SAP ABAP - Polymorphism

The term polymorphism pterally means ‘many forms’. From an object-oriented perspective, polymorphism works in conjunction with inheritance to make it possible for various types within an inheritance tree to be used interchangeably. That is, polymorphism occurs when there is a hierarchy of classes and they are related by inheritance. ABAP polymorphism means that a call to a method will cause a different method to be executed depending on the type of object that invokes the method.

The following program contains an abstract class class_prgm , 2 sub classes (class_procedural and class_OO), and a test driver class class_type_approach . In this implementation, the class method start allow us to display the type of programming and its approach. If you look closely at the signature of method start , you will observe that it receives an importing parameter of type class_prgm. However, in the Start-Of-Selection event, this method has been called at run-time with objects of type class_procedural and class_OO.

Example

Report ZPolymorphism1. 
CLASS class_prgm Definition Abstract. 
PUBLIC Section. 
Methods: prgm_type Abstract, 
approach1 Abstract. 
ENDCLASS. 

CLASS class_procedural Definition 
Inheriting From class_prgm. 
PUBLIC Section. 
Methods: prgm_type Redefinition, 
approach1 Redefinition. 
ENDCLASS. 

CLASS class_procedural Implementation. 
Method prgm_type. 
Write:  Procedural programming . 

EndMethod. Method approach1. 
Write:  top-down approach . 

EndMethod. ENDCLASS. 
CLASS class_OO Definition 
Inheriting From class_prgm. 
PUBLIC Section. 
Methods: prgm_type Redefinition, 
approach1 Redefinition. 
ENDCLASS. 

CLASS class_OO Implementation. 
Method prgm_type. 
Write:  Object oriented programming . 
EndMethod. 

Method approach1. 
Write:  bottom-up approach .
EndMethod. 
ENDCLASS. 

CLASS class_type_approach Definition. 
PUBLIC Section. 
CLASS-METHODS: 
start Importing class1_prgm 
Type Ref To class_prgm. 
ENDCLASS. 

CLASS class_type_approach IMPLEMENTATION. 
Method start. 
CALL Method class1_prgm→prgm_type. 
Write:  follows . 

CALL Method class1_prgm→approach1. 
EndMethod. 
ENDCLASS. 

Start-Of-Selection. 
Data: class_1 Type Ref To class_procedural, 
class_2 Type Ref To class_OO. 

Create Object class_1. 
Create Object class_2. 
CALL Method class_type_approach⇒start 
Exporting 

class1_prgm = class_1. 
New-Line. 
CALL Method class_type_approach⇒start 
Exporting 
class1_prgm = class_2.  

The above code produces the following output −

Procedural programming follows top-down approach  
Object oriented programming follows bottom-up approach

ABAP run-time environment performs an imppcit narrowing cast during the assignment of the importing parameter class1_prgm. This feature helps the start method to be implemented generically. The dynamic type information associated with an object reference variable allows the ABAP run-time environment to dynamically bind a method call with the implementation defined in the object pointed to by the object reference variable. For instance, the importing parameter class1_prgm for method start in the class_type_approach class refers to an abstract type that could never be instantiated on its own.

Whenever the method is called with a concrete sub class implementation such as class_procedural or class_OO, the dynamic type of the class1_prgm reference parameter is bound to one of these concrete types. Therefore, the calls to methods prgm_type and approach1 refer to the implementations provided in the class_procedural or class_OO sub classes rather than the undefined abstract implementations provided in class class_prgm .