ABAP OOP UpCasting And DownCasting

REPORT zcl_test.
CLASS a1 DEFINITION.
PUBLIC SECTION.
DATA: num1 TYPE i VALUE 100.
METHODS:m1.
ENDCLASS.

CLASS a1 IMPLEMENTATION.
METHOD m1.
WRITE: 'a1:',num1.
ENDMETHOD.
ENDCLASS.

CLASS b1 DEFINITION INHERITING FROM a1.
PUBLIC SECTION.
METHODS:m2, m1 REDEFINITION.
ENDCLASS.

CLASS b1 IMPLEMENTATION.
METHOD m1.
num1 = num1 .
WRITE: 'b1:',num1.
ENDMETHOD.

METHOD m2.
WRITE: 'M2 in class b1'.
ENDMETHOD.
ENDCLASS.

START-OF-SELECTION.

DATA: a TYPE REF TO a1.
DATA: b TYPE REF TO b1.
data: c type REF TO c1.


****************************************************************
CREATE OBJECT b.
a = b. "upcasting
CALL METHOD a->m1( ).
*call METHOD a->m2( ). " we can't access the own sub class methods using super class ref.
NEW-LINE.

b ?= a. " down casting
CALL METHOD b->m1( ).
NEW-LINE.
CALL METHOD b->m2( ).

*****************************************************************
" error null ref
*create OBJECT a.
*b ?= a. "down casting still its giving dump
*call METHOD b->m1( ).

*****************************************************************
*
*CREATE OBJECT a.
*TRY.
* b ?= a. "u r attempted to use a 'NULL' object reference dump
*CATCH cx_sy_move_cast_error.
* CALL METHOD b->m1( ).
*ENDTRY.

*****************************************************************

*CREATE OBJECT a.
*CREATE OBJECT b.
*TRY.
* b ?= a.
*CATCH cx_sy_move_cast_error.
* CALL METHOD b->m1( ).
* call METHOD b->m2( ).
*ENDTRY.

*****************************************************************
" creating a object for sub class eg: 'b' and assigining sub class object
" towards super class reference eg: 'c'