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

4.15 Operations, Services, Methods 111
whether they are examined or not. This helps to achieve completeness. Moreira differentiates
in ROOA [Mor94] between Offered Services and Required Services. An offered
service is defined as a capability that an object exports and which can be used (called) by
other objects in the model. A required service is a service required from another object
[Mor94].
4.15.3 Methods
We define a method as a POOSL behaviour description of one or more operations. Methods
are specified for object classes. They give an algorithm in the form of POOSL
statements. Notice that POOSL behaviour descriptions are still specifications in the
sense that this behaviour must finally be implemented in software or hardware. This
definition is different from traditional ones, where methods are the actual implementation
of an operation in a class. The concept of method has completely different meanings
for data objects and process objects. This is described in the following paragraph.
4.15.3.1 Data Object Methods
The operations that can be performed by a data object correspond one to one to the
methods defined in its class. Operation and service is the same concept for a data object.
The names of the methods are the names of the messages that the object is responding
to. A method may need data that must come with the message to perform the required
operation. This data is transferred with the message in the form of a collection of
parameters. A method of a data class is user defined or primitive:
m(u1 : C1 ¥¡ ¡ ¡ ¥ un : Cn) : C ¡ v1 : C¡1 ¡ ¡ vm : C¡m ¡ E
¡ m(u1 : C1 ¥¡ ¡ ¡ ¥ un : Cn) : C primitive
A reception of a message with name m leads to the evaluation of expression E, and the
returning of the result to the sender of the message. The result is an object of class C.
The behaviour of some methods cannot be expressed as expressions. So-called primitive
methods are specified in the form of axioms in the semantics of the language (See Chapter
8). Besides the parameters u1 ¥¡ ¡ ¡ ¥ un that should come with the message, a method may
have local variables v1 ¥¡ ¡ ¡ ¥ vm to store intermediate results during the operation. Each
variable is assigned a type in the form of a name of a data class. Variable u1, for instance,
will refer to an object of class C1.
4.15.3.2 Process Object Methods
Whereas data object methods follow object-oriented traditions, the methods of process
objects differ strongly. There is no direct relation between message names and methods.
The message interface of a class is defined by:
message interface l1 ¡ ¡ lm
in which each li is either of the form ch!m ¢
¥¡ ¡ ¡ ¥ C1 £ Cn or of the form ch?m ¢
¥¡ ¡ ¡ ¥ Cn£ C1 . This
denotes that message m can be sent respectively be received with n data objects of classes