Software Engineering Exam Chapter 5

UML diagram types

Activity diagrams-which show the activities involved in a process or in data processing. Use case diagrams- show the interactions between a system and its environment. Sequence diagrams, which show interactions between actors and the system and between system components. Class diagrams, which show the object classes in the system and the associations between these classes. State diagrams, which show how the system reacts to internal and external events.

Object class aggregation models

An aggregation model shows how classes that are collections are composed of other classes. Aggregation models are similar to the part-of relationship in semantic data models.

System perspectives

An external perspective, where you model the context environment of the system. An interaction perspective, where you model the interactions between a system and its environment, or between the components of a system. A structural perspective, where you model the organization of a system or the structure of the data that is processed by the system. A behavioral perspective, where you model the dynamic behavior of the system and how it responds to events.

Use of graphical models

As a means of facilitating discussion about an existing or proposed system. Incomplete and incorrect models are ok as their role is to support discussion. As a way of documenting an existing an existing system Models shouls be an accurate representation of the system but need not complete. As a detailed system description that can be used to generate a system implementation. Models have to be both correct and complete.

Behavioral models

Behavioral models are models of the dynamic behavior of a system as it is executing. They show what happens or what is supposed to happen when a system responds to a stimulus from its environment. You could think of these stimuli as being of two types: data- some data arrives that has to be processed by the system. Events- Some event happens that triggers system processing. Events may have associated data, although this is not always the case.

Class diagrams

Class diagrams are used when developing an object-oriented system model to show the classes in a system and the associations between these classes. An object can be though of as a general definition of one kind of system object. An association is a link between classes that indicates that there is some relationship between these classes. When you are developing models during the early stages of the software engineering process, objects represent something in the real world, such as a patient, a prescription, doctor, etc.

Context models

Context models are used to illustrate the operational context of a system- they show what lies outside the system boundaries. Social and organisational concerns bay affect the decision on where to position system boundaries. Architectural models show the system and its relationship with other systems.

Structural models

Display the organization of a system in terms of the components that make up that system and their relationships. Structural models may be static models, which chow the structure of the system design, or dynamic models, which show the organization of the system when it is executing. You create structural models of a system when you are discussing and designing the system architecture.

Generalization

Generalization is an everyday technique that we use to manage complexity. Rather than learn the detailed characteristics of every entity that we experience, we place these entities in more general classes and learn the characteristics of these classes. This allows us to infer that different members of these classes have some common characteristics e.g. squirrels and rats are rodents.

Generalization continued.

In modeling systems, it is often useful to examine the classes in a system to see if there is scope for generalization. If changes are proposed, then you do not have to look at all classes in the system to see if they are affected by the change. In object-oriented languages, such as Java, generalization is implemented using the class inheritance mechanisms built into the language. In generalization, the attributes and operations associated with higher-level classes are also associated with higher level classes are also associated with the lower-level classes. The lower-level classes are subclasses that inherit the attributes and operations from their superclasses. These lower-level classes then add more specific attributes and operations.

Data driven modeling

Many business systems are data-processing systems that are primarily driven by data. They are controlled by the data input to the system, with relatively little external even processing. Data-driven models show the sequence of actions involved in processing input data and generating associated output. They are particularly useful during the analysis of requirements as they can be used to show end-to-end processing in a system.

Interaction models

Modeling user interaction is important as it helps to identify user requirements. Modeling system-to-system interaction highlights the communication problems that may arise. Modeling component interaction helps us understand if a proposed system structure is likely to deliver the required system performance and dependability. Use case diagrams and sequence diagrams may be used for interaction modeling.

Event-driven modeling

Real-time systems are often event-driven, with minimal data processing. For example, a landline phone switching system responds to events such as 'receiver off hook' by generating a dial tone. Event driven modeling shows how a system responds to external and internal events. It is based on the assumption that a system has a finite number of states and that events may cause a transition from one state to another

Sequence diagrams

Sequence diagrams are part of the UML and are used to model the interactions between the actors and the objects within a system. A sequence diagram shows the sequence of interactions that take place during a particular use case or use case instance. The objects and actors involved are listed along the top of the diagram, with a dotted line drawn vertically from these. Interactions between objects are indicated by annotated arrows.

System Modeling

System modeling is the process of developing abstract models of a system, with each model presenting a different view or perspective of that system. System modeling has now come to mean representing a system using some kind of graphical notation, which is now almost always based on notations in the Unified Modeling Language. System modelling helps the analyst to understand the functionality of the system and models are used to communicate with customers.

State machine

These model the behavior of the system in response to external and internal events. They show the system's responses to stimuli so are often used for modeling real-time systems. State machine models show system states as nodes and events as arcs between these nodes. When an event occurs, the system moves from one state to another. Statecharts are an integral part of the UML and are used to represent state machine models.