OOP ABAP Part 8: Interfaces and Factory Methods

📌 KEY POINTS
✅ A factory method creates and returns objects typed as an interface; callers never write CREATE OBJECT for the concrete class.
✅ The factory decides which implementation to create (by parameter, customizing or country).
✅ CREATE PRIVATE + static factory = the only way to get an instance (used for singletons).
⚠️ Singletons keep state for the whole session — use them only for truly session-wide objects.

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▶ WHERE WE ARE
Parts 3 and 7 created implementations directly in the report. Real programs should not know all concrete classes — a factory hides that decision.

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▶ 1. WHY FACTORIES

• Callers depend only on the interface (ZIF_…).
• Adding an implementation changes only the factory.
• Creation logic (validation, reading customizing, caching) lives in one place.
• Tests can make the factory return a test double (Part 9).

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▶ 2. STATIC FACTORY METHOD
CLASS-METHODS create IMPORTING iv_country TYPE land1 RETURNING VALUE(ro_tax) TYPE REF TO zif_tax_calculator.
Inside: CASE iv_country → CREATE OBJECT ro_tax TYPE zcl_tax_in / zcl_tax_de / zcl_tax_default.
Note the syntax CREATE OBJECT ref TYPE class — the reference is interface-typed, the object is concrete.

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▶ 3. CUSTOMIZING-DRIVEN SELECTION
Instead of a CASE, a Z-table maps country → class name. The factory reads it and uses dynamic CREATE OBJECT ro_tax TYPE (lv_classname). New countries need only a new class and a table entry. Catch CX_SY_CREATE_OBJECT_ERROR for wrong names.

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▶ 4. SINGLETON
A class with CREATE PRIVATE, a private static attribute go_instance and a static get_instance( ) that creates it once. Typical for session-wide buffers or loggers. Downsides: global state, harder tests.

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▶ 5. SAP'S OWN FACTORIES

• cl_salv_table=>factory( IMPORTING r_salv_table = … CHANGING t_table = … ) — ALV objects.
• GET BADI — the enhancement framework picks the active implementations.
• cl_abap_typedescr=>describe_by_data( ) — RTTI.
• cl_bcs=>create_persistent( ) — send requests.

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▶ 6. FACTORY VS DEPENDENCY INJECTION
A factory decides what to create; dependency injection (Part 9) decides who passes it in. They work together: the factory creates, the constructor receives.

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▶ STEP BY STEP

1. Run the code example with countries IN and DE.
2. Add a US class and extend only the factory.
3. Replace the CASE with a dynamic class name and try an invalid name.

💡 Modern (7.40+): ro_tax = NEW lcl_tax_in( ) for static creation; dynamic creation by class name still uses CREATE OBJECT … TYPE (lv_classname).

✅ CHECKPOINT: callers ask a factory for an interface, and implementations can be added without touching them.

A global rollout added country-specific e-invoicing: India, Germany and Mexico each needed different XML formats. The first version had IF country = … branches in the output program, and every new country meant changes and regression tests in shared code.
The team introduced ZIF_EINVOICE_BUILDER, one class per country and a factory reading a customizing table (country → class). Adding Italy later required one new class and one table entry transported separately — the main program was not touched.

• CREATE OBJECT of concrete classes scattered in callers.
• Factories returning concrete types instead of interfaces.
• Singletons for data that should be per document.
• Dynamic class names without catching CX_SY_CREATE_OBJECT_ERROR.
• Business logic inside the factory.

• Return interface references from factories.
• Keep the factory as the only place that knows implementations.
• Use customizing tables for extensible selection.
• CREATE PRIVATE for controlled instantiation.
• Use singletons sparingly and allow resetting them for tests.

Questions: "What is a factory method and why use it?", "How do you implement a singleton in ABAP?", "Examples of SAP factories?"
Strong answer: a static method that creates and returns an interface-typed object, hiding the concrete class; enables adding implementations without changing callers, often driven by customizing with dynamic CREATE OBJECT TYPE (name); singleton = CREATE PRIVATE, private static instance, get_instance; SAP examples: CL_SALV_TABLE=>FACTORY, GET BADI, CL_BCS=>CREATE_PERSISTENT.