Slides from my session at Software Craftsmanship 2011 Conference

1. How Object-Oriented Are You
Feeling Today?
Software Craftsmanship 2011
26th May, Bletchley Park
Krzysztof Jelski

2. Krzysztof Jelski
PRAGMATISTS,PL

3. Agenda
INTRO
WRAP-UP

4. The Nine Rules
1. Use One level of indentation per method
2. Don't use the else keyword
3. Wrap all primitives and strings
4. Use only one dot per line
5. Don't abbreviate (one­ or two­words names)
6. Keep all entities small (50 lines/class, 10 classes/package)
7. Don't use any classes with more than two instance variables
8. Use first class collections
9. Don't use any getters, setters or properties

5. Bank Accounts Kata
 Think of your personal bank account experience
 When in doubt, go for the simplest solution

6. Features
 Deposit, Withdrawal
 Transfer
 Account Statement (date, amount, balance)
 Statement printing
 Statement filters

7. You can start with
 Bank Accounts Kata sample code
http://bit.ly/jtOP6F
 if you need joda­time
http://bit.ly/kSpwNj

8. Wrap-up
Which rules:
 were simplest to follow?
 hardest?
 had biggest impact on design?
 you think you could use in production?

9. Rule 1: Use One Level of
Indentation per Method
 Method ­> does exactly one thing
 Reuse
 Readability
 Implementation matches name ­> easier to find bugs

10. Rule 2: Don't Use the else
Keyword
 Simple cases ­> guard clauses, early returns
 Complex cases ­> polymorphism (Strategy, Null Object)

11. Rule 3: Wrap All Primitives and
Strings
 Express intent
 Type­safe
 Place for behaviour

12. Rule 4: Use Only One Dot per
Line
 Place responsibilities properly
 The Law of Demeter
 your toys
 toys that you make
 toys someone gives you

13. Rule 5: Don't Abbreviate
 Remove duplication
 Place responsibilities properly

14. Rule 6: Keep All Entities Small
 Single Responsibility Principle
 Cohesive packages

15. Rule 7: Don't Use Any Classes
with More Than Two Instance
 Class cohesion
Variables
 Find commonality of instance variables
 Object Model decomposition

16. Rule 8: Use First-Class
Collections
 Home for collection­related behaviour

17. Rule 9: Don't Use Any
Getters/Setters/Properties
 Strong encapsulation boundaries
 “Tell, don't ask”

18. References
 Object Calisthenics, Jeff Bay in: The ThoughtWorks Anthology,
Pragmatic Bookshelf 2008.
Pics
 http://www.flickr.com/photos/jiheffe/3462940215/
 http://www.flickr.com/photos/singapore2010/4916726882