Definition of Classes and Interfaces

In ABAP Objects, you can define classes and interfaces either globally or locally. You define global classes and interfaces with the Class Builder tool in the ABAP Workbench; they are then stored centrally in the class library in the repository. All ABAP programs in an AS ABAP can access these global classes and interfaces. Conversely, you can only use local classes and interfaces in the program that defines them. When an ABAP program uses a class, the system searches first for a local class, then for a global class of the same name. Otherwise, there is no difference in the use of local and global classes or interfaces.

Since local classes are only used by one program, it is generally sufficient to define the public interface - that is, the outwardly visible components appropriate for this program. Conversely, each global class is available throughout the system, which means that its public interface can only be typed with reference to data types that are themselves visible throughout the system.

The following sections describe classes and interfaces using the syntax for the definition of local classes and interfaces in an ABAP program. For the definition of global classes and interfaces, all language elements have a counterpart in the operating elements of the Class Builder. For more information, see the documentation for the Class Builder.

Defining Local Classes

Local classes consist of ABAP source code enclosed between the CLASS - ENDCLASS statements. A complete class definition consists of a declaration part and, if necessary, an implementation part.

The declaration part of a class class consists of one statement block:

CLASS class DEFINITION.
...
ENDCLASS.

This declaration part contains the declaration of the components (attributes, methods, events) of a class. All the components of a class must be assigned explicitly to a visibility section (PUBLIC SECTION, PROTECTED SECTION, PRIVATE SECTION), which defines from where each component can be accessed. In global classes, the declaration part belongs to the global data declarations and should be at the start of the program.

If methods are declared in the declaration part of a class, the class needs an implementation part, which consists of one statement block:

CLASS class IMPLEMENTATION.
...
ENDCLASS.

The implementation part of a class contains the implementation of all the methods of this class. Methods are procedures - that is, processing blocks of an ABAP program. The position of the implementation part in the source code is thus unimportant. For clarity, however, you should either put all the implementation parts of local classes at the end (as in subroutines), or directly after the relevant definition part. If you do the latter, note that you must then assign subsequent non-declarative statements explicitly to a processing block, such as START-OF-SELECTION, so that they can be accessed.

Defining Local Interfaces

You define the local interface intf using the

INTERFACE intf. and
...
ENDINTERFACE. statements.

The definition contains all the components (attributes, methods, events) of the interface. In interfaces, you can define the same components as in classes. You cannot assign the components of a interface explicitly to a visibility section, because interface components always extend the public area of the class. There is no implementation part for interfaces, since the methods of an interface must be implemented in the same class that implements the interface.

Object Orientation

Object orientation (or, more correctly, object-oriented programming) is a problem-solving method that represents the real world in a series of software object.

Object-oriented programming is based on a progamming model in which data and functions are united in objects. The remaining language scope of ABAP mainly supports procedural programming, where the data is stored at other places than the objects and where programs that are modularized by procedures access this data.

This document defines a few general terms that are widely used in object orientation and in ABAP Objects.

Objects

Objects represent abstract or concrete objects of the real world. An object is a section of program code that has data (called attributes) and provides services called methods (sometimes also known as operations or functions). Methods typically work with private data in the object (attributes, also known as the object state), that are only visible within the object. This guarantees the internal consistency of the object, since the data is only changed by the methods, not by the user. This ensures that the object is consistent in itself.

Classes

Classes are program code that describes objects. Technically, an object is an instance of a class. In theory, you can create an infinite number of objects from a single class definition. Each instance of a class (object) has its own values for its attributes.

Object References

In a program, you identify and address objects using a unique object reference. They allow you to access the attributes and methods of an object.

In object-oriented programming, objects usually have the following characteristics:

Encapsulation

Objects restrict the external visibility of their resources (attributes and methods). Each object hat an interface that determines how other objects or applications can use it. The implementation of the object is encapsulated (not visible outside the class).

Polymorphism

Methods with the same name can behave differently in different classes. In object-oriented programming, you can use interfaces to address methods with the same name in different objects. The form of address always remains the same, but the actual method implementation is class-specific, and can be different in each class.

Inheritance

You can derive a class from another class. A derived class (subclass) inherits the data and methods of its superclass. You can add new methods to a subclass, or redefine existing methods. Redefined methods have the same name and interface as the original method. Their classes are therefore polymorphous, too.

Uses of Object Orientation

Object-orientation has the following advantages:

• Complex software systems become easier to understand, since an object-oriented architecture resembles reality more closely than other programming techniques.
  

• Changes in object-oriented systems should be possible locally (at class level), without further changes being necessary in other parts of the system. This reduces the amount of maintenance required.
  

• Polymorphism and inheritance enable many individual components to be reused.
  

• Object-oriented systems require less adjustment and maintenance, because the majority of problems can be discovered and corrected in the design and development phases.
  

Achieving these goals requires:

• Object-oriented Programming Languages
  
  Object-oriented programming techniques do not necessarily require object-oriented programming languages. However, they do depend on the implementation of object-oriented constructions in the system kernel.

• Object-oriented Tools
  
  Object-oriented tools help you create object-oriented programs in object-oriented languages. They allow you to store and visualize your program objects and the relationship between them.

• Object-oriented Modeling
  
  Object-oriented modeling of a software system is the most important, most time consuming, and most difficult task required to achieve the above goals. Object-oriented design encompasses more than just object-oriented programming, and offers logical advantages that are independent of the eventual implementation.
  
  
  

Classes

The type of an object is known as its class. A class is an abstract representation, or, metaphorically speaking, a set of instructions for building objects. So that you can describe the properties of objects, classes contain components, which define the state and behavior of objects.

Global and Local Classes

You can define classes globally in the class library in the repository or locally in an ABAP program.

Global classes are coded in a special ABAP program, a class pool, whereas local classes can be coded in almost every ABAP program. Global classes can be used and are visible in all ABAP programs, which is the case for local classes only in their own program. When a global class is used for the first time, the class pool is loaded into the internal mode of the user. The local classes of a class pool can be used by the pool's global classes.

Except for the storage type and the visibility, there are hardly any conceptual differences between global and local classes. One of the few differences is that in the public interface of a global class only references to public types are possible. These types are subdivided into global and local friends.

In principle, it also does not make any difference if you call a method of a local or global class. This is the reason why classes that are used by several programs are exclusively created in the class library. You should avoid, if possible, to reuse local classes using include programs.

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Interview Question on BAPI, RFC, ABAP Objects, Tables

What is the difference between RFC and BAPI ?
What are subclasses and super classes in BAPI and also what are the methods in BAPI ?
2) Is it possible to connect SAP to Non-SAP systems to retrieve data using RFC alone with out using BAPI ?
3) What is the difference between Function module and BAPI ?
4) What are the types of tables?
5) What are pooled table ?
6) What are Hashed Tables ?
7) What are advantages of using ABAP objects?
8) What is the advantage of using ABAP objects in Reports ?

1) BAPI are RFC enabled function modules. the difference between RFc and BAPI are business objects. You create business objects and those are then registered in your BOR (Business Object Repository) which can be accessed outside the SAP system by using some other applications (Non-SAP) such as VB or JAVA. in this case u only specify the business object and its method from external system in BAPI there is no direct system call. while RFC are direct system call Some BAPIs provide basic functions and can be used for most SAP business object types. These BAPIs should be implemented the same for all business object types. Standardized BAPIs are easier to use and prevent users having to deal with a number of different BAPIs. Whenever possible, a standardized BAPI must be used in preference to an individual BAPI.

The following standardized BAPIs are provided:
Reading instances of SAP business objects
GetList ( ) With the BAPI GetList you can select a range of object key values, for example, company codes and material numbers.

The BAPI GetList() is a class method.
GetDetail() With the BAPI GetDetail() the details of an instance of a business object type are retrieved and returned to the calling program. The instance is identified via its key. The BAPI GetDetail() is an instance method. BAPIs that can create, change or delete instances of a business object type

The following BAPIs of the same object type have to be programmed so that they can be called several times within one transaction. For example, if, after sales order 1 has been created, a second sales order 2 is created in the same transaction, the second BAPI call must not affect the consistency of the sales order 2. After completing the transaction with a COMMIT WORK, both the orders are saved consistently in the database.

Create( ) and CreateFromData! ( )
The BAPIs Create() and CreateFromData() create an instance of an SAP business object type, for example, a purchase order. These BAPIs are class methods.

Change( )
The BAPI Change() changes an existing instance of an SAP business object type, for example, a purchase order. The BAPI Change () is an instance method.

Delete( ) and Undelete( ) The BAPI Delete() deletes an instance of an SAP business object type from the database or sets a deletion flag.

The BAPI Undelete() removes a deletion flag. These BAPIs are instance methods.

Cancel ( ) Unlike the BAPI Delete(), the BAPI Cancel() cancels an instance of a business object type. The instance to be cancelled remains in the database and an additional instance is created and this is the one that is actually canceled. The Cancel() BAPI is an instance method.

Add ( ) and Remove ( ) The BAPI Add adds a subobject to an existing object inst! ance and the BAPI and Remove removes a subobject from an object instance. These BAPIs are instance methods.

2) No it is not possible to connect SAP to Non-SAP systems to retrieve data using RFC alone. RFC can acces the SAP from outside only through BAPI and same is for vice versa access.

3) Each Bapi Object has Interface, Key Fields, Attributes,Methods and Events.
Bapi Function Modules can be attached to these Bapi objects .Function module has a single bound functionality while a BAPI object can contain many functionalities

4) Transparent table, Pool table and cluster table are data dictionary table objects sorted table, indexed table and hash table are internal tables.

5)Table pools (pools) and table clusters (clusters) are special table types in the ABAP Dictionary. The data from several different tables can be stored together in a table pool or table cluster. Tables assigned to a table pool or table cluster are referred to as pooled tables or cluster tables.

A table in the database in which all records from the pooled tables assigned to the table pool are stored corresponds to a table pool.

The definition of a pool consists essentially of two key fields (Tabname and Varkey) and a long argument field (Vardata).
Table Clusters Several logical data records from different cluster tables can be stored together in one physical
record in a table cluster.

A cluster key consists of a series of freely definable key fields and a field (Pageno) for distinguishing continuation records. A cluster also contains a long field (Vardata) that contains the contents of the data fields of the cluster tables for this key. If the data does not fit into the long field, continuation records are created. Control information on the structure of the data string is still written at the beginning of the Vardata field.

6) Hashed tables
This is the most appropriate type for any table where the main operation is key access. You cannot access a hashed table using its index. The response time for key access remains constant, regardless of the number of table entries. Like database tables, hashed tables always have a unique key. Hashed tables are useful if you want to construct and use an internal table which resembles a database table or for processing large amounts of data.

Sample Prog: This does nothing.
REPORT Z_1 .
tables: mara.
data: i type hashed table of mara with unique key matnr

7) and 8) ABAP objects are root for your program and reports.

RFC Vs BAPI

BAPI are RFC enabled function modules. the difference between RFc and BAPI are business objects. You create business objects and those are then registered in your BOR (Business Object Repository) which can be accessed outside the SAP system by using some other applications (Non-SAP) such as VB or JAVA.

In this case you only specify the business object and its method from external system in BAPI there is no direct system call. while RFC are direct system call Some BAPIs provide basic functions and can be used for most SAP business object types. These BAPIs should be implemented the same for all business object types. Standardized BAPIs are easier to use and prevent users having to deal with a number of different BAPIs. Whenever possible, a standardized BAPI must be used in preference to an individual BAPI. It is not possible to connect SAP to Non-SAP systems to retrieve data using RFC alone. RFC can acces the SAP from outside only through BAPI and same is for vice versa access.

RFC is the protocol used by SAP for remote communication, that is, for communications between remote (independent) systems. RFC is used for communications between two independent SAP systems, or for communications between an SAP system and a non-SAP system, such as an external application. It can also be used for communications between modules on the same system. Using the RFC interfaces you can extend the functionality of R/3 applications from an external program.

What is the different btween clear and refresh?

There are 2 types of clear statements we can use:
Clear ITAB : This Statement will clear the Internal Table Header content.
To clear the Intertal Table Hearder as well Body we can use Clear ITAB [ ] statement.

Refresh will deletes the Internal Table content but still memory is not freed.

What is the pick statment do?

Pick Statemnt will captures the UserAction.
Eg: if SY-UCOMM = 'X'.
This type of user actions can be captured thru PICK Statement only.

What is the different between sesstion method and call transaction method and suppose I used one method to transform the data next time suppose I want transform the data which method I can use.

There are many difference between Session method and Call Transaction.
Which method we have to use will be found based on Volume Of data and Accuracy of data given. Mainly Call Transaction used for small sets of data because in Call Transaction we have to Handle the Processing errors explicitly. We can do this by BDCMSGCOLL structure.
In Session method seperate session will crate for errors. Calll Transaction will update the Database fastly compare with Session Method.

What is single and upto one row different?

The Major difference between Select Single and Select UPTO 1 rows is The Usage Of Buffer for each.
Select Single will search for all the satisfied data and bring all that data into Buffer and later it will give to that data to the program.
Select UPTO 1 Rows will end the search after getting the 1st satisfied record and gives that record to the program.
Thus Select Single will take much processing time when compare with Select UPTO 1 rows.

What is the differences between selection screen and selection screen out?

The difference between Selection Screen and Selection Screen Output is we use the Selection Screen Output Event before the Selection Screen will displayed. Eg: Suppose if we want to disable some fields and enable some fields in the Selection Screen then we can code that in Section-Screen Output event.
Selection-Screen Event will trigger after section-screen is displayed and user action takes place on that.