Understanding ABAP Object
 
Published: 20 Feb 2007
Abstract
The programming language like ABAP (Advanced Business Application Programming) is specially created for developing SAP applications. ABAP Objects is a new concept in R/3 System and you can find its two distinct meanings, one stands for the entire ABAP runtime environment and the other represents the new object-oriented generation of this language.
by Arindam Ghosh
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Introduction

The object orientation can be defined as a problem-solving method in which the software solution reflects objects in the real world. Therefore, a comprehensive introduction to object orientation would go far beyond the limits of this introduction to ABAP Objects as a whole.

Object

An object can be defined as a section of source code that contains data and provides services. The attributes of the object are formed by the data. The services are known as methods (also known as operations or functions). Typically, it is seen that the methods operate on private data (the attributes, or state of the object), which is only visible to the methods of the object. Thus, the attributes of an object cannot be changed directly by the user, but by the methods of the object only. This guarantees the internal consistency of the object.

Classes

The classes are very important, as they describe the objects. From a technical point of view, the objects are runtime instances of a class. One can create any number of objects based on a single class in theory. Each instance (object) of a class has a unique identity and its own set of values for its attributes.

Features of Object Orientation

Encapsulation

According to this property, objects restrict the visibility of their resources (attributes and methods) to other users. Every object has an interface which plays a very important role. It determines how other objects can interact with it. It is found that the implementation of the object is encapsulated, which means that it is invisible outside the object itself.

Polymorphism

According to this property, identical (identically-named) methods behave differently in different classes. Object-oriented programming contains constructions, which are called interfaces. They enable you to address methods with the same name in different objects. The implementation of the method is specific to a particular class, though the form of address is always the same.

Inheritance

According to this property, one can use an existing class to derive a new class. The data and methods of the super class are inherited by the derived classes. However, they can overwrite existing methods and add new ones.

Uses of Object Orientation

The object-oriented programming is quite useful in many ways. It has various advantages, some of which are mentioned below.

Since object-oriented structuring provides a closer representation of reality than other programming techniques, complex software systems become easier to understand.

In object-oriented system, it should be possible to implement changes at class level, without having to make alterations at other points in the system. This reduces the overall amount of maintenance required.

The object-oriented programming allows reusing individual components through polymorphism and inheritance. 

In this system, the amount of work involved in revising and maintaining the system is reduced as many problems can be detected and corrected in the design phase.

To achieve these goals, the followings are required:

Object-oriented programming languages: The efficiency of object-oriented programming depends directly on how object-oriented language techniques are implemented in the system kernel.

Object-oriented tools: Object-oriented tools allow you to create object-oriented programs in object Oriented languages. They also allow you to model and store development objects and the Relationships between them.

Object-oriented modeling: The object-orientation modeling of a software system is the most important, most time-consuming, and most difficult requirement for attaining the above goals. These designs involve more than just object-oriented programming and logical advantages that are independent of the actual implementation are provided by it.

ABAP Objects

ABAP Objects is a new concept in R/3 System and you can find its two distinct meanings --- one is for the entire ABAP runtime environment and the other represents the new object-oriented generation of this language.

The Runtime Environment

ABAP Objects, for the entire ABAP runtime environment, are an indication of how SAP has, for sometime, been moving towards object orientation. Object-oriented techniques have been used exclusively in system design. The ABAP language did not support these techniques earlier.

The ABAP Workbench will allow creating R/3 Repository objects in this regard. These objects are programs, authorization objects, lock objects, customizing objects, and so on and so forth. By using function modules, one can also encapsulate functions in separate programs with a defined lnterface. The Business Object Repository (BOR) allows you to create SAP Business Objects for internal and external use.

The Object-Oriented Language Extension

ABAP Objects support object-oriented programming. The ABAP Objects is a complete set of object-oriented statements, which has been introduced into the ABAP language. This object-oriented extension of ABAP builds on the existing language and is fully compatible with it.

The Object Orientation (OO), also known as the object-oriented paradigm, is a programming model that unites data and functions in objects. You can not only use ABAP Objects in existing programs, but also work with and use a conventional ABAP in new ABAP Objects programs. The rest of the ABAP language is primarily intended for structured programming, where data is stored in a structured form in database tables and function-oriented programs access and work with it.

Moreover, we should know that the object-oriented enhancement of ABAP is based on the models of Java and C++. It is compatible with external object interfaces such as DCOM and CORBA. The implementation of object-oriented elements in the kernel of the ABAP language has considerably increased response times when you work with ABAP Objects. Some other objects, such as SAP Business Objects and GUI objects, which are already object-oriented by themselves, are also benefiting from being incorporated into ABAP Objects.

About Classes

The classes are templates for objects. An abstract description of an object is the class. You could say it is a set of instructions for building an object. The attributes of objects are defined by the components of the class, which describe the state and behavior of objects.

Local and Global Classes

In ABAP Objects, classes can be declared either globally or locally. You define global classes and interfaces in the Class Builder (Transaction SE24.} in the ABAP Workbench. In the R/3 Repository, they are stored centrally in class pools in the class library. In an R/3 System, all of the ABAP programs can access the global classes. The local classes are defined within an ABAP program. Local classes and interfaces can only be used in the program in which they are defined. When you use a class in an ABAP program, the system first searches for a local class with the specified name. If it does not find one, then it looks for a global class. Apart from the visibility question, there is no difference between using a global class and using a local class.

However, there is a significant difference in the way that local and global classes are designed. If you are defining a local class that is only used in a single program, then to define the outwardly visible components so that it fits into that program is usually sufficient. On the other hand, global classes must be able to be used anywhere. Since the system must be able to guarantee any program using an object of a global class, it can recognize the data type of each interface parameter and then certain restrictions are applied at the time of defining the interface of a global class.

Defining Local Classes

Local classes consist of ABAP source code, where the ABAP statements CLASS...ENDCLASS are enclosed. A complete class definition consists of the following parts, a declaration part and, if required, an implementation part. It is found that the declaration part of a class <class> is a statement block:

It contains the declaration for all components (attributes, methods, events) of the class. When you define local classes, the declaration part belongs to the global program data. Therefore, it should be placed at the beginning of the program. If you declare methods in the declaration part of a class, then you must write an implementation part for it. This consists of a statement block:

CLASS <class> IMPLEMENTATION
¦
ENDCLASS

The implementation part of a class actually contains the implementation of all methods of the class because it is observed that the implementation part of a local class is a processing block. Therefore, subsequent coding itself is not a part of a processing block and is not accessible.

Class Structure

A class contains components where each component is assigned to a visibility section. Moreover, the classes implement methods. Let us define these components one by one.

Components

This is the first component and is declared in the declaration part of the class. The components define the attributes of the objects in a class. When you define a class, each component is assigned to one of the three visibility sections. This defines the external interface of the class and all of the components of a class are visible within the class. All components are in the same namespace. This means that all components of the class must have names that are unique within the class. There are two kinds of components in a class:

One that exists separately for each object in the class.

The other consists of those that exist only once for the whole class, regardless of the number of instances.

Attributes

It is the instance-specific components, which are known as instance components. The static components are those components that are not instance-specific. The classes can define the following components in ABAP Objects.

The attributes are internal data fields that can have any ABAP data type within a class. The state of an object is determined by the contents of its attributes. One kind of attribute is the reference variable. The reference variables are also very important as they allow you to create and address objects. Reference variables can be defined in classes, which allow you to access objects from within a class.

Instance Attributes

The instance-specific state of an object is defined by the contents of instance attributes. They can be declared by using the DATA statement.

Static Attributes

The contents of static attributes define the state of the class that is valid for all instances of the class. The static attributes exist once for each class. It is declared using the CLASSDATA statement. They are accessible for the entire runtime of the class. All of the objects in a class can access its static attributes. If you change a static attribute in an object, then the change is visible in all other objects of the class.

Methods

The methods are internal procedures that define the behavior of an object in a class. They can access all the attributes of a class. This allows them to change the data content of an object. They also have a "parameter interface," whose role is to help the users by supplying them with values when calling them and receiving values back from them. The private attributes of a class can only be changed by methods in the same class. The definition and parameter interface of a method is similar to that of function modules. Using the following processing block, you define a method <meth> in the definition part of a class and implement it in the implementation part:

METHOD <meth>
ENDMETHOD

In the same way as in other ABAP procedures (subroutines and function modules), you can declare local data types and objects in methods. You call the following methods by using the CALL METHOD statement.

Instance Method

You can declare instance methods by using the METHODS statement. They play a very important role as they can access all of the attributes of a class and can trigger all of the events of the class.

Static Methods

You can declare static methods by using the CLASS-METHODS statement. They are important and can only access static attributes and trigger static events.

Special Methods

Apart from normal methods, there are two special methods which you call using CALL METHOD. These are called CONSTRUCTOR and CLASS _CONSTRUCTOR. They are automatically called when you create an object (Constructor) or when you first access the components of a class (CLASS _CONSTRUCTOR).

Events

Objects or classes can use events to trigger event handler methods in other objects or classes. One method can be called by any number of users in a normal method call. When an event is triggered, any number of event handler methods can be called. Until runtime, the link between the trigger and the handler is not established. The calling program determines the methods that it wants to call in a normal Method call and it must exist. With events, the handler determines the events to which it wants to react. It is not necessary that for every event there has to be a handler method registered.

With the help of RAISE EVENT statement, the events of a class can be triggered in the methods of the same class. You can declare a method of the same or a different class as an event handler method for the event <evt> of class <class> by using the addition FOR EVENT <evt> OF <class>. Events have a similar parameter interface to methods, but only have output parameters. These parameters are passed by the trigger (RAISE EVENT statement) to the event handler method, which receives them as input parameters.

Using the SET HANDLER statement, the link between trigger and handler is established dynamically in a program. The trigger and handlers can be objects or classes, depending on whether you have instance or static events and event handler methods. When an event is triggered, the corresponding event handler methods are executed in all registered handling classes.

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.

Conclusion

I have explained object orientation which is such an important aspect in SAP. The discussion began with the features of Object Orientation and includes the runtime environment, the language extension, the classes and the class components. Apart from that I have also discussed for new first time readers event handling, inheritance encapsulation and polymorphism.

 

 



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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