composition_over_inheritance

Discussion and solutions to "composition over inheritance"

https://keremkoseoglu.wordpress.com/2024/01/08/suggestions-for-better-object-oriented-design/

Chapter "Avoid inheritance"

goal

Build a realistic example that has nothing to with animals or flight data

show and discuss solutions with these paterns:

• inheritance 01
• composition
• servant

further changes

the solutions might stand the following changes:

• new method get_revenue_by_country

know inadequacies

the methods concerning the documents should be encapsuled in a separate class that manages the table access and actions. But this would make the example even more complex.

example

The following assumptions are made to define an example that is on one hand simple enough to understand and on the other hand complex enough to show the advantages and disadvantages of a proposed solution.

There is an organizational object like "area". There are objects and structures linked to this area. The objects are abstract to keep the example simple.

the following actions can be executed to an area object

• get_name display the name of the area

• get_documents return all documents assigned to this area

• add_document add a document to the area

• delete_document remove a document

• get_responsible_person

• get_assigned_countries

• display display all information needed to this area

• Factory method to create an instance for each area

report for testing

PROG ZCOI01_AREA_DEMO01: Enter area code and execute. The report will create an instance of the desired area class, add some documents, call the display method and list all documents.

general actions

get_Documents

reads documents from table documents for specified area

add_document

adds document to table with specific area

delete_document

deletes the specified document

get_assigned_countries

returns the specified countries.

display

display:

• area name
• responsible persons
• assigned countries
• number of documents

specific actions

area A

Name Avaloria

assigned countries: DE, CH, AT

Responsible person: Aaron

get_documents: only documents having status released

Display: additional display of number of documents status

• released
• open

area B

Name Brythoria

Assigned countries: ES, PT, FR

delete_document not allowed in FR due to legal reasons

Responsible person: Bernard

Display: Display telephone number of responsible person

area C

Name Celestria

Assigned countries: PL, RO

Responsible person: Carolina

add_documents: force status draft. only allowed if there are no other documents with status draft

area D

Name Draconia

Assigned countries: IT, GR

Responsible person: Dania

Documents: has always two templates in status released:

• template for technician
• template for customer

organization

there are 4 areas: A, B, C and D

structures

document

• doc_number (key)
• title
• status
  • open
  • released
  • draft