The Unified Modeling Language User Guide SECOND EDITION [Electronic resources] نسخه متنی

Getting Started

A building is a living thing. Although every building is constructed of static stuff, such as bricks, mortar, lumber, plastic, glass, and steel, those things work together dynamically to carry out behavior that is useful to those who use the building. Doors and windows open and close. Lights turn on and off. A building's furnace, air conditioner, thermostat, and ventilation ducts work together to regulate the building's temperature. In intelligent buildings, sensors detect the presence or absence of activity and adjust lighting, heating, cooling, and music as conditions change. Buildings are laid out to facilitate the flow of people and materials from place to place. More subtly, buildings are designed to adapt to changes in temperature, expanding and contracting during the day and night and across the seasons. All well-structured buildings are designed to react to dynamic forces, such as wind, earthquakes, and the movement of its occupants, in ways that keep the building in equilibrium.

Software-intensive systems are the same way. An airline system might manage many terabytes of information that sit untouched on some disk most of the time, only to be brought to life by outside events, such as the booking of a reservation, the movement of an aircraft, or the scheduling of a flight. In reactive systems, such as those found on the computer in a microwave oven, objects spring to life and work gets carried out when the system is stimulated by such events as a user pushing a button or by the passage of time.

In the UML, you model the static aspects of a system by using such elements as class diagrams and object diagrams. These diagrams let you visualize, specify, construct, and document the things that live in your system, including classes, interfaces, components, nodes, and use cases and their instances, together with the way those things sit in relationship to one another.

In the UML, you model the dynamic aspects of a system by using interactions. Like an object diagram, an interaction statically sets the stage for its behavior by introducing all the objects that work together to carry out some action. Going beyond object diagrams, however, interactions also introduce messages that are dispatched from object to object. Most often, messages involve the invocation of an operation or the sending of a signal; messages may also encompass the creation and destruction of other objects.

You use interactions to model the flow of control within an operation, a class, a component, a use case, or the system as a whole. Using interaction diagrams, you can reason about these flows in two ways. First, you can focus on how messages are dispatched across time. Second, you can focus on the structural relationships among the objects in an interaction and then consider how messages are passed within the context of that structure.

The UML provides a graphical representation of messages, as Figure 16-1 shows. This notation permits you to visualize a message in a way that lets you emphasize its most important parts: its name, parameters (if any), and sequence. Graphically, a message is rendered as a directed line and almost always includes the name of its operation.