Object Identity in Java

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Object Identity
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One of the most important properties of an object is that of having an identity, different from the identity of any other object, and immutable during the object's lifetime. Object identifiers usually are not directly visible and accessible by database users; they are internally used by the system. In addition to the object identifier, an object can be characterized by one or more names that are meaningful to the programmer or enduser. Object names are used for convenience, especially to refer to root objects, which provide entry points into the database. The notion of an object identifier is quite different from the notion of a key used in the relational model to uniquely identify each tuple in a relation. A key is defined as the value of one or more attributes, which can be modified, whereas an OID is independent from the value of an object's state. In particular, two different objects have different OIDs even when their states are identical. Moreover, a key is unique with respect to a relation, whereas an OID is unique within the entire database. The use of OIDs, as an identification mechanism, has a number of advantages with respect to the use of keys. First of all, because the OIDs are implemented by the system, the application programmer does not have to select the appropriate keys for the various sets of objects. Moreover, because the OIDs are implemented at a low level by the system, better performance is achieved. A disadvantage in the use of OIDs with respect to keys could be the fact that no semantic meaning is associated with them. Note, however, that very often in relational systems, for efficiency reasons, users adopt semantically meaningless codes as keys, especially when foreign keys need to be used. The ODMG Object Model supports the notion of a key, but only as a semantic integrity constraint, which is not used to identify objects. An important difference between the object and relational model is represented by the fact that the object model is identity-based, whereas the relational model is value-based. Thus, keys in the relational model, besides establishing integrity constraints, are also used to refer tuples through the foreign key mechanism. Although the first use of keys is meaningful in the object context, the second is not, since objects are always referred through their identity. OIDs are used in OODBMSs to identify objects and to support object references through object property values. Objects can thus be interconnected and can share components. The notion of object identity introduces at least two different notions of object equality:
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OODBMS History and Concepts
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Equality by identity. Two objects are identical if they are the same object; that is, if they have the same identifier. Equality by value. Two objects are equal if their states are recursively equal. Obviously, two identical objects are also equal, whereas the converse does not hold. Some object-oriented data models also provide a third kind of equality, known as shallow value equality, where two objects are equal, though not identical, if they share all attributes.
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Object State
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In an object-oriented database the value associated with an object (its state) consists of its values for a number of attributes and of its relationships with other objects. Attribute values can be complex; that is, they can be values built from other objects and values, using some type constructors. Complex (or structured) values are obtained by applying those constructors to simpler objects and literals. The minimal set of constructors that a system should provide includes sets, lists, and tuples (records). Constructors can be arbitrarily nested. A complex value can contain, as components, references to objects. Many OODBMSs support storage and retrieval of nonstructured values of a large size, such as character strings or bit strings. Those values are passed "as is" to the application program for interpretation, and are commonly known as BLOBs (binary large objects). Those values are not structured in that the DBMS does not know their semantic structure. The application using them knows how to interpret them. For example, the application may contain some functions to display an image or to search for some keywords in a text. Relationships hold between objects. Object models support only binary relationships; that is, relationships between two object types, and literals cannot participate in relationships. One-to-many and many-to-many relationships can be represented using set or list collection constructors.
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