Reviews : Java Books :
Beginning Java Objects : Chapters 2 and 3

Buy this book Title: Beginning Java Objects ISBN: 1861004176 US Price: $ 39.99 Canadian Price: C$ 59.95 UK Price: £ 28.99 Publication Date: November 2000 Pages: 800 © Wrox Press Limited, US and UK.

Objects vs. Database Records

As we mentioned in Chapter 2, when people are learning a new concept, they automatically try to map that concept to a mental model with which they are already familiar. Folks who are knowledgeable about relational databases (Sybase, Oracle, SQL Server, Access, DB2, and the like), therefore, have a tendency to relate the concept of an object to that of a record in a database. (If you are not familiar with relational databases, please skip this section.)

The object-record analogy holds up to a point:

• Each class of object is analogous to a table in a relational database
• Each record, or row, in the table is analogous to a different object instance
• Each column in a table corresponds to a different attribute of the associated class

This correlation between objects and records has several flaws, however:

• Objects exhibit behavior, whereas records typically do not.

  One can argue that the behavior of an object can be modeled by relational database triggers or stored procedures, but this is really the database management system operating on the data versus an individual record taking action on its own behalf.

• Objects have unique identities, which is the characteristic that distinguishes one object from all others, even if all of their attribute values are the same. For example, if we happened to have two professors with the same name, and the same address, and the same birthdate, and so on, then the two objects representing these two professors could still exist as separate instances of the Professor class. In a relational database, on the other hand, identical records cannot, strictly speaking, coexist in the same table, so we'd be forced to add an attribute/column known as a primary key to the table — perhaps inventing a unique professor ID number — so that we could differentiate the two professors. In an object-oriented system, objects need not have a unique key - objects inherently have identity.

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