(containment) instance variables of the class. â Passed has argument. â Ask to a third party object (mappingâ¦) â Create the object and interact with it (coded in.
Arthur Riel Design Heuristics from Object-Oriented Design Heuristics by Arthur Riel • Read the Heuristics… •
– Find reasons why – We will discuss them during the lectures
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«ChapterNr».1
Different Relationship Uses • Contains • Specializes •
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«ChapterNr».2
Hidding Data •
All data should be hidden within its class
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«ChapterNr».3
No Dependence on Clients •
Users of a class must be dependent on its public interface, but a class should not be dependent on its users
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«ChapterNr».4
Support Class = one Clear Responsibility •
Minimize the number of messages in the protocol of a class
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«ChapterNr».5
Supporting Polymorphism and Communication Implement a minimal interface that all classes understand • To send the same message to different objects • To be able to substitute them Remember: grObjects do: [:each | each translate: 10@10] •
•
Example: Object>>printString, Object>>copy…
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«ChapterNr».6
Clear Public Interface •
Do not put implementations details such as common-code private functions into the public interface of a class
•
Example:
•
Do not clutter the public interface of a class with items that clients are not able to use or are not interested in using
– Private/protected in C++ – Private method categories in Smalltalk
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«ChapterNr».7
Minimize Classes Interdependencies •
A class should only use operations in the public interface of another class or have nothing to do with that class
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«ChapterNr».8
Support a Class = one Responsibility •
A class should capture one and only one key abstraction
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«ChapterNr».9
Strengthen Encapsulation Keep related data and behavior in one place • Spin off non related information into another class •
•
-> Move Data Close to Behavior
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Object: a Cohesive Entity •
Most of the methods defined on a class should be using most of the instance variables most of the time
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«ChapterNr».11
Roles vs. Classes •
Be sure the abstractions you model are classes and not the roles objects play
Are mother and father classes or role of Person? • No magic answer: Depend on the domain • Do they have different behavior? So they are more distinct classes •
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«ChapterNr».12
Support one Class = one Responsibility •
Distribute system intelligence horizontally as uniformly as possible, i.e., the top-level classes in a design should share the work
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«ChapterNr».13
Support one Class = one Responsibility •
Do not create god classes/objects (classes that control all other classes). Be very suspicious of class whose names contains Driver, Manager, System, SubSystem
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«ChapterNr».14
Model and Interfaces •
Model should never be dependent on the interface that represents it. The interface should be dependent on the model
•
What is happening if you want two different Uis for the same model?
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«ChapterNr».15
Basic Checks for God Class Detection •
Beware of classes that have many accessor methods defined in their public interface. May imply that data and behavior is not being kept at the same place
•
Beware of classes having methods that only operate on a proper subset of the instance variables.
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«ChapterNr».16
One Class: One Responsibility •
One responbility: coordinating and using other objects – OrderedCollection maintains a list of objects sorted by arrival order: two indexes and a list
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Class should not contain more objects than a developper can fit in his short-term memory. (6 or 7 is the average value)
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«ChapterNr».17
Classes Evaluation •
Model the real world whenever possible
•
Eliminate irrelevant classes
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Eliminate classes that are outside of the system
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A method is not a class. Be suspicious of any class whose name is a verb or derived from a verb, especially those that only one piece of meaningful behavior
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«ChapterNr».18
Minimizing Coupling between Classes Minimize the number of classes with which another class collaborates • Minimize the number of messages sent between a class and its collaborators •
– Counter example: Visitor patterns
•
Minimize the number of different messages sent between a class and its collaborators
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«ChapterNr».19
About the Use Relationship •
When an object use another one it should get a reference on it to interact with it
•
Ways to get references – (containment) instance variables of the class – Passed has argument – Ask to a third party object (mapping…) – Create the object and interact with it (coded in class: kind of DNA)
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«ChapterNr».20
Containment and Uses •
If a class contains object of another class, then the containing class should be sending messages to the contained objects (the containment relationship should always imply a uses relationships)
•
A object may know what it contains but it should not know who contains it.
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«ChapterNr».21
Representing Semantics Constraints •
How do we represent possibilities or constraints between classes? – Appetizer, entrée, main dish… – No peas and corn together…
•
It is best to implement them in terms of class definition but this may lead to class proliferation
•
=> implemented in the creation method
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«ChapterNr».22
Objects define their logic •
When implementing semantic constraints in the constructor of a class, place the constraint definition as far down a containment hierarchy as the domain allows
=> Objects should contain the semantic constraints about themselves
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«ChapterNr».23
Third party constraint holder •
If the logic of a constraint is volatile, then it is better to
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«ChapterNr».24
Classes - Subclasses •
Superclass should not know its subclasses
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Subclasses should not use directly data of superclasses
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If two or more classes have common data and behavior, they should inherit from a common class that captures those data and behavior
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«ChapterNr».25
Controversial •
All abstract classes must be base classes
•
All base classes should be abstract classes – > Not true they can have default value method
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«ChapterNr».26
**Fundamental**: Avoid Type Checks •
Explicit case analysis on the type of an objects is usually an error.
•
An object is responsible of deciding how to answer to a message
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A client should send message and not discriminate messages sent based on receiver type
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