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

4.4 Data Objects and Process Objects 71
4.4.2.5 Equality and Equivalence
The equality operation, denoted as ==, is a predefined operation in POOSL, that can be
used in any object class. The operation == determines whether two objects refer to the
same object. We explain the meaning with an example, which also shows some aspects
of data object communication and behaviour expression evaluation. The following
supervisor
X
scheduler
feedServer
A
cardInjector
C
Figure 4.8: Data Object References
expression checks whether objects A and C in Figure 4.8 refer both to the same object B.
supervisor whatIsYourFeedServer ¡ scheduler whatIsYourCardInjector
This expression is part of the behaviour description of object X. The evaluation of the
expression causes a sequence of actions. The first part of the expression must be read
as: send to supervisor the message whatIsYourFeedServer. Object X has a reference to A,
stored in variable supervisor. So object X sends the message named whatIsYourFeedServer
to object A. Object A receives message whatIsYourFeedServer. Object A performs an
operation that corresponds to the reception of this message and returns the result to
object X. This result is object B. So this communication causes that object X gets (a
temporary reference to) object B. This reference has not been shown in Figure 4.8. The
expression is partially evaluated now to:
B ¡ scheduler whatIsYourCardInjector
This means that message == scheduler whatIsYourCardInjector must be sent to object B.
The message name is == and the parameter sent with the message is scheduler whatIsYour-
CardInjector. The message == can be sent because X has a reference to B. The parameter
scheduler whatIsYourCardInjector has to be evaluated before the sending. This part of the
expression must be read as: send to object scheduler the message whatIsYourCardInjector.
This message is sent to the object that is stored in variable scheduler. This object is object
B