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Inheritance Structures

Oracle Tips by Burleson Consulting

High Performance Data Warehousing

Database Architectures For The 1990s

Inheritance

Inheritance is defined as the ability of a lower-level object to inherit or access the data structures and behaviors associated with all classes above it in the class hierarchy. Multiple inheritance refers to the ability of an object to inherit data structures and behaviors from more than one superclass.

To illustrate, let's look at an application of this system for a vehicle dealership. Occurrences of items to a dealership are vehicles; beneath the vehicle class, we may find subclasses for cars and boats. The car class may be further partitioned into truck, van, and sedan classes.
The vehicle class would contain the data items unique to vehicles, including the vehicle ID and the year of manufacture. The car class, because it IS-A vehicle, would inherit the data items of the vehicle class. The car class might contain data items, such as the number of axles and the gross weight of the vehicle. Because the van class IS-A car, which in turn IS-A vehicle, objects of the van class will inherit all data structures and behaviors relating to the car and vehicle classes.

Important facts about Inheritance

It is critical to the understanding of inheritance to note that inheritance happens at different times during the life of an object.

* Inheritance of data structures--At object creation time, inheritance is the mechanism whereby the initial data structure for the object is created. It is critical to note that only data structures are inherited--never data. It is a common misconception that data is inherited, such that an order may inherit the data items for the customer that placed the order. We must understand that inheritance is only used to create the initial, empty data structures for the object. In our example, all vehicles would inherit data definitions in the vehicle class, while an object of a lower-level class (say, sailboat) would inherit data structures that only apply to sailboats--as in sail_size.

* Inheritance of methods--Inheritance also happens at runtime when a call to a method (stored procedure) is made. For example, assume that the following call is made to sailboat object:

SAILBOAT.compute_rental_charges();

The database will first search for the compute_rental_charges in the sailboat class; if it is not found, the database will search up the class hierarchy until compute_rental_charges is located.


This is an excerpt from "High Performance Data Warehousing", copyright 1997.

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