Interface attributes can be initiated with values at the time of Interface declaration in any class definition. 

REPORT zinterface_valueassign.

*----------------------------------------------------------------------*
*       INTERFACE inf1
*----------------------------------------------------------------------*
*
*----------------------------------------------------------------------*
INTERFACE inf1.
  DATA: v_txt1 TYPE char40,
        v_txt2 TYPE char40.
  METHODS mit.
ENDINTERFACE.                    "inf1

*----------------------------------------------------------------------*
*       CLASS cls DEFINinf1ION
*----------------------------------------------------------------------*
*
*----------------------------------------------------------------------*
CLASS cls DEFINITION.
  PUBLIC SECTION.
* Value assignment to the inf1erface attribute
    INTERFACES inf1 DATA VALUES
      v_txt1 = 'SAP ABAP'
      v_txt2 = 'Object Oriented'.
ENDCLASS.                    "cls DEFINITION

*----------------------------------------------------------------------*
*       CLASS cls IMPLEMENTATION
*----------------------------------------------------------------------*
*
*----------------------------------------------------------------------*
CLASS cls IMPLEMENTATION.
  METHOD inf1~mit.
    WRITE: / inf1~v_txt1,
           / inf1~v_txt2.
  ENDMETHOD.                    "inf1~minf1
ENDCLASS.                    "cls IMPLEMENTATION

* Start of selection
START-OF-SELECTION.

* Data Declaration
  DATA obj TYPE REF TO cls.

* Object Creation
  CREATE OBJECT obj.
  CALL METHOD obj->inf1~mit.

Output

If we give values like class attributes then compile error will come as follows: