Specification of Reactive Hardware/Software Systems - Electronic ...

4.4 Data Objects and Process Objects 69
The behaviour of the objects in a Data Class is specified in a Class definition. A class
definition is a formal behaviour description in the form of a POOSL Data Class. Many
properties of data classes in POOSL are equal to that of classes in Smalltalk [GR89]. They
have instance variables, local variables, and methods. Instance variables implement
attributes found during analysis into the formal class definition. Methods describe an
object’s responses to the reception of messages. POOSL is an imperative language. So,
the behaviour specification is ’implemented’ in POOSL in the form of statements.
Data object A
4.4.2.2 Sequential Behaviour
message
reply
Figure 4.6: Data Objects
Data object B
Data objects model passive entities. They must receive a message to become active.
Data objects that exchange messages perform together a sequential behaviour. Only one
object is actively performing operations at the same time. When the sender of a message
is another data object this sender becomes passive. Figure 4.6 illustrates data object 1
communication. To be able to send a message, data object A must be activated by the
reception of a message which has not been shown. A becomes passive after sending
a message to B. Subsequently B becomes active, and processes the message. After
returning a reply B becomes passive again, and A becomes active.
4.4.2.3 References
A data object can communicate with another data object when it ’knows’ that object’s
name. The latter object is said to be an acquaintance of the first object. An object ’knows’
its acquaintances by having references to it. References to objects can be passed between
objects. In this way objects can extend or limit their collection of acquaintances. This
enables very flexible communication. References are stored in instance variables. Figure
4.7 shows three objects. Both object A and object C know object B. Both have a reference
to object B. Object A has a reference to object B, which is stored in variable feedServer.
Object C has such a reference in variable cardInjector.
References are encapsulated in the objects and cannot be addressed directly by other
1 The circle symbol is for illustration purposes only. It has no formal status in the method.