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Encapsulation of Operations, Methods, and Persistence

Encapsulation is related to the concepts of abstract data types and information hiding in programming languages. Here the main idea is to define the behavior of a type of object based on the operations that can be externally applied to objects of that type. The internal structure of the object is hidden, and the object is only accessible through a number of predefined operations. Some operations may be used to create or destroy objects; other operations may update the object value and other may be used to retrieve parts of the object value or to apply some calculations to the object value.

The external users of the object are only made aware of the interface of the object, which defines the names and arguments of each operation. The implementation of the object is hidden from the external users; it includes the definition of the internal data structure of the object and the implementation of the operations that access these structures.

In object oriented - OO terminology, the interface part of each operation is called the signature, and the operation implementation is called a method. A method is invoked by sending a message to the object to execute the corresponding method.

Not all objects are meant to be stored permanently in the database. Transient objects exist in the executing program and disappear once the program terminates.

Persistent objects are stored in the database and persist after program terminates. The typical mechanism for persistence involves giving an object a unique persistent name through which it can be retrieved.

Inheritance

Inheritance is deriving objects from existing objects. The derived objects inherit properties from their parent object. Parent objects are those objects from which other objects are derived. Inheritance is a way of reusing the existing code.

Polymorphism

Polymorphism concept allows the same operator name or symbol to be bound to two or more different implementation of the operator, depending on the type of objects to which the operator is applied.

Major features of Object Oriented databases

Object Oriented databases store persistent objects permanently on secondary storage, and allow the sharing of these objects among multiple programs and applications.

Object Oriented databases provide a unique system-generated object identifier for each object. Object Oriented databases maintain a direct correspondence between real-world and database objects so that objects don’t lose their integrity and identify and can be easily be identified and operated upon.

In Object Oriented databases, objects can be very complex in order to contain all significant information that may be required to describe the object completely.

Object Oriented databases allow us to store both the class and state of an object between programs. They take the responsibility for maintaining the links between stored object behavior and state away from the programmer, and manage objects outside of programs with their public and private elements intact. They also simplify the whole process of rendering objects persistent by performing such tasks invisibly.

Persistence has to do with time i.e. a persistent object can exist beyond the program that created it. It also has to do with space (the location of the object may vary between processors, and even change its representation in the process).

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