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

4.2 Objects 59
An object can mean the thing it represents or the concept of object. The actual meaning
must become clear from the context. This can be further clarified with an example.
What do we mean with ’the properties of objects’. It can mean two things. Firstly, an
object models an entity from a problem domain. Such an entity has various properties
which are modelled by the object. Therefore the object is said to have those properties.
Secondly, an object is a concept that has the power to describe entities from a problem
domain. An object has this power because the concept of object is constructed properly.
The properties of the concept of object must be such that an object in a system model
can adequately represent all relevant properties of the thing (entity or object) it models.
4.2.2 Definition
The common use of the term object is to refer to a thing, a commodity, an article, a matter,
a manifestation, a something, a recipient or a target. These common meanings show
that the notion of object is suitable to refer to many things in a problem domain. Coad
and Yourdon refer to Webster’s dictionary (1977) to define an object: ’A person or thing
to which action, thought, or feeling is directed. Anything visible or tangible; a material product
or substance’ [CY91a].
For system analysis we define an object to be a model of a concept, or of an entity
from a problem domain. It is characterised by its externally observable behaviour and
its internal state. This definition matches with definitions from the world of software
engineering, because it is kept abstract and general. The semantics of objects are language
or method dependent. Properties such as internal concurrency, message passing,
persistence, and degree of encapsulation, determine the semantics. These, and other
properties, will be described later.
4.2.3 Alternative Definitions
For a better understanding of the concept of object, alternative definitions add various
aspects:
1. Rumbaugh et al. [R 91] describe an object as ’a concept, abstraction, or thing with
crisp boundaries and meaning for the application.’ They distinguish two sorts. Concrete
objects refer to physical things. Conceptual objects refer to a policy to do
something.
2. Coad and Yourdon [CY91a, CY91b] describe an object as ’an abstraction of something
in the problem domain (or its implementation) reflecting the capabilities of a system to keep
information about it, interact with it, or both.’ A second definition they give is: ’an
encapsulation of attribute values and their exclusive services.’ A third is: ’a run-time
instance of some processing and values, defined by a static description called a ’class’.’
3. Jacobson [J 92] defines an object as ’an entity able to save a state (information) and
which offers a number of operations (behaviour) to examine or affect this state.’