Object-Oriented Databases (Cont.)

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3. Objects with a common structure and common behavior are grouped into classes. Each object is an instance of one class.


4. The state of an object can be accessed by passing messages.
   1. If an object receives a message that it understands, the execution of an associated method is initiated.
   2. Objects communicate by exchanging messages (message passing).
   
   

   For example, a School object asks the Student object for its name by sending it a message asking for its name.

   
   

   The receiving Student object returns the name back to the sending object.



5. Classes are arranged in a hierarchy which is implied by defining a class as a specialization of one or several other classes.
   1. The subordinate classes are called subclasses and the superordinate classes are referred to as superclasses.
   2. Subclasses inherit the structure and behavior of their superclasses.
   3. If a subclass provides a specific structure and behavior, this will override inheritance.
   4. Inheritance conflicts may arise if multiple inheritance takes place. They must be solved by the programmer or the system.