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CS 683 Emerging Technologies: Embracing Change
Spring Semester, 2001
Coding Patterns
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© 2001, All Rights Reserved, SDSU & Roger Whitney
San Diego State University -- This page last updated 15-Feb-01
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Contents of Doc 7, Coding Patterns
References
This
document is taken from the following two references. All the good ideas here
are from Beck and Johnson. Quoted text in this lecture is from Beck's book.
My
contribution is just to realize that their ideas are well worth stealing.
Johnson lecture on coding style alone is worth the price of his on-line course.
Beck's text is a must own for a Smalltalk programmer.
CS
497 Object-Oriented Programming & Design, Lecture notes, Ralph Johnson,
Department of Computer Science, UIUC,
http://st-www.cs.uiuc.edu/users/cs497/lectures.html
Smalltalk
Best Practice Patterns, Kent Beck, Prentice Hall, 1997
Coding
Patterns
Kent
Beck's Predictors of Good Style
-
- "In
a program written with good style,
- everything
is said once and only once"
-
- "Good
code invariable has small methods and small objects"
- "When
you can extend a system solely by adding new objects without modifying any
existing objects, then you have a system that is flexible and cheap to maintain."
-
- Object
should be easily moved to new contexts
-
- "Don't
put two rates of change together"
Coding
Standards
Consistency
makes the code easier to read
A
poor standard is better than mixing several good styles
Names
Names
have big impact on the readability of code
Names
should mean something
- at:put:,
size, printString
Use
standard naming conventions
Class
Names
Smalltalk
class names:
- Use
complete words, no abbreviations
-
- Names
are read 100 to 1000 times more often than typed
- Abbreviations
waste more time (reading) than they save
- First
character of each word is capitalized
-
- SmallInteger,
LimitedWriteStream, LinkedMessageSet
Simple
Superclass Name
Superclass
names
- Simple
words
- One
word preferred, two at maximum
- Convey
class purpose in the design
Number
Collection
Magnitude
Model
Qualified
Subclass Name
- Unique
simple name that conveys class purpose
-
- If
name is in common use
-
- Array,
Number, String
- If
the purpose is more important that class hierarchy
- Prepend
an adjective to superclass name
-
- If
the class hierarchy is important
- Subclass
is conceptually a variation on the superclass
- OrderedCollection,
LargeInteger, CompositeCommand
Method
Names
If
a standard name exists, use of over following these rules
Search
the image for similar methods
Intention
Revealing Selector
- Name
methods after what the method does
- Don't
name methods after how it works
- linearSearchFor:
indicates how the method works
- searchFor:
is better
- includes:
is even better
To
test a name
- Imagine
a second very different implementation
- Will
the name work for both implementations?
Types
of Methods
General
Types
- Modifies
receiver
- Modifies
argument
- Returns
a value
Specialized
Types with special rules
- Accessing
methods
- Query
methods
- Boolean
property setting
- Converter
methods
Names
for Methods that
Modify
the Receiver
Use
a verb phrase for the name
- add:
- flush
- translateBy:
- rotate
- at:put:
Names
for Methods that
Modify
an Argument
Use
a verb phrase ending with preposition like:
- displayOn:
- displayOn:at:clippingBox:rule:fillColor:
- addTo:
- printOn:
- storeOn:
Names
for Methods that
Return
a value
Use
a noun phrase or adjective
Use
description not a command
- size
- capacity
- translatedBy:
Accessing
State
How
does a method access instance variable?
deposit: aFloat
balance := balance + aFloat
deposit: aFloat
self balance: (self balance + aFloat)
Direct
access
- Simpler
- Easier
to read
- Better
information hiding
Indirect
access
- Makes
subclassing easier
- Permits
lazy evaluation
Accessing
Methods
Methods
setting and getting the state of an object
Use
the name of the variable
- x
x:
- balance
balance:
Java
prepends set and get to these methods
This
is not the Smalltalk convention
Boolean
Property Accessing
Don't
make accessing methods with just a boolean argument
For
setter use two methods starting with "make"
- makeVisible,
makeInvisible
- makeClean,
makeDirty
Add
a "toggle" method if needed
- toggleVisible
For
getters use methods starting with "is"
- isVisible
- isDirty
- isClean
Query
Methods
Query
methods
- Tests
a property of an object
- Return
a boolean
Prefix
the name with a form of "be"
- isNil
- isActiveDirectory
- isEmpty
Use
common English testing words
- includes:
If
the negation of a query method is common provide an inverse method
- notNil
- notEmpty
Place
query methods in testing category (or protocol)
Converter
Methods
Converter
methods convert the receiver to another object with the same protocol
Examples
- Converting
float to an integer
- Converting
a string to a bag
Prepend
"as" to the name of the class of the object returned
- asSet
- asString
- asFloat
- asSortedCollection
Converter
Constructor Method
Converting
an object to another with different protocol
- Converting
a string to a date object
Provide
a class method that
- Takes
the object to be converted
- Returns
new object
- Place
in the class of the new object
- Place
the method in "instance creation" category
Name
the method by prepending "from" to the class of the original object
Example
- in the Date class have:
- fromString:
aString
- "Returns
a date object represented by aString"
Composed
Method
Methods
should perform one identifiable task
- Method's
operations should be at same level of abstraction
- Method
should be a few lines long
This
minimizes
- Code
copying in subclasses
- Number
of methods needing changes in subclass
How
to Use Composed MethodTop-Down
While
writing a method
- Invoke
several smaller methods
- The
smaller method need not yet exist
Bottom-Up
Factor
common code in a single method
Put
long loop bodies into separate method
If
lines of code need a comment, place the code in a method with Intention
Revealing Selector
If
you send two or more messages to an object in a method, add a method in that
object with Intention Revealing Selector
Variable
Names
Key
pieces of information about a variable
- Its
purpose or role it plays
- Its
type
Roles
or purpose
- Communicates
intent
- Harder
to understand than type
Role
Suggesting Instance Variable Name
Name
instance variables for the role they play
Use
a plural name if the variable is a collection
Comment
the type in the class comment
Type
Suggesting Parameter Name
Keywords
indicate the parameter's role
Name
parameters after the most general expected type
at: anInteger put: anObject
add: aCharacter
If
multiple parameters have same type, precede the class with descriptive name
Temporary
Variable Names
Name
temporaries after the role they play
Methods
are simpler when they don't use temporaries
Don't
avoid using temporaries, try to write methods that don't need them
Behavior
& State
Explicit
Initialization
How
to initialize an instance variable to a default value?
Implement
an "initialize" method
- Sets
all the variable explicitly
- Call
the initialize method from the class method "new"
Object subclass: #Timer
instanceVariableNames: 'count period '
Instance
Methods
Category:
initialize
initialize
count := 0.
period := self defaultMillisecondPeriod
Category:
private
defaultMillisecondPeriod
^1000
Class
Methods
Category:
instance creation
new
^self basicNew initialize
Lazy
Initialization
How
to initialize an instance variable to a default value?
Lazy
initialization uses
- A
getter method for all accesses to a variable
- Getter
sets the value of the variable with default value method
Explicit
initialization favors readability
Lazy
initialization favors
- Subclasses
can easily change defaults
- Initialization
is done only when needed
In
timer class remove initialize method and add:
period
period isNil ifTrue: [period := self defaultMillisecondPeriod].
^period
count
count isNil ifTrue: [count := self defaultCount].
^count
defaultCount
^0
Choosing
Message
"The
long term health of a system is all about managing themes and variations"
Avoid
using conditional logic (if statement)
Replace
if statement with message sends
Replace:
responsibility := (entry isKindOf: Film)
ifTrue: [anEntry producer]
ifFalse: [anEntry author]
With:
Responsibility := anEntry responsibility
Where
in the Film class we have
responsibility
self producer
And
in the Entry class we have
responsibility
self author
Method
Comment
"Communicate
important information that is not obvious from the code in a comment at the
beginning of the method"
Types
of information difficult to convey in code
-
- Sometimes
another method must be called before this one
-
- If
you need to change a method written by someone else comment why
Formatting
Methods
Consistency
of code layout improves readability
Squeak's
system browser formats code, use it
To
improve your code layout and understanding of the issues involved read Kent
Beck's book: Smalltalk Best Practice Patterns
Exercises
1.
Find examples of code in the Squeak image that follow the guidelines given here.
2.
Find examples of code in the Squeak image that violate the guidelines given here.
3.
Contrast the readability and flexibility of the examples found in 1 & 2.
Copyright ©, All rights reserved.
2001 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
OpenContent license defines the copyright on this document.
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