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

Chapter 22. State Machines

In this chapter

• States, transitions, and activities

• Modeling the lifetime of an object

• Creating well-structured algorithms

Using an interaction, you can model the behavior of a society of objects that work together. Using a state machine, you can model the behavior of an individual object. A state machine is a behavior that specifies the sequences of states an object goes through during its lifetime in response to events, together with its responses to those events.

You use state machines to model the dynamic aspects of a system. For the most part, this involves specifying the lifetime of the instances of a class, a use case, or an entire system. These instances may respond to such events as signals, operations, or the passing of time. When an event occurs, some effect will take place, depending on the current state of the object. An

effect is the specification of a behavior execution within a state machine. Effects ultimately resolve into in the execution of actions that change the state of an object or return values. A state of an object is a period of time during which it satisfies some condition, performs some activity, or waits for some event.

You can visualize the dynamics of execution in two ways: by emphasizing the flow of control from activity to activity (using activity diagrams) or by emphasizing the potential states of the objects and the transitions among those states (using state diagrams).