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

60 Abstraction of a Problem Domain
4. Wegner [Weg87]: describes objects as ’autonomous entities that respond to messages
or operations and share a state. An object is a modular computing agent. In contrast with
functions, objects have memory.’
5. Verschueren [Ver92] notes that objects do not always have a fixed location within
a system. ’Passive travelling objects move and are operated upon by other objects, e.g. a
’data packet’. Active travelling objects move and perform operations at the location where
they are. They can move themselves.’
4.2.4 Object-Oriented Analysis
The main difference between object-oriented analysis and a functional approach is respectively
the union and the separation of functions and data. SASD methods support a
functional approach. They model data separate from functions. Data is modelled in
stores and on flows. Functions are modelled as processes (transformations). In contrast,
object-oriented analysis uses objects that encapsulate data in a shell together with the
operations on that data. This data is private to the object, and can only be transformed
or examined by services (operations) that the object offers via its message interface.
Messages activate well defined internal behaviour of the object. Possible results will
be returned to the sender of a message. So an object hides and combines data with
its accompanying operations. The behaviour of an object may differ at various times
depending on the communication that happened before. Objects give a completely dif-
Object D
Object A
Object
to be
defined
Object C
Object B
Figure 4.1: Analyst Visualises to be in an Object
ferent feeling and attitude towards the problem domain analysis, when compared to a
functional approach. Various items, such as things, commodities, concepts, and targets
can be modelled as objects. For human thinking and reasoning it is important that we
can identify ourselves with an object. We can imagine ourselves to be in an object, look
outside, and wonder what we want from other objects (see Figure 4.1). We can also