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Lesson 7 — Polymorphism & Method Redefinition ​

Polymorphism lets one reference type work with several concrete object types.

text
REF TO Product
      │
      ├── Book object
      └── Movie object

When a subclass redefines a method, ABAP executes the implementation belonging to the actual object.

Learning objectives ​

  • declare redefinable superclass methods;
  • use REDEFINITION in subclasses;
  • explain dynamic dispatch;
  • use superclass references for subclass objects;
  • create internal tables of superclass references;
  • process heterogeneous objects uniformly;
  • call super->method when extending superclass behavior.

1. Add a polymorphic method to Product ​

In ZCL_PRODUCT_BASE_### add:

abap
METHODS get_description
  RETURNING VALUE(description) TYPE string.

Implementation:

abap
METHOD get_description.
  description = |Generic product - Price: { price }|.
ENDMETHOD.

2. Redefine in Book ​

In ZCL_BOOK_###:

abap
METHODS get_description REDEFINITION.

Implementation:

abap
METHOD get_description.
  description = |Book: { title } by { author } - Price: { price }|.
ENDMETHOD.

The method signature cannot be changed by the redefinition.

3. Redefine in Movie ​

abap
METHODS get_description REDEFINITION.

Return movie-specific content.

4. Dynamic dispatch ​

abap
DATA item TYPE REF TO zcl_product_base_###.

item = NEW zcl_book_###( ... ).
out->write( item->get_description( ) ).

Even though item is typed as Product, ABAP calls Book's redefined implementation because the actual object is a Book.

5. Reassign the same reference ​

abap
item = NEW zcl_movie_###( ... ).
out->write( item->get_description( ) ).

Now the Movie implementation runs.

6. Table of superclass references ​

abap
TYPES tt_products
  TYPE STANDARD TABLE OF REF TO zcl_product_base_###
  WITH EMPTY KEY.

Create:

abap
DATA products TYPE tt_products.

APPEND NEW zcl_book_###( ... ) TO products.
APPEND NEW zcl_movie_###( ... ) TO products.

7. Polymorphic loop ​

abap
LOOP AT products INTO DATA(product).
  out->write( product->get_description( ) ).
ENDLOOP.

One loop, one method call, different implementations.

8. super->method ​

A redefined method can reuse superclass behavior:

abap
METHOD get_description.
  DATA(base) = super->get_description( ).
  description = |{ base }; Book: { title }|.
ENDMETHOD.

Use this when the subclass wants to extend rather than fully replace behavior.

9. Why polymorphism matters ​

Without polymorphism:

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IF object is Book ...
ELSEIF object is Movie ...

With polymorphism:

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product->get_description( )

The object decides the implementation.

10. Summary ​

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REDEFINITION
dynamic dispatch
superclass reference
subclass object
table of references
polymorphic loop
super->method

Lesson 8 introduces interfaces: polymorphism based on a contract rather than a shared superclass.