Classes, Interfaces and Polymorphism

Explanation

Prefer composition — a class holding an interface reference — over deep inheritance. Interfaces (IF_…) are the ABAP contract mechanism and let you inject test doubles. Polymorphism lets a single caller work with any implementation of the interface, enabling strategies like payment provider selection.

Code example

INTERFACE if_payment.  METHODS charge IMPORTING iv_amount TYPE p RAISING zcx_payment.ENDINTERFACE. CLASS lcl_stripe DEFINITION.  PUBLIC SECTION. INTERFACES if_payment.ENDCLASS.

Real project scenario

A billing engine used inheritance for every provider variant. Switching to an interface + factory removed 12 subclasses and simplified testing.

Common mistakes

Deep inheritance for reuse; leaking implementation types through public methods; missing FINAL/ABSTRACT modifiers.

Best practices

Program to interfaces; keep classes FINAL by default; expose only interface types from factories.

Interview angle

When is inheritance the right choice over composition?