Object Oriented Design Principle Class Design Principles

1. Mental Log - Week 3
Object Oriented Design Principles
Class Design Principles
Dinh Hoang Long
Mental Log
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2. Outline
Overview
Bad Design
Class Design Principles
Single Responsibility Principle
Open-Closed Principle
Liskov’s Substitution Principle
Interface Segregation Principle
Dependency Inversion Principle
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3. Overview
Design Principles
“Software design principles represent a set of guidelines that
helps us to avoid having a bad design.”
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4. Overview
Design Principles
“Software design principles represent a set of guidelines that
helps us to avoid having a bad design.”
Identified by Robert C. Martin (Uncle Bob) in the early 2000s.
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5. Overview
Design Principles
“Software design principles represent a set of guidelines that
helps us to avoid having a bad design.”
Identified by Robert C. Martin (Uncle Bob) in the early 2000s.
Gathered in ”Agile Software Development: Principles,
Patterns, and Practices” (2003).
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6. Overview
Design Principles
“Software design principles represent a set of guidelines that
helps us to avoid having a bad design.”
Identified by Robert C. Martin (Uncle Bob) in the early 2000s.
Gathered in ”Agile Software Development: Principles,
Patterns, and Practices” (2003).
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7. Overview
Design Principles
“Software design principles represent a set of guidelines that
helps us to avoid having a bad design.”
Identified by Robert C. Martin (Uncle Bob) in the early 2000s.
Gathered in ”Agile Software Development: Principles,
Patterns, and Practices” (2003).
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8. Object Oriented Design Principles
Class Design Principles (5)
Package Cohesion Principles (3)
Package Coupling Principles (3)
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9. Object Oriented Design Principles
Clas Design Principles
Single Responsibility Principle
Open-Closed Principle
Liskov’s Substitution Principle
Interface Segregation Principle
Dependency Inversion Principle
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10. Object Oriented Design Principles
Package Cohesion Principles
Release Reuse Equivalency Principle
Common Closure Principle
Common Reuse Principle
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11. Object Oriented Design Principles
Package Coupling principles
Acyclic Dependencies Principle
Stable Dependencies Principle
Stable Abstractions Principle
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12. Bad Code
Rigidity - It is hard to change because every change affects
too many other parts of the system.
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13. Bad Code
Rigidity - It is hard to change because every change affects
too many other parts of the system.
Fragility - When you make a change, unexpected parts of the
system break.
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14. Bad Code
Rigidity - It is hard to change because every change affects
too many other parts of the system.
Fragility - When you make a change, unexpected parts of the
system break.
Immobility - It is hard to reuse in another application because
it cannot be disentangled from the current application.
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15. Class Design Principles
Single Responsibility Principle
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16. Class Design Principles
Single Responsibility Principle
Open-Closed Principle
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17. Class Design Principles
Single Responsibility Principle
Open-Closed Principle
Liskov’s Substitution Principle
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18. Class Design Principles
Single Responsibility Principle
Open-Closed Principle
Liskov’s Substitution Principle
Interface Segregation Principle
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19. Class Design Principles
Single Responsibility Principle
Open-Closed Principle
Liskov’s Substitution Principle
Interface Segregation Principle
Dependency Inversion Principle
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20. Class Design Principles
Single Responsibility Principle
Open-Closed Principle
Liskov’s Substitution Principle
Dependency Inversion Principle
Interface Segregation Principle
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21. Single Responsibility Principle
Figure 1: Real World
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22. Single Responsibility Principle
Figure 1: Real World
Intent:
“A class should have only one reason to change.”
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23. Single Responsibility Principle
Another Definition:
“Every software module should have only one reason to change”
Software Module - Class, Function,...
Reason to change - Responsibility
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24. Single Responsibility Principle
Question
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25. Single Responsibility Principle
Question
A class will have single responsibility.
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26. Single Responsibility Principle
Question
A class will have single responsibility.
A function will have single responsibility.
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27. Single Responsibility Principle
Question
A class will have single responsibility.
A function will have single responsibility.
A class will have more than one function.
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28. Single Responsibility Principle
Question
A class will have single responsibility.
A function will have single responsibility.
A class will have more than one function.
→ A class will have more than one responsibility?
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29. Single Responsibility Principle
Figure 2: Factory Implementation
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30. Single Responsibility Principle
Drawbacks?
The so called ’Class Explosion’. Our application may end
up with too many classes to manage.
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31. Single Responsibility Principle
Drawbacks?
The so called ’Class Explosion’. Our application may end
up with too many classes to manage.
There are a lot of class, so when I want to change
something, I can’t find the code I need to change.
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32. Single Responsibility Principle
The Single-Responsibility Principle is one of the simplest
principles but one of the most difficult to get right.
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33. Single Responsibility Principle
The Single-Responsibility Principle is one of the simplest
principles but one of the most difficult to get right.
Recommendation
“The first rule of functions is that they should be small. The
second rule of functions is that they should be smaller than
that.” (Clean Code, Chapter 3: Functions)
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34. Single Responsibility Principle
The Single-Responsibility Principle is one of the simplest
principles but one of the most difficult to get right.
Recommendation
“The first rule of functions is that they should be small. The
second rule of functions is that they should be smaller than
that.” (Clean Code, Chapter 3: Functions)
“The first rule of classes is that they should be small. The
second rule of classes is that they should be smaller than
that.” (Clean Code, Chapter 10: Classes)
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35. Single Responsibility Principle
The Single-Responsibility Principle is one of the simplest
principles but one of the most difficult to get right.
Recommendation
“The first rule of functions is that they should be small. The
second rule of functions is that they should be smaller than
that.” (Clean Code, Chapter 3: Functions)
“The first rule of classes is that they should be small. The
second rule of classes is that they should be smaller than
that.” (Clean Code, Chapter 10: Classes)
With functions we measured size by counting physical lines.
With classes we count responsibilities.
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36. Open-Closed Principle
Intent:
Software entities (classes, modules, functions, etc.) should be
open for extension but closed for modification.
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37. Open-Closed Principle
Intent:
Software entities (classes, modules, functions, etc.) should be
open for extension but closed for modification.
“Open For Extension”
The behavior of the module can be extended.
We can make the module behave in new and different ways as the
requirements of the application change, or to meet the needs of new
applications.
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38. Open-Closed Principle
Intent:
Software entities (classes, modules, functions, etc.) should be
open for extension but closed for modification.
“Open For Extension”
The behavior of the module can be extended.
We can make the module behave in new and different ways as the
requirements of the application change, or to meet the needs of new
applications.
“Closed For Modification”
The source code of such a module is inviolate. No one is allowed to
make source code changes to it.
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39. Open-Closed Principle
Abstraction is the Key
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40. Open-Closed Principle
Abstraction is the Key
Figure 3: Graph Editor (1)
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41. Open-Closed Principle
Abstraction is the Key
Figure 4: Graph Editor (2)
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42. Open-Closed Principle
Abstraction is the Key
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43. Open-Closed Principle
Abstraction is the Key
Well designed code can be extended without modification
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44. Open-Closed Principle
Abstraction is the Key
Well designed code can be extended without modification
In well designed program, new features are added by adding
new code, rather than by change old, already working code
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45. Open-Closed Principle
Abstraction is the Key
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46. Open-Closed Principle
Abstraction is the Key
What are the characteristics of the best inheritance
hierarchies?
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47. Open-Closed Principle
Abstraction is the Key
What are the characteristics of the best inheritance
hierarchies?
Liskov’s Substitution Principle
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48. Liskov’s Substitution Principle
Barbara Liskov, “Data Abstraction and Hierarchy” (1988)
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49. Liskov’s Substitution Principle
Barbara Liskov, “Data Abstraction and Hierarchy” (1988)
What is wanted here is something like the following substitution property:
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50. Liskov’s Substitution Principle
Barbara Liskov, “Data Abstraction and Hierarchy” (1988)
What is wanted here is something like the following substitution property:
If for each object o1 of type S, there is an object o2 of type T
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51. Liskov’s Substitution Principle
Barbara Liskov, “Data Abstraction and Hierarchy” (1988)
What is wanted here is something like the following substitution property:
If for each object o1 of type S, there is an object o2 of type T
such that
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52. Liskov’s Substitution Principle
Barbara Liskov, “Data Abstraction and Hierarchy” (1988)
What is wanted here is something like the following substitution property:
If for each object o1 of type S, there is an object o2 of type T
such that
for all programs P defined in terms of T,
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53. Liskov’s Substitution Principle
Barbara Liskov, “Data Abstraction and Hierarchy” (1988)
What is wanted here is something like the following substitution property:
If for each object o1 of type S, there is an object o2 of type T
such that
for all programs P defined in terms of T,
the behavior of P is unchanged
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54. Liskov’s Substitution Principle
Barbara Liskov, “Data Abstraction and Hierarchy” (1988)
What is wanted here is something like the following substitution property:
If for each object o1 of type S, there is an object o2 of type T
such that
for all programs P defined in terms of T,
the behavior of P is unchanged
when o1 is substituted for o2
then S is a subtype of T.
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55. Liskov’s Substitution Principle
Robert Martin
Function that use pointers or references to base classes must be
able to use object of derived classes without knowing it.
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56. Liskov’s Substitution Principle
Robert Martin
Function that use pointers or references to base classes must be
able to use object of derived classes without knowing it.
Intent: Derived classes should be substitutable for base classes.
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57. Liskov’s Substitution Principle
Derived classes should be substitutable for base classes.
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58. Liskov’s Substitution Principle
Derived classes should be substitutable for base classes.
We can always write
BaseClass b = new DerivedClass();
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59. Liskov’s Substitution Principle
Derived classes should be substitutable for base classes.
We can always write
BaseClass b = new DerivedClass();
Why would such a principle be made?
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60. Liskov’s Substitution Principle
class Rectangle{
private int width;
private int height;
public setWidth(int width_in);
public setHeight(int height_in);
public int getWidth ();
public int getHeight ();
public int getArea( return width * height );
}
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61. Liskov’s Substitution Principle
class Rectangle{
private int width;
private int height;
public setWidth(int width_in);
public setHeight(int height_in);
public int getWidth ();
public int getHeight ();
public int getArea( return width * height );
}
Clearly, a square is a rectangle for all normal intents and purposes.
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62. Liskov’s Substitution Principle
class Rectangle{
private int width;
private int height;
public setWidth(int width_in);
public setHeight(int height_in);
public int getWidth ();
public int getHeight ();
public int getArea( return width * height );
}
Clearly, a square is a rectangle for all normal intents and purposes.
class Square extends Rectangle{
// Code specific to square
}
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63. Liskov’s Substitution Principle
class Rectangle{
private int width;
private int height;
public setWidth(int width_in);
public setHeight(int height_in);
public int getWidth ();
public int getHeight ();
public int getArea( return width * height );
}
Clearly, a square is a rectangle for all normal intents and purposes.
class Square extends Rectangle{
// Code specific to square
}
class RectangleFactory (){
public static Rectangle createRectangle (int type){
switch(type){
case 1: return new Rectangle ();
case 2: return new Square ();
}
}
}
Rectangle a = RectangelFactory :: createRectangle (2);
a.setWidth (10);
a.setHeight (5);
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64. Liskov’s Substitution Principle
class Rectangle{
private int width;
private int height;
public setWidth(int width_in);
public setHeight(int height_in);
public int getWidth ();
public int getHeight ();
public int getArea( return width * height );
}
class Square extends Rectangle{
public setWidth(int width_in){
Rectangle :: setWidth(width_in);
Rectangle :: setHeight(width_in);
}
public setHeight(int height_in){
Rectangle :: setWidth(height_in);
Rectangle :: setHeight(height_in);
}
}
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65. Liskov’s Substitution Principle
class Rectangle{
private int width;
private int height;
public setWidth(int width_in);
public setHeight(int height_in);
public int getWidth ();
public int getHeight ();
public int getArea( return width * height );
}
class Square extends Rectangle{
public setWidth(int width_in){
Rectangle :: setWidth(width_in);
Rectangle :: setHeight(width_in);
}
public setHeight(int height_in){
Rectangle :: setWidth(height_in);
Rectangle :: setHeight(height_in);
}
}
Rectangle a = RectangleFactory : createRectangle (2);
a.setWidth (10); // both width and height are set to 10
a.setHeight (5); //now , both width and height are 5
println(a.getArea ());
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66. Liskov’s Substitution Principle
Violating LSP!
Changing the behavior of Width and Height properties in derived
class.
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67. Liskov’s Substitution Principle
Violating LSP!
Changing the behavior of Width and Height properties in derived
class.
Consider postcondition of Rectangle::setWidth(int width in):
assert (( width == width_in) && (height == old.height))
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68. Liskov’s Substitution Principle
Design by Contract: Introduced by Bertrand Meyer, in Object
Oriented Software Construction, 1988.
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69. Liskov’s Substitution Principle
Design by Contract: Introduced by Bertrand Meyer, in Object
Oriented Software Construction, 1988.
Preconditions: The preconditions must be true in order for the
method to execute.
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70. Liskov’s Substitution Principle
Design by Contract: Introduced by Bertrand Meyer, in Object
Oriented Software Construction, 1988.
Preconditions: The preconditions must be true in order for the
method to execute.
Postconditions: Upon completion, the method guarantees
that the postconditions will be true.
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71. Liskov’s Substitution Principle
Design by Contract: Introduced by Bertrand Meyer, in Object
Oriented Software Construction, 1988.
Preconditions: The preconditions must be true in order for the
method to execute.
Postconditions: Upon completion, the method guarantees
that the postconditions will be true.
...when redefining a routine [in a derivative], you may only
replace its precondition by a weaker one, and its postcondition
by a stronger one.
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72. Dependency Inversion Principle
Intent:
High Level Modules should not depend upon low level
modules. Both should be depend upon abstractions.
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73. Dependency Inversion Principle
Intent:
High Level Modules should not depend upon low level
modules. Both should be depend upon abstractions.
Abstractions should not depend upon details. Details should
depend upon abstractions.
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74. Dependency Inversion Principle
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75. Dependency Inversion Principle
Figure 5: The Copy Program
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76. Interface Segregation Principle
Intent:
The clients should not be forced to depend upon interfaces that
they do not use.
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77. Interface Segregation Principle
Intent:
The clients should not be forced to depend upon interfaces that
they do not use.
Another definition:
Many client specific interfaces are better than one general
purpose interface.
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78. Summary
Single Responsibility Principle
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79. Summary
Single Responsibility Principle
Open-Closed Principle
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80. Summary
Single Responsibility Principle
Open-Closed Principle
Liskov’s Substitution Principle
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81. Summary
Single Responsibility Principle
Open-Closed Principle
Liskov’s Substitution Principle
Interface Segregation Principle
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82. Summary
Single Responsibility Principle
Open-Closed Principle
Liskov’s Substitution Principle
Interface Segregation Principle
Dependency Inversion Principle
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83. Summary
Single Responsibility Principle
Open-Closed Principle
Liskov’s Substitution Principle
Interface Segregation Principle
Dependency Inversion Principle
A principle is only a principle. In order to avoid bad design and
make a flexible design, we should spend a lot of time and effort.
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84. Thank you for watching!
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