1 CS 501 Spring 2005 CS 501: Software Engineering Lecture 15 Object Oriented Design 1

2 CS 501 Spring 2005 Administration Projects Now is the time to press ahead with final design and implementation. Allow plenty of time for testing, for making small changes, and for documentation and packaging.

3 CS 501 Spring 2005 The Waterfall Model Requirements analysis System design Testing Operation & maintenance Program design Coding Acceptance Requirements Design Implementation Feasibility study

4 CS 501 Spring 2005 Program Design The task of program design is to represent the software system functions in a form that can be transformed into one or more executable programs. Given a system architecture, the program design specifies: programs, components, packages, classes and class hierarchies interfaces, protocols security mechanisms, operational procedures

5 CS 501 Spring 2005 The Unified Modeling Language UML is a standard language for modeling software systems Serves as a bridge between the requirements specification and the implementation. Provides a means to specify and document the design of a software system. Is process and programming language independent. Is particularly suited to object-oriented program development.

6 CS 501 Spring 2005 UML Models A UML model consists of: (a)A diagram. This gives a general overview of the model, showing the principal elements and how they relate to each other. (b)A specification. This provides details about each element of the model. Specification for models used in program design should have sufficient detail that they can be used to write code from. A diagram without a specification is not a complete model, but may be very useful as a design tool.

7 CS 501 Spring 2005 Diagrams in UML A diagram is the graphical representation of a set of elements, usually rendered as a connected graph of vertices (things) and arcs (relationships). Class diagram shows a set of classes, interfaces, and collaborations with their relationships. Object diagram shows a set of objects and their relationships. Use case diagram shows a set of use cases and actors (a special kind of class) and their relationships.

8 CS 501 Spring 2005 Diagrams in UML (continued) An interaction diagram shows an interaction, consisting of a set of objects and the relationships, including the messages that may be dispatched among them. => A sequence diagram emphasizes the time ordering. => A collaboration diagram emphasizes the structural organization of the objects that send and receive messages.

9 CS 501 Spring 2005 Diagrams in UML (continued) Statechart diagram shows a state machine consisting of states, transitions, events, and activities. Activity diagram is a statechart diagram that shows the flow from activity to activity within a system. Component diagram shows the organization and dependencies among a set of components. Deployment diagram shows the configuration of processing nodes and the components that live on them.

10 CS 501 Spring 2005 Annotation some text note A note is a symbol for rendering constraints and comments attached to an element or a collection of elements.

11 CS 501 Spring 2005 Class Diagrams Window origin size open() close() move() display() name attributes operations responsibilities (optional text) A class is a description of a set of objects that share the same attributes, operations, relationships and semantics.

12 CS 501 Spring 2005 Notation: Grouping A package is a general-purpose mechanism for organizing elements into groups. Business rules

13 CS 501 Spring 2005 Packaging Classes applet awt lang HelloWorld java Graphics package

14 CS 501 Spring 2005 The "Hello, World!" Example import java.awt.Graphics; class HelloWorld extends java.applet.Applet { public void paint (Graphics g) { g.drawString ("Hello, World!", 10, 10); } Example from: BJR

15 CS 501 Spring 2005 The HelloWorld Example HelloWorld paint() class name operations

16 CS 501 Spring 2005 Abstraction for HelloWorld HelloWorld paint() g.drawString ("HelloWorld", 0, 10)" class name operations annotation

17 CS 501 Spring 2005 Notation: Relationships A dependency is a semantic relationship between two things in which a change to one may effect the semantics of the other * employer employee An association is a structural relationship that describes a set of links, a link being a connection among objects.

18 CS 501 Spring 2005 Relationships ParkingGarage ParkingSpace location is_available() *

19 CS 501 Spring 2005 Notation: Relationships (continued) A generalization is a specialization/generalization relationship is which objects of the specialized element (child) are substitutable for objects of the generalized element (parent). childparent A realization is a semantic relationship between classifiers, wherein one classifier specifies a contract that another classifier guarantees to carry out.

20 CS 501 Spring 2005 Generalization Applet HelloWorld paint() Graphics generalization dependency Note that the Applet and Graphics classes are shown elided, i.e., just the name is shown, not the attributes or operations.

21 CS 501 Spring 2005 Notation: Interface An interface is a collection of operations that specify a service of a class or component, i.e., the externally visible behavior of that element. ISpelling

22 CS 501 Spring 2005 Class Inheritance Diagram Object Component Container Panel Applet HelloWorld ImageObserver interface

23 CS 501 Spring 2005 Modeling Classes Given a real-life system, how do you decide what classes to use? What terms do the users and implementers use to describe the system? They are candidates for classes. Is each candidate class crisply defined? For each class, what is its set of responsibilities? Are the responsibilities evenly balanced among the classes? What attributes and operations does each class need to carry out its responsibilities? Aim for high cohesion within classes and weak coupling between them.

24 CS 501 Spring 2005 Noun Identification: A Library Example The library contains books and journals. It may have several copies of a given book. Some of the books are reserved for short-term loans only. All others may be borrowed by any library member for three weeks. Members of the library can normally borrow up to six items at a time, but members of staff may borrow up to 12 items at one time. Only members of staff may borrow journals. The system must keep track of when books and journals are borrowed and returned and enforce the rules.

25 CS 501 Spring 2005 Noun Identification: A Library Example The library contains books and journals. It may have several copies of a given book. Some of the books are reserved for short-term loans only. All others may be borrowed by any library member for three weeks. Members of the library can normally borrow up to six items at a time, but members of staff may borrow up to 12 items at one time. Only members of staff may borrow journals. The system must keep track of when books and journals are borrowed and returned and enforce the rules.

26 CS 501 Spring 2005 Candidate Classes Librarythe name of the system Book Journal Copy ShortTermLoanevent LibraryMember Weekmeasure MemberOfLibraryrepeat Itembook or journal Timeabstract term MemberOfStaff Systemgeneral term Rulegeneral term

27 CS 501 Spring 2005 Relations between Classes Bookis anItem Journalis anItem Copyis a copy of a Book LibraryMember Item MemberOfStaffis aLibraryMember Is Item needed?

28 CS 501 Spring 2005 Operations LibraryMemberborrowsCopy LibraryMemberreturnsCopy MemberOfStaffborrowsJournal MemberOfStaffreturnsJournal Item not needed yet.

29 CS 501 Spring 2005 Class Diagram MemberOfStaffBookCopyJournal is a copy of 1..* 1 LibraryMember 1 0..* on loan

30 CS 501 Spring 2005 Rough Sketch: Wholesale System A wholesale merchant supplies retail stores from stocks of goods in a warehouse. What classes would you use to model this business?

31 CS 501 Spring 2005 Rough Sketch: Wholesale System RetailStore Warehouse Order Invoice Product Shipment Merchant

32 CS 501 Spring 2005 Rough Sketch: Wholesale System Warehouse Order Invoice Product Merchant RetailStore name address contactInfo financialInfo Shipment Responsibilities -track status of shipped products Reversal damaged() return() wrongItem() responsibility (text field)

33 CS 501 Spring 2005 Expanding a Class: Modeling Financial Information RetailStore Transaction 1 * association Invoice Payment Which class is responsible for the financial records for a store?

34 CS 501 Spring 2005 Modeling Invoice Shipment Invoice invoiceNumber +goodsShipped() -sendInvoice() goodsShipped PartsList adornments + public - private RetailStore ??? invoiceRecord

35 CS 501 Spring 2005 Lessons Learned Design is empirical. There is no single correct design. During the design process: Eliding: Elements are hidden to simplify the diagram Incomplete: Elements may be missing. Inconsistency: The model may not be consistent The diagram is not the whole design. Diagrams must be backed up with specifications.

36 CS 501 Spring 2005 Levels of Abstraction The complexity of a model depends on its level of abstraction: High-levels of abstraction show the overall system. Low-levels of abstraction are needed for implementation. Two approaches: Model entire system at same level of abstraction, but present diagrams with different levels of detail. Model parts of system at different levels of abstraction. *