CS 501: Software Engineering Lecture 16 Object Oriented Design 1

Administration

The Waterfall Model Requirements Requirements Analysis System design Program design Implementation Coding Unit & Integration Testing System Testing Acceptance Testing Operation & Maintenance

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

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.

UML Models A UML model consists of: A diagram. This gives a general overview of the model, showing the principal elements and how they relate to each other. 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.

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.

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.

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.

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

Class Diagrams Window name origin attributes size open() operations 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.

Notation: Grouping Business rules A package is a general-purpose mechanism for organizing elements into groups.

Packaging Classes java applet HelloWorld package awt Graphics lang

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

The HelloWorld Example class HelloWorld paint() name operations

Abstraction for HelloWorld class HelloWorld paint() name annotation g.drawString ("HelloWorld", 0, 10)" operations

Notation: Relationships A dependency is a semantic relationship between two things in which a change to one may effect the semantics of the other. 0..1 * employer employee An association is a structural relationship that describes a set of links, a link being a connection among objects.

Relationships Parking 1 0 ... 1 ParkingSpace location is_available()

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

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

Notation: Interface ISpelling 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.

Class Inheritance Diagram Object Panel interface Component ImageObserver Applet Container HelloWorld

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.

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.

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.

Candidate Classes Library the name of the system Book Journal Copy ShortTermLoan event LibraryMember Week measure MemberOfLibrary repeat Item book or journal Time abstract term MemberOfStaff System general term Rule general term

Relations between Classes Book is an Item Journal is an Item Copy is a copy of a Book LibraryMember Item MemberOfStaff is a LibraryMember Is Item needed?

Operations LibraryMember borrows Copy LibraryMember returns Copy MemberOfStaff borrows Journal MemberOfStaff returns Journal Item not needed yet.

Class Diagram MemberOfStaff LibraryMember 1 1 on loan on loan 0..* 0..12 Journal Copy is a copy of 1..* 1 Book

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?

Rough Sketch: Wholesale System RetailStore Order Merchant Product Warehouse Shipment Invoice

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

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

Modeling Invoice Shipment ??? RetailStore invoiceRecord goodsShipped invoiceNumber +goodsShipped() -sendInvoice() PartsList adornments + public - private

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.

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.