NI ©UNITEN 2000/01 Rational Rose Lab1 Rational Rose Lab Topic 4 Object and Class.

Rational Rose Lab 1NI ©UNITEN 2000/01

Rational Rose LabTopic 4

Object and Class


What is an Object?

• Informally, an object represents an entity, either physical, conceptual or software.

• Example:• Physical entity : Car• Conceptual entity: Chemical process• Software entity: Linked list.

A more formal definition:

• An object is a concept abstraction, or thing with sharp boundaries and meaning for an application. An object is something that has:

• State• Behaviour• Identity


An Object Has State

• The state of an object is one of the possible conditions (values of its all attributes at a given time) in which an object may exist.

• The state of an object normally changes over time.

• The state of an object is usually implemented by a set of properties (called attributes), with the values of properties, plus the links the object may have with other objects.

• Example :

Object name: PencilState : Sharp


An Object Has Behaviour

• Behaviour determines how an object acts and reacts

• Behaviour defines how an object reacts to requests from other objects

• The visible behaviour of an object is modelled by the set of messages it can respond to (the operations the object can perform)

Example:• Object name: Car engine• Behaviour: start(), spark(),ignition(),getGas(), getAir()


Object has Identity

• Each object has a unique identity, even if its state is identical to that of another object.

• It characterises its own existence

• Distinguish any object in an unambiguous way, and independently from its state.

Colour - Blue

H/power - 240bph

Model -MercedesCLK340

Plate Num-WDG 997

Colour - Blue

H/power - 240bph

Model -MercedesCLK340

Plate Num-WFN 338


What are Classes?

• A class is a description of a group of objects with common properties (attributes), common behaviour (operations), common relationships to other objects (associations and aggregations) and common semantic.

• A class is an abstract definition of an object.– It defines the structure and behaviour of each object in the class

– It serves as a template for creating objects.

• Abstraction help us deal with complexity


Naming Classes

• A class should capture one and only one key abstraction

• Bad abstraction: Student class which know student information and the student schedule for the current semester

• Good abstraction: Separate classes for Student and Student Schedule.

• A class name should be a singular noun that best characterises the abstraction.

• Difficulty in naming a class may be an indication pf a poorly defined abstraction.

• Name should come directly from the vocabulary of the domain.


Naming Classes cont...

• After naming a class, a brief concise description of the class should be made– Focus on the purpose of the class not on the implementation.

• Example:– Name: Student

– Working Definition: Information about a person registered to attend classes at the University.

Look for the “WHATS” and ignore the “HOWS”


Representing Classes

• A class is represented using a compartment rectangle

• A class is comprised of three sections– The first section contain the class name– The second section shows the structure (attributes)– The third section shows the behaviour (operations)

• The second and third sections may be suppressed if they need not to be visible on the diagram.

SportsCar

+ revenue-model+ accelerate()

+ brake()

SportsCar SportsCar


Attribute

• An attribute is a data definition held by instances of class (objects)

• Attributes do not have behaviour – they are not object• Attribute names are simple noun or nouns phrase• Each attribute should have a clear, concise definition• An attribute value is the value of an attribute for a particular

object• Each object has a value for every attribute define for its class• For example: for an object of the “Lecturer class”

– Attribute:

– Name : Zainab Ali

– Staff ID : 215548

– Subject : Mathematics


Derived attributes

• A derived attribute is an attribute whose value may be calculated based on the value of other attribute(s)

• Used when it is not enough time to re-calculate the value every time it is needed

• Trades off runtime performance vs. memory required

Rectangle

LengthWidthArea


Operations

• A class has a set of responsibilities which define the behaviour of the objects in the class

• The responsibilities of a class are carried by its operations

• An operation (method) is a service that can be requested from an object to effect behaviour

• An operation should perform a simple, cohesive function.


Naming operation

• Operation should be names to indicate their outcome, not the steps behind the operation.

• Example:

– CalculateBalance() Poorly named: Indicates that the balance must be calculated – this

is an implementaion decision

– GetBalance() Well named: Indicates the outcomes only


Naming operation cont…

• Operation should be named from the perspective of the supplier not the client

• In an oil station,oil is received from the pump

• One operation from the pump has this responsibility – what should it be called? Good names – dispense(), giveOil() Bad name – receiveOil()

The pump gives the oil – it does not receive the oil


Class Stereotypes

• A stereotype is a new type of modeling that extends the semantics of the metamodel.

• Every class may have at most one stereotype

• Common stereotype:– Boundary class– Entity class – Control class– Exception class– Utility class

• Stereotypes are shown in the class name compartment enclosed in << class stereotype name >>


Boundary Class

• A boundary class models communication between the system’s surroundings and its inner workings

• Responsible for the exchanges of information between the software and one or more of its actors

• The exchange are done according to a given format and a protocol.

• Typical boundary classes– Windows (user interface), Communication protocol (system

interface), printer interface, sensor and etc.


Interfaces to other systems

• A boundary class is also used to model an interface to another system

• The important characteristics of this type of boundary class are– The information to be passed to the other system

– The communication protocol used to ‘talk’ to other system

• In the ‘Card Validation” scenario,information must be sent to the external Bank System– A class called CardValidation is created to hold the interface to the external

system

<<boundary>>CardValidation


Entity Class

• An entity class models information and associated behaviour that is generally long-lived (persistent)– i.e. Entity class holds persistent information

• It can reflects a real-life phenomenon• It may also needed for the internal tasks of the system• The values of its attributes are often provided by an actor• The behaviour is surroundings-independent

• Entity classes in the “Withdraw Money” use case:– account <<entity>>

account


Control Class

• A control class models control behaviour specific to one or more use cases

• A control class– creates, initialises and deletes controlled objects

– controls the sequencing or co-ordination of execution of controlled objects

– controls concurrency issues for the controlled classes

– is, most of the time, the implementation of an intangible object

• In the Withdraw Money’s scenario, a WithdrawMgr class controls the process of cash withdrawal

<<control>>WithdrawMgr


Exercise 4

From the ATM case study

1. Identify all possible objects/classes involved

2. For each of them, classify their – stereotypes, either boundary, control or entity

– attributes

– operations

3. Create the classes identified using Rational Rose

3. Save your rational model in the diskette

NI ©UNITEN 2000/01 Rational Rose Lab1 Rational Rose Lab Topic 4 Object and Class.

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