The concept of a class hierarchy in object oriented database is similar to

All computer systems must possess the properties of their architecture to be considered that kind of a computer system.  For example, a system must have tables to be considered a relational architecture.

An object-oriented database is no exception.  An object-oriented database must contain some basic object architecture properties.  However, in the real world many of these properties are under debate, and some, such as multiple inheritance are viewed as enhancements to the object model, rather than a part of the core foundation.  For example in the object-oriented language Smalltalk, multiple inheritance is not supported, even though multiple inheritance is consider a part of the object architecture.   The following section explores the object-oriented terminology and explains the basic precepts of the object model.    

Classes

A class characterizes one or more objects that have common methods, variables and relationships (figure 1.3).  Here we see that that ?Publisher? is the class and ?Jones Publishing? and ?Brown Publishing? are objects within this class.

Classes are hierarchical in nature. This means that a class can be broken down into other classes, and  when this is done they follow a hierarchical chart.    In a class hierarchy, a ?derived? class is called a subclass while the original class is called a superclass. A subclass can also be decomposed where a subclass becomes a superclass for all classes below it.  On the other hand, a superclass is a combination of all of its? subclasses.  If you combine the sub-classes you would form a superclass as shown in Figure 1-4.  In figure 1-4 Vehicle is the superclass and automobile, boat, and airplanes would be the subclasses.

Classes are the templates for objects.  They serve as the ?blueprint? to 'stamp-out? objects when they are created.  A class contains attributes (sometime called variables), and these attributes become a part of the object when it is created.   When defined in a class definition, attributes describe the class; when and object is created, these descriptions become variables.  In addition to data definitions, classes also contain the behaviors (sometimes called ?methods?)  that can be applied to the variables inside an object.

In figure 1-4 we see that VEHICLE has the  variables serial number, weight, cost and number of passengers.  The class AUTOMOBILE  has the variables engine size and number of tires, The BOAT class has the variables width and length, and AIRPLANE has the variables wingspan and number of engines.

The methods for a class define the set of operations that can be performed upon an object.  For example, when a method is applied to an object it will either return a value  or the method will perform some operation to update the values in an object.  In figure 1-4 we see that a method for VEHICLE might be used to compute the cost of the vehicle, returning the dollar value of the vehicle.  Sometimes methods do not return values.  If a method was designed to update the number of passengers for a vehicle, no value would be returned but a data item inside the target object would change value.

Objects

Objects are the fundamental concept in an object-oriented database.  In essence, objects are an abstract representation of real world things that are stored in an object-oriented database.   An object is an instance of a class, in the sense that it is stamped-out from the class definition.  The object contains all the values of the variables from the class or classes. The methods are stored with the class definition, and the objects know how to access the class definition so that they can be used to perform their functions on the data in the object.  Data in an object can only be accessed by using one of the objects methods.  When we say that an object is an instance of a class, we mean that all objects of the same class contain the same data items, but each data item may contains different values.

You can think of an object as a self contained package that has three parts:

1.      It's own private information (data values).

2.      It's own private procedures that will manipulate the objects data values. (via the class definition)

3.      A public interface so that this object can communicate with other objects.

An important point about objects is to remember that objects don't do something, they are something.  Objects are given a system generated unique id called the Object IDentifier (OID) when they are created.  OID's are explained in chapter 2, but basically the OID that is assigned to an object stays with the object for the life of the object and will never be re-used, even after the object has been deleted.  The OID is always unique and no matter what object methods are applied to the values inside the object, the OID will never change.  One thing that an OID lets us do is distinguish between two or more objects.  Using the OID we can determine if two objects are equal, that is, if two objects contain the same values.  Object-oriented databases allow you two have two or more objects with the same values unlike other databases like a relational database that does not allow duplicate rows.   

Which type of database is similar to hierarchical and allow for many to many relationships?

What is class in object

How the use of a database class is related to object

What are the concepts of object