This document discusses object-oriented programming and design principles. It begins by describing the classical and modern views of objects, with the classical view being that an object represents a physical thing and the modern view being that an object is a collection of related behaviors. The document then discusses how polymorphism and encapsulation are better approaches than inheritance alone. It introduces the SOLID principles for creating high-quality APIs, including the single responsibility, open/closed, Liskov substitution, interface segregation and dependency inversion principles. Finally, it discusses how to recognize "bad" or "stupid" code that violates these principles.

1. Don’t Be STUPID
Grasp SOLID!
Anthony Ferrara

2. Let’s Talk
Object
Oriented
Programming

3. What
Is An
Object?

4. Classic View
Object == Physical “Thing”

5. Classic View
Object == Physical “Thing”
Methods == Actions on “Thing”

6. Classic View
Object == Physical “Thing”
Methods == Actions on “Thing”
Properties == Description of “Thing”

7. Animal
MammalBird Fish
CatCow Dog
Lion Feline Cheetah

9. Classic View
$lion = new Lion;
$lion->roar();
$lion->walkTo($point);
$lion->hunt($zebra);
$lion->sleep();

10. Is This Realistic?

11. Classic View
$lion = new Lion;
$lion->roar();
$lion->walkTo($point);
$lion->hunt($zebra);
$lion->sleep();

12. (9 Months Later)

13. Classic View
$lion = new Lion;
$lion->roar();
$lion->walkTo($point);
$lion->hunt($zebra);
$lion->sleep();

14. Classic View
$lion = new Lion;
$lion->roar();
$lion->walkTo($point);
$lion->hunt($zebra);
$lion->sleep();
Does A Lion Have
A Button To Make
It Roar?

15. Classic View
$lion = new Lion;
$lion->roar();
$lion->walkTo($point);
$lion->hunt($zebra);
$lion->sleep();
Does A Lion Have
A Button To Make
It Roar?What Does It
Mean For An
Object To “Hunt”?

16. Classic View
$lion = new Lion;
$lion->roar();
$lion->walkTo($point);
$lion->hunt($zebra);
$lion->sleep();
Does A Lion Have
A Button To Make
It Roar?What Does It
Mean For An
Object To “Hunt”?
What Is A Lion In
Relation To Our
Application?

17. The Classical
Model Is Easy To
Understand

19. The Classical
Model Is
Completely
Impractical

20. “Modern” View
Object == Collection Of (Related)
Behaviors

21. “Modern” View
Object == Collection Of (Related)
Behaviors
Methods == Behavior

22. “Modern” View
Object == Collection Of (Related)
Behaviors
Methods == Behavior
Properties == Details Of Behavior

23. Classic View == “(conceptually) is a”
Modern View == “behaves as a”

24. interface Number {
function getValue();
function __toString();
function add(Number $n);
function subtract(Number $n);
function equals(Number $n);
function isLessThan(Number $n);
function isGreaterThan(Number $n);
}

25. Number
IntegerFloat Decimal
longshort long long
unsigned signed

26. But Wait!

27. We Don’t Even
Need Inheritance

28. All We Need Is
Polymorphism
And Encapsulation

30. Polymorphism
Behavior Is Determined Dynamically
“Dynamic Dispatch”

31. Procedural Code
if ($a instanceof Long) {
return new Long($a->getValue() + 1);
} elseif ($a instanceof Float) {
return new Float($a->getValue() + 1.0);
} elseif ($a instanceof Decimal) {
return new Decimal($a->getValue() +
1.0);
}

32. Polymorphic Code
return $int->add(new Integer(1));

33. Polymorphic Code
class Integer implements Number {
public function add(Number $a) {
return new Integer(
$this->getValue() +
(int) $a->getValue()
);
}
}

34. Polymorphic Code
class Float implements Number {
public function add(Number $a) {
return new Float(
$this->getValue() +
(float) $a->getValue()
);
}
}

36. Encapsulation
Behavior Is Completely Contained By
The Object’s API
(Information Hiding)

37. Procedural Code
if (5 == $number->value) {
print “Number Is 5”;
} else {
print “Number Is Not 5”;
}

38. Encapsulated Code
if ($number->equals(new Integer(5))) {
print “Number Is 5”;
} else {
print “Number Is Not 5”;
}

39. Encapsulated Code
class Decimal implements Number {
protected $intValue;
protected $exponent;
public function equals(Number $a) {
if ($a instanceof Decimal) {
// Compare Directly
} else {
// Cast
}
}

41. Behavior Is
Defined By The
API

42. Two Types Of Primitive APIs
Interfaces (Explicit)

43. Two Types Of Primitive APIs
Interfaces (Explicit)
Duck Typing (Implicit)

44. If an Object Is A
Collection Of
Behaviors...

45. What Is A
Collection Of
Classes/Objects?

46. APIs
Method

47. APIs
Method MethodMethod
Class

48. APIs
Method MethodMethod
Class ClassClass
Package

49. APIs
Method MethodMethod
Class ClassClass
Package PackagePackage
Library

50. APIs
Method MethodMethod
Class ClassClass
Package PackagePackage
Library
Framework
LibraryLibrary

52. What Makes A
Good API?

53. A Good API
Does One Thing

54. A Good API
Never Changes

55. A Good API
Behaves Like Its
Contract

56. A Good API
Has A Narrow
Responsibility

57. A Good API
Depends Upon
Abstractions

58. And That’s
SOLID
Code

59. S - Single Responsibility Principle
O-
L -
I -
D-
A Good API
Does One
Thing

62. S - Single Responsibility Principle
O- Open / Closed Principle
L -
I -
D-
A Good API
Never Changes

68. S - Single Responsibility Principle
O- Open / Closed Principle
L - Liskov Substitution Principle
I -
D-
A Good API
Behaves Like
Its Contract

72. S - Single Responsibility Principle
O- Open / Closed Principle
L - Liskov Substitution Principle
I - Interface Segregation Principle
D-
A Good API
Has A Narrow
Responsibility

74. S - Single Responsibility Principle
O- Open / Closed Principle
L - Liskov Substitution Principle
I - Interface Segregation Principle
D- Dependency Inversion Principle
A Good API
Depends Upon
Abstractions

77. S - Single Responsibility Principle
O- Open / Closed Principle
L - Liskov Substitution Principle
I - Interface Segregation Principle
D- Dependency Inversion Principle

78. Note That SOLID
Does Not Dictate
What Is Good OOP

79. SOLID Emerges
From Good OOP

81. So, What Makes
A Bad API?

82. Global Variables
(Spooky Action
At A Distance)

83. Depending On
Specifics Of An
Implementation

84. Hidden
Dependencies

85. Unhealthy Focus
On Performance

86. Poorly Named
APIs

87. Duplication

88. And That’s
STUPID
Code

89. S - Singletons
T -
U -
P -
I -
D-
Global Variables
(Spooky Action
At A Distance)

91. S - Singletons
T - Tight Coupling
U -
P -
I -
D-
Depending On
Specifics Of An
Implementation

93. S - Singletons
T - Tight Coupling
U - Untestable Code
P -
I -
D-
Hidden
Dependencies

95. S - Singletons
T - Tight Coupling
U - Untestable Code
P - Premature Optimization
I -
D-
Unhealthy Focus
On Performance

97. S - Singletons
T - Tight Coupling
U - Untestable Code
P - Premature Optimization
I - Indescriptive Naming
D-
Poorly Named
APIs

99. S - Singletons
T - Tight Coupling
U - Untestable Code
P - Premature Optimization
I - Indescriptive Naming
D- Duplication
Duplication
Duplication
DuplicationDuplication
Duplication
Duplication
Duplication
Duplication

101. S - Singletons
T - Tight Coupling
U - Untestable Code
P - Premature Optimization
I - Indescriptive Naming
D- Duplication

102. STUPID
Embodies
Lessons Learned
From Bad OOP

103. What Good OOP Gives You
Modular Code
Reusable Code
Extendable Code
Easy To Read Code
Maintainable Code
Easy To Change Code
Easy To Understand Code
Clean Abstractions (mostly)

104. What Good OOP Costs You
Tends To Have More “Layers”
Tends To Be Slower At Runtime
Tends To Have Larger Codebases
Tends To Result In Over-Abstraction
Tends To Require More Effort To Write
Tends To Require More Tacit Knowledge

105. Let’s Look At
Some Code!

106. interface Car {
public function turnLeft();
public function turnRight();
public function goFaster();
public function goSlower();
public function shiftUp();
public function shiftDown();
public function start();
}

107. interface Steerable {
public function steer($angle);
}
interface Acceleratable {
public function accelerate($amt);
}
interface Shiftable {
public function shiftDown();
public function shiftUp();
}

108. Let’s Look At
Drupal Code!

109. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}

110. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}
What Responsibility?

111. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}
What Responsibility?
Formatting Messages

112. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}
What Responsibility?
Formatting Messages
Encoding Messages

113. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}
What Responsibility?
Formatting Messages
Encoding Messages
Assembling Headers

114. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}
What Responsibility?
Formatting Messages
Encoding Messages
Assembling Headers
Calling Sendmail

115. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}
What Responsibility?
Formatting Messages
Encoding Messages
Assembling Headers
Calling Sendmail
Setting INI settings…?

116. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}
What Responsibility?
Open For Extension?

117. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}
What Responsibility?
Open For Extension?
Edits Require
Copy/Paste

118. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}
What Responsibility?
Open For Extension?
Liskov Substitution...

119. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}
What Responsibility?
Open For Extension?
Liskov Substitution...
One Interface...
Many Responsibilites

120. interface MailSystemInterface {
public function format(array $message);
public function mail(array $message);
}
What Responsibility?
Open For Extension?
Liskov Substitution...
One Interface...
What Dependencies?

121. interface MessageFormatter {
public function format(Message $message);
}
interface MessageEncoder {
public function encode(Message $message);
}
interface MessageTransport {
public function send(Message $message);
}

122. class MailSystem {
public function __construct(
MessageFormatter $messageFormatter,
MessageEncoder $messageEncoder,
MessageTransport $messageTransport
) {}
public function mail(Message $message);
}

123. Principle Of
Good
Enough

129. Anthony Ferrara
bit.ly/prague-solid-talk
@ircmaxell
blog.ircmaxell.com
me@ircmaxell.com
youtube.com/ircmaxell