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<h1>Data Abstraction</h1>
<p>Data Abstraction enables isolation of the object's
implementation details from the way it is used. Data Abstraction groups the
pieces of data that describe some entity, so that programmers can manipulate
that data as a unit. It helps a
programmer to cope with the complexity of data by hiding the details.</p>
<p>In the object database model, each single entity is called
an object instance. Each object has a unique unchanging identity. Object
identity is characterized by the following features:</p>
<ul><li>object
identity is independent from the data contained in the object - internal
data values are not used to generate an identity;</li><li>object
identities are generates by the object system and are not controlled by a
programmer or a database system;</li><li>object
identity lives as long as the object itself, it does not change with the
modifications of the objects data content.</li></ul>
<p>Objects are described by classes. A class defines a
structure and attributes (fields) of an object. Classes are also used to define
hierarchical properties: child and parent relationships.</p><p>
Object model directly supports references. Note that
references establish a connection between objects using their identities. For
example:</p><p><img src="Classes1.Jpg"/></p><p>In this example Pilot object is referenced from
a Car object.<br>
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This revision (2) was last Modified 2007-06-02T19:28:21 by Tetyana.
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