May-June 2001 ISISTAN Research Institute – Tandil, Argentina
Software Design Methodologies: UML in Action
Dr. Mohamed Fayad, J.D. Edwards Professor
Department of Computer Science & Engineering
University of Nebraska, Lincoln
Ferguson Hall, P.O. Box 880115
Lincoln, NE 68588-0115
http://www.cse.unl.edu/~fayad
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S2 Object Identification- 2
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Lesson 5:Object Identification - 2
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S3 Object Identification- 2
Lesson Objectives
Objectives
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+ Learn how to identify:
Associations and aggregations
Attributes
Behaviors
Inheritance
+ Understand how to use the following approaches:
Use Case
CRC
Questioning Techniques
+ Understand how to refine objects and associations
+ Learn how to define responsibility & collaborations
+ Learn how to eliminate unnecessary classes, associations, and attributes
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S4 Object Identification- 2
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Automated Teller Machine (ATM)
Stop bothering me!I told you I don’t have
any money!
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S5 Object Identification- 2
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Actors
Customer
Bank System
Automated Teller
Machine
ATM Operator
In Use Cases, Everything that interacts with the system will be modeled as an actor, such as persons as well as machines.
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S6 Object Identification- 2
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Use Cases in ATM
Customer
Bank System
Cash Withdrawal
Transfer Funds
Deposit Funds
Automated Teller Machine
Balance Inquiry
System Start
ATM Operator
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S7 Object Identification- 2
Use Case: Cash Withdrawal When a customer inserts a card in the ATM, the machine
reads the code from the card and checks if it is a valid card. If the card is valid then the machine queries the customer for a PIN number, else the card is ejected.
When the machine matches customer coded in the PIN number, the machine checks the validity of the PIN number. If the PIN number is correct and matches the card number then the machine asks for the desired transaction the customer wishes to perform.
When the customer selects cash withdrawal the machine asks for the desired amount with a warning indicating only multiple of $10 is allowed.
When .... 7
A Use Case Description: Cash Withdrawal
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S8 Object Identification- 2
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Candidate Objects
Account ATM Balance Inquiry Bank Card Reader Cancel Key Cash Dispenser Deposit Slot Deposit Funds Display Screen (Bank System
Interface) Menu (Graphical User Interface)
• User Message
• Numeric Keypad
• Numeric Input Key
• PIN
• Cash
• Receipt Printer
• Special Input Key
• Transfer Funds
• Cash Withdrawal
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S9 Object Identification- 2
Define Responsibilities
– What are the goals of the system
– What must objects know to meet goals
– What steps must each object accomplish
Determine Collaborations
– Decompose responsibilities into interactions among objects
– Define clients and servers
– Where should knowledge be held 9
System Responsibilities & Collaborations
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S10 Object Identification- 2
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CRC Cards General
– Each class is described on a separate 3X5 or 4X6 card The cards are known as CRC card. They have 3 sections:
– Class
– Responsibilities
– Collaborations
ATM (role)
Responsibility Collaboration
Server Clients
Access & modify account balance Account (role) Balance Inquiry Deposit Transaction Funds Transfer Withdrawal Transaction
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S11 Object Identification- 2
Generalize and Specialize objects
Associate Objects
Recognize Accidental Objects
Challenging and Testing Objects
Ask Questions 11
Other Techniques Help Refine Objects
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S12 Object Identification- 2
Generalization exposes commonalities
Exercise helps to identify new classes
Considerations for generalizations and specializations
– Is it in the problem domain?
– Is it within the system’s responsibilities?
– Will there be inheritance? 12
Explore Generalizations and Specializations
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S13 Object Identification- 2
Essential objects represent genuine high-level abstractions
Accidental objects represent qualitative judgments
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Avoid Accidental Objects
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S14 Object Identification- 2
Needed Remembrance -- attributes
Needed Behavior -- methods
Usually Multiple Services per Object
Usually More than One Object per Class
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Challenge Objects
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S15 Object Identification- 2
Uniformity Test– Each instance must have the same set of characteristics
and be subject to the same rules - Car license More than a Name Test
– Every object has attributes, if not it is probably an attribute of another object -- home address
Or Test– If inclusion criteria should not use “OR” in any
significant way -- driver’s license number or learner’s permit number
More Than a List Test– If inclusion criteria is only a list of instances -- decadent
foods includes croissant , cappuccino, chocolate pie, ice cream.
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Other Object Tests
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S16 Object Identification- 2
Something universal and real for reuse
Should encapsulate some reasonably complex behavior to justify existence
Methods that don’t make use of its current class’s own attributes is probably encapsulated in the wrong object.
Small and simple stable interfaces
Self sufficient and complete 16
Final Object Checklist
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S17 Object Identification- 2
Play Twenty Questions– Is it animal, vegetable, or mineral?– Does it have fur or feathers?– Can it fly?
Define Boundaries– What else?– What about..?
Quantify Qualities as Attributes – How fast?– How hot? 17
Questioning Techniques Help Elicit Domain Knowledge
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S18 Object Identification- 2
Irrelevant Associations– outside problem domain
Implementation Associations– Examples: concurrent process,
contains a list
Associations Between Eliminated Classes
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Eliminating Unnecessary Associations
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S19 Object Identification- 2
Actions or Transient Events– Examples: “Interacts with the Robot”, “ATM
accepts cash card.”
Ternary Associations– Decompose as binary associations or rephrase
to one binary association.
Derived Associations– These are redundant
– Examples: “Younger than ..” derived from age 19
Eliminating Unnecessary Associations (cont’d)
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S20 Object Identification- 2
Choose meaningful association names Add role names where appropriate Add attributes or associations which qualify existing
associations– Example: “Standard Oil of Ohio” uses state attribute to
qualify company name. Specify one-to-many and many-to-many associations
in the class diagram Add missing associations
– Not in problem statement– from knowledge of application domain
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Refine Association List
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S21 Object Identification- 2
Attributes can be thought as a simple association with a value which is not an object
– Examples: name, age, weight
Usually corresponding to nouns followed by possessive phrases
– Examples: “color of the car”, “age of the donor”
Less likely to be fully described in the problem statement
Included in the class box diagram Not as relevant to the problem structure as
associations 21
Identifying Attributes
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S22 Object Identification- 2
Descriptive Attributes
Naming Attributes
Referential Attributes
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Attribute Types
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S23 Object Identification- 2
Provide facts intrinsic to each
instance of the object.
– Examples Account.balance, Cat.weight
– If the value of a descriptive attribute
changes, it means only that some aspect
of an instance has changed, but the
instance is still the same instance. 23
Descriptive Attributes
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S24 Object Identification- 2
Are used to name or label instances.
– Examples:Account.number, Flight.number
– Names are typically somewhat arbitrary
– Naming attributes are frequently used as an
identifier or part of an identifier.
– If the vale of a naming attribute changes, it means
only that a new name has been given to exactly
the same instance 24
Naming Attributes
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S25 Object Identification- 2
Are used to tie an instance of one
object to an instance of another.
– Examples: Cat.owner name indicates which person
owns this cat.
– If the vale of a referential attribute changes, it means
that different instances are now being associated.
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Referential Attributes
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S26 Object Identification- 2
First Rule: One instance of an object or a class has exactly one value
for each attribute at any given time.
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Rules of Attributes
Employee M/S Phone
M. Fayad 171 4356
G. Smee 23 3456 7890
L. Harris 1234
OK
Not OK
Not OK
[Shlaer-Mellor 90]
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S27 Object Identification- 2
Second Rule:
An attribute must contain no internal structure
Examples:
– Age, balance, size are all OK.
– A name consists of first name, middle initial, and last name (Not OK)
– An address contains house number, street name, city, state, zip code, and country name (Not OK) 27
Rules of Attributes (cont’d)
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S28 Object Identification- 2
Third Rule: When an object has a compound identifier -- that is, one
made up of two or more attributes -- every attribute that is not part of the identifier represents a characteristic of the entire object.
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Rules of Attributes (cont’d)
Juice Transfer
storage Tank IDcooking Tank IDgallonsplannedTime
The juice Transfer.gallons attribute means that the number of gallons transferred from the storage tank to the cooking tank and not the number of gallons in either the storage tank or the cooking tank.
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S29 Object Identification- 2
Fourth Rule: Each attribute is not part of an identifier that represents
a characteristic of the instance named by the identifier and a characteristic of some other non-identifier attribute
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Rules of Attributes (cont’d)
Batch
batch IDrecipe IDgallonscookingTime
The Batch.cookingTime attribute mustrepresent the actual time the batch was cooked, and not the cooking time specified by the recipe
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S30 Object Identification- 2
Do not keep attributes that have an object as a value, they are associations
Do not keep attributes that depend on a context, these are qualifiers for associations
If an object can have more that one name, then the name qualifies an association of that object with another
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Eliminating Unnecessary Attributes
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S31 Object Identification- 2
Do not put attributes of the association in one or the other of the objects involved in the association, put the attributes in the association itself
Eliminate attributes which are only used internally by the object
Keep initial analysis of attributes at a high level
Eliminate attributes which are too low level
Attributes which are in some instances of a class, but not in others, indicate that the class should be split into two or more classes 31
Eliminating Unnecessary Attributes (cont’d)
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S32 Object Identification- 2
Identify classes which share common information
Three basic approaches:
Bottom Up
– Look for classes with repeated associations, attributes or behaviors, and group together into higher level classes
– This approach is easier for inexperienced modelers
Top Down
– Look for Noun phrases describing different kinds of things in the problem statement.
– Examples: Family cars, Sports cars, Luxury cars
Combination of the two approaches works the best.
– Do Top Down when doing initial analysis
– Identify repeated information in the late passes.32
Identifying Inheritance
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S33 Object Identification- 2
Always use the “AKO” test
– All inheritance specifications should identify one or more classes which are “A Kind Of” a higher level class.
– NEVER use inheritance for “Part / Part-of” relationships
Use multiple inheritance only when necessary
– Some object-oriented programming languages do not even have this feature.
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Identifying Inheritance (cont’d)
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S34 Object Identification- 2
Done in latest stages
List of behaviors can become large and get detailed quickly.
May correspond to queries about attributes and associations
– Operations to read or write attribute or association value
– Examples: user name, property value, etc.
May correspond to events or activities
– Examples: begin simulation, alert, calculate balance, computer distance 34
Identifying Behaviors
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S35 Object Identification- 2
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Experience and Domain Knowledge
Good objects come from language of domain– If you are not an expert -- consult users
Experience will tune decisions– Slowly at first– Much faster later
Just do it!
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S36 Object Identification- 2
• Define with examples: CRC cards, associate objects, referential attributes.
• Describe the third norm for testing objects
• What are the differences between essential objects and accidental objects
• What are questioning techniques and their purposes?
• Describe how do you identify: associations , aggregations, inheritance, attributes, and behaviors
• Describe how to refine objects and associations
• Explain how to define responsibilities and collaborations 36
Discussion Questions
May-June 2001 ISISTAN Research Institute – Tandil, Argentina -- M.E. Fayad
L5-S37 Object Identification- 2
Define:
– Guidelines
– Heuristics
What do you think of these heuristics
– A designer should distribute system intelligence uniformly among the top level classes in the system.
– A designer should have 4.6 top level classes per 1,000 lines of code.
– Eliminate classes that are outside the system 37
Questions for the Next Lecture