What are the Key Concepts?
OT models systems using 2 key concepts:
It defines the business world (or domain)
that a system encompasses in terms of objects.
Objects may be physical objects (a car) or abstract ideas (commission scheme)
or events (a delivery).
It classifies the objects into similar concepts
by looking for their common characteristics (or properties) and groups them
Common characteristics need only be defined once.
In order to then build the system, OT uses 4 key principles:
Abstraction - the describing
of objects by defining their unique and relevant characteristics (properties).
Whilst an object may have 100s of properties normally only those properties
of importance to the situation are described. (e.g. life policies premiums
are normally important; whereas the time of day a policy was purchased is
not usually of value).
Encapsulation - the decoupling of the external
use of an object from the object itself.
Objects have behaviours (e.g. formula to work out
monthly premium) with individual behaviours called services (e.g. provide
monthly premium). Services are invoked by sending messages (inputs) which
in return produce output. Involving a service causes a state change (e.g.
monthly premium has been paid). Encapsulation leads to more stable systems.
Inheritance - Inheritance is
the ability to write one definition as an extension of another.
Only the properties and behaviours which are different need be described
in the child object. (e.g. a monthly savings plan and a single premium bond
have different collection frequencies). All the properties and behaviours
which are unchanged are obtained by reference to the parent.
Polymorphism - the ability to
make one object able to be coupled with a wide variety of others, relying
on their all having a common external interface.
Polymorphism separates the different uses of an object (e.g. print text from
print a drawing). New uses (e.g. print a picture) can be added with no impact
on the existing uses of the object.
This can get rather technical, so don't worry if it seems obtuse. For those
that are interested in knowing more, there are links at then end of the page.
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