The Object-Oriented (OO) Model

Increasingly complex real-world problems demonstrated a need for a data model that more closely represented the real world. In the object-oriented data model (OODM), both data and their relationships are contained in a single structure known as an object. In turn, the OODM is the basis for the object-oriented database management system (OODBMS)

An OODM reflects a very different way to define and use entities. Like the relational model’s entity, an object is described by its factual content. But quite unlike an entity, an object includes information about relationships between the facts within the object, as well as information about its relationships with other objects. Therefore, the facts within the object are given greater meaning. The OODM is said to be a semantic data model because semantic indicates meaning.

Subsequent OODM development has allowed an object to also contain all operations that can be performed on it, such as changing its data values, finding a specific data value, and printing data values. Because objects include data, various types of relationships, and operational procedures, the object becomes self-contained, thus making the object—at least potentially—a basic building block for autonomous structures

The OO data model is based on the following components:

  • An object is an abstraction of a real-world entity. In general terms, an object may be considered equivalent to an ER model’s entity. More precisely, an object represents only one occurrence of an entity. (The object’s semantic content is defined through several of the items in this list.)
  • Attributes describe the properties of an object. For example, a PERSON object includes the attributes Name, Social Security Number, and Date of Birth
  • Objects that share similar characteristics are grouped in classes. A class is a collection of similar objects with shared structure (attributes) and behavior (methods). In a general sense, a class resembles the ER model’s entity set. However, a class is different from an entity set in that it contains a set of procedures known as methods. A class’s method represents a real-world action such as finding a selected PERSON’s name, changing a PERSON’s name, or printing a PERSON’s address. In other words, methods are the equivalent of procedures in traditional programming languages. In OO terms, methods define an object’s behavior
  • Classes are organized in a class hierarchy. The class hierarchy resembles an upside-down tree in which each class has only one parent. For example, the CUSTOMER class and the EMPLOYEE class share a parent PERSON class.
  • Inheritance is the ability of an object within the class hierarchy to inherit the attributes and methods of the classes above it. For example, two classes, CUSTOMER and EMPLOYEE, can be created as subclasses from the class PERSON. In this case, CUSTOMER and EMPLOYEE will inherit all attributes and methods from PERSON.
  • Object-oriented data models are typically depicted using Unified Modeling Language (UML) class diagrams. Unified Modeling Language (UML) is a language based on OO concepts that describes a set of diagrams and symbols that can be used to graphically model a system. UML class diagrams are used to represent data and their relationships within the larger UML object-oriented system’s modeling language. For a more complete description of UML see Appendix H, Unified Modeling Language (UML).

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