Chapter 5 Class Design

The Deliverables of the Class Design Process Class diagrams are further refined in this phase of development Object diagrams are created Interaction diagrams are created Class skeletons are created to embody all analysis and design information created to this point in the development process

Class Skeletons We will preview class skeletons to better understand the objectives of design Class skeletons are partial class definitions Class skeletons should be heavily commented, so that the purpose of all attributes, methods, and constructors is clear Class skeletons are the basis for the implementation phase of development

Contents of Class Skeletons A list of the roles the class plays within the system Information concerning when objects of the class are created and deleted (information maintenance) For each role, the semantics of the class All attributes with access modifiers, types, names, and semantics For all constructors and methods, their signature, semantics, preconditions and postconditions

LMS Class Skeleton public class Patron { // Class Semantics and roles // Library Patrons function in two primary // roles, as researchers who use index, // reference and database materials, and as // borrowers of loanable resources. // Information maintenance // Creation: new patrons are introduced // into the system by library staff when // presented with a library membership // application or from information // retrieved from a web-based application

More LMS Class Skeleton // Information maintenance continued // Deletion: patrons are removed from the // library database 3 years after their // membership expires // // Instance variables private String name; // Patron name in // last, first, middle initial format private long PatronID; // Patron library ID // number. Automatically generated...( See deliverable 5.1 for other instance variables )

More LMS Class Skeleton // Class variables private static long nextPatronID; // Keeps // track of next patronID to be assigned // Constructors public Patron(String n, long home, Date m, Date e, String street, String city, String state, long zip) { // Parameters: n = name, home = homephone // PatronID = getnextPatronID() // street,city, state, and zip are used // to create an address object for // homeAddress

More LMS Class Skeleton // Constructors continued // Precondition: for constructor: // Library database can accept an // additional entry and memory allocation // succeeds // Postcondition: Library database will // contain an additional Patron and Address // entry } // Static methods public static long getnextPatronID() { return nextPatronID; nextPatronID++;}

More LMS Class Skeleton // Non-static methods public boolean validatePatron(Date e) { // ensure membership is not expired // Precondition: expireDate != null // if expireDate <= Today return false // else return true }...( See deliverable 5.1 for other non-static methods ) } // end class Patron

System Decomposition Adds detail to the previous system representation Can be done iteratively or in a traditional, waterfall manner Each phase in the system development decomposes the system further Leads to a blueprint for implementation

More UML Access Modifiers + means public - means private # means protected Constraints (restriction on the class) { } E.g {Students may check out at most 25 items} Tagged values also use { } E.g. {Requirement #5}

More UML: Multiplicity Multiplicity or cardinality is represented above by the 0..1 and 0..* The above diagram indicates that a resource is checked out by 0 or 1 patrons and that each patron may check out 0 to many resources PatronResource 0..1 checks out 0..*

More UML: Aggregation Patron Resource The solid diamond indicates that the Overdue form letter class consists of Patron and Resource objects The diamond is solid to indicate that the Patron and Resource classes exist in their own right Overdue form letter

Interaction Diagrams Interaction diagrams model dynamic aspects of the system by specifying the interaction among objects to produce a particular behavior Two types of interaction diagrams are defined in UML –Collaboration diagrams, which emphasize the structural organization of objects that send and receive messages –Sequence diagrams, which emphasize the time ordering of the messages passed between objects

Notational Elements of Interaction Diagrams Object Link Message method(parameters) Object: class The object name is optional in the depiction of an object in UML notation An object is distinguished from a class in UML notation by the colon and underlining of the class name

LMS Case Study: Collaboration Diagram(Partial) : Patron : Library System : LibraryDatabase Checkout(ResourceID) validatePatron(MemDate) ( See deliverable 5.3 for entire diagram ) update(Patron) create(LibraryDatabase) getResource(ResourceID) getPatron(PatronID)

Steps for Creating Collaboration Diagrams Identify a behavior to model Identify participating class and their relevant interrelationships Identify a specific scenario to model determine necessary message passing to carry out the behavior Introduce solution for object persistence, if needed

Sequence Diagrams Like collaboration diagrams, sequence diagrams model dynamic aspects of the system by specifying the interaction among objects to produce a particular behavior Sequence diagrams specify the time ordering of messages Sequence diagrams show the life span of each object

LMS Case Study: Sequence Diagram(Partial) : Patron : Library System : LibraryDatabase create(LibraryDatabase) getResource(ResourceID) getPatron(PatronID) validatePatron (MemDate) (See deliverable 5.5 for entire diagram )

Evaluating Design Modeling software helps us produce correct, well- structured systems The resultant models can also be scrutinized for potential data integrity problems For example, in the LMS system, having update methods execute separately for the Patron and Resource objects may result in data integrity errors if system failure occurs between the initiation of the first method and the termination of the second method

Object Diagrams Models a set of objects and their interrelationships during a system snapshot A system snapshot is the state of the software system at a selected moment of time Object diagrams model another static perspective of the system Unlike other diagrams, object diagrams may contain multiple instances of the same class

LMS Case Study: Object Diagram (partial) currentP: Student: List name=“Gert Stein” libraryID= homephone= workphone= membership= expire= :Book name = “SOTY” author=“b. hooks” ISBN=... :Book name = “FOF” author=“Ehrenreich” ISBN=... (See deliverable 5.7 for entire diagram )

Steps for Creating Object Diagrams Identify a system snapshot within a scenario to model Identify participating classes and their interrelationships Identify all allocated objects at the time of the snapshot Show the state of each object in the snapshot Determine all interobject links

Code Reuse Collaboration diagrams are of particular use in pattern scavenging Pattern scavenging involves studying the various diagrams produced during analysis and class design to identify patterns of class interaction Once such patterns are found, they should be evaluated to determine if they can be effectively reused

Reuse in LMS Resource Checkable Resource Reserve Resource BookElectronic Media

Guidelines for Class Design Always keep data private Always initialize data in a constructor Do not use too many related primitives Not all attributes need individual accessor or mutator methods Order elements comprising class definitions consistently Break up overly complex classes into multiple classes Name classes, methods and attributes well

Verification of the Class Design All system requirements developed during analysis must be addressed during design –All design documents must cross reference requirements from the requirements specification All required attributes and methods must be used properly –E.g. data integrity of attributes must be enforced by update methods The modules comprising the system must work together properly