The Object-Oriented Model

1. The object-oriented model is based on a collection of objects, like the E-R model.
   • An object contains values stored in instance variables within the object.
   • Unlike the record-oriented models, these values are themselves objects.

   • Thus objects contain objects to an arbitrarily deep level of nesting.
   • An object also contains bodies of code that operate on the the object.

   • These bodies of code are called methods.
   • Objects that contain the same types of values and the same methods are grouped into classes.

   • A class may be viewed as a type definition for objects.
   • Analogy: the programming language concept of an abstract data type.

   • The only way in which one object can access the data of another object is by invoking the method of that other object.
   • This is called sending a message to the object.

   • Internal parts of the object, the instance variables and method code, are not visible externally.
   • Result is two levels of data abstraction.

   For example, consider an object representing a bank account.

   • The object contains instance variables number and balance.
   • The object contains a method pay-interest which adds interest to the balance.

   • Under most data models, changing the interest rate entails changing code in application programs.
   • In the object-oriented model, this only entails a change within the pay-interest method.
2. Unlike entities in the E-R model, each object has its own unique identity, independent of the values it contains:
   • Two objects containing the same values are distinct.
   • Distinction is created and maintained in physical level by assigning distinct object identifiers.