CHECK vs. RETURN

There is no consensus on whether you should use CHECK or RETURN to exit a method if the input doesn’t meet expectations.

While CHECK definitely provides the shorter syntax

METHOD read_customizing.
  CHECK keys IS NOT INITIAL.
  " do whatever needs doing
ENDMETHOD.

the statement’s name doesn’t reveal what happens if the condition fails, such that people will probably understand the long form better:

METHOD read_customizing.
  IF keys IS INITIAL.
    RETURN.
  ENDIF.
  " do whatever needs doing
ENDMETHOD.

You could avoid the question completely by reversing the validation and adopting a single-return control flow. This is considered to be an anti-pattern because it introduces unnecessary nesting depth.

METHOD read_customizing.
  " anti-pattern
  IF keys IS NOT INITIAL.
    " do whatever needs doing
  ENDIF.
ENDMETHOD.

In any case, consider whether returning nothing is really the appropriate behavior. Methods should provide a meaningful result, meaning either a filled return parameter, or an exception. Returning nothing is in many cases similar to returning null, which should be avoided.

The section Exiting Procedures in the ABAP Programming Guidelines recommends using CHECK in this instance. Community discussion suggests that the statement is so unclear that many people will not understand the program’s behavior.