Abstract Class

• Defined with CLASS ... DEFINITION ABSTRACT.
• Can have:
  • Attributes (data)
  • Concrete methods (implemented)
  • Abstract methods (declared but not implemented — must be redefined in subclass)
• Cannot be instantiated directly — only subclasses can be created.
• Supports inheritance (a subclass inherits implementation + attributes).
• Can implement interfaces too.

Think of an abstract class as a partially implemented blueprint.

Example :

CLASS cl_vehicle DEFINITION ABSTRACT.
PUBLIC SECTION.
METHODS: start ABSTRACT, ” must be implemented in subclass
stop. ” already implemented
ENDCLASS.

CLASS cl_vehicle IMPLEMENTATION.
METHOD stop.
WRITE: / ‘Vehicle stopped.’.
ENDMETHOD.
ENDCLASS.

Interface

• Defined with INTERFACE ....
• Can contain:
• Method declarations only (no implementation).
• Constants, types, attributes (but no data storage).
• No concrete code inside (implementation happens in the class that implements it).
• A class can implement multiple interfaces (like multiple inheritance).
• Cannot be instantiated or inherited — only implemented.

Think of an interface as a contract: “any class implementing me must provide these methods.”

Example:

INTERFACE if_flyable.
METHODS fly.
ENDINTERFACE.

INTERFACE if_swimmable.
METHODS swim.
ENDINTERFACE.

Key Differences:

When to Use

• Use an abstract class when:

  • You want to share implementation across subclasses.

  • You want to define default behavior that can be reused.

  • Example: CL_VEHICLE (base) → CL_CAR, CL_TRUCK.

• Use an interface when:

  • You just need a common contract across unrelated classes.

  • You want to achieve multiple inheritance of behavior.

  • Example: IF_FLYABLE, IF_SWIMMABLE, IF_SERIALIZABLE.