Giáo trình Kỹ nghệ phần mềm - Bài 7: Thiết kế hướng đối tượng

essential communication mechanism.
Process stages
• Highlights key activities without being tied to 
any proprietary process such as the RUP.
– Define the context and modes of use of the 
system;
– Design the system architecture;
– Identify the principal system objects;
– Develop design models;
– Specify object interfaces.
Weather system description
A weather mapping system is required to generate weather 
maps on a regular basis using data collected from remote, 
unattended weather stations and other data sources such as 
weather observers, balloons and satellites. Weather stations 
transmit their data to the area computer in response to a 
request from that machine.
The area computer system validates the collected data and 
integrates it with the data from different sources. The 
integrated data is archived and, using data from this archive 
and a digitised map database a set of local weather maps is 
created. Maps may be printed for distribution on a special-
purpose map printer or may be displayed in a number of 
different formats.
System context and models of use
• Develop an understanding of the relationships 
between the software being designed and its 
external environment
• System context
– A static model that describes other systems in the 
environment. Use a subsystem model to show other 
systems. Following slide shows the systems around the 
weather station system.
• Model of system use
– A dynamic model that describes how the system interacts 
with its environment. Use use-cases to show interactions
Layered architecture
Subsystems in the weather mapping system
Use-case models
• Use-case models are used to represent each 
interaction with the system.
• A use-case model shows the system features 
as ellipses and the interacting entity as a stick 
figure.
Use-cases for the weather station
Use-case description
System Weather station
Use-case Report
Actors Weather data collection system, Weather station
Data The weather station sends a summary of the weather data that has been
collected from the instruments in the collection period to the weather
data collection system. The data sent are the maximum minimum and
average ground and air temperatures, the maximum, minimum and
average air pressures, the maximum, minimum and average wind speeds,
the total rainfall and the wind direction as sampled at 5 minute intervals.
Stimulus The weather data collection system establishes a modem link with the
weather station and requests transmission of the data.
ResponseThe summarised data is sent to the weather data collection system
Comments Weather stations are usually asked to report once per hour but
this frequency may differ from one station to the other and may be
modified in future.
Architectural design
• Once interactions between the system and its environment 
have been understood, you use this information for designing 
the system architecture.
• A layered architecture as discussed in Chapter 11 is 
appropriate for the weather station
– Interface layer for handling communications;
– Data collection layer for managing instruments;
– Instruments layer for collecting data.
• There should normally be no more than 7 entities in an 
architectural model.
Weather station architecture
Object identification
• Identifying objects (or object classes) is the 
most difficult part of object oriented design.
• There is no 'magic formula' for object 
identification. It relies on the skill, experience 
and domain knowledge of system designers.
• Object identification is an iterative process. 
You are unlikely to get it right first time.
Approaches to identification
• Use a grammatical approach based on a natural language 
description of the system (used in Hood OOD method).
• Base the identification on tangible things in the application 
domain.
• Use a behavioural approach and identify objects based on 
what participates in what behaviour.
• Use a scenario-based analysis. The objects, attributes and 
methods in each scenario are identified.
Weather station description
A weather station is a package of software controlled instruments 
which collects data, performs some data processing and transmits 
this data for further processing. The instruments include air and 
ground thermometers, an anemometer, a wind vane, a barometer 
and a rain gauge. Data is collected periodically. 
When a command is issued to transmit the weather data, the 
weather station processes and summarises the collected data. The 
summarised data is transmitted to the mapping computer when a 
request is received.
Weather station object classes
• Ground thermometer, Anemometer, Barometer
– Application domain objects that are ‘hardware’ objects related to the 
instruments in the system.
• Weather station
– The basic interface of the weather station to its environment. It 
therefore reflects the interactions identified in the use-case model.
• Weather data
– Encapsulates the summarised data from the instruments.
Weather station object classes
Further objects and object refinement
• Use domain knowledge to identify more objects and 
operations
– Weather stations should have a unique identifier;
– Weather stations are remotely situated so instrument 
failures have to be reported automatically. Therefore 
attributes and operations for self-checking are required.
• Active or passive objects
– In this case, objects are passive and collect data on request 
rather than autonomously. This introduces flexibility at the 
expense of controller processing time.
Design models
• Design models show the objects and object 
classes and relationships between these 
entities.
– Static models describe the static structure of the 
system in terms of object classes and 
relationships.
– Dynamic models describe the dynamic 
interactions between objects.
Examples of design models
• Sub-system models that show logical groupings of 
objects into coherent subsystems.
• Sequence models that show the sequence of object 
interactions.
• State machine models that show how individual 
objects change their state in response to events.
• Other models include use-case models, aggregation 
models, generalisation models, etc.
Subsystem models
• Shows how the design is organised into 
logically related groups of objects.
• In the UML, these are shown using packages -
an encapsulation construct. This is a logical 
model. The actual organisation of objects in 
the system may be different.
Weather station subsystems
Sequence models
• Sequence models show the sequence of 
object interactions that take place
– Objects are arranged horizontally across the top;
– Time is represented vertically so models are read 
top to bottom;
– Interactions are represented by labelled arrows, 
Different styles of arrow represent different types 
of interaction;
– A thin rectangle in an object lifeline represents the 
time when the object is the controlling object in 
the system.
Data collection sequence
Statecharts
• Show how objects respond to different service 
requests and the state transitions triggered by these 
requests
– If object state is Shutdown then it responds to a Startup() 
message;
– In the waiting state the object is waiting for further 
messages;
– If reportWeather () then system moves to summarising 
state;
– If calibrate () the system moves to a calibrating state;
– A collecting state is entered when a clock signal is received.
Weather station state diagram
Object interface specification
• Object interfaces have to be specified so that the 
objects and other components can be designed in 
parallel.
• Designers should avoid designing the interface 
representation but should hide this in the object 
itself.
• Objects may have several interfaces which are 
viewpoints on the methods provided.
• The UML uses class diagrams for interface 
specification but Java may also be used.
Weather station interface
interface WeatherStation {
public void WeatherStation();
public void startup();
public void startup(Instrument i);
public void shutdown();
public void shutdown(Instrument i);
public void reportWeather();
public void test();
public void test(Instrument i);
public void calibrate(Instrument i);
public int getID();
} //WeatherStation
Design evolution
• Hiding information inside objects means that 
changes made to an object do not affect other 
objects in an unpredictable way.
• Assume pollution monitoring facilities are to 
be added to weather stations. These sample 
the air and compute the amount of different 
pollutants in the atmosphere.
• Pollution readings are transmitted with 
weather data.
Changes required
• Add an object class called Air quality as part of 
WeatherStation.
• Add an operation reportAirQuality to 
WeatherStation. Modify the control software 
to collect pollution readings.
• Add objects representing pollution monitoring 
instruments.
Pollution monitoring
Key points
• OOD is an approach to design so that design 
components have their own private state and 
operations.
• Objects should have constructor and inspection 
operations. They provide services to other objects.
• Objects may be implemented sequentially or 
concurrently.
• The Unified Modeling Language provides different 
notations for defining different object models.
Key points
• A range of different models may be produced 
during an object-oriented design process. 
These include static and dynamic system 
models.
• Object interfaces should be defined precisely 
using e.g. a programming language like Java.
• Object-oriented design potentially simplifies 
system evolution.
Bài tập về nhà
• Trả lời câu hỏi chương 14: 14.4, 14.7, 14.9, 
14.10, sách Software Engineering, by Ian 
Summerville
– Có thể sử dụng công cụ Enterprise Architecture 
của SparxSystems.com
• Có ví dụ kèm theo sau khi cài đặt.