Understanding ABAP Object
page 11 of 12
by Arindam Ghosh
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Features

Encapsulation

The three visibility areas (public section, protected section, private section) are the basis for one of the important features of object orientation, Encapsulation. You should take great care in designing the public components and try to declare as few public components as possible when you define a class. Once you have released the class, the public components of global classes may not be changed.

Public attributes are visible externally and form a part of the interface between an object and its users. If you want to encapsulate the state of an object fully, then you cannot declare any public attributes. Apart from defining the visibility of an attribute, you can also protect it from changes using the READ-ONLY addition.

Inheritance

Inheritance allows you to derive a new class from an existing class. It is done by using the INHERITING FROM addition in the statement:

CLASS <subclass> DEFINITION INHERITING FROM <superclass>

All of the components of the existing class <superclass> are inherited by the new class <subclass>. The new class is called the subclass of the class from which it is derived. The original class is called the superclass of the new class. It contains the same components as the superclass if you do not add any new declarations to the subclass. However, in the subclass only the public and protected components of the superclass are visible. The private components of the superclass are not visible though they exist in the subclass. You can declare private components in a subclass that have the same names as private components of the superclass. It is seen that each class works with its own private components. The another point that we note is that methods which a subclass inherits from a superclass use the private attributes of the superclass and not any private components of the subclass with the same names.

The subclass is an exact replica of the superclass if the superclass does not have any private visibility section. However, you can add a new component to the subclass because it allows you to turn the subclass into a specialized version of the superclass. If a subclass is itself the superclass of further classes, then you can introduce a new level of specialization.

Polymorphism

Reference variables are defined with reference to a superclass or an interface defined with reference to it can also contain references to any of its subclasses. A reference variable defined with reference to a superclass or an interface implemented by a superclass can contain references to instances of any of its subclasses, since subclasses contain all of the components of all of their superclasses and also convey that the interfaces of methods cannot be changed. In particular, you can define the target variable with reference to the generic class OBJECT.

Using the CREATE OBJECT statement, when you create an object and a reference variable typed with reference to a subclass then you can use the TY PE addition to which the reference in the reference variable will point.

A reference variable can be used by a static user to address the components visible to it in the superclass to which the reference variable refers. However, any specialization implemented in the subclass cannot be addressed by it.

Depending on the position in the inheritance tree at which the referenced object occurs, you can use a single reference variable to call different implementations of the method. This is possible only if you redefine an instance method in one or more subclasses. This concept is called polymorphism, in which different classes can have the same interface and, therefore, be addressed using reference variables with a single type.


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User Comments

Title: understanding ABAP Objects..   
Name: Anony
Date: 2008-10-06 8:28:45 AM
Comment:
Its worth reading and after this we need some lessons with examples to work it out and see abap objects in action...
Keep up the good work.

Thanks.
Title: Understanding ABAP Object   
Name: Reena
Date: 2008-07-15 12:41:28 PM
Comment:
Hi Arindam Ghosh,

I have read the entire article. Its indeed very useful and self explanatory.
Thank you providing the information so elaborately.
If given an example, it would have been even more appreciable...

Thanks and REgards

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