When it comes to Object Oriented Programming, there is no shortage of guidelines and principles for how to properly design an OO system. There is also no shortage of acronyms to describe these principles: DRY, SRP, LSP, LoD, ISP, OCP, etc. However, there are two acronyms that really shine through to describe how to, and how not to do OOP well. The two acronyms are SOLID and STUPID (respectively).

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 ($a->isLong()) {
return new Long($a->getValue() + 1);
} elseif ($a->isFloat()) {
return new Float($a->getValue() + 1.0);
} elseif ($a->isDecimal()) {
return new Decimal($a->getValue() +
1.0);
}

32. Polymorphic Code
return $number->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
}
}

42. Behavior Is
Defined By The
API

43. Two Types Of Primitive APIs
Interfaces (Explicit)

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

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

46. What Is A
Collection Of
Classes/Objects?

47. APIs
Method

48. APIs
Method MethodMethod
Class

49. APIs
Method MethodMethod
Class ClassClass
Package

50. APIs
Method MethodMethod
Class ClassClass
Package PackagePackage
Library

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

53. What Makes A
Good API?

54. A Good API
Does One Thing

55. A Good API
Never Changes

56. A Good API
Behaves Like Its
Contract

57. A Good API
Has A Narrow
Responsibility

58. A Good API
Depends Upon
Abstractions

59. And That’s
SOLID
Code

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

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

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

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

75. 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

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

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

80. SOLID Emerges
From Good OOP

82. So, What Makes
A Bad API?

83. Global Variables
(Spooky Action
At A Distance)

84. Depending On
Specifics Of An
Implementation

85. Hidden
Dependencies

86. Unhealthy Focus
On Performance

87. Poorly Named
APIs

88. Duplication

89. And That’s
STUPID
Code

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

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

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

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

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

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

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

104. STUPID
Embodies
Lessons Learned
From Bad OOP

105. 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)

106. 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

107. Let’s Look At
Some Code!

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

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

110. Let’s Look At
Drupal Code!

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

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

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

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

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

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

117. 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…?

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

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

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

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

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

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

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

125. Principle Of
Good
Enough

133. Know the rules well
so you can break them
effectively...
Dalai Lama XIV

134. Anthony Ferrara
@ircmaxell
blog.ircmaxell.com
me@ircmaxell.com
youtube.com/ircmaxell