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Lesson 6 — Object References & Inheritance ​

Inheritance models a generalization/specialization relationship.

text
          Product
          /    \
       Book    Movie

A Book is a Product, but has additional Book-specific state/behavior.

Learning objectives ​

  • declare reference variables with REF TO;
  • understand static reference type vs actual object type;
  • create a superclass;
  • create subclasses with INHERITING FROM;
  • use PROTECTED SECTION deliberately;
  • call super->constructor;
  • reuse inherited public behavior;
  • decide whether inheritance fits an “is-a” relationship.

1. Create the superclass ​

Create:

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ZCL_PRODUCT_BASE_###

Do not mark it FINAL, because it must allow subclasses.

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CLASS zcl_product_base_### DEFINITION
  PUBLIC
  CREATE PUBLIC.

  PUBLIC SECTION.
    METHODS constructor
      IMPORTING price TYPE decfloat34.

    METHODS get_price
      RETURNING VALUE(price) TYPE decfloat34.

  PROTECTED SECTION.
    DATA price TYPE decfloat34.

ENDCLASS.

2. Why protected? ​

PROTECTED SECTION is visible inside:

text
the class itself
its subclasses

but not to arbitrary external callers.

Use protected state carefully; private state provides stronger encapsulation. Here it is useful to demonstrate subclass access.

3. Implement superclass constructor ​

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METHOD constructor.
  IF price >= 0.
    me->price = price.
  ELSE.
    me->price = 0.
  ENDIF.
ENDMETHOD.

4. Create Book subclass ​

Create:

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ZCL_BOOK_###

Definition:

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CLASS zcl_book_### DEFINITION
  PUBLIC
  INHERITING FROM zcl_product_base_###
  FINAL
  CREATE PUBLIC.

Book-specific private state:

text
title
author

5. Subclass constructor ​

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METHODS constructor
  IMPORTING
    price  TYPE decfloat34
    title  TYPE string
    author TYPE string.

Implementation:

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METHOD constructor.
  super->constructor( price = price ).
  me->title = title.
  me->author = author.
ENDMETHOD.

super->constructor initializes the superclass portion of the object.

6. Create Movie subclass ​

Create:

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ZCL_MOVIE_###

It inherits Product price behavior and adds:

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title
director
category

7. REF TO ​

Reference variable:

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DATA product_ref TYPE REF TO zcl_product_base_###.

It can point to a Product object or an object of a compatible subclass.

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product_ref = NEW zcl_book_###(
  price  = '15.00'
  title  = 'ABAP Basics'
  author = 'A. Developer'
).

8. Static type vs dynamic type ​

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Reference variable static type:
  REF TO ZCL_PRODUCT_BASE_###

Actual object dynamic type:
  ZCL_BOOK_###

Through the superclass-typed reference, only components visible through the static type can be called directly.

9. Inherited methods ​

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out->write( product_ref->get_price( ) ).

get_price was defined in Product, but the Book object inherits it.

10. “Is-a” test ​

Inheritance is appropriate when the specialized object can meaningfully be used wherever the superclass is expected.

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Book is a Product      reasonable
Movie is a Product     reasonable
Inventory is a Product no

Do not use inheritance merely to reuse code between unrelated concepts.

11. Summary ​

You now know:

text
REF TO
static reference type
dynamic object type
superclass
subclass
INHERITING FROM
PROTECTED SECTION
super->constructor
inherited behavior

Lesson 7 uses this hierarchy to introduce polymorphism and method redefinition.