Objects of Desire
A Few Questions (and Answers) on
Modern Object-Oriented Development
Kevlin Henney
kevlin@curbralan.com

Agenda
• Intent
Some answers concerning OO and related topics
• Content
A brief history of objects
Linguistics
Encapsulation
Class hierarchies
Patterns
Testing
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A Brief History of Objects
History rarely happens in the right order or at the right time, but the job of aa
History rarely happens in the right order or at the right time, but the job of
historian is to make it appear as if it did.
historian is to make it appear as if it did.
James Burke
James Burke
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Back to the Future
• How old is OO?
40 years old this year (Simula 67, Norway, 1967)
• When did OO become mainstream?
Around the mid 1990s (1997 is often quoted)
• Where is OO used?
Everywhere... and then some
• What about the future?
Objects will remain mainstream, but they are not
the only game in town
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Linguistics
Language is not simply aareporting device for experience but aadefining
Language is not simply reporting device for experience but defining
framework for it.
framework for it.
Benjamin Whorf
Benjamin Whorf
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General Linguistics
• Is there a relationship between language and
OO style?
In spite of rumours to the contrary, there is a strong
relationship between language features and
appropriate OO style
• What can we learn from other languages?
A lot!
• Which is the best OO language?
It depends
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Type Systems
• How do static and dynamic type systems
compare?
They each both strengths and weaknesses
• How does PHP5's type system measure up?
It is inherently dynamic, so some of the features
borrowed from static type systems are surprising
• What about type hints?
These appear to combine all the problems of static
typing with all the weaknesses of dynamic typing
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Encapsulation
encapsulate enclose (something) in or as if in aacapsule.
encapsulate enclose (something) in or as if in capsule.
express the essential feature of (someone or something) succinctly.
express the essential feature of (someone or something) succinctly.
enclose (a message or signal) in aaset of codes which allow use by or
enclose (a message or signal) in set of codes which allow use by or
transfer through different computer systems or networks.
transfer through different computer systems or networks.
provide an interface for (a piece of software or hardware) to allow or
provide an interface for (a piece of software or hardware) to allow or
simplify access for the user.
simplify access for the user.
The New Oxford Dictionary of English
The New Oxford Dictionary of English
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In a Capsule
• How do you use public, protected and private?
Declare data private, steer clear of protected, and only
make methods public if you know why and where
you're going to use them and how to test them
• So, encapsulation is about data privacy?
No, it relates to stability, coherence and ease of use
of an object — data privacy is a by-product
• How big should a class be?
Big enough, but not too big
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Train Wrecks and Vampires
• Is chaining calls on function results useful?
PHP4's model was, quite frankly, surprising
• What's the Law of Demeter?
Don't talk to strangers
• What's a train wreck?
A violation of the Law of Demeter
• But what about collections?
Return iterators and offer focused methods, rather
than uncontrolled access to private collections
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Class Hierarchies
The history of all hitherto existing society is the history of class struggles.
The history of all hitherto existing society is the history of class struggles.
Karl Marx and Friedrich Engels
Karl Marx and Friedrich Engels
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Substitutability
• What is LSP?
The Liskov Substitution Principle
• What is the Liskov Substitution Principle?
Organise class hierarchies according to type
substitutability, not according to commonality of
implementation
• What about reuse of implementation?
Favour delegation, steer clear of ad hoc subclassing
— spaghetti inheritance is hard to digest
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Interfaces
• What is the common role of interfaces?
In statically typed languages, the interface construct,
or equivalent, promotes polymorphism and reduces
dependencies — inheritance of implementation is
the strongest form of coupling in an OO system
In dynamically typed languages, the notion of
usage type is normally left as an informal protocol
or is expressed as predicate or constraint on objects
• What is the role of interfaces in PHP5?
Good question
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Patterns
The enormous success of design patterns is aatestimonial to the commonality
The enormous success of design patterns is testimonial to the commonality
seen by object programmers. The success of the book Design Patterns, however,
seen by object programmers. The success of the book Design Patterns, however,
has stifled any diversity in expressing these patterns.
has stifled any diversity in expressing these patterns.
Kent Beck
Kent Beck
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Shameless Plug
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Pitter Pattern Brain Drop
• What is a pattern?
A good pattern captures a proven solution practice
along with the problem it addresses and the context
in which it applies
• Are patterns only about OO design?
No
• How do you apply patterns?
Unconsciously and tacitly or consciously and
explicitly — but watch out for patternitis
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The Good, the Bad and the Ugly
• What are the top five patterns to know?
It depends
• If dependencies are so bad, why is Decorator
recommended?
They are, and it isn't: it was documented as an OO
pattern nearly 15 years ago, but Interceptor is now
considered better for much of what Decorator does
• Do patterns make code more complicated?
Only if used to make code more complicated
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The Epicycle Problem
• What is the trouble with Singleton?
It is normally used by coincidence, it introduces a
centralised point of coupling, it complicates testing
and it comes with various lifecycle problems
• What are the alternatives?
Focus on making essential object relationships
explicit, e.g. using patterns like Context Object
• What about the Monostate pattern?
This is also known as the Borg pattern, which tells
you everything you need to know
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Testing
Science is what we have learned about how to keep from fooling ourselves.
Science is what we have learned about how to keep from fooling ourselves.
Richard Feynman
Richard Feynman
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Motivation for Testing
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More Motivation
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And More
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Testing, Testing, One, Two, One, Two
• How soon should testing be introduced?
Either as soon as you know what to test — why
wait? — or as soon as you don't know what to test
— use testing to help drive detailed design and
clarification of requirements
• What is the relationship between testing and
design?
A test is a proposition about how something is
used, so it can be used as a form of specification and
clarification of needs and knowledge
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In Closing...
The only thing to do with good advice is to pass it on. It is never any use to oneself.
The only thing to do with good advice is to pass it on. It is never any use to oneself.
Oscar Wilde
Oscar Wilde
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