1. Collaborations and Hierarchies
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Chapters 5 and 6 of
Wirfs-Brock, R., Wilkerson, B., and Wiener, L., Designing Object-Oriented Software, Prentice Hall, 1990.
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2. Outline Collaborations Hierarchies Identifying collaborations
Recording collaborations
Hierarchies
Hierarchy graphs
Venn diagrams
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3. Motivation for Collaborations
Two ways to perform responsibilities ?
Collaboration is
Request from one object to another in order to fulfill a responsibility.
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4. Motivation (Cont.) Why identify collaborations?
Collaborations represents flow of control and information through the system,
May identify misplaced responsibilities, and
May identify missing responsibilities.
In sum, shows dynamics of the system.
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5. Finding Collaborations
Look at each responsibility of each class:
Is the class capable of fulfilling this responsibility by itself?
If not, where can it get what it needs?
Look at each class:
What other classes needs what this class does or knows?
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6. Finding More Collaborations
Examine relationships between classes, especially:
The “is-part-of” relationship, and
The “has-knowledge-of” relationship.
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7. “Is-part-of” Relationship
May imply responsibilities for maintaining information.
May fulfill responsibilities by delegating them.
Two of relationships:
Composite
Aggregate (or container)
Q: Which relationship is more likely to require collaborations?
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8. “Has-knowledge-of” Relationship
May know other classes that are not in part-of relationships (i.e., associations in UML).
May imply responsibilities to know information, and thus collaborations.
Q: Can you think of an example?
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9. Recording Collaborations
Write the name of the server (or helper) class on the CRC card of the client.
Write the name directly to the right of the responsibility the collaboration fulfills.
Client
Class: Person
Responsibilities Collaborations
Knows name
Knows address AddressBook
Knows phone number PhoneBook …
Server
(or helper)
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10. Collaboration Graph Graphical representation of collaborations
Arrow from client to a “contract” of the server, denoted by a semicircle
Contract: group of responsibilities (more on this later)
AddressBook 1
Person
PhoneBook 2
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11. Example: Mail-Order System
Class Responsibilities Collaborators
Catalog Create a catalog
Mail a catalog
Order Process an order
Track an order
Product Ship a product
Payment
Supplier
Customer
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12. Other Tools - UML UML interaction diagrams Message sequence diagram
Communication diagram (a.k.a. collaboration diagram)
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13. Message Sequence Diagram
object
lifetime
control
message
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14. Message Sequence Diagram
Collaboration
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15. Communication Diagram
object
link
message
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16. Communication Diagram
Collaboration
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17. Outline Collaborations Hierarchies Hierarchy graph Venn diagram
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18. Review on CRC Process Exploratory phase Analysis phase
Identify a preliminary list of classes, responsibilities and collaborations.
Analysis phase
Obtain a more global understanding of the design, e.g., by using tools such as:
Hierarchy graphs,
Venn diagrams, and
Contracts.
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19. Hierarchy Graph
A graphical representation of inheritance between related classes.
A hierarchy graph is a general tree.
The nodes are classes and the arcs represent inheritance.
Ancestor nodes are superclasses of descendent nodes, which are subclasses.
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20. Example - Notation of [Wirfs-Brock-etal90]
PartTime
FullTime
Employee
- Notation of [Wirfs-Brock-etal90]
- Leaf nodes are often concrete classes.
- Non-leaf nodes are often abstract classes.
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21. In UML …
Employee
{abstract}
PartTime
FullTime
Employee
(or )
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22. Abstract Classes Classes that cannot be instantiated.
Designed only to be inherited from, thus allowing reuse and avoiding duplication.
Used to factor common behaviors among classes.
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23. Finding Abstract Classes
At the exploratory stage, all identified classes are probably concrete and some may have been identified as abstract.
But, do you have all your abstract classes? That is, have you used the power of abstraction (factoring behavior)?
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24. Another Example of Hierarchy Graph
Ordered
Collection
Indexable
Collection
Array
Magnitude
Matrix
String
Date
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25. Venn Diagram Another tool to understand inheritance relationships.
Views a class as a set of responsibilities.
Thus, an intersection among classes:
Denotes common responsibilities, and thus
May indicates an abstract superclass.
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26. Example set of responsibilities PartTime Employee FullTime
set of objects
PartTime
FullTime
Employee
set of responsibilities
PartTime
FullTime
Employee
PartTime
FullTime
Employee
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27. Exercise Draw a Venn diagram for the following hierarchy graph.
Ordered
Collection
Indexable
Collection
Array
Magnitude
Matrix
String
Date
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28. In Sum, Hierarchies …
Are for reviewing the inheritance relationships, and
Use hierarchy graphs and Venn diagrams as a notation and an analysis tool
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29. Building Good Hierarchies
Model “is-a” relationships.
Factor common responsibilities as high as possible.
Make sure abstract classes do not inherit from concrete classes.
Eliminate classes that do not add functionality.
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30. Example – Using Venn Diagram To Check Hierarchy
If B supports only a part of responsibilities defined for A, what
does the Venn diagram look like? How to fix this? A B
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31. Fixing the Problem
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32. Example – Factoring Responsibilities
Drawing
Element
Text
Element
Line
Element
Ellipse
Element
Rectangle
Element
Group
Element
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33. Factoring (Cont.)
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34. Factoring (Cont.)
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35. Group Exercise Suppose that you are designing a new Java compiler
and you identified the following concrete classes for
various kinds of declarations allowed in Java.
Class Declaration
Interface Declaration
Field Declaration
Method Declaration
Local Variable Declaration
Method Parameter Declaration
Organize the above classes into a class hierarchy.
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