Strategy Design Pattern in ABAP

Purpose

This is a series that will investigate various design patterns and how they can be used in ABAP Objects.

History

After reading the excellent series of articles by Andreas Blumenthal and Horst Keller, I decided that my next project would be written using ABAP Objects.  At the same time, I am learning about design patterns to help me better design and maintain my programs.  As I read about a pattern, I try to think about the practical applications of the pattern.  Then I try to find some code that I can rewrite with this application in mind.

I thought that I would share my experience with you so that you could share your experience with me.  As I am just learning about ABAP objects and design patterns, perhaps I am not applying the principles properly, or someone sees something that would be better.  I hope that you enjoy the series and that we can all learn from each other.

Pattern

The Strategy pattern defines a family of algorithms, encapsulates each one and makes them interchangeable.  Strategy lets the algorithm vary independently from clients that use it.(1)

Guiding Principles

Encapsulate the concept that varies.  Program to an interface, not an implementation.

Actions

In order to implement the strategy pattern you need to follow these steps:

• Implement a Strategy interface for your strategy objects.
• Implement ConcreteStrategy classes that implement the Strategy interface.
• In the Context class, maintain a private reference to a Strategy object.
• In the Context class, implement the methods for the Strategy object.

In my case I have a file that comes from our cash register machines.  The file has a header line and various record lines.  The data on the record line varies.  Some record lines show sales by the hour.  Some show sales by clerk.  Others show the materials sold.  And still other lines hold information on the machine itself, like its name and the date and time of the close out.

This file needs to be loaded into SAP so that sales orders can be created and the revenue recognized.  Following the first guiding principle, I figured that this would be the perfect candidate for the strategy pattern, where the record line would be the context and the parsing logic and data upload could be handled in the ConcreteStrategy classes.  I could go further, because each record line then has various fields with different field types.  But I think this is enough for my poor brain for now.

Here are two UML diagrams.  One shows a generic implementation of the strategy pattern(2), and the other shows how it is used with the file upload.

UML Diagrams

Coding

Here is the ABAP Code to implement the strategy.

  * 1. Implement a Strategy interface for your strategy objects.   INTERFACE zif_upload_strategy.     METHODS: read_data.   ENDINTERFACE.                    “zif_upload_strategy    

  * 2. Implement ConcreteStrategy classes that implement the Strategy interface.   *———————————————————————*   *       CLASS zcl_headerline   *———————————————————————*   *   *———————————————————————*   CLASS zcl_headerline DEFINITION.     PUBLIC SECTION.       INTERFACES zif_upload_strategy.   ENDCLASS.                    “zcl_headerline       *———————————————————————*   *       CLASS zcl_ecr_information DEFINITION   *———————————————————————*   *   *———————————————————————*   CLASS zcl_ecr_information DEFINITION.     PUBLIC SECTION.       INTERFACES zif_upload_strategy.   ENDCLASS.                    “zcl_ecr_information DEFINITION       *———————————————————————*   *       CLASS zcl_plu_by_clerk DEFINITION   *———————————————————————*   *   *———————————————————————*   CLASS zcl_plu_by_clerk DEFINITION.     PUBLIC SECTION.       INTERFACES zif_upload_strategy.   ENDCLASS.                    “zcl_plu_by_clerk DEFINITION   *———————————————————————*   *       CLASS zcl_headerline IMPLEMENTATION   *———————————————————————*   *   *———————————————————————*   CLASS zcl_headerline IMPLEMENTATION.     METHOD zif_upload_strategy~read_data.       ” Parse the data according to the header line structure       ” Upload the header line into a header table     ENDMETHOD.                    “zif_upload_strategy~read_data   ENDCLASS.                    “zcl_headerline IMPLEMENTATION       *———————————————————————*   *       CLASS zcl_ecr_informatin IMPLEMENTATION   *———————————————————————*   *   *———————————————————————*   CLASS zcl_ecr_information IMPLEMENTATION.     METHOD zif_upload_strategy~read_data.       ” Parse the data according to the cash register line structure       ” Upload the cash register line into a record table     ENDMETHOD.                    “zif_upload_strategy~read_data   ENDCLASS.                    “zcl_ecr_informatin IMPLEMENTATION       *———————————————————————*   *       CLASS zcl_plu_by_clerk IMPLEMENTATION   *———————————————————————*   *   *———————————————————————*   CLASS zcl_plu_by_clerk IMPLEMENTATION.     METHOD zif_upload_strategy~read_data.       ” Parse the data according to the PLU by Clerk line structure       ” Upload the PLU by Clerk line into a record table     ENDMETHOD.                    “zif_upload_strategy~read_data   ENDCLASS.                    “zcl_plu_by_clerk IMPLEMENTATION    

  * 3. In the Context class, maintain a private reference to a Strategy object.   CLASS zcl_file_line DEFINITION.     PRIVATE SECTION.       DATA: file_line         TYPE REF TO zif_upload_strategy,             next_line_type    TYPE char2,             header_line       TYPE char2 VALUE ’00’,             ecr_line          TYPE char2 VALUE ’02’,             plu_by_clerk_line TYPE char2 VALUE ’05’.       METHODS: read_next_line.   ENDCLASS.                    “zcl_file_line DEFINITION    

  * 4. In the Context class, implement the methods for the Strategy object.   *———————————————————————*   *       CLASS zcl_file_line IMPLEMENTATION   *———————————————————————*   *   *———————————————————————*   CLASS zcl_file_line IMPLEMENTATION.     METHOD read_next_line.       CASE next_line_type.         WHEN header_line.           CREATE OBJECT file_line TYPE zcl_headerline.           file_line->read_data( ).         WHEN ecr_line.           CREATE OBJECT file_line TYPE zcl_ecr_information.           file_line->read_data( ).         WHEN plu_by_clerk_line.           CREATE OBJECT file_line TYPE zcl_plu_by_clerk.           file_line->read_data( ).       ENDCASE.     ENDMETHOD.                    “read_next_line   ENDCLASS.                    “zcl_file_line IMPLEMENTATION

Bibliography

(1)Head First Design Patterns, Eric Freeman and Elisabeth Freeman

(2)DO Factory (http://www.dofactory.com/Patterns/PatternStrategy.aspx)