Smalltalk and Object Orientation: an Introduction - Free

• A conditional indicator. This is a question mark (?), optionally followed by a Boolean
expression in parentheses. The iteration and conditional indicators are mutually exclusive.
3. A return value name followed by an assignment sign (“:=“). If present this indicates that the
procedure returns a value designated by the given name. The use of the same name elsewhere in
the diagram designates the exact same value. If no return value is specified, then the procedure
operates by side effects.
4. The name of the message . This is an event name or operation name. It is unnecessary to specify
the class of an operation since this is implicit in the target object.
5. The argument list of the message . The arguments are expressions defined in terms of input
values of the nesting procedure, local return values of other procedures and attribute values of
the object sending the message.
Argument values and return values for messages may optionally be shown graphically using small
data flow tokens near a message. Each token is a small circle, with an arrow showing the direction of
the data flow, labeled with the name of the argument or result.
18.2.3 State machine diagrams
Scenarios are used to help understand how the objects within the system collaborate, where as state
diagrams illustrates how these objects behave internally. That is, state diagrams relate events to state
transitions and states. The transitions change the state of the system and are triggered by events. The
notation used to document state diagrams is based on that developed by Harel [Harel et al 1987, Harel
1988] and termed Statecharts.
Statecharts are a vari ant of the finite -state machine formalism which reduces the apparent
complexity of a graphical representation of a finite -state machine. This is accomplished through the
addition of a simple graphical representation of certain common patterns of finite sta te machine usage.
As a result, what might be a complex subgraph in a “basic” finite state machine is replaced by a single
graphical construct.
Each state diagram (statecharts are referred to as state diagrams in UML) has a start point a t which
the state is entered and may have an exit or termination point at which the state is terminated. The state
may also contain concurrency as well as synchronization of concurrent activities.
Figure 18.4 illustrate s a typical state diagram. This state diagram describes a (very) simplified
remote control locking system. The chart indicates that the system first checks the identification code of
the hand held transmitter. If it is the same as that held in the memory i t will allow the car to be locked
or unlocked. For the car locking situation, the windows are also closed and the car is alarmed.
Remote locking
Start
[transmittedID = memoryID]
Close
accepted
Open
lock
close
unlock
car lock
requested
car locked
window close
requested
car unlock
requested
car unlocked
car alarmed
car locked and alramed
Figure 18.4: An example State diagram
A state diagram consists of a start point, events, a set of transitions, a set of variables, a set of states
and a set of exist points. These are described briefly below:
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