2. Implementation
Patterns
By Kent Beck
• 157 Pages / 77 Implementation Patterns
• Not a “patterns” book
• Java programming habits for creating readable code
• Conscious approach
• what do I want to tell the reader about this code?
• what does the code communicate what I am
thinking to others?
3. This book is not ...
• Not a style guide
• Not a design book
• Not a patterns book (!?)
• Not a language guide
• Not for beginners (despite what Amazon
thinks ...)
4. This book is ...
• About writing good code for others -
• to read, to understand, to learn from and to
extend
• For those who think life is too short to waste
on bad code
• For those who delight in writing beautiful code
• Like spending a few hours inside Kent Beck’s
head while he’s hacking JUnit
5. Premise
• “I have seen too much ugly code make too
much money to believe that quality of code
is either necessary or sufficient for
commercial success or wide-spread use.”
• Communication - “ ... programming so
other people can understand your code.”
• Does good code matter?
6. Book Overview
Introduction
• Patterns
• Values & Principles
• Motivation
Similar Logic Class Different Data
Behavior State
Dividing Logic Multi-Valued Data
Methods Collections
Frameworks
7. 1. Introduction
• Overview
• Question minute-to-minute decisions
• Why am I so sure this method should be
called foo?
• Program consciously, for others
• Format - essays, diagrams, teaching story,
examples
8. Implementation
Patterns
Every few
seconds
Design Pattern Language Guide
Few per day
Why choose a
Regulate object
particular
interaction
construct?
9. 2. Patterns
• Programming laws (forces)
1. Read more often than written
2. No such thing as done
3. Structured using basic set of state and control
flow concepts
4. Readers need to understand in detail and in
concept
• Patterns illustrate a point of view about the
relative priorities of these forces
10. 3. A Theory of Programming
• Values
• Over-arching universal themes of
programming
• Communication, Simplicity, Flexibility
• Principles
• Bridge the values
11. 3. A Theory of Programming
Principles
Values Patterns
Translate motive
Motivation What to do
into action
12. 3. A Theory of Programming
• Principles
• Local consequences
• Minimize repetition
• Logic and data together
• Symmetry
• Declarative expression
• Rate-of-change
13. 4. Motivation
• cost = cost
total develop+ cost
maintain
• Increase up front costs does not help
• Implementation patterns satisfy human and
economic needs
14. 5. Class
• Bundles related state
• Class
• Simple Superclass Name
• Interface
• Value Object
• Inner Class
• Conditional
15. 6. State
• Communicate use of state
• Objects package:
• Behaviour (presented to outside world)
• State (supports behaviour)
• Objects make analysing state easier
16. 6. State
• Patterns
• Access (Direct, Indirect)
• State (Common,Variable, Extrinsic)
• Variable (Local, Field, Parameter)
• Role Suggesting Name
• Declared Type
• Initialisation (Eager, Lazy)
17. The best intentions ...
x = 10;
setX(10);
openGarageDoor(10);
openGarageDoor() {
setX(10);
}
18. 7. Behaviour
• How to express behaviour of a program
• Control Flow
• Message (Choosing, Double Dispatch,
Reversing, Inviting, Explaining)
• Guard Clause
• Exception
19. Guards, Guards!
void initialise() { void initialise() {
if (!isInitialised()) { if (isInitialised()) {
... return;
} }
} ...
}
• One entry point, one exit point
• Express simple and local exceptional
situations with purely local consequences
• Highlight the difference between normal and
exceptional processing
20. 8. Methods
• Why divide logic into chunks?
• Readability, critical flow, understanding,
communication ...
• Logic into methods Communicate
• Methods into classes connection of
logic
• Classes into packages
21. 8. Methods
• Deciding what goes where is hard
• Compose Method
• Intention-Revealing Name
• Method Visibility
• Method Object
• Collection Accessor
• Query Method
• Setting Method
