State machine diagrams captures the life cycles of
objects, sub-systems and systems. They indicate what states an
object can have and how different events affect those states over
time.
State machine diagrams should be attached to classes that have
clearly identifiable states and is governed by complex
behaviour.
A State is displayed as a rounded rectangle with
optional compartments for attributes, events and internal
activities. State flows or transitions are drawn between States,
usually with guard conditions and rules governing how and when an
object may transition from one state to
another.
States are usually named according to their
condition, for example 'Checking', 'Waiting' and 'Dispatching' are
all active conditions an object can be in while waiting to
transition to another state or end the cycle
completely.
Start and end nodes represented as solid and empty
circles are used to represent the beginning and end of all
transitions. State machine diagrams can have a start point and
several end points.
State transitions may be triggered by events.
These events may have keywords (guards) associated with it to
clarify the event. It is not always necessary to show these
events.
States can be nested. This implies that states
(sub states) can exist in an overall state. Parallel states can
also be defined where an object may have several states at the same
time. For example: A person has at any time many parallel states.
These may be: walking, thinking, young,
etc.
Consider the following state chart of an invoice
class:
