Object Oriented Design Chapter 7

Design Models

• Show objects and object classes, as well as relationships between them
• Two types of design models:
  • Structural models that describe the static structure of the system
  • Dynamic models that describe the dynamic interactions between objects

Examples of Design Models

• Subsystem 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

• Show how the design is organized into logically related groups of objects
• In the UML, these are shown using packages, an encapsulation construct
• Logical model, actual organisation of objects in the system may be different

Sequence Models

• Show the sequence of object interactions that take place
• Objects are arranged horizontally across the top, time is represented vertically
• 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

Process Stages

• Variety of object-oriented design processes depending on the organization
• Common activities include:
  • Defining the context and modes of use of the system
  • Designing the system architecture
  • Identifying the principal system objects
  • Developing design models
  • Specifying object interfaces

System Context and Interactions

• Understanding the relationships between the software and its external environment is essential for deciding how to provide required system functionality
• 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

Context and Interaction Models

• A system context model is a structural model that demonstrates the other systems in the environment of the system being developed
• An interaction model is a dynamic model that shows how the system interacts with its environment as it is used

State Diagrams

• Used to show how objects respond to different service requests and the state transitions triggered by these requests
• Useful high-level models of a system or an object’s run-time behavior
• Not usually needed for all objects in the system, as many objects are relatively simple

Interface Specification

• Object interfaces have to be specified so that 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