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Don't mix stateful and stateless in the same class

Don’t mix the stateless and the stateful programming paradigms in the same class.

In stateless programming, methods get input and produce output, without any side effects, resulting in methods that produce the same result no matter when and in what order they are called.

CLASS /clean/xml_converter DEFINITION PUBLIC FINAL CREATE PUBLIC.
  PUBLIC SECTION.
    METHODS convert
      IMPORTING
        file_content  TYPE xstring
      RETURNING
        VALUE(result) TYPE /clean/some_inbound_message.
ENDCLASS.

CLASS /clean/xml_converter IMPLEMENTATION.
  METHOD convert.
    cl_proxy_xml_transform=>xml_xstring_to_abap(
      EXPORTING
        xml       = file_content
        ext_xml   = abap_true
        svar_name = 'ROOT_NODE'
      IMPORTING
        abap_data = result ).
   ENDMETHOD.
ENDCLASS.

In stateful programming, we manipulate the internal state of objects through their methods, meaning it is full of side effects.

CLASS /clean/log DEFINITION PUBLIC CREATE PUBLIC.
  PUBLIC SECTION.
    METHODS add_message IMPORTING message TYPE /clean/message.
  PRIVATE SECTION.
    DATA messages TYPE /clean/message_table.
ENDCLASS.

CLASS /clean/log IMPLEMENTATION.
  METHOD add_message.
    INSERT message INTO TABLE messages.
  ENDMETHOD.
ENDCLASS.

Both paradigms are okay and have their applications. However, mixing them in the same object produces code that is hard to understand and sure to fail with obscure carry-over errors and synchronicity problems. Don’t do that.

From SAP/styleguides under CC BY 3.0 — more details
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Prefer composition to inheritance