Object-Oriented Design and Implementation, Slides of Software Engineering

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Chapter 7 – Design and Implementation

Topics covered  Object-oriented design using the UML  Design patterns  Implementation issues  Open source development

Build or buy  In a wide range of domains, it is now possible to buy off- the-shelf systems (COTS) that can be adapted and tailored to the users’ requirements. ▪ For example, if you want to implement a medical records system, you can buy a package that is already used in hospitals. It can be cheaper and faster to use this approach rather than developing a system in a conventional programming language.  When you develop an application in this way, the design process becomes concerned with how to use the configuration features of that system to deliver the system requirements.

Object-oriented design using the UML

Process stages  There are a variety of different object-oriented design processes that depend on the organization using the process.  Common activities in these processes include: ▪ 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.  Process illustrated here using a design for a wilderness weather station.

System context and interactions  Understanding the relationships between the software that is being designed and its external environment is essential for deciding how to provide the required system functionality and how to structure the system to communicate with its environment.  Understanding of the context also lets you establish the boundaries of the system. Setting the system boundaries helps you decide what features are implemented in the system being designed and what features are in other associated systems.

System context for the weather station

Weather station use cases

High-level architecture of the weather station

Architecture of data collection system

Approaches to identification  Use a grammatical approach based on a natural language description of the system.  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 object classes  Object class identification in the weather station system may be based on the tangible hardware and data in the system: ▪ Ground thermometer, Anemometer (Wind Speed), Barometer (atmospheric pressure)

• 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 summarized data from the instruments.

Design models  Design models show the objects and object classes and relationships between these entities.  There are two kinds of design model: ▪ Structural 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  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.