This code can be helpful to get fields names of an Internal Table at Runtime.

 
report  ztest_internal_tab_comp line-size 100.type-pools: abap.
 *Declare the type of the internal table
types: begin of x_final,
 matnr type matnr,
 werks type werks_d,
 flag type c length 1,
 value type p length 10 decimals 2,end of x_final.
 data:*The Internal table whose components are to found
i_data type sorted table of x_final with unique key matnr,*Table to hold the components
tab_return type abap_compdescr_tab,*Work area for the component table
components like line of tab_return.
 *Call Perform to get the Int. Table Components
perform get_int_table_fields using    i_data
                            changing  tab_return.*Display Components
loop at tab_return into components.
  write: / components-name,  components-type_kind,
           components-length,components-decimals.endloop.
 
 
 
 *&---------------------------------------------------------------------*
*&      Form  get_int_table_fields
*&---------------------------------------------------------------------*
*       Get the Components of an internal table
*----------------------------------------------------------------------*
*      -->T_DATA     text
*      -->T_RETURN   text
*----------------------------------------------------------------------*
form get_int_table_fields  using    t_data type any table
                           changing t_return type abap_compdescr_tab.
 
  data:
  oref_table type ref to cl_abap_tabledescr,
  oref_struc type ref to cl_abap_structdescr,
  oref_error type ref to cx_root,
  text type string.*Get the description of data object type
  try.
      oref_table ?=
      cl_abap_tabledescr=>describe_by_data( t_data ).
    catch cx_root into oref_error.
      text = oref_error->get_text( ).
      write: / text.
      exit.
  endtry.*Get the line type
  try.
      oref_struc ?= oref_table->get_table_line_type( ).
    catch cx_root into oref_error.
      text = oref_error->get_text( ).
      write: / text.
      exit.
  endtry.
 
  append lines of oref_struc->components to t_return.
 endform.                    " GET_INT_TABLE_FIELDS
 
 
Serkan AKKAVAK
Computer Engineer
ABAP Developer & SAP MM SD Consultant
Contact : serkurumsal@yandex.com