Modularization Part 6: Methods and Local Classes

📌 KEY POINTS
✅ A class has a DEFINITION (what) and an IMPLEMENTATION (how).
✅ Static methods (CLASS-METHODS) are called via class=>method; instance methods via object->method after creating an object.
✅ Visibility sections PUBLIC / PROTECTED / PRIVATE hide internal details.
💡 For report logic, one local class with small methods is the modern replacement for FORM routines.

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▶ WHERE WE ARE
Parts 1–5 covered classic units. Classes are the modern way to modularize — this part is the practical introduction; the OOP ABAP topic goes deeper (inheritance, interfaces, polymorphism).

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▶ 1. DEFINITION AND IMPLEMENTATION
CLASS lcl_flight_service DEFINITION.

PUBLIC SECTION.

METHODS constructor IMPORTING iv_carrid TYPE s_carr_id.
METHODS get_flights RETURNING VALUE(rt_flights) TYPE ty_flights.

PRIVATE SECTION.

DATA mv_carrid TYPE s_carr_id.

ENDCLASS.

CLASS lcl_flight_service IMPLEMENTATION.
METHOD constructor. mv_carrid = iv_carrid. ENDMETHOD.
METHOD get_flights. SELECT … WHERE carrid = mv_carrid. ENDMETHOD.

ENDCLASS.

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▶ 2. INSTANCE VS STATIC

• Instance: each object has its own attributes (mv_carrid). Create with CREATE OBJECT lo_service EXPORTING iv_carrid = 'LH'. or NEW lcl_flight_service( 'LH' ).
• Static: CLASS-METHODS / CLASS-DATA, one per class. Good for stateless helpers (conversions, calculations).
• Use instance methods when the object holds state or must be replaceable in tests.

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▶ 3. CALLING METHODS

• Classic: CALL METHOD lo_service->get_flights RECEIVING rt_flights = lt_flights.
• Modern functional: lt_flights = lo_service->get_flights( ).
• With several parameters: lo->m( EXPORTING a = 1 IMPORTING b = lv_b ).

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▶ 4. CONSTRUCTORS

• constructor — runs when an object is created; initializes attributes.
• class_constructor — runs once before first use of the class (static initialization).

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▶ 5. ATTRIBUTES AND NAMING
Common prefixes: mv_/mt_ (member variable/table), gv_ for globals, iv_/ev_/cv_/rv_ for parameters. Follow your team's convention.

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▶ 6. FROM FORMS TO A CLASS
Each FORM becomes a method; global variables become private attributes or parameters; START-OF-SELECTION creates the object and calls run( ).

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▶ STEP BY STEP
Run the code example: a local flight service class with constructor, instance method, static helper and functional calls.

✅ CHECKPOINT: you can build and use a simple class instead of FORM routines.

A team's reports used dozens of FORM routines sharing global tables. When two reports needed the same "flight availability" logic, it was copied, and the copies drifted apart.
They moved the logic into a small global class ZCL_FLIGHT_SERVICE with instance methods for data access and static helpers for calculations. Both reports used the same class; a later rule change was made once and verified with unit tests.

• Making everything static even when state or test replacement is needed.
• Public attributes that anyone can change.
• Huge methods that are just FORMs renamed.
• Forgetting to create the object before calling instance methods (null reference dump).
• Mixing CALL METHOD and functional style inconsistently.

• Small methods with clear parameters.
• Private attributes, public methods.
• Static methods for stateless helpers; instances for stateful services.
• Functional call style for readability.
• Constructors to initialize required state.

Questions: "Instance vs static method?", "What is a constructor?", "How do you replace FORMs with classes?"
Strong answer: instance methods work on an object's attributes and need an object (CREATE OBJECT/NEW), static methods belong to the class (class=>method) and suit stateless helpers; constructor initializes objects, class_constructor the class; replace FORMs by methods of a local class with parameters and private attributes, calling run( ) from START-OF-SELECTION.