Object-Oriented Analysis & Design Dr. M.E. Fayad, Professor Computer Engineering Department, Room #283I College of Engineering San José State University One Washington Square San José, CA 95192-0180 http://www.engr.sjsu.edu/~fayad 2003 SJSU -- CmpE

Object Identification - 2 Lesson 10: Object Identification - 2 2 2003 SJSU – CmpE -- M.E. Fayad

3 Lesson Objectives + 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 3 2003 SJSU – CmpE -- M.E. Fayad

Automated Teller Machine (ATM) Stop bothering me! I told you I don’t have any money! 4 2003 SJSU – CmpE -- M.E. Fayad

5 Actors Automated Teller Machine Customer Automated Teller Machine Bank System ATM Operator In Use Cases, Everything that interacts with the system will be modeled as an actor, such as persons as well as machines. 5 2003 SJSU – CmpE -- M.E. Fayad

6 Use Cases in ATM Automated Teller Machine Customer Bank System Cash Withdrawal Customer Transfer Funds Bank System Deposit Funds Balance Inquiry 6 System Start ATM Operator 2003 SJSU – CmpE -- M.E. Fayad

A Use Case Description: Cash Withdrawal 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 2003 SJSU – CmpE -- M.E. Fayad

8 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 8 2003 SJSU – CmpE -- M.E. Fayad

System Responsibilities & Collaborations 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 2003 SJSU – CmpE -- M.E. Fayad

10 CRC Cards ATM (role) Collaboration Responsibility Clients Server 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 Clients Server Access & modify account balance Account Balance Inquiry Deposit Transaction Funds Transfer Withdrawal Transaction 10 2003 SJSU – CmpE -- M.E. Fayad

Other Techniques Help Refine Objects Generalize and Specialize objects Associate Objects Recognize Accidental Objects Challenging and Testing Objects Ask Questions 11 2003 SJSU – CmpE -- M.E. Fayad

Explore Generalizations and Specializations 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 2003 SJSU – CmpE -- M.E. Fayad

Avoid Accidental Objects Essential objects represent genuine high-level abstractions Accidental objects represent qualitative judgments 13 2003 SJSU – CmpE -- M.E. Fayad

14 Challenge Objects Needed Remembrance -- attributes Needed Behavior -- methods Usually Multiple Services per Object Usually More than One Object per Class 14 2003 SJSU – CmpE -- M.E. Fayad

15 Other Object Tests Uniformity Test More than a Name Test Or 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. 15 2003 SJSU – CmpE -- M.E. Fayad

Final Object Checklist 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 2003 SJSU – CmpE -- M.E. Fayad

Questioning Techniques Help Elicit Domain Knowledge 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 2003 SJSU – CmpE -- M.E. Fayad

Eliminating Unnecessary Associations Irrelevant Associations outside problem domain Implementation Associations Examples: concurrent process, contains a list Associations Between Eliminated Classes 18 2003 SJSU – CmpE -- M.E. Fayad

Eliminating Unnecessary Associations (cont’d) 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 2003 SJSU – CmpE -- M.E. Fayad

Refine Association List 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 20 2003 SJSU – CmpE -- M.E. Fayad

Identifying Attributes 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 2003 SJSU – CmpE -- M.E. Fayad

22 Descriptive Attributes Naming Attributes Referential Attributes Attribute Types Descriptive Attributes Naming Attributes Referential Attributes 22 2003 SJSU – CmpE -- M.E. Fayad

Descriptive Attributes 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 2003 SJSU – CmpE -- M.E. Fayad

24 Naming Attributes 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 2003 SJSU – CmpE -- M.E. Fayad

Referential Attributes 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. 25 2003 SJSU – CmpE -- M.E. Fayad

26 Rules of Attributes First Rule: One instance of an object or a class has exactly one value for each attribute at any given time. Employee M/S Phone M. Fayad 171 4356 G. Smee 23 3456 7890 L. Harris 1234 OK Not OK 26 [Shlaer-Mellor 90] 2003 SJSU – CmpE -- M.E. Fayad

Rules of Attributes (cont’d) 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 2003 SJSU – CmpE -- M.E. Fayad

Rules of Attributes (cont’d) 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. Juice Transfer storage Tank ID cooking Tank ID gallons plannedTime 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. 28 2003 SJSU – CmpE -- M.E. Fayad

Rules of Attributes (cont’d) 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 Batch batch ID recipe ID gallons cookingTime The Batch.cookingTime attribute must represent the actual time the batch was cooked, and not the cooking time specified by the recipe 29 2003 SJSU – CmpE -- M.E. Fayad

Eliminating Unnecessary Attributes 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 30 2003 SJSU – CmpE -- M.E. Fayad

Eliminating Unnecessary Attributes (cont’d) 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 2003 SJSU – CmpE -- M.E. Fayad

Identifying Inheritance 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 2003 SJSU – CmpE -- M.E. Fayad

Identifying Inheritance (cont’d) 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. 33 2003 SJSU – CmpE -- M.E. Fayad

Identifying Behaviors 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 2003 SJSU – CmpE -- M.E. Fayad

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! 35 2003 SJSU – CmpE -- M.E. Fayad

Discussion Questions 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 2003 SJSU – CmpE -- M.E. Fayad

Questions for the Next Lecture Define: Type Type vs. Class Type specifications Interface Signatures Elements of behavior 37 2003 SJSU – CmpE -- M.E. Fayad