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7.1 States
As discussed in Chapter 2, as elements communicate
with one another within a society of objects, each element has a
lifecycle in
which it is created, knows something, can
do something, can communicate with other elements to request
processing of those other elements, can have other elements
communicate with it to request processing of it, and is destroyed. A
state is a specific
condition
or situation of an element during its lifecycle. Define the states
for your elements. The current state of an element is called its
active state, and the element is
said to be
"in" that state. There are various
types of states, including simple, initial, and final states. The
next few sections discuss these different types of states.
7.1.1 Simple States
A simple state indicates a condition
or
situation of an element. For example, the project management system
may be in one of the following simple states:
Inactive
Indicates that the project management
system
is not
available to its users, because it is not started or has been shut
down
Active
Indicates that the project management
system
has been
started and is available to its users
Suspended
Indicates that the project management
system
has
encountered some severe error, perhaps because it is running low on
secondary storage and requires user intervention before becoming
active again
In the UML, a simple state is shown as a rectangle with rounded
corners and labeled with the name of the state or a description of
the situation of the element. Figure 7-1 shows the
various states associated with the project management system.
Figure 7-1. Simple states
7.1.2 Initial and Final States
An initial state indicates the state of an
element when it is created. In the UML,
an initial state is shown using a small solid filled circle. A
final state indicates the state of an element
when it is destroyed. In the UML, a final state is shown using a
circle surrounding a small solid filled circle (a
bull's eye). Figure 7-2 updates
Figure 7-1 with an initial state and final state. A
state diagram may have only one initial state, but may have any
number of final states.
Figure 7-2. Simple, initial, and final states
