An interface can only be declared in the public section of any class. Any other section will cause a syntax error in the system.

Here is a program where we have declared an Interface it. We have defined attribute and method in this interface. Next, we have defined a class where we have declared the interface in the protected section. After implementing the method we compile the program and a syntax error has come as follows. 

Now Comment the protected section and execute the program

REPORT zinterface_public.

*----------------------------------------------------------------------*
*       INTERFACE it
*----------------------------------------------------------------------*
*
*----------------------------------------------------------------------*
INTERFACE it.
  DATA v_txt TYPE char50.
  METHODS meth.
ENDINTERFACE.                    "it

*----------------------------------------------------------------------*
*       CLASS cls DEFINITION
*----------------------------------------------------------------------*
*
*----------------------------------------------------------------------*
CLASS cls DEFINITION.
  PUBLIC SECTION.
    DATA date TYPE sy-datum.
*  PROTECTED SECTION.
    INTERFACES it.
ENDCLASS.                    "cls DEFINITION

*----------------------------------------------------------------------*
*       CLASS cls IMPLEMENTATION
*----------------------------------------------------------------------*
*
*----------------------------------------------------------------------*
CLASS cls IMPLEMENTATION.
  METHOD it~meth.
    date = sy-datum.
    WRITE: / it~v_txt,
           / 'Today''s Date is', date DD/MM/YYYY.
  ENDMETHOD.                    "it~meth
ENDCLASS.                    "cls IMPLEMENTATION

START-OF-SELECTION.
  DATA obj TYPE REF TO cls.
  CREATE OBJECT obj.

  obj->it~v_txt = 'SAP ABAP Object Oriented'.
  CALL METHOD obj->it~meth.

Output