| 摘要 |
<p>An interface provides a level of abstraction between the structure of a database system and application programs which access that system. The database system is represented by a model comprised of objects which correspond to the components of the database system. An object at a higher level encapsulates information contained in these other objects regarding the structure of the database. Whenever an application program requires access to the database, it sends a message to the higher level encapsulation object. The lower-level objects implement methods which automatically generate appropriate database commands. When the encapsulation object receives a call from an application program requesting data in the database, it instructs table objects to obtain the required data. In response, the table objects invoke field objects to identify how to represent data in each of the database fields to which they correspond. The table object concatenates the responses received from each of the field objects to construct a command that is presented to the database to retrieve the desired data.</p> |
