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Certainly not all of these
properties are necessary to solve the administration problem. Only some of
them are problem specific. Consequently you create a model of an
employee for the problem. This model only implies properties which are needed
to fulfill the requirements of the administration, for instance name, date of
birth and social number. These properties are called the data of the
(employee) model. Now you have described real persons with help of an
abstract employee.
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Of course, the pure
description is not enough. There must be some operations defined with which
the administration is able to handle the abstract employees. For example,
there must be an operation which allows you to create a new employee once a
new person enters the institution. Consequently, you have to identify the
operations which should be able to be performed on an abstract employee. You
also decide to allow access to the employees' data only with associated
operations. This allows you to ensure that data elements are always in a
proper state. For example you are able to check if a provided date is valid.
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To sum up, abstraction is
the structuring of a nebulous problem into well-defined entities by defining
their data and operations. Consequently, these entities combine data
and operations. They are not decoupled from each other.
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