;
};
void DrawSquare(struct Square*)
#include <Shape.h>
#include <Circle.h>
#include <Square.h>
typedef struct Shape* ShapePtr;
void
DrawAllShapes(ShapePtr list[], int n)
{
int i;
for( i=0; i< n, i++ )
{
ShapePtr s = list[i];
switch ( s->itsType )
{
case square:
DrawSquare((struct Square*)s);
break;
case circle:
DrawCircle((struct Circle*)s);
break;
}
}
}
Shape.h
Circle.h
Square.h
DrawAllShapes.c
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![A Closed Implementation
Class Shape
{
public:
virtual void Draw() const =0;
};
#include <Shape.h>
void
DrawAllShapes(Shape* list[],int n)
{
for(int i=0; i< n; i++)
list[i]->draw();
}
Shape.h
DrawAllShapes.cpp
Circle.h
Square.h
Class Square: public Shape
{
public:
virtual void Draw() const;
};
Class Circle: public Shape
{
public:
virtual void Draw() const;
};
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Lec15a1-Object-Oriented Development.ppt
- 1. COMP 2710 Software Construction Object-Oriented Development Dr. Xiao Qin Auburn University http://www.eng.auburn.edu/~xqin xqin@auburn.edu Some slides are adapted from notes by Robert C. Martin 1-1
- 3. First Version All designs start well The program is an overnight success! How could it be more simple, elegant, and maintainable? ReadKeyboard Copy WritePRinter void copy(void) { int ch; while( (ch=ReadKeyboard()) != EOF) WritePrinter(ch); } 1-3
- 4. Second Version ⢠We sometimes want to read from paper tape reader. ⢠We could put a parameter in the call, but we have hundreds of users already! ⢠No big deal, this is just an exception⌠we can make it work. Oh, no! Nobody said the requirements might change!
- 5. Second Version Design ReadKeyboard Copy WritePrinter ReadTape bool GtapeReader = false; // remember to clear void copy(void) { int ch; while( (ch=GtapeReader ? ReadTape() : ReadKeyboard()) != EOF) WritePrinter(ch); } 1-5
- 6. Third Version How unexpected! Requirements changed again! bool GtapeReader = false; Bool GtapePunch = false; // remember to clear void copy(void) { int ch; while( (ch=GtapeReader ? ReadTape() : ReadKeyboard()) != EOF) GtapePunch ? WritePunch(ch) : WritePrinter(ch); } It seems that sometimes we need to write to a paper tape punch. Weâve had this problem before, and just added a flag. Looks like it should work again. 1-6
- 7. Example of a Good Design Why it is a good design? First and only version. void Copy() { int c; while( (c=getchar()) != EOF) putchar(c); } But wait! Arenât we supposed to be learning OO design? This isnât OO is it? 1-7
- 8. âŚis it? ⢠FILE is an abstraction â It represents some kind of byte stream â It has many variations ⢠It has methods â Read, Write, getchar, putchar, etc â The methods are *dynamically* bound It is a small program based on abstractions! FILE is a class, just implemented differently. 1-8
- 9. Rephrased in OO Copy Reader Writer KeyboardReader PrinterWriter ÂŤinterfaceÂť ÂŤinterfaceÂť class Reader { public: char read(); }; class Writer { public: void write(char c); } class Copy { Copy(Reader r, Writer w) { itsReader = r; itsWriter = w; } public: void copy() { int c; while( (c==itsReader.read()) != EOF ) itsWriter.write(c); } private: Reader itsReader; Writer itsWriter; }; 1-9
- 10. Class Design Principles ⢠SRP: The Single Responsibility Principle ⢠OCP: The Open/Closed Principle ⢠LSP: The Liskov Substitution Principle ⢠ISP: The Interface Segregation Principle ⢠DIP: The Dependency Inversion Principle Reference: Agile Software Development: Principles, Patterns, and Practices. Robert C. Martin, Prentice Hall, 2002. 1-10
- 11. The Single Responsibility Principle ⢠A class should have one, and only one, reason to change. + CalcPay + ReportHours + WriteEmployee Employee Payroll 1-11
- 12. The Single Responsibility Principle. (SRP) + CalcPay Employee Payroll Report Writer Employee Repository 1-12
- 13. Open/Closed Principle ⢠A principle which states that we should add new functionality by adding new code, not by editing old code. ⢠Defines a lot of the value of OO programming ⢠Abstraction is the key âModules should be open for extension, but closed for modificationâ -Bertrand Meyer 1-13
- 14. Abstraction is Key ⢠Client/Server relationships are âopenâ ⢠Changes to servers cause changes to clients ⢠Abstract servers âcloseâ clients to changes in implementation. Abstraction is the most important word in OOD Client Server Client Abstract Server Concrete Server 1-14
- 15. The Shape Example ⢠Procedural (not closed) implementation ⢠OO (closed) implementation 1-15
- 16. Procedural (open) version enum ShapeType {circle, square}; struct Shape {enum ShapeType itsType;}; struct Circle { enum ShapeType itsType; double itsRadius; Point itsCenter; }; void DrawCircle(struct Circle*) struct Square { enum ShapeType itsType; double itsSide; Point itsTopLeft; }; void DrawSquare(struct Square*) #include <Shape.h> #include <Circle.h> #include <Square.h> typedef struct Shape* ShapePtr; void DrawAllShapes(ShapePtr list[], int n) { int i; for( i=0; i< n, i++ ) { ShapePtr s = list[i]; switch ( s->itsType ) { case square: DrawSquare((struct Square*)s); break; case circle: DrawCircle((struct Circle*)s); break; } } } Shape.h Circle.h Square.h DrawAllShapes.c 1-16
- 17. What is wrong with the code? ⢠DrawAllShapes is not closed. â Switch/case tend to recur in diverse places. â If we add a shape, we add to the switch/case â All switch/case statements must be found and editd. â Switch/Case statements are seldom this tidy â When we add to the enum, we must rebuild everything ⢠The software is both rigid and brittle It can be demonstrated to work. Isnât that the important thing? 1-17
- 18. A Closed Implementation Class Shape { public: virtual void Draw() const =0; }; #include <Shape.h> void DrawAllShapes(Shape* list[],int n) { for(int i=0; i< n; i++) list[i]->draw(); } Shape.h DrawAllShapes.cpp Circle.h Square.h Class Square: public Shape { public: virtual void Draw() const; }; Class Circle: public Shape { public: virtual void Draw() const; }; 1-18
- 19. Open/Closed Review ⢠What does the open/closed principle say? ⢠What does that mean practically? ⢠How can it be achieved? ⢠What is strategic closure? â How can this be achieved in design? â What if you canât close completely? 1-21