Stoop sed-smells
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This document discusses several basic design principles and smells: - A class should have a single responsibility and minimize access to its data. - Inheritance should represent an "is-a" relationship, not be used for implementation convenience. - Methods should not expose implementation details and overuse of conversions should be avoided. - Missing information should be handled gracefully rather than raising errors. - Nil values should be avoided by returning empty collections instead. - Objects rather than strings or tuples should be used to represent data. - The tell-don't-ask principle of using polymorphism rather than conditionals on types should be followed. - Encapsulation should be maintained by not overusing accessors and
![S.Ducasse 7
Hiding missing information
Dictionary>>at: aKey
This raises an error if the key is not found
Dictionary>>at: aKey ifAbsent: aBlock
Allows one to specify action aBlock to be done when the
key does not exist.
Do not overuse it:
nodes at: nodeId ifAbsent:[ ]
This is bad because at least we should know that the
nodeId was missing](https://image.slidesharecdn.com/stoop-sed-smells-101026142500-phpapp01/85/Stoop-sed-smells-7-320.jpg)
![S.Ducasse 8
Avoid returning nil
Avoid to return special results as nil
messages := self fetchMessages.
messages isNil
ifFalse: [ messages dispatchFrom: self ]
What if we would simply return an empty collection in
fetchMessages instead of nil?
Less conditional and ugly tests!!](https://image.slidesharecdn.com/stoop-sed-smells-101026142500-phpapp01/85/Stoop-sed-smells-8-320.jpg)
![S.Ducasse 12
Tell, Don’t Ask!
MyWindow>>displayObject: aGrObject
aGrObject displayOn: self
•And not:
MyWindow>>displayObject: aGrObject
aGrObject isSquare ifTrue: […]
aGrObject isCircle ifTrue: […]
…](https://image.slidesharecdn.com/stoop-sed-smells-101026142500-phpapp01/85/Stoop-sed-smells-12-320.jpg)
Stoop sed-smells
- 1. S.Ducasse 1 QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture. Stéphane Ducasse Stephane.Ducasse@univ-savoie.fr http://www.iam.unibe.ch/~ducasse/ Elements of Design - Simple Smells Simple Smells
- 2. S.Ducasse 2 Basic Design Mistakes
- 3. S.Ducasse 3 A Class should have Class Person { String getName(); void setName(String name); int getAge(); void setAge(int age); Car getCar(); void setCar(Car car); } What do we see ? A class should have one main responsibility and some behavior not just holding state Minimal access to its data!
- 4. S.Ducasse 4 Confusing Class City extends Place { … } Class Jerusalem extends City implements Capital { … } Class TelAviv extends City { … } What is wrong here? Confusing inheritance and instantiation Too much inheritance?
- 5. S.Ducasse 5 Do not expose implementation
- 6. S.Ducasse 6 Do not overuse conversions nodes asSet removes all the duplicated nodes (if node knows how to compare). But a systematic use of asSet to protect yourself from duplicate is not good nodes asSet asOrderedCollection returns an ordered collection after removing duplicates Look for the real source of duplication if you do not want it!
- 7. S.Ducasse 7 Hiding missing information Dictionary>>at: aKey This raises an error if the key is not found Dictionary>>at: aKey ifAbsent: aBlock Allows one to specify action aBlock to be done when the key does not exist. Do not overuse it: nodes at: nodeId ifAbsent:[ ] This is bad because at least we should know that the nodeId was missing
- 8. S.Ducasse 8 Avoid returning nil Avoid to return special results as nil messages := self fetchMessages. messages isNil ifFalse: [ messages dispatchFrom: self ] What if we would simply return an empty collection in fetchMessages instead of nil? Less conditional and ugly tests!!
- 9. S.Ducasse 9 Objects not strings! • Strings are dead objects • You can only concatenate strings • Use objects not their textual representation
- 10. S.Ducasse 10 Objects not tuples! • spec first • spec second • spec third • spec action • spec selector • spec menuItem • And add a printing • aSpec(‘open’, #openBrowser,‘open (O)’)
- 11. S.Ducasse 11 Tell, Don’t Ask • no condition and case based on the receiver type • Use polymorphism as much as possible to avoid type checking
- 12. S.Ducasse 12 Tell, Don’t Ask! MyWindow>>displayObject: aGrObject aGrObject displayOn: self •And not: MyWindow>>displayObject: aGrObject aGrObject isSquare ifTrue: […] aGrObject isCircle ifTrue: […] …
- 13. S.Ducasse 13 Don’t violate encapsulation No overuse of accessors Encapsulation principle: minimize data representation dependencies Offer complete interface