code: public static void checkDirectoryArgument(File fileName, String variableName) {
      checkNullArgument(fileName, variableName);
      if (!fileName.isDirectory()) {
        throw new IllegalArgumentException(ExceptionMessageMap.getMessage("000007")
            + "  { variableName=[" + variableName + "] }");
      }
    }
docs: Checks a <code>File</code> argument to make sure that it is a directory. If not
  <code>IllegalArgumentException</code> is thrown.
  
  @param fileName
  The <code>File</code> to be checked.
  @param variableName
  The name of the variable being checked; for use in exception messages.