ors will be included.The identity of an object is what makes it different from all the other objects. This allows theobjects to be independent of their values. Therefore the notion of identical objects isintroduced: two objects are equal if they have the same values, but are identical if they havethe same object identity. The fact that each object possesses an identity facilitates thehandling of composite objects since it makes the common use of objects possible and itprotects the consistency of the database. If a component object is changed, this changeaffects all the composite objects that reference it. Due to the object identity, there is noneed for replicates, and that is how the consistency of the database is protected.The mechanism of encapsulation allows the hiding of the internal state of the objects. Theinternal state of an object is not liable to direct access. It can only be accessed by itsmethods. Objects that have this ability are called encapsulated objects. There are manytypes of encapsulation including: full, write, and partial. Using full encapsulation, all theoperations on objects are done via message sending and method execution. In writeencapsulation, the internal state of the objects is visible only for reading operations. Partialencapsulation involves allowing direct access for reading and writing for only a part of theinternal state (private and public part). The use of the same message for different methodsthat belong to different classes can facilitate the design of the database as well as of theapplications that access it.In general, since the internal structure of an object is not visible by the other objects, we canassign to methods with the same functionality the same message even if theirimplementation is different. This is called overloading of the message. Since a message cancorrespond to more than one method, the code of the method that has to be executed canonly be found at run time. That means that whi...
