Class Design Another Look – Part 1

1. Class DesignAnother Look – Part 1

2. Note: there is so much more than below… • More facts regarding Class DesignRe-look at Persistent ClassesRe-look at Class Operations • Scope of OperationsMethodsStates – State ChartsAttributes • Defining Dependenciesand Associations • GeneralizationsMultiple InheritancePolymorphism

3. We want to: • Refine relationships, operations, and attributes • Focus on fleshing out the details of a particular class – operations needed and allocated to classes and how they collaborate to support the responsibilities allocated to the class. • Address non- functional requirements • We will look at Design patterns in the very near future.

4. Class Design in Context* Architectural Analysis Review the Architecture Describe Architectural Describe Architecture Reviewer Architect Concurrency Design Distribution Subsystem Design Use-Case Analysis Review the Use-Case Design Design Design Designer Reviewer Class Design This is where we stand. Recall: Architectural Design is where we decide what the infrastructure is (pieces and parts of the architecture and how they interact). Use Case Designis where theresponsibilities of the system are allocated to the pieces/parts; Subsystem and Class design are where we detail the specifics of the pieces/parts.

5. Class Design in Context* Architectural Analysis Review the Architecture Describe Architectural Describe Architecture Reviewer Architect Concurrency Design Distribution Subsystem Design Use-Case Analysis Review the Use-Case Design Design Design Designer Reviewer Class Design • During Class Design, we consider: implementation and deployment environments. • May need to adjust the classes to the particular products in use, the programming languages, • distribution, performance, use of component architectures like COM or CORBA, and other • constraints • Frequent iteration between Class Design, Subsystem Design and Use Case Design. • Class Design - performed for each class in current iteration.

6. How Many Classes Are Needed? •  Many, simple classes means that each class • Encapsulates less of the overall system intelligence • Is more reusable • Is easier to implement •  Few, complex classes means that each class • Encapsulates a large portion of the overall system intelligence • Is less likely to be reusable • Is more difficult to implement • Proper size may depend heavily on implementation environment – classes should map directly to some phenomenon in the implementation language in such a way that the mapping results in good code. A class should have a single well focused purpose. A class should do one thing and do it well!

7. DropDownList MainWindow SubWindow Button MainForm Recall: Boundary Classes – External System Interface:* • Note: • Usually model as subsystem (recall Billing System interface?) • Oftentimes these interfaces have complex internal behavior (hence the modeling as a subsystem)

8. Recall: Entity Classes (1 of 3) • Entity objects are often passive and persistent • In Analysis, we identified entity classes. • May have been associated with analysis mechanisms for persistence representing manipulated units of information. • Performance concerns may suggest re-factoring of persistent classes, causing changes to the Design Model. (Re-factoring covered ahead) • Maybe only ‘parts’ of objects need persistency….

9. << entity >> FatClass FatClass -transientBookeeping - transientBookeeping + commonlyUsedAtt1 + getCommonlyUsedAtt1() + commonlyUsedAtt2 + getCommonlyUsedAtt2() + rarelyUsedAtt3 + getRarelyUsedAtt3() + rarelyUsedAtt4 + getRarelyUsedAtt4() FatClassDataHelper FatClassLazyDataHelper + commonlyUsedAtt1 + rarelyUsedAtt3 + commonlyUsedAtt2 + rarelyUsedAtt4 Entity Classes - Sample of Re-factoring • Have persistent class with five attributes. • One attribute is not really persistent – used during runtime • Use Cases tell us two attributes used a lot; two others less. • In design, we would like to retrieve commonly used attributes right away but defer others until asked for. • But we don’t want a complex design for the client. • So: Analysis Design 1 1

10. << entity >> FatClass FatClass -transientBookeeping - transientBookeeping + commonlyUsedAtt1 + getCommonlyUsedAtt1() + commonlyUsedAtt2 + getCommonlyUsedAtt2() + rarelyUsedAtt3 + getRarelyUsedAtt3() + rarelyUsedAtt4 + getRarelyUsedAtt4() FatClassDataHelper FatClassLazyDataHelper + rarelyUsedAtt3 + commonlyUsedAtt1 + rarelyUsedAtt4 + commonlyUsedAtt2 Entity Classes • From a data standpoint, will consider the FatClass to be a proxy in front of the two real persistent data classes. • It will retrieve FatClassDataHelper from database when it is first retrieved. Will retrieve FatClassLazyDataHelper in rare occasion when a client asks for one of these attributes. • This is a view from a data-oriented perspective while retaining a logical object-oriented view for clients to use. • It is tuning for performance too. Analysis Design So, which would you rather retrieve?? FatClass or FatClassDataHelper? 1 1

11. Recall: Control Classes • What Happens to Control Classes? • Are they really needed? Split them? • If they seem like just ‘pass throughs’ from the boundary to the entity classes, eliminate them. • Control Classes may become true design classes for any of following reasons: • Encapsulate significant control flow behavior • High probability of Change • Behaviors are to be distributed across multiple processes and/or processors (often JSP, servlets…) • The behavior they encapsulate requires some transaction management • A single analysis control class can easily become two classes in design

12. Class Design Steps – Identify Persistent Classes • In Use-Case Analysis, • a vague notion that certain classes need to be persistent • But this is just the 'tip of the iceberg' of system design. • Now, in Class Design, get really specific about • what the classes are, (are there more?) • what their behaviors are (parameters, et al), and • what attributes these classes really have. • Now, we must be certain • which classes will have persistent instances, and • that all persistent classes should be mapped to a storage mechanism.

13. Course Student Client Class Analysis Mechanism (Conceptual) Design Mechanism (Concrete) Implementation Mechanism (Actual) Legacy Data Persistency RDBMS JDBC to Ingres New Data ObjectStore OODBMS Persistency • Persistent class? Any instance of the class that requires its state to be preserved. • A persistent class may have both persistent and transient instances • labeling a class 'persistent' means merely that some instances of the class may need to be persistent.

14. Important Note. • Persistent classes may not only come from entity classes. • Could also be other classes required to handle other non-functional requirements in general. • Examples: • Persistent objects needed to maintain information relevant to process control, or • Persistent objects needed to maintain state information between transactions.

15. Re-look at Class Operations • Purpose • Map responsibilities (analysis) to operations (design) that implement them • Things to consider : • Operation name, signature, and description • Operation visibility • Operation scope • Class operation vs. instance operation • Operations: define at most primitive level to promote reusability and maintainability.

16. Operations: Name and Describe • Appropriate operation names Indicate the outcome – e.g. getBalance(). Consistent across classes • Define operation signatures • operationName(parameter : class,..) : returnType • Best to specify operations and their parameters using implementation language syntax and semantics. • Thus the interfaces will already be specified in terms of the implementation language when coding starts. •  Always provide short textual description, including meaning of all parameters for an operation!!!

17. Operation Signatures: Guidelines • In addition to a short description of the parameter, be sure to include: • Parameters passed by-value or by-reference? • If by value, parameter cannot be changed. • If by reference, is parameter(s) changed? • Parameters optional? • Default parameter values? • Valid parameter ranges? • The fewer the parameters, the better. •  less coupling; more understandable and maintainable. •  Pass objects instead of “data bits” – a rich strength of OO.

18. Class2 Class3 Discovering Additional Classes and Relationships ClassA op1(var1:Class2): Class3 • Parameters and return types may lead to discovery • of other classes. • Operation parameters and return classes denote • a relationship between these classes and the • parameter class and/or the return class. • In many cases, the relationships added to • support operation signatures are dependency • relationships. • Dependency relationships are discussed ahead. • This applies to attributes as well as operations.. What does the class diagram above tell you??? Additional classes and relationships may be added to support signature

19. Class - privateAttribute # protectedAttribute +publicOp() # protectedOp() - privateOp() How Is Visibility Noted? • The following symbols are used to specify export control for attributes and operations: • + Public access • # Protected access • - Private access • In Java we also have package visibility. Symbol??

20. More on Classes

21. Class - classifierScopeAttribute - instanceScopeAttribute classifierScopeOperation() instanceScopeOperation() Scope of Operation and Attributes • Determines number of instances of the attribute or operation. • Instance scope: one instance for each class instance • Class scope: one instance for all class instances • Class scope: underline attribute/operation name • Cannot do this in Rose, but can use stereotype <<class>> to indicate class scope • Generally, we have instance scope; but can have class scope for may other practical reasons: counters, etc. • Class scoped operations can only access class-scoped attributes.

22. <<entity>> Student - name - address - studentID - nextAvailID : int + addSchedule(theSchedule : Schedule, forSemester : Semester) + getSchedule(forSemester : Semester) : Schedule + hasPrerequisites(forCourseOffering : CourseOffering) : boolean # passed(theCourseOffering : CourseOffering) : boolean + getNextAvailID() : int Example: Scope • Here, have a single classifier scoped attribute, nextAvailID; single classifier • scoped operation, getNextAvailID(). • These support the generation of a unique ID for each Student. • Each Student instance has it’s own unique Student-ID, whereas, there is only • one nextAvailID for all Student instances. • The getNextAvailID() classifier scoped operation can only access nextAvailID.

23. <<utility>> MathFunctions Utility Classes • What is a Utility Class? • Utility is a class stereotype • Used for a class that contains a collection of free subprograms • Free subprograms are nonmember functions, that is, functions that do not belong to a particular class. • Why use it? • To provide services that may be (re)useful in a variety of contexts (e.g. common algorithmic services) • Or, to wrap non object-oriented libraries or applications • In Use • No ‘instances’ of a utility class; all attributes / ops are considered classifier scoped (class scope). • Utility classes are not formal UML constructs, but can be defined for programming convenience. (Will see more ahead)

24. <<utility>> MathPack -randomSeed : long = 0 -pi : double = 3.14159265358979 +sin (angle : double) : double +cos (angle : double) : double +random() : double Example: Utility Classes* Discuss syntax of class… Note: all properties and methods are class-scoped.

25. Example: Portion of VOPC for the Register for Courses Use Case Realization <<Interface>> <<control>> ICourseCatalogSystem RegistrationController 1 (from External System Interfaces) 0..* (from Registration) + getCourseOfferings() + submitSchedule() + initialize() + saveSchedule() + getCourseOfferings() : CourseOfferingList + getCurrentSchedule(forStudent : Student, forSemester : Semester) : Schedule + deleteCurrentSchedule() +currentSchedule <<class>> + new(forStudent : string) 0..1 <<entity>> + getStudent(withID : string) : Student Schedule 0..1 (from University Artifacts) 0..1 0..* 0..* 0..* +registrant 0..1 <<entity>> +alternateCourses 1 Student. +primaryCourses (from University Artifacts) 0..4 + getTuition() : double 0..2 + addSchedule(theSchedule : Schedule) + getSchedule(forSemester : Semester) : Schedule <<entity>> + deleteSchedule(forSemester : Semester) CourseOffering + hasPrerequisites(forCourseOffering : CourseOffering) : boolean (from University Artifacts) # passed(theCourseOffering : CourseOffering) : boolean Note: the <<class>> operations. Note: + classes (invoked by clients); # - only invoked by defining class/subclasses, (usually correspond to reflexive operations on interaction diagrams; (more ) <<class>> + getNextAvailID() : int + getStudentID() : int + getName() : string + getAddress() : string

26. Example: Portion of VOPC for the Register for Courses Use Case Realization* <<Interface>> <<control>> ICourseCatalogSystem RegistrationController 1 (from External System Interfaces) 0..* (from Registration) + getCourseOfferings() + submitSchedule() + initialize() + saveSchedule() + getCourseOfferings() : CourseOfferingList + getCurrentSchedule(forStudent : Student, forSemester : Semester) : Schedule + deleteCurrentSchedule() +currentSchedule <<class>> + new(forStudent : string) 0..1 <<entity>> + getStudent(withID : string) : Student Schedule 0..1 (from University Artifacts) 0..1 0..* 0..* 0..* +registrant 0..1 <<entity>> +alternateCourses 1 Student. +primaryCourses (from University Artifacts) 0..4 + getTuition() : double 0..2 + addSchedule(theSchedule : Schedule) + getSchedule(forSemester : Semester) : Schedule <<entity>> + deleteSchedule(forSemester : Semester) CourseOffering + hasPrerequisites(forCourseOffering : CourseOffering) : boolean (from University Artifacts) # passed(theCourseOffering : CourseOffering) : boolean Dependency from Student to CourseOfferingClass was added to support the inclusion of the CourseOffering as a parameter to operations within the Student Class. Do you understand what this means?? <<class>> + getNextAvailID() : int + getStudentID() : int + getName() : string + getAddress() : string

27. Example: Portion of VOPC for the Register for Courses (Attribute compartments supressed here) <<Interface>> <<control>> ICourseCatalogSystem RegistrationController 1 (from External System Interfaces) 0..* (from Registration) + getCourseOfferings() + submitSchedule() + initialize() + saveSchedule() + getCourseOfferings() : CourseOfferingList + getCurrentSchedule(forStudent : Student, forSemester : Semester) : Schedule + deleteCurrentSchedule() +currentSchedule <<class>> + new(forStudent : string) 0..1 <<entity>> + getStudent(withID : string) : Student Schedule 0..1 (from University Artifacts) 0..1 0..* 0..* 0..* +registrant 0..1 <<entity>> +alternateCourses 1 Student. +primaryCourses (from University Artifacts) 0..4 + getTuition() : double 0..2 + addSchedule(theSchedule : Schedule) + getSchedule(forSemester : Semester) : Schedule <<entity>> + deleteSchedule(forSemester : Semester) CourseOffering + hasPrerequisites(forCourseOffering : CourseOffering) : boolean (from University Artifacts) # passed(theCourseOffering : CourseOffering) : boolean Semester is included as the type for several parameters in Student ops. For Course Registration System, it is considered to be an abstract data type that has no significant behavior and thus is not modeled as a separate class. <<class>> + getNextAvailID() : int + getStudentID() : int + getName() : string + getAddress() : string ?

28. A PackageB PackageA B Class A1 +Class B1 Class A3 Class A2 -Class B2 Review: Package Element Visibility Only public classes can be referenced outside of the owning package Can specify visibility for package elements in same way as class attributes / operations (can protect classes) Shows how other packages can access the elements owned by the package. (Have visibility symbols for packages) Class has Public visibility Class has Private visibility OO Principle: Encapsulation

29. A PackageB PackageA B Class A1 +Class B1 Class A3 Class A2 -Class B2 Review: Package Element Visibility Public classes can be accessed outside of owning package. Protected classes only be accessed by owning package & any packages that inherit from owning package. Private classes can only be accessed by classes within owning package. Public elements of package constitute package’s interface. Class has Public visibility Class has Private visibility OO Principle: Encapsulation

30. Defining Dependenciesand Associations

31. Client Supplier Define Dependency • What Is a Dependency? • A relationship between two objects • In analysis, we assumed relationships were ‘structural’ that is, associations or aggregations- parts, numbers, coincident lifetimes, one – to – many, etc. • In design, we must decide what type of communication pathway is required. • A dependency relationship denotes a semantic relationship between model elements, where a change in the supplier may cause a change in the client. • Need to • Determine where structural relationships are NOT required • What causes the supplier to be visible to the client

32. Supplier1 Supplier2 Client Dependencies vs. Associations* • Read these carefully: • Associations are structural relationships • Dependencies are non-structural relationships Dependency Association

33. Communication pathways to suppliers*** • Four communications pathways to supplier… • Local variable reference – supplier object is declared locally (created temporarily during execution of an operation) • Parameter reference – supplier object is a parameter to, or the return class of, an operation in the client object. • Global reference – supplier object is global. • Field reference – The supplier object is a data member in the client object.

34. Supplier1 Supplier2 Client Dependencies vs. Associations: Look at relationships: What are they going to be/become? • Associations and aggregations are structuralrelationships (field visibility). • We’re talking about ‘Association relationships’ realized by variables that exist in the data member section of the class definition. • Dependency is a type of communication pathway that is a more temporary type of relationship –global, parameter, local visibility •  Any relationships not associations are dependency Association Dependency

35. ClassA op1 () ClassB Local Variable Visibility  Dependency • The op1() operation contains a local variable of type ClassB. Hence there is a dependency between these two classes.

36. ClassA op1 (param1: ClassB) ClassB Parameter Visibility  Dependency • The ClassB instance is passed to the ClassA instance – hence a dependency.

37. ClassA op1 () ClassUtility utilityOp () Global Visibility  Dependency • The ClassUtility instance is visible because it is global. Clear dependency.

38. Identifying Dependencies: Considerations • Strive for real-world relationships • Strive for the lightest relationships possible (dependency) • Dependency is the cheapest to keep, easiest to utilize and benefit from encapsulation. •  Sometimes a relationship may be more permanent (aggregation) and sometimes the same kind of relationship might be a dependency.

39. Identifying Dependencies: Considerations • Is relationship “permanent”? Use association (field visibility) • Will I need this relationship again and again over time, or do I just need it to do some work and then I throw it away? • If I need it again and again, i.e., if a thing appears to remain related to another thing even across the execution of one or more operations, then it is likely as association and therefore should benefit from field visibility.

40. Identifying Dependencies: Considerations • Is relationship “temporary”? Use dependency • Multiple objects at run time share the same instance again and again, probably should be passing it as a parameter…(parameter visibility) • Or if there is only one in existence in whole process, set it up as a managed global (Singleton design pattern – next week) • If same instance is not shared, then a local copy should suffice; i.e., multiple objects don’t share the same instance – use local visibility.

41. Identifying Dependencies: Considerations •  No exact recipe for decision • How long does it take to create/destroy? • Expensive to connect/disconnect every time I need it? • If so, use field, parameter (association), or global visibility (dependency). •  All variations of dependencies are dependent on situations. •  Choice? Design it however it works best in your circumstances…

42. <<Interface>> ICourseCatalogSystem (from External System Interfaces) + getCourseOfferings(forSemester : Semester) : CourseOfferingList 1 courseCatalog 0..* <<entity>> <<control>> Schedule RegistrationController (from University Artifacts) (from Registration) - semester currentSchedule + // submit schedule() + submit() 0..1 0..1 + // save schedule() + // save() + // create schedule with offerings() # any conflicts?() + // getCourseOfferings(forSemester) : CourseOfferingList + // create with offerings() 0..* 0..1 0..* 0..* alternateCourses primaryCourses 1 registrant 0..2 0..4 0..1 <<entity>> <<entity>> CourseOffering Student (from University Artifacts) (from University Artifacts) - number : String = "100" - name - startTime : Time - address - endTime : Time - StudentID : int - days : Enum + addSchedule(theSchedule : Schedule, forSemester : Semester) + addStudent(studentSchedule : Schedule) + getSchedule(forSemester : Semester) : Schedule + removeStudent(studentSchedule : Schedule) + hasPrerequisites(forCourseOffering : CourseOffering) : boolean + new() # passed(theCourseOffering : CourseOffering) : boolean + setData() Example:Define Dependencies (before) VOPC Register for Courses Use Case Up to here, most relationships have been associations and aggregations. Now, will see how some of these are refined into dependencies. The dependency shown (next slide) was previously defined in the Define Ops section to support the Schedule operation signatures. All associations /aggregations should be examined to see if they are dependencies.

43. <<Interface>> ICourseCatalogSystem (from External System Interfaces) + getCourseOfferings(forSemester : Semester) : CourseOfferingList Global visibility <<entity>> <<control>> Schedule RegistrationController (from University Artifacts) (from Registration) - semester currentSchedule + // submit schedule() + submit() 0..1 0..1 + // save schedule() + // save() + // create schedule with offerings() Field visibility # any conflicts?() + // getCourseOfferings(forSemester) : CourseOfferingList + // create with offerings() 0..* 0..1 0..* 0..* Field visibility alternateCourses primaryCourses 1 registrant 0..2 0..4 0..1 <<entity>> <<entity>> CourseOffering Student (from University Artifacts) (from University Artifacts) - number : String = "100" - name - startTime : Time - address - endTime : Time - StudentID : int - days : Enum + addSchedule(theSchedule : Schedule, forSemester : Semester) + addStudent(studentSchedule : Schedule) + getSchedule(forSemester : Semester) : Schedule + removeStudent(studentSchedule : Schedule) + hasPrerequisites(forCourseOffering : CourseOffering) : boolean + new() # passed(theCourseOffering : CourseOffering) : boolean + setData() Parameter visibility Example: Define Dependencies (after) Changed one association to a dependency relationship. (This change discussed on ‘next’ slide) Here, during a registration session, the Registration Controller works with a singleStudent, the registrant, and oneSchedule, the current Schedule for the Student. These instances need to be accessed by more than one of the Registration Controller’s, operations so Field Visibility is chosen from Registration Controller to Student and from Registration Controller to Schedule. Thus relationships remain associations. (more ‘permanent’) A Student manages its own Schedules, so Field visibility is chosen from Student to Schedule – and relation remains aggregation. Again, more ‘permanent.’ More 

44. <<Interface>> ICourseCatalogSystem (from External System Interfaces) + getCourseOfferings(forSemester : Semester) : CourseOfferingList Global visibility <<entity>> <<control>> Schedule RegistrationController (from University Artifacts) (from Registration) - semester currentSchedule + // submit schedule() + submit() 0..1 0..1 + // save schedule() + // save() + // create schedule with offerings() Field visibility # any conflicts?() + // getCourseOfferings(forSemester) : CourseOfferingList + // create with offerings() 0..* 0..1 0..* 0..* Field visibility alternateCourses primaryCourses 1 registrant 0..2 0..4 0..1 <<entity>> <<entity>> CourseOffering Student (from University Artifacts) (from University Artifacts) - number : String = "100" - name - startTime : Time - address - endTime : Time - StudentID : int - days : Enum + addSchedule(theSchedule : Schedule, forSemester : Semester) + addStudent(studentSchedule : Schedule) + getSchedule(forSemester : Semester) : Schedule + removeStudent(studentSchedule : Schedule) + hasPrerequisites(forCourseOffering : CourseOffering) : boolean + new() # passed(theCourseOffering : CourseOffering) : boolean + setData() Parameter visibility Example: Define Dependencies (after) Course Offerings are part of semantics of what defines a Schedule (courses Student has selected). Thus Field visibility is chosen from Schedule to CourseOffering; relationships remain associations. The Student class has several operations where CourseOffering appears in the parameter list. Thus, Parameter visibility is chosen from Student to CourseOffering. It is envisioned the course Catalog System may need to be accessed by multiple clients in the system, so Global visibility was chosen – and relationship becomes a dependency.