Smalltalk and Object Orientation: an Introduction - Free

Operations can be of the following types:
• entry (executed when the state is entered). These are the same as specifying an operation on a
transition. They are useful if all transitions into a state perform the same operation (rather than
need to specify the same operation on each transition). Such operations are considered to be
instantaneous.
• exit (executed when the state is exited). T hese are less common than entry actions and indicate
an operation performed before any transition from the state occurs.
• do (executed while the state is active). These are operations which start on entry to the state and
terminate when the state is exited.
• event. A specified event can also trigger off an operation while within a particular state. For
example, the event help could trigger the help operation which in the state active.
Each operation is separated from its type by a forward slash (“/”). The ordering of operations is:
operation on incoming transitions -> entry operations -> do operations -> exit operations ->
operations on outgoing transitions.
State diagrams allow a state to be a single state variable, or a set of substates. This allows for
complex hierarchical models to be developed gradually as a series of nested behavior patterns. This
means that a state can actually be a State diagram in its own right. For example, car alarmed is a single
state, however the car locked state, is actually another State diagram. Notice that the transition from car
alarmed to accepted jumps from an inner state to an outer state.
The dotted line down the middle of car locked state indicates that the two halves of that state run
concurrently. That is, the car is locked as the windows are closed.
A special type of state, called a history state, is used to represent a state which must be remembered
and used the next time the (outer) state is entered. The symbol for a history state is an H in a circle.
18.2.3.6 A set of exit points
Exit points are used to specify the result of the State diagram. They also terminate the execution of
the State diagram.
18.3 Deployment diagrams
PC
*
Order
Entry
Fax
1
PC
1
Receivin
g
1
Server
Purchasin
g
1
Printer
PC
1
Figure 18.6: Nodes in a Deployment Diagram
The elements in Figure 18.6 are called nodes and represent processors (such as PC’s and servers) and
devices (such as Printers and Faxes). A node is thus a resource in the real world upon which we can
distribute and execute elements of the (logical) design model. A node is drawn as a three -dimensional
rectangular solid with no shadows. The stereotype designation of the Fax and Printer
indicates that these nodes are not processors. That is, they do not ha ve any processing ability (at least
not from the point of view of the model being constructed). It is also possible to show how many nodes
are likely to be involved in the system. Thus the Order Entry PC is of order * ( 0 or more), while there
will be exactly one server, printer, fax etc. Finally, we have also indicated the roles of the associations
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