Understanding the concepts of Object Oriented Programming

What is Object Orientation?
In the past, information systems used to be defined primarily by their functionality: Data and functions were kept separate and linked together by means of input and output relations.

The object-oriented approach, however, focuses on objects that represent abstract or concrete things of the real world. These objects are first defined by their character and their properties, which are represented by their internal structure and their attributes (data). The behavior of these objects is described by methods (functionality).

Comparison between Procedural and Object Oriented Programming

Features	Procedure Oriented approach	Object Oriented approach
Emphasis	Emphasis on tasks	Emphasis on things that does those tasks.
Modularization	Programs are divided into smaller programs known as functions	Programs are organized into classes and objects and the functionalities are embedded into methods of a class.
Data security	Most of the functions share global data	Data can be hidden and cannot be accessed by external sources.
Extensibility	Relatively more time consuming to modify for extending existing functionality.	New data and functions can be easily added whenever necessary

Object Oriented Approach - key features

1. Better Programming Structure.

2. Real world entity can be modeled very well.

3. Stress on data security and access.

4. Reduction in code redundancy.

5. Data encapsulation and abstraction.

What are Objects and Classes?

Objects: An object is a section of source code that contains data and provides services. The data forms the attributes of the object. The services are known as methods (also known as operations or functions). They form a capsule which combines the character to the respective behavior. Objects should enable programmers to map a real problem and its proposed software solution on a one-to-one basis.

Classes: Classes describe objects. From a technical point of view, objects are runtime instances of a class. In theory, you can create any number of objects based on a single class. Each instance (object) of a class has a unique identity and its own set of values for its attributes.

Local and Global Classes

As mentioned earlier a class is an abstract description of an object. Classes in ABAP Objects can be declared either globally or locally.

Global Class: Global classes and interfaces are defined in the Class Builder (Transaction SE24) in the ABAP Workbench. They are stored centrally in class pools in the class library in the R/3 Repository. All of the ABAP programs in an R/3 System can access the global classes

Local Class: Local classes are define in an ABAP program (Transaction SE38) and can only be used in the program in which they are defined.

	Global Class	Local Class
Accessed By	Any program	Only the program where it is defined.
Stored In	In the Class Repository	Only in the program where it is defined.
Created By	Created using transaction SE24	Created using SE38
Namespace	Must begin with Y or Z	Can begin with any character

Local Classes

Every class will have two sections.

(1) Definition. (2) Implementation

Definition: This section is used to declare the components of the classes such as attributes, methods, events .They are enclosed in the ABAP statements CLASS ... ENDCLASS.

CLASS DEFINITION.
...
ENDCLASS.

Implementation: This section of a class contains the implementation of all methods of the class. The implementation part of a local class is a processing block.

CLASS IMPLEMENTATION.
...
ENDCLASS.

Structure of a Class

The following statements define the structure of a class:

1. A class contains components
2. Each component is assigned to a visibility section
3. Classes implement methods

1. Components of a Class are as follow:

· Attributes:- Any data,constants,types declared within a class form the attribute of the class.

· Methods:- Block of code, providing some functionality offered by the class. Can be compared to function modules. They can access all of the attributes of a class.

Methods are defined in the definition part of a class and implement it in the implementation part using the following processing block:

METHOD .

...

ENDMETHOD.

Methods are called using the CALL METHOD statement.

· Events:- A mechanism set within a class which can help a class to trigger methods of other class.

· Interfaces:- Interfaces are independent structures that you can implement in a class to extend the scope of that class.

Instance and Static Components:

§ Instance components exist separately in each instance (object) of the class and are referred using instance component selector using ‘à’.

§ Static components only exist once per class and are valid for all instances of the class. They are declared with the CLASS- keywords

§ Static components can be used without even creating an instance of the class and are referred to using static component selector ‘ =>’ .

2. Visibility of Components

Each class component has a visibility. In ABAP Objects the whole class definition is separated into three visibility sections: PUBLIC, PROTECTED, and PRIVATE.

· Data declared in public section can be accessed by the class itself, by its subclasses as well as by other users outside the class.

· Data declared in the protected section can be accessed by the class itself, and also by its subclasses but not by external users outside the class.

· Data declared in the private section can be accessed by the class only, but not by its subclasses and by external users outside the class.

CLASS DEFINITION.
PUBLIC SECTION.
...
PROTECTED SECTION.
...
PRIVATE SECTION.
...
ENDCLASS.

We shall see an example on Visibility of Components once we become familiar with attributes of ABAP Objects.

The yellow block of code is CLASS Definition

The Green block of code is CLASS Implementation

The Grey block of code is for object creation. This object creation includes two steps:

Step1 is Create a reference variable with reference to the class.

Syntax: DATA :

Step 2 : Create an object from the reference variable:-

Syntax: CREATE OBJECT

Output for the above code is

Attributes of Object Oriented Programming:

• Inheritance.

• Abstraction.

• Encapsulation.

• Polymorphism

Inheritance is the concept of adopting the features from the parent and reusing them . It involves passing the behavior of a class to another class. You can use an existing class to derive a new class. Derived classes inherit the data and methods of the super class. However, they can overwrite existing methods, and also add new ones.

Inheritance is of two types: Single Inheritance and Multiple Inheritance

Single Inheriting: Acquiring the properties from a single parent. (Children can be more).

Example for Single Inheritance

Multiple inheritance: Acquiring the properties from more than one parent.

Example

Syntax : CLASS DEFINITION INHERITING FROM .

Let us see a very simple example for creating subclass(child) from a superclass(parent)

Multiple Inheritance is not supported by ABAP.

Output is as follows :

Abstraction: Everything is visualized in terms of classes and objects.

Encapsulation The wrapping up of data and methods into a single unit (called class) is known as Encapsulation. The data is not accessible to the outside world only those methods, which are wrapped in the class, can access it.

Polymorphism: Methods of same name behave differently in different classes. Identical (identically-named) methods behave differently in different classes. Object-oriented programming contains constructions called interfaces. They enable you to address methods with the same name in different objects. Although the form of address is always the same, the implementation of the method is specific to a particular class.