Peter Kofler is a software developer and code mentor who helps development teams improve code quality through mentoring techniques like pair programming and coding dojos. The document summarizes Object-Oriented Programming concepts like abstraction, encapsulation, polymorphism, and inheritance. It also describes "Object Calisthenics" rules for writing high-quality object-oriented code, such as wrapping primitives, short methods, and avoiding getters/setters. The agenda involves a discussion of OOP concepts and a coding exercise on implementing LCD number display following the Object Calisthenics rules.

1. Object Calisthenics
October 2016
Peter Kofler, ‘Code Cop’
@codecopkofler
www.code-cop.org
Copyright Peter Kofler, licensed under CC-BY.

2. Peter Kofler
• Ph.D. (Appl. Math.)
• Professional Software
Developer for 15+ years
• “fanatic about code quality”
• Freelance Code Mentor
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

3. I help development teams with
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY
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Professionalism
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Quality and
Productivity
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Continuous
Improvement

4. Mentoring
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY
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Pair Programming
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Programming
Workshops
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Deliberate
Practice, e.g.
Coding Dojos

5. Developing Quality
Software Developers

6. Agenda
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Concepts of OO
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Encapsulation
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Coupling and
Cohesion
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Coding Exercise
“Object Calisthenics”
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

7. Discussion
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What is Object Orientation?
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What are its
basic features?
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

8. OOP is based on
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Abstraction
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Encapsulation
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Polymorphy
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Inheritance
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Coupling
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Cohesion
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

9. Abstraction
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“A class should capture one and only
one key abstraction.”
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Avoid Primitive Obsession.
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Single Responsibility Principle (SRP).
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Wrap All Primitives And Strings. (OC#3)
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First Class Collections. (OC#4)
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Keep All Entities Small. (OC#7)
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

10. Encapsulation
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“Objects are defined by what they do, not
what they contain.“
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All data should be hidden within its class.
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Tell, Don't Ask.
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e.g.
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All fields must be private.
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No Getters/Setters/Properties. (OC#9)
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

11. Polymorphy
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Avoid switch statements (Type Code).
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“Explicit case on the type of an object
(instanceof) is usually an error.”
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Open Closed Principle (OCP).
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e.g.
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Avoid switch statements.
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Don’t Use The else Keyword. (OC#2)
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

12. Inheritance
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„Inheritance should only be used to
model a specialization hierarchy.“
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“IS A ...“ vs. „HAS A ...“ means the
subclass “IS exactly the same AS …”
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Inheritance hierarchies should be
shallow. (2-3)
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See Liskov Substitution Principle (LSP).
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

13. Composition
over
Inheritance

14. Low Coupling
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“Minimize the number of classes with which
another class collaborates.“
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“Minimize the number of messages sent
between a class and its collaborator.“
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Law of Demeter (“Only talk to friends”).
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See Interface Segregation Principle (ISP).
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e.g.
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One Dot (dereference) Per Line. (OC#5)
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

15. High Cohesion
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“Most of the methods defined on a class
should be using most of the data members
most of the time.“
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Related data and behaviour in one place.
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e.g.
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Don’t Abbreviate (long names). (OC#6)
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Keep All Entities Small. (OC#7)
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Max Two Instance Variables. (OC#8)
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

16. Jeff Bay's Object Calisthenics
1. Only One Level Of Indentation Per Method.
2. Don’t Use The else Keyword.
3. Wrap All Primitives And Strings.
4. First Class Collections.
5. One Dot/Arrow (dereference) Per Line.
6. Don’t Abbreviate (long names).
7. Keep All Entities Small. (50 LoC per class)
8. Not More Than Two Instance Variables.
9. No Getters/Setters/Properties.
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

17. Try it yourself
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

18. Coding Dojo Mindset
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Safe place outside
work
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We are here to learn
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Need to slow down
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Focus on doing it right
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Collaborative Game
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

19. Rules
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

20. Pair Programming & TDD
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regular Pair Programming
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do not talk for too long
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do not interrupt the other
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no “keyboard hugging“
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use TDD (or at least “sort of” TDD)
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write a test before you write code
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refactor mercilessly
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no debugger
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

21. LCD Numbers
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

22. Assignment
● Create an LCD string representation of an integer value using
a 4x7 grid of space, minus and pipe characters for each digit.
● Each digit is shown below (dot instead of space)
.--......--..--......--..--..--..--..--.
|..|...|...|...||..||...|......||..||..|
|..|...|...|...||..||...|......||..||..|
.........--..--..--..--..--......--..--.
|..|...||......|...|...||..|...||..|...|
|..|...||......|...|...||..|...||..|...|
.--......--..--......--..--......--..--.
● Bar size should be adjustable. The default value is 2.
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

23. “Calisthenics” Constraint
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

24. Jeff Bay's Object Calisthenics
1. Only One Level Of Indentation Per Method.
2. Don’t Use The else Keyword.
3. Wrap All Primitives And Strings.
4. First Class Collections.
5. One Dot/Arrow (dereference) Per Line.
6. Don’t Abbreviate (long names).
7. Keep All Entities Small. (50 LoC per class)
8. Not More Than Two Instance Variables.
9. No Getters/Setters/Properties.
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

25. Prepare
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Find a pair.
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Choose a programming language.
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Get the Java project or create new one.
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Implement LCD Numbers.
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

26. Don't Focus on
Getting it Done.
F0cus on Doing
It Perfectly.

27. →Practice

28. Closing Circle
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What did you learn today?
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What surprised you today?
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What will you do
differently in the
future?
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

29. Peter Kofler
@codecopkofler
www.code-cop.org
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY

30. CC Images
● Bruce http://www.flickr.com/photos/sherpas428/4350620602/
● pairing http://www.flickr.com/photos/dav/94735395/
● agenda http://www.flickr.com/photos/24293932@N00/2752221871/
● wants you http://www.flickr.com/photos/shutter/105497713/
● city http://www.flickr.com/photos/42311564@N00/2411037726/
● hands https://www.flickr.com/photos/ninahiironniemi/497993647/
● dojo http://www.flickr.com/photos/49715404@N00/3267627038/
● rule http://www.flickr.com/photos/phunk/4188827473
● LCD https://www.flickr.com/photos/ta3/3275907319/
● calisthenics https://www.flickr.com/photos/snickclunk/18522120656/
PETER KOFLER, CODE-COP.ORG FANATIC ABOUT CODE QUALITY