OOABAP Wide Casting or Up Casting

OOABAP Concept Wide Casting or Up Casting

Definition: - The assignment of a super class instance to a reference variable of the type "points to sub class" is described as a Wide casting, because here we are switching from a more generalized view to a one with more detail (Specialized View).

Note:- In order to implement wide casting first we need to implement the narrow casting.

      Steps:-

Here I am taking the example of calculation of area of rectangle and area of cube.

 a) Definition and implementation of Super Class:-

     CLASS class_super DEFINITION.
        PUBLIC SECTION.
          DATA:
             w_area TYPE i.
          METHODS:
             area IMPORTING w_length TYPE i 

                                           w_breadth TYPE i.
     ENDCLASS.                            "class_super DEFINITION
     CLASS class_super IMPLEMENTATION.
         METHOD area.
            w_area = w_length * w_breadth.
           WRITE:/ 'Area of rectangle = ',

                         w_area.
         ENDMETHOD.                    "area
     ENDCLASS.      

In the above picture the class class_super is the super class and has one method area which is to find the area of rectangle. The method has two parameters w_length, w_breadth for length and breadth.

 b) Definition and implementation of sub class:-

    CLASS class_sub DEFINITION INHERITING FROM class_super.
        PUBLIC SECTION.
           DATA:
             w_height TYPE i.
           METHODS:
              area REDEFINITION.
   ENDCLASS.                            "class_sub DEFINITION
  CLASS class_sub IMPLEMENTATION.
      METHOD area.
        w_area = 6 * w_length * w_breadth * w_height.
        WRITE:/ 'Area of the cube ='.
        WRITE: w_area.
      ENDMETHOD.                    "area
  ENDCLASS.                            "class_sub IMPLEMENTATION

In the above picture the class class_sub is the sub class which is inheriting from the super class class_super. This sub class has one method area which is same as that of super class and redefined for calculating the area of cube. This method has one more parameter W_height along with w_length and w_breadth.  This method finds the area of cube.

 c) Declaration of reference variables:-

  DATA:
      object_superclass   TYPE REF TO class_super,
      object_subclass      TYPE REF TO class_sub,
      object2_superclass TYPE REF TO class_super.

In above screen two reference variables have been declared for super class and one reference variable is declared for sub class.

 d) Creation of objects:-

  START-OF-SELECTION.
     CREATE OBJECT object_superclass.
     CREATE OBJECT object2_superclass.
     CREATE OBJECT object_subclass.

Now create the objects for the super and base class. Call the method Area of super class as below:

    CALL METHOD object_superclass->area
       EXPORTING
         w_length   = 10
         w_breadth = 5.

 e) Implementation of Narrow casting:- As I discussed in introduction in order to implement wide casting  we need to first implement narrow casting.

    "Narrow casting
      object_subclass->w_height = 10.
      object_superclass = object_subclass.

     CALL METHOD object_superclass->area
        EXPORTING
          w_length    = 10
          w_breadth = 10.

We are assigning the value ‘10’ to ‘w_height’ instance variable of subclass object.

In above picture we are assigning the sub class instance reference variable to super class instance reference variable. This is known as narrow casting.

Now the instance of the super class(object_superclass) now points or refers to the object of the subclass, thus the modified method 'area' of the subclass is accessible. Now we can call the method Area of subclass class_sub to calculate the area of cube. 

              

  f) Implementation of Wide casting:- From below screen shot we implement the wide casting.

  "Wide casting
     object_superclass ?= object2_superclass.

     CALL METHOD object_superclass->area
        EXPORTING
          w_length  = 10
          w_breadth = 5.

The object 'object_superclass' now refers to the object of the subclass 'object_subclass'. Thus the 'area' method of the superclass cannot be called by this object. In order to enable it to call the 'area' method of the superclass, wide casting has to be performed.

  "Wide casting
    object_superclass ?= object2_superclass.

So right hand side of wide casting always has object of super class. Left side of wide casting always is object reference declared as ‘type ref to’ to super class.  Now method area of super class is called again to find the area of rectangle.

Note:- Wide Casting cannot be performed on sub class objects. It can only be performed on super class objects that have narrow casting.

      Summary:-  The assignment of a super class instance to a reference variable of the type  "points to sub class" is described as a Wide casting, because here we are switching from a more generalized view to a one with more detail(Specialized View).

      Source Code:- 

REPORT  yh_wide NO STANDARD PAGE HEADING.CLASS class_super DEFINITION.
  PUBLIC SECTION.
    DATA:
      w_area TYPE i.
    METHODS:
      area IMPORTING w_length TYPE i w_breadth TYPE i.
ENDCLASS.                             "class_super DEFINITION
CLASS class_super IMPLEMENTATION.
  METHOD area.
    w_area = w_length * w_breadth.

    WRITE:/ 'Area of rectangle = '.
    WRITE: w_area.
  ENDMETHOD.                      "area
ENDCLASS.                            "class_super IMPLEMENTATION
CLASS class_sub DEFINITION INHERITING FROM class_super.
  PUBLIC SECTION.
    DATA:
      w_height TYPE i.
    METHODS:
      area REDEFINITION.
ENDCLASS.                          "class_sub DEFINITION
CLASS class_sub IMPLEMENTATION.
  METHOD area.
    w_area = 6 * w_length * w_breadth * w_height.
    WRITE:/ 'Area of the cube =',
                  w_area.
  ENDMETHOD.                    "area
ENDCLASS.                          "class_sub IMPLEMENTATION

  DATA:
    object_superclass   TYPE REF TO class_super,
    object_subclass      TYPE REF TO class_sub,
    object2_superclass TYPE REF TO class_super.

START-OF-SELECTION.
  CREATE OBJECT object_superclass.
  CREATE OBJECT object2_superclass.
  CREATE OBJECT object_subclass.CALL METHOD object_superclass->area
    EXPORTING
      w_length    = 10
      w_breadth  = 5.
"Narrow casting
  object_subclass->w_height = 10.
  object_superclass = object_subclass.CALL METHOD object_superclass->area
   EXPORTING
      w_length     = 10
      w_breadth  = 10.
"Wide casting
object_superclass ?= object2_superclass.CALL METHOD object_superclass->area
   EXPORTING
      w_length   = 10
      w_breadth = 5.

Output:-