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An Example of ObjectOriented Analysis and Design

A Dice Game

Play Rules / Business Rules:

A dice game in which a player rolls two dice.

If the total is seven, they win; otherwise, they lose.

The dice game is very simple problem, presented in order tofocus on some of the steps and artifacts in object oriented

analysis and design rather than on the problem domain.


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Most essential and commonly used steps in OOAD

Define usecases

Define

conceptual

model

Definecollaboration

diagrams

Define design

class diagrams

System Behaviordiagram

Analysis Design


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Understanding the requirements includes, in part, understanding the

domain processes and the external environment external actors who

participate in processes.

These domain processes can be expressed in use cases

narrative descriptions of domain processes in a structured text format

For example, in the dice game, here is the Play Game High level

usecase

Use case: Play a Game

Actors: PlayerDescription: This use case begins when then

the player picks up and rolls the

dice. If the dice total seven, they

win; other wise, they lose

Define use cases


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268Expanded Usecase:

SI

NoActors Action SI

NoSystem Response

1 This use case beginswhen the Playerstarts

the game by picking

up the diceand then

playerrolls the the

dice

2

2a

Die Gamewill collect face

value of each diceand then

adds up to give the sum ofthe face value(total) for a

particular roll.

If the dice totalis 7 then

system will indicate theplayerthat he wins, and

request theplayerto

continue the game.

Alternate path to 2a: the total is 7 then the player looses the game


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Usecase Diagram


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A decomposition of the problem domain involves an identification of

concepts

attributes

associations

in the domain that are considered important.

The result can be expressed in a conceptual model, which is

illustrated in a set of diagrams that depict concepts (objects).

Define conceptual model


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Conceptual model of the Dice game

The conceptual model is not a description

of software components; it represents

concepts in the real world

problem domain.


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Defining Collaboration Diagrams

Collaboration diagrams show the flow of messages betweeninstances and the invocation of methods

Object - oriented design is concerned with defining logical software

specificationsthat fulfill the functional requirementsbased on

decompositionby classes of objects.

An essential step in this phase is the allocation of responsibilitiesto

objects and illustration how they interact via messages, expressed in

collaboration diagrams.


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Defining Collaboration Diagrams Illustrating messages

between software objects

For example, assume that a simulation in software of the dice game

is desired. This collaboration diagram in Figure illustrates the

essential step of playing by sending messages to instances of the

Player and die classes.


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Defining Design Class DiagramsThese illustrate class definitions that are to be implemented

in software

In order to define a class, several questions must be answered How do objects connect to other objects?

What are the methods of a class?

To answer these questions, inspect the collaboration diagrams,

which suggests the necessary connections between objects,and the methods that each software class must define


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Design class diagram of software components.

For example, in the dice game, an inspection of the collaboration

diagram leads to the following design class diagram.

In contrast to the conceptual model, this diagram does not

illustrate real world concepts; it describes software components

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Design class diagram of software components........

Since a play message is sent to a Player instance, Playerrequires a play method, while Die requires a roll method.

To illustrate how objects connect to each other via

attributes, a line with an arrow at the end may suggest anattribute.

For example, Dice Game has an attribute that points to aninstance of a Player.(visibility issues)

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